Basic measurements with a Vector Network Analyser
In our wireless world the need of RF component testing is one of a key factors to bring a product to market. Devices are getting smaller and are containing more and more complex components. It is a must to have knowledge of complex impedance (or admittance) and reflection / transmission parameters to bring the most optimum functionality to the RF device. RF components like filters, resonators, etc. can be calculated according to capacitance and inductive values. Software simulators can take these values and help fine tune the design. But at the end of the day, the quality and performance needs to be measured. For several applications, a scalar network analyser might be adequate but for some specific design work phase information is required. A vector network analyser [VNA] has the possibility to measure amplitude and phase over specified frequency range.
The vector network analysis allows for the measurement of complex scattering parameter [Sxx] of a device under test [DUT] over a specified frequency range. Vector network analysis allows for the characterisation of a scattered matrix with reflection [S11] and transmission [S21] factors. These parameters are required to design e.g. a matching circuit for an amplifier. With phase information it is also possible to calculate the time range where additional failures at different positions can be analysed. Due to the complex (vector) characteristic it possible to make an accurate correction with calibration routines.
RIGOL’s VNA solution in RSA5000N and RSA3000N [RSAxN] series can perform three different measurements these include reflection [S11], transmission [S21] and Distance-To-Fault [DTF] measurements. All three of these measurements have several different views which allows engineers to easily determine a DUT’s frequency response, phase, SWR, Smith Charts and Polar Plane measurements. In figure 1 the principle of S-Parameter measurement is visible. These parameters can be calculated with the complex factors ax and bx. For example, a1 refers to the incident wave into the DUT and b1 refers to the reflected wave. The transmitted factor after DUT is referring to b2. At RSAxN version an incident wave can only be generated by port 1. Therefore, a2 is 0.
The principle of S Parameter Measurement in a network:

S11 Measurements
The reflection measurement is an important key to specifying the performance of complex systems (e.g. wireless communication system) Reflection factor r describes the ratio of incident and the reflected wave. There are several different tools that can be used to perform this measurement but one of the most useful tools is the Smith Chart because it contains the most information, like:
• Complex impedance and tools to determine how to match the (compensation of inductive / capacitive reactance)
• Complex reflection factor
• Impact of real / capacitance or inductive
• Influence of frequency range and displaying frequency response
• Q Factor of RF components
• Influence of the cable length
• Determination of cable loss
In RSAxN the Smith Chart can display impedance [Ω] (components in series) or admittance area [1/ Ω] (parallel connection of components). A universal Smith Chart is visible in figure 2. “Universal” means it can be used for each system impedance. In this example a 50 Ω reference is used (which can be modified to a different impedance, like 75 Ω if required). The reference is used to center the chart for better visualisation. A complex impedance of Z = 50 Ω + j25 Ω is transformed with that reference into 1 + j0,5 to make manual calculations easier. But in the end the calculation for real complex impedance has to be done after the measurement has been finalised. In RSAxN it is possible to measure the transformed values via a marker and display impedance value (in the example above: 50 Ω + j25 Ω).
Taking into consideration to have a serial connection of impedance, capacitance and inductivity, the impedance is calculated as follow:

In this formula it is visible that the inductive imaginary component is positive and capacitive imaginary component is negative. The lower half of Smith Chart is referring to capacitance and upper half is referring to inductance. On the outer diameter of Smith Chart, the length of line referring to Wavelength λ is displayed. On the Smith Chart it is visible that a turn of 360° results into 0.5 x l/λ. The second value which is visible on the outer diameter is the angle ϕ of a complex reflection factor r. There is a 100% reflection of incident wave with either an Open termination (right side of the chart; when the real and imaginary impedances are close to ∞ Ω) or with a Short termination (left side of the chart; when these impedances are closed to 0 Ω). In the center of Smith Chart the impedance of 50 Ω is visible. It is possible to measure out the complex reflection factor in Smith Chart, but it is easier to use a marker in the Polar Plane in RSAxN to get this value.

The Smith Chart and Polar Plane are useful tools to analyse complex impedance and reflection factor on a network for a specific frequency range. In figure 3 the capacitance in series with a resistor was measured at 541 MHz. The same configuration was measured again with a cable at ~16 cm. In this example it is visible that the impedance position is changing when using an additional cable at a specific frequency. The reflection factor remains very close to the origin point (cable has attenuation which has for this measurement a very small influence. As higher the cable attenuation, the more their has influence).

The marker on the Smith Chart calculates the correct impedance when using the reference of 50 Ω. Then the same configuration was tested again without the additional cable over a different frequency range (see figure 4).

In this measurement the frequency response curve is visible for the adjusted frequency range. With the marker it is visible that the curve is moving clockwise with increasing frequency. In RSAxN version different intermediate frequency bandwidth [IF BW] can be used for testing (1 kHz to 10 MHz in 1-3-1 steps) to realise the frequency resolution as required.
For complex networks one of the top uses is of the impedance to the network (here: to realise 50 Ω at network input) at the required center frequency. Different possibilities can be used in which can be shown on the Smith Chart.

First, with a series impedance of 20 Ohm can be set to 50 Ohm. As a next step an inductive component in series could be used to bring the impedance level to 50 Ω without an imaginary component (see figure 5). The problem of this theory is that the inductive component (in this case: 67 pH) is very small and hard to realise. Discrete inductive or capacitive elements can only be used for maximum frequency of several 100 MHz. For higher frequency ranges, different methods (e.g. microstrip solutions) needs to be used. One of the approaches might be using a serial 50 Ω stub to compensate the -j27 Ω (length of stub with short: l = 0,078 λ, with open: l = 0,328 λ). For the stub, the dielectric constant is required to evaluate the correct wavelength.
For S11 it is also possible to display return loss and (voltage) standing wave ratio [(V)SWR] over frequency range. If “V” is used, then the ratio is defined to voltage level of a standing wave at a line.
VSWR is referring to the maximum and minimum voltage values that is being transmitted and reflected by the component. The difference to reflection factor is, that there is no relation to phase.

For deeper analysis it is often necessary to use logarithmic values to have a deeper view of smaller modification compare to bigger values. In RSAxN it is also possible to display the return loss value ardB in log scale over the frequency range.

Linear distortion occurs in all linear networks and components. Linear distortion could have an impact deviation in phase, in amplitude and / or in a constant group delay. When measuring a filter with the RSAxN, it is possible to measure amplitude flatness, the deviation in phase and group delay (Group delay is a deviation from linear phase). Thereby the VNA using phase over frequency and adds it to a positive constant phase over frequency. The difference of both results is the phase deviation over frequency and the group delay is calculated as follow:

Each signal will be delayed with transmission over a component like a filter, amplifier, etc. different group delay results in a non-linear delay of signals at different frequency components and distorts the signal, which is not ideal and not desired. If the group delay is constant over the frequency range, all frequency components will have the same shift and, in that case, the ideal system would be free of distortion and the group delay would be a constant value. The aperture step width [df] can be adjusted in RSAxN according to their need. In S11 (and S21) measurement of RSAxN, phase and group delay can be measured and displayed over the desired frequency range (see figure 7).

S21 Parameter Measurements
S21 parameter defines the insertion loss over a specified frequency range which can be measured with high accuracy after a Through calibration. The measurement of frequency response can be used to measure the 3 dB bandwidth of a bandpass filter (see figure 8) or characterise amplifiers.

Similar to S11 measurement, also phase over frequency range and group delay can be measured with RSAxN (see figure 9).

Distance-to-Fault Parameter Measurements
In RF measurement normally frequency range will be selected because it has more significance in characterisation in this area. For example, a filter will be characterised in frequency range. But for some cases it is very useful to take also a look into the time range to evaluate impulse response of a DUT.
One big advantage when phase information is available is, that the frequency range can be transformed (via Inverse Fast Fourier Transformation [IFFT]) into time. The time view has different advantages, it can be used to localise a defect on cables due to measurement of impulse response, localisation and characterisation of discontinuities or getting a better view of physical characteristic of a DUT. In the formula below it is visible that S11(t) is the impulse response of reflection factor S11(ω):

Figure 10 shows the frequency range (S11) and DTF measurement of a DUT (two cables with connectors in between and a 50 Ω match at the end). In the frequency range, the discontinuities can only be captured in summary. But in DTF the reflection points are easily visible and can the exact distance of reflection points (e.g. due to connectors or cable defects) can be measured with marker. It is necessary to perform the same calibration like for S11 measurement.

Calibration
One important part of accurate measurements is the calibration procedure. Each measurement contains different failure mechanisms, with calibration routines these can be minimized, and the quality of measurement accuracy can be increased.
S11 / DTF Calibration:
D = Directive Error: coming from imperfect signal split of coupler.
MS = Source Match Error: coming from imperfect source matching of VNA
TR = Tracking Error: coming from frequency response of components used for signal split (like directive coupler) and mixer and internal detector.
Here is an error model for a one port measurement:

Load calibration: With using a 50 Ω impedance [load], S11A is 0 and S11M = D (Directivity Error from directivity coupler is measured). VNA is now minimising the directivity error [D] over the adjusted frequency range. After this calibration, the directivity error of RSA5000N is ~40 dB.
Short / Open calibration: From the DUT’s view there is a mismatch of source [MS] which creates a reflection loop between the DUT and the system. This failure is visible when the DUT shown a mismatch. Additionally, the frequency response failures [TR] due to connectors, cables, internal coupler, detectors occurring. With open (S11A = 1) and short (S11A = -1) calibration there will be two equation with two factors Ms and TR and the VNA knows these values.
The calibration standards Open / Load / Short and Through should be ideal to reach e.g. with Short r = -1, but they aren’t. E.g. an Open contains stray capacitances or Short contains inductivities. This is not a problem, when the non-ideal behaviour of standards is known. For RIGOL’s calibration kit CK106A (DC – 6.5 GHz) and the CK106E (DC – 1.5 GHz) the parameters are known and already integrated into the RSAxN versions. With regards to these values an accurate calibration is now possible. If an additional calibration kit is used, then these parameters needs to be customized according to this kit.

For DTF measurement, the velocity factor of cable (e.g. 70% 0.7) and the cable loss needs to be integrated to extend the accuracy of the measurement. Both values are defined in cable specification.
S21 Calibration:
For S21 (transmission factor) measurement, a Through calibration is required to flatten the frequency response of cabling and connection (needed to connect DUT to VNA) from VNA source to VNA input. Figure 13 displays the curve before and after Through calibration.

The RSA5000N and RSA3000N series have four additional application modes, in addition to the new VNA function. These four modes include RTSA (real-time spectrum analyser up to a maximum bandwidth of 40 MHz), GPSA (sweep-based spectrum analyser with outstanding performance), EMI (pre-compliance tests according to CISPR specifications) and VSA (vector signal analysis for different digital demodulation and bit error measurement, only RSA5000N). With the addition of the VNA application mode the RSA5000N and the RSA3000N series are some of the most complete RF testing platforms on the market.

Products Mentioned In This Article:

• RSA3000N Series Please See HERE
• RSA5000N Series Please See HERE

Real-Time Spectrum Analyser vs Spectrum Analyser
Today the RF industry has to face more and more the open question, how to transport the data from my test device (DUT) to different receiver spots (like to transmit data into World Wide Web). For IoT applications the most common way is, to use wireless transmission of data via common standards like Bluetooth, Wi-Fi or Zigbee. A more complex test system than a spectrum analyser is required to evaluate the results in a short time. Wireless transmission works with digitalisation of data. These digital data will then be modulated to an RF carrier via complex modulation schemes. This process results in a very fast and dynamic signal change over time and frequency band. Speed becomes more and more an important factor in frequency analysis. So it is not enough to use a sweep based spectrum analysers with FFT or superposition principle. Rigols new outstanding Real-Time Spectrum Analyser RSA5000 series will give the answer to that question and combines an elegant design with full flexibility and speed during test.

RSA5000 series can be switched between a common superposition spectrum analyser [SA] and a real- time spectrum analyser [RT-SA]. The RSA5000 is working like a SA of the DSA800 series but with better RF performance. This document will describe the difference of analyser techniques and will display the advantages:

The complete RF input signal will be set to an intermediate frequency via a swept local oscillator in superposition technique. In other words a signal trace of SA will be sweep between start / stop frequency according adjusted center frequency and span. Sweep time is depending of adjusted parameter like RBW, VBW, Span. This measurement technology can be perfectly used to get a fast overview of a wide range spectrum with good amplitude accuracy and for insertion loss or VSWR measurements. Additionally a common SA is a very useful tool to perform RF measurements with a big dynamic range and good performance.

For measurement of low level signals it is important to have a good dynamic range. Some standards have a low reference sensitivity level below -120 dBm which is lower than the noise level. Therefore it is necessary to have a test device with possibility to decrease the noise level as low as possible. The DANL of Rigols RSA5000-SA is specified with -165 dBm/1 Hz (typ.)1. Low signal measurements can be performed with following parameter adjustments2.

The negative aspect is that only the sweep point is measuring at a time. The rest of the trace is not updated at the same time. With SA blind time occurs where signal information is lost (see figure 1).

Sweep result of spectrum analyser with blind time

For example a fast changing frequency hop signal like Bluetooth can be measured with SA. One trace can be set to maximum hold. A second trace can be set to clear write. With one sweep it is not possible to capture all signal components. Several sweeps are necessary and they are only visible with maximum hold function (see figure 2). But not all frequency components are visible. There is no time information available and it is not possible to detect that this signal is a frequency hopping spread spectrum signal.

Figure 2: Bluetooth signals are only visible via max hold function with SA

Signals which are only randomly available and very fast cannot be detected. Frequency, Span and RBW has a direct influence to sweep time on common spectrum analyser. If a better frequency resolution is required, then RBW needs to be decreased. This results in a lower sweep time and capturing of fast signals is more difficult and time consuming.

The real-time spectrum analysis uses FFT technology and works without a sweep. But the calculation form is different comparing to normal FFT. In normal FFT form, calculation time needs more time than FFT process. The result is, that some parts of time signal will be lost based on the gap between the FFT acquisitions (see figure 3, below). For example this kind of FFT analysis can’t be used for measurement of pulsed signals because part of pulses could be in the gap between FFT acquisition and the frequency result will be different with each FFT acquisition.
Normal FFT Analyser example:

In real-time acquisition the calculation will be performed in parallel to FFT process and the calculation itself is very fast. The calculation is faster than FFT acquisition and contents all operation until displaying the trace to the display. The display data will be changed with very high and constant speed. The result is that time acquisition of different FFT blocks is gap free (see figure 4 below). Speed will be not changed with using of different RBW adjustment.
Gap free FFT example in real-time operation:

Figure 4: FFT in Real-time spectrum analyser without gaps

A fixed number of 1024 samples are used for one FFT time acquisition. Each FFT calculation is using a window function. Windowing is important to define a discrete number of time points for calculation. Size of window can be varied and is not fixed in time domain. A variation of window size will have a direct influence of real-time resolution bandwidth [RBW] or the other way around: with changing the RBW, size of window will be changed.
Slew rate, sharp of window and number of window points has an influence to leak effect3, frequency- and amplitude accuracy. Therefore several windows are available in RSA5000 series to use the device for a wide range of applications.
Negative aspect of a filter is, that some signal information will be lost due to amplitude suppression at begin and end of a filter (see figure 5).

Figure 5: unfiltered time signal but with lost amplitude information

The position of a time signal like a pulse needs to be in the center of FFT window to transform it correctly into frequency range. In case that a pulse is in between two FFT events, then amplitude is suppressed by filter side loops and is no longer correct (see figure 6).

Figure 6: Amplitude is wrong if signal is located in between of two FFT blocks

An overlapping process of FFT events will be used in RSA5000 series to avoid losing signal information. Overlapping has the effect that more spectrums are available over a time period and time resolution is higher. Smaller events can be measured (see figure 7) and signal suppression of single FFT acquisition occurred due windowing is eliminated with overlapping.

Figure 7: Overlapping process in real-time spectrum analyser

In other words, overlapping process of FFT events has a direct influence of smallest pulse width which can be measured with a real-time spectrum analyser. The RT-SA RSA5000 is working with a FFT rate of 146.484 FFT/sec. which results into a calculation speed (Tcalc) of 6,82 µsec.:

Depending on real-time span there are 4 different sample rates available. The maximum sample rate is 51,2 MSa/sec4. With that sample rate and the fixed number of samples (NFix = 1024), used for one FFT acquisition, the duration can be calculated as follow:

An overlap of FFT frames is not possible during calculation progress. Therefore the overlapping time of FFT frames can be calculated with that formula:
𝑇𝑜𝑣𝑒𝑟𝑙𝑎𝑝 = 𝑇𝑎𝑐𝑞 – 𝑇𝑐𝑎𝑙𝑐
For example with sample rate of 51,2 MSa./sec. the overlap time is 13,18 µsec or 65,86% which results into NOverlap = 674 sample points.
Probability of Intercept [POI]
POI specify the smallest pulse duration which can be measured with 100 % amplitude accuracy. Furthermore POI defines the minimum pulse width where each pulse will be captured (see figure 8). The smallest POI of RSA5000 is 7.45 µsec5.

Figure 8: Measurement of a pulse of 7.45 µsec. (period: 1 sec.) with amplitude of -35 dBm. Each pulse is captured with correct amplitude.

These small pulse events can’t be measured constantly with a normal SA. A RT-SA is necessary for that kind of short events. RSA5000 can measure a minimum event of 25 nsec., but not with 100% amplitude accuracy and not all pulses will be measured (see figure 9).

Figure 9: Measurement of a pulse of 25 nsec. (period: 1 sec.) with amplitude of -35 dBm. Not all pulses are captured. Amplitude is wrong amplitude.

POI depends on FFT rate, used RBW and adjusted Span. The principle of POI is described with a span of 40 MHz (=51,2 MSa/sec.) and RBW of 3.21 MHz (Kaiser Window) in figure 10. Due to calculation time, second FFT acquisition starts after 6.82 µsec. Window size is depending of RBW in real-time mode:

Figure 10: Example with RT-Span of 40 MHz, sample rate of 51.2 MSa/s and RBW of 3.21 MHz (Kaiser Filter)

With that POI and speed it is now possible to measure a Bluetooth signal with the RT-SA mode of RSA5000 series. Usage of maximum hold is no longer needed. It is possible to set 6 different RBW settings in RT-SA mode and speed is not affected. RSA5000 provides different measurement modes for the analysis:

• Normal Trace Analysis
• Density Analysis
• Spectrogram
• Power vs Time
  In normal mode the trace information of current time is visible. It looks like a trace of a SA but due to the real-time sweep more information is visible at the same time compare to SA. Normal trace analysis is a 2D measurement (power over frequency).
  Density Analysis is the same result like normal trace analysis. But with density analysis it is additionally possible to analyse the repetition rate of a signal. Density is working with a colour scheme (from blue = 0% to red = 100%, see figure 8). As more often the signal hits a single pixel point within a certain time, as higher is the percentage which defines the colour of this pixel. For example a constant wave [CW] signal would be visible in red colour. A very short single signal event would be visible in blue colour. The colour percentage can be calculated as follow:

This could be a signal with a pulse width of 30 msec. With acquire time of e.g. 60 msec. n = 50% which would be result into the colour ‘yellow’ (see figure 11, below). The normal trace in density has the colour white. Density analysis is a 3D measurement (power over frequency over repetition rate).

Figure 11: Density example with colour scheme examples

In normal and density mode it is possible to activate a spectrogram measurement. Spectrogram is a waterfall measurement frequency over time and bring the possibility to measure out duration of pulses (like for Bluetooth signals). A spectrogram also works with a colour scheme for signal level (DANL: 0% = blue, Reference level: 100% = red). With waterfall spectrogram it is possible to analyse on / off scenarios of signals. Density in combination with spectrogram is a 4D measurement (power over frequency over repetition rate and power over time, see figure 12 with a Bluetooth example).

Figure 12: Bluetooth signal measured with density spectrogram

In Power vs Time (PvT) it is possible to display the time domain of a signal within adjusted real-time bandwidth. The acquisition time can be changed in this measurement. The Power vs Time analysis is displayed for the used real-time bandwidth and not to RBW like in SA with zero-span configuration. Signal bursts of modulated signals and pulses can be displayed to measure duty cycle and amplitude of a pulse or to display pulse trains over certain time. PvT can be used in combination of normal trace analysis (frequency spectrum) and spectrogram (see figure 13).

Figure 13: normal trace vs spectrogram vs PvT of a Bluetooth signal

Comparing the measurement result of Bluetooth signal in figure 12 and figure 13 and the result of SA (figure 2) test engineer has much more information available now. Within the adjusted real-time bandwidth all frequency components can be measured. Time information can be displayed in parallel of spectrum measurement. In spectrogram it is visible that this signal is a frequency hopping spread spectrum signal and the length of data block can be analysed. The Power vs Time is no longer depended on RBW bandwidth like in SA and frequency domain and time domain can be displayed in one time.

Products Mentioned In This Article:

• DSA800 Series please see HERE
• RSA5000 Series please see HERE

Wireless Communication and Function
The internet of Things (IoT) is one of the fastest growing areas of technology today. In this increasing competitive field, the greatest challenge facing IoT device manufacturers is determining how to create the smallest devices possible without sacrificing performance in terms of range or power. For IoT devices that utilise radio frequency (RF) communication, one of the greatest opportunities to improve the design of the device while also improving the range and quality; that is, it provides the greatest possible range given the available power and size constraints.
Determining Operating Frequency
To Determine if an antenna is performing as efficiently as possible, the first step is knowing the exact frequency bandwidth that is intended to be used to transmit information. This is generally determined by the communication protocol that will be used, along with the kind of RF circuit that is being designed. This will affect the natural properties of the signal; in general, the higher the frequency the more power is needed to transmit the same information.
Antenna Testing Methodologies
Once the frequency range has been
determined there are several simple tests that can be performed to help identify or verify which antenna will be best suited for the application, including a bandwidth test (or tracking generator test) and a voltage standing waveform ratio (VSWR) test. These two tests will determine the operational bandwidth of a matched set of antennas, and how efficient they are at the desired frequency bandwidth. With a more efficient antenna, less power is required to transmit a signal over a given distance. This can help increased the battery life, reduce the size of the IoT product or add addition range to the product.
Performing a bandwidth test will help to determine if a pair of matched antennas will operate properly at the desired frequency bandwidth. This test must be performed on a spectrum analyser that can operate at the desired frequency bandwidth and has a tracking generator built in; this allows the user to inspect the operating bandwidth as a function of its own power level. This is done by connecting the antenna to the front of a spectrum analyser that has the tracking generator enabled. When the test is performed the spectrum analyser, will transmit a sine wave that is being swept across the desired frequency bandwidth while listening for the signal with the input of the spectrum analyser, and then compare the power level of the transmitted signal. To perform this, test the entire range of the spectrum analyser was used from 9kHz to 7.5GHz to determine the best operating bandwidth for the antennas. See Figure 1.

Figure 1: A bandwidth test being performed on two matched antennas that are rated for the 2.4GHz bandwidth.

As shown in Figure 1, the frequency ranges where the antenna received the most power at 900MHz bandwidth, 1.5GHz bandwidth and at 2.4GHz bandwidth: therefor this antenna is capable of being used efficiently at these three bandwidths. This makes this antenna and excellent choice for either Wi-Fi or Bluetooth applications which operate at 2.4 GHz bandwidth.

The next test requires the use of a spectrum analyser with a tracking generator as well as a VSWR bridge. The VSWR test is designed to determine the reflection coefficient of a given antenna to help determine the most efficient antenna at a given frequency. VSWR is determined by measuring the voltage standing waves along a transmission line leading to an antenna, it is the ratio of the peak amplitude of a standing wave and the minimum amplitude of a standing wave. When an antenna’s impedance is not matched with the transmission line, power is reflected reducing the amount of transmitted power. The ideal VSWR would be equal to 1 (no power being reflected) at the desired frequency. To perform this, test the desired frequency (2.4GHz) was selected and then the antenna was attached the bridge. A VSWR bridge measures the amount of power that is transmitted into antenna and compares it with the reflect power. Due to the design of a VSWR bridge most of the power that is reflected at a given frequency is the reflected power from the antenna. See Figure 2.

Figure 2: The image shows a VSWR test being performed on an antenna that is meant to operate at 2.4 GHz and has a VSWR of 1.17.

Based on Figure 2, the antenna has a VSWR of 1.17 at a frequency of 2.4GHz which makes this antenna almost an ideal antenna for Wi-Fi and Bluetooth communication. With a more efficient antenna, less power is required for transmission, which can increase the effective range of the device. Focusing on improving this one aspect of the IoT device can vastly improve the functionality of the device and address a number of challenges inherent in the overall design.
Additional Areas of Interest
Range is another critical aspect of an IoT device’s design. Sufficient range is required to facilitate wireless communication between IoT devices or with the internet. It is a critical aspect of the initial design and protocol selecting process. For instance, Near-Field Communication (NFC) is a communication technique that is used to transmit information over a couple of inches whereas Bluetooth is used to transmit information for tens of feet. Range requirements will also affect what type of antenna is being used to transmit data, along with determine whether to use a highly directional antenna or an omni directional antenna. Another consideration is the effective range required will determine the amount of power that is needed for transmission; thus, an antenna that makes more efficient use of the available power will help to ensure that the device will have the desired range.

Another area of interest to improve the functionality of an IoT device by eliminating or reducing interference to the RF signal. Interference can be generated by the electronic components used in the device itself such as the PCB, battery or user interface, or it can also be generated by the enclosure used to both protect the electronic components and improve the appearance of the device.

Conclusion and Key Learnings
With more Internet of Things devices being conceptualized every day, creating reliable Radio Frequency communication is an essential step towards creating a device that will stand out among the crowd. Successful device manufacturers must overcome the most difficult efficient antenna possible. Key test and measurement equipment will help facilitate basic antenna testing and interference testing that will help overcome these RF communication challenges.

Solution: This application note is designed to show FM data transmission and modulation using some common test instruments.

An FM transmission consists of a wave who’s frequency is modulated, or changed, with respect to an input signal. If the input signal frequency changes, so does the carrier frequency.

The modulating signal affects the FM deviation. The total deviation is a function of the modulating signal’s frequency and amplitude.

Here is a typical FM signal shown on an amplitude vs. time graph.

In this demonstration, we are going to use an arbitrary waveform generator to create an FM signal. The FM signal will be modulated by a audio source.
This signal will be transmitted from the output of the generator through an antenna to the input of a spectrum analyser. There, the signal will be displayed on the spectrum analyser, demodulated, and played back through the internal speaker of the instrument.
Required Hardware:
1. A Spectrum Analyser with demodulation capabilities: The Rigol DSA800, 1000, and 1000A series of spectrum analysers all support this function. This demo will use the DSA-1030A, but the process is basically the same.
2. Arbitrary Waveform Generator: Rigol’s DG4000 series is perfect for this experiment. We use the DG4162 160MHz model in this demo.
3. Music source with a headphone jack. In this demo, we will use a cell phone with music source.
4. Adapter cable from music source to BNC cable. In this demo, we made a cable by splicing a 3.5mm audio jack to a 50 ohm coaxial cable with a BNC termination.

5. N-Type to BNC adapter. This is used to adapt the DSA input to the antenna.

6. Qty 2 FM Radio Antennas with matching frequency ranges. In this demo, we use (2) BNC terminated center loaded whip antennas from Radio Shack.

Configure the Arbitrary waveform generator:
In this setup, we want to use the Sine function of the arbitrary waveform to create the carrier wave that supports the signal that we want to transmit and . The audio input signal will be used to modulate the carrier frequency by the deviation that we set.
In the US, commercial radio stations transmit from 87.9 to 107.9MHz. The FM deviation, or amount that the carrier frequency is modulated, is less than or equal to 150kHz, or +/- 75kHz around the carrier frequency.
1. Power on the DG4000.
2. Connect the BNC-to-Phone jack to the CH1 Mod/FSK/TRIG input on the back of the DG4000.
3. Connect the antenna to the Channel 1 output.

4. Connect your music source and start the music. Make sure to enable the sound on the device.. check that the volume level is > 0.
5. Configure the DG4000 to output a sine by pressing Sine on the Waveform area of the front panel.

6. Set the frequency of the sine wave to 100.00MHz.

NOTE: You may need to change the frequency by a few 100kHz, if the local radio stations are interfering with the band you are transmitting in. You can simply change the carrier a few 100kHz.
7. Set the amplitude to 2Vpp by pressing the Amp button and then entering 2Vpp on the keypad. Alternately, you can use the scroll wheel to change the values.

8. Press the Mod key to enable modulation.
• Set type to FM
• Set source to external
• Set deviation to 150kHz

Configure the Spectrum Analyser:
The arbitrary waveform generator is used to create the FM signal and the spectrum analyser is used to capture and demodulate the transmission.
In the US, commercial radio stations transmit from 87.9 to 107.9MHz. The FM deviation, or amount that the carrier frequency is modulated, is less than or equal to 150kHz, or +/- 75kHz around the carrier frequency.
1. Power on the DSA.
2. Connect the N-type adapter and antenna to the RF input

3. BNC-to-Phone jack to the CH1 Mod/FSK/TRIG input on the back of the DG4000.
4. Set the center frequency equal to the carrier frequency of the arbitrary waveform generator. In this example, we have set the DG4000 to output at 100.00MHz.

• Press Freq > Center Frequency > 100.00MHz

NOTE: You may need to change the frequency by a few 100kHz, if the local radio stations are interfering with the band you are transmitting in. You can simply change the carrier a few 100kHz.
5. Set the span of the DSA to 200kHz.
• Press Span > set span to 200kHz using the keypad

• You should see the signal present on the DSA. You can observe the signal by turning the DG4000 output on and off. The DSA signal should go flat when the DG4000 is off and then rise when the output is on.

6. Now, we can demodulate the FM signal and listen to the transmission.
• Press Demod

• Enable Demod

• Press FM

• Press Demod Setup

• Turn the Speaker ON
• Set volume to 100
• Set Demo Time to 10s

Products Mentioned In This Article:

• DSA800 Series please see HERE
• DG4000 Series please see HERE

Solution: This document provides step-by-step instructions on using the Rigol DSA800 series of Spectrum Analysers to measure the characteristics of an RF Bandpass filter.
NOTE: The DSA must have a Tracking Generator to effectively perform the following test.
Normalise the trace (Optional)
Many elements in an RF signal path can have nonlinear characteristics. In many cases, these nonlinear effects on your base measurements can be minimised by normalising the instrument.
1. Connect tracking generator output to RF input using the same cabling that you will be using to test your device. Any element, like an adapter, used during normalisation should also be used during device measurement as any changes to the RF signal path could effect the accuracy of the measurement.
2. Enable the tracking generator by pressing the TG button > TG On
3. Store the reference trace by pressing the TG button > Normalise > Stor Ref
4. Enable normalisation by pressing the TG button > Normalise > Normalise on

Measure the filter
1. Connect the tracking generator output to the filter input using the appropriate cabling and connectors.
2. Connect the filter output to the instrument RF input.

3. Set the tracking generator amplitude by pressing the TG button and the TG Amplitude. You can use the keypad or wheel to enter the correct value.

NOTE: If your instrument is equipped with a Preamplifier, you can enable it to lower the displayed noise floor by pressing the following sequence:
Amplitude button > Down Arrow > RF Preamp On
4. Enable the Tracking generator by pressing the TG button > RF Source ON You can see the small bump in the figure below.

5. You can use the Auto button to center and zoom on the waveform. You can also use the Freq and Span buttons to manually manipulate the displayed data.

6. You can now enable the Marker function to measure the bandwidth and attenuation or passband characteristics of the filter.

7. Press Marker Fctn > N dB BW and set the function to the amplitude of interest. In this example, we are measuring the 3dB Bandwidth of our filter.

Products Mentioned In This Article:

• DSA800 Series please see HERE

How to measure an RF Amplifier using a DSA800 Series Spectrum Analyser

Solution: This document provides step-by-step instructions on using the Rigol DSA800 series of Spectrum Analysers to measure the characteristics of an RF Amplifier.
In addition to the DSA-800 Spectrum Analyser, you will need an RF Source, cabling, and adapters.
Measure the amplifier
1. Connect the RF generator output to the RF input of the instrument using the appropriate cabling and connectors.
NOTE: If your instrument is equipped with a Preamplifier, you can enable it to lower the displayed noise floor by pressing the following sequence:
Press AMP button > Down Arrow > RF Preamp On

Figure 1: Preamplifier off

Figure 2: Preamplifier on

2. You can use the Auto button to center and zoom on the waveform. You can also use the Freq and Span buttons to manually manipulate the displayed data.

• Another option to center the waveform is to press Freq button > Peak → CF. This will automatically align the center of the display with the peak of the trace.
3. Freeze the unamplified trace by pressing Trace > Trace Type > Freeze. You can use the Marker button to create a marker. This can be used to find the peak frequency and amplitude of the displayed waveform.
4. Disable the RF Generator output.
5. Disconnect RF generator from the instrument RF Input and connect it to the Amplifier input.
6. Connect the Amplifier output to the instrument RF Input.
7. Enable the RF generator.

8. Enable a second trace to visualise the amplified signal by pressing the Trace button > Select Trace 2.
9. Set the trace type to Clear/Write by pressing Trace > Type > Clear/Write.
10.You can use the Auto button or manually center the trace using the Freq, Span, and Amp buttons.
11.Readjust the amplitude scale by pressing Amp > Auto
12.You can enable an additional marker for the new trace by pressing Marker > Select the marker you would like to use
13. Now, select the trace you want to mark by pressing Marker > Marker Trace > select trace of interest
• Be sure Normal is selected to enable the marker

14.You can also enable a marker table by pressing Marker > Down Arrow > Mkr Table ON. This allows a convenient way to compare markers and values between traces.

15. Alternately, you can use the Trace Math function to create a Trace difference on the screen.
16. Enable Trace Math by pressing Trace > Trace Math
17. Set Function to A-B
18. Set A = T1
19. Set B = T2
20. Set Operate > On

21.Set Amplitude by pressing AMPL > Auto
NOTE: New trace appears. This represents Trace 1 – Trace 2.
22. Set Marker to Math Trace by pressing Marker > select Marker 3
23.Set Marker Trace to Math by pressing Marker > Marker Trace > Math

• You can then move the marker to the smoothest portion of Trace 3.

Products Mentioned In This Article:

• DSA800 Series please see HERE

Why is Pre-Compliance Testing done?
Almost any electronic design slated for commercial use is subject to EMC (Electromagnetic Compatibility) testing. Any company intending to sell these products into a country must ensure that the product is tested versus specifications set forth by the regulatory body of that country. In the USA, the FCC specifies rules on EMC testing. CISPR and IEC test definitions are also commonly used throughout the world.
To be sold legally, a sample of the electronic product must pass a series of tests. In many cases, companies can self- certify, but they must have detailed reports of the test conditions and data. Many companies choose to have these tests performed by an accredited compliance company. This full compliance testing can be expensive with many labs charging thousands of dollars for a single day of testing. Testing a product for full compliance can also require specialized testing environments. Any failures in compliance testing require that the design heads back to Engineering for analysis and possible redesign. This can cause delays in product release and an obvious increase in design costs.
One of the best methods to lower the additional costs associated with EMC compliance is to perform EMC testing throughout the design process well before sending the product off for full compliance testing. This pre-compliance testing can be cost effective and can be tailored to closely match the conditions used for compliance testing. Pre- compliance Testing can range from basic signal visualization with a spectrum analyser to validation testing against limits or standards and even to interactive debugging. These types of pre-compliance testing require different probes, setups, and tools. Basic visualization, comparison versus standard limits or previous results, and debugging all play important roles in EMI testing to improve your designs, lowers your test costs, and speed your time to market. In this application note, we will compare the advantages, requirements, and available solutions for each type of pre-compliance analysis.
Basic Visualisation
Near Field Probes for Radiated Emissions
The simplest pre-compliance measurements involve visualizing and analysing the magnetic fields resulting from emissions. These test setups start with a spectrum analyser, like the RIGOL RSA3030 (Figure 1). For radiated emissions, use near field magnetic (H field) probes as shown in Figure 2.

Figure 1: Rigol RSA3000 Spectrum Analyser with EMI

Figure 2: Near Field E and H Probes used to identify EMI Sources

Near field probes pick up emissions that pass through the small loop at the end of the probe. These magnetic probes are relatively inexpensive and make it possible to capture signals only in close proximity to the probe, hence the name ‘near field’ probe. This makes them well suited to basic visualization because engineers can quickly scan a new board or enclosure looking for problems by passing the probe over the area as demonstrated in Figure 3. A basic configuration of a near field radiated emissions test would be simply configuring the analyser to use the peak detector and set the RBW and Span for the area of interest per the regulatory requirements for your device. The Peak detector will provide you with a “worst case” reading on the radiated RF and it is the quickest path to determining the problem areas. Then select the proper H field probe for your design and scan over the surface of the design. Larger probes will give you a faster scanning rate, albeit with less spatial resolution.
The probes act as an antenna, picking up radiated emissions from seams, openings, traces, and other elements that could be emitting RF. A thorough scan of all of the circuit elements, connectors, knobs, openings in the case, and seams is crucial. Chambers and shielding are usually not needed for this type of EMI visualization since the probe registers only very close signals. Engineers can often determine the source of an emission by orienting and locating the probe along the test device. The downside to this approach is that there is no easy correlation between near field measurements and compliance results and making repeatable measurements is difficult due to probe positioning. Therefore, engineers must think critically about detected emissions and determine whether they are worth worrying about before going to the expense of further validation.

Figure 3: Using an H field probe to test a power supply

RF Current Probes for Conducted Emissions
Conducted emissions are unwanted signals that travel via cables. The most common issue is when devices send RF signals back into the power line. There are specific limits associated with these types of emissions designed to protect the power grid and other devices on the circuit. When visualizing conducted emissions engineers use an RF current probe like those in Figure 4. Current flow in a cable placed within the probe is shown on the spectrum analyser. This makes it possible to visualize signals being coupled into a communication or power cable from either the device under test or from external sources. The conducted emissions from a simple LED light fixture are shown in Figure 5. Even simple electronics like this can couple significant power switching frequencies back into the power line. After visualizing these signals care can be taken to filter or improve them if needed before further analysis.
When combined with a spectrum analyser both near field probes and RF current probes are low cost basic visualization solutions for EMI signals. They provide insight without the cost of a more complete EMI system.

Figure 4: RF Current Probes for Conducted Emissions

Figure 5: Conducted Emission profile of an LED light fixture

Pre-Compliance Validation Testing
Limits, Standards, Detectors, and Data Management
Once the testing turns to validation, near field measurements still provide important insights, but the wand style probes can be frustrating since slight changes in position or orientation will affect the results. This makes it impossible to compare to standardized test results or even gauge improvement from one version to another. A full compliance setup with calibrated antennas, an EMI receiver, and a chamber could make final compliance tests, but the cost to setup and maintain this setup can be overwhelming. Fortunately, there are tools that help bridge this gap by making relative analysis simpler. With the right setup engineers can evaluate new designs versus known good designs and compare versus established standards. Getting the most out of this type of pre-compliance validation testing requires additional instrumentation capabilities as well as a more stable test setup.
For this type of pre-compliance testing spectrum analysers should include the standard EMI bandwidths and CISPR detectors. The analyser also needs to be able to segment scans using preferred settings for different areas of the spectrum to optimize sweep time and required accuracy. Most importantly, a spectrum analyser must include standard limit lines as well as the ability to customize limit and margin levels. This is critical because without a calibrated chamber alterations must be made for the test environment in order to build confidence in the results. Lastly, the analyser must be able to create, archive, and compare tests and reports so engineers can problem solve any issues or concerns that arise later in the design process. While not an EMI Receiver, RIGOL’s RSA3000 and RSA5000 series spectrum analysers with the EMI application mode (Figure 6) include all of these features in a single box validation solution.

Figure 6: Rigol RSA5000 Series Real-Time Spectrum Analyser with EMI

The EMI measurement mode provides the RSA Series spectrum analysers with limits and CISPR detector modes including Quasi-Peak, CISPR Average, and RMS Average. Engineers also have access to the standard EMI resolution bandwidths (200 Hz, 9 kHz, 120 kHz, and 1 MHz). EMI mode (Figure 7) operates entirely within the instrument from the touch screen or with a mouse and keyboard. This makes it easy to archive reports, run scans, and jump to a signal of interest and immediately debug when needed. The bar graph on the right shows real-time measurements at a given frequency of interest. This utility is made to quickly move to a signal of interest right after a scan without having to change modes. It can show live measurements on up to 3 detectors at the frequency of interest providing an easy transition to debugging and further analysis.
For engineers using a common EMI test software platform, their software toolkits provide flexibility by integrating components from multiple test vendors. Many of these pre- compliance software packages support spectrum analysers including models made by RIGOL. All RIGOL spectrum analysers can be programmed over USB or Ethernet using a standard SCPI instruction set.
EMI Mode on the RSA family of spectrum analysers is a powerful solution providing all the capabilities of a complete EMI validation software package within the instrument.

Figure 7: EMI mode using limits, multiple detectors, and meters on a RSA series, Real-Time Spectrum Analyser

TEM Cell setups for repeatable measurements
For a test setup with more repeatable measurements than near field probes we can look to TEM Cells. A TEM (Transverse Electro-Magnetic) Cell (Figure 8) is a low cost alternative to measurements in an anechoic chamber. A TEM Cell is a near field device for radiated and immunity measurements. Because the device under test sits at a fixed location within the cell test results are easier to compare and repeat over time than with just a probe. While not directly correlated to far field chamber measurements, a TEM Cell takes repeatable measurements. When used with a complete EMI application tool, new designs can be compared to known good devices and custom limits can be established or developed from known standards that incorporate background emissions, limitations of the test setup, and correction tables. Used in this way, a TEM cell setup with RIGOL’s EMI Mode builds confidence in final compliance results by making it easy to compare good devices, failed devices, and new designs against corrected limit lines with appropriate margin. A repeatable test setup and RIGOL’s EMI Application mode provide an affordable test bed for EMI pre-compliance validation and comparison.

Figure 8: A TEM Cell for repeatable radiated emissions testing

EMI Debugging
Real-Time
Once validation tests are completed on a new design, areas of concern are identified and require additional debugging or trouble shooting. Emission issues not captured by basic visualization are often dynamic or dependent on the operating state of the design being tested. These issues can be difficult to capture and understand in a typical swept EMI mode. The combination of a repeatable test setup and a real-time spectrum analyser provide additional debugging capabilities. The RIGOL RSA Series analysers provide multiple views valuable in debugging signals that change over time including density, spectrogram, and power vs time (shown in Figure 9 and Figure 10). In real-time mode the RSA is capable of making seamless measurements. Capture a spectrum without sweeping or missing critical signal activity. Debugging in real-time means that infrequency emissions that might affect compliance are easy to characterize, and with a sense of time, it is much easier to establish the ultimate cause in the design.

Figure 9: Debugging emissions with the density display (top) and the spectrogram waterfall chart (bottom)

Figure 10: Debugging with a combined view of the spectrum (bottom), power over time (top), and spectogram (left)

Debugging with Time Correlation
Additionally, the RSAs have an IF Output. For advanced time correlation of emissions events with embedded signals this IF Output can be input into a mixed signal oscilloscope like the RIGOL MSO7054. This brings the RF signal down to a carrier visible to the scope. In this configuration the RF signal can be viewed alongside digital channels to debug embedded code and communications. To learn more about debugging with the IF output go to our Multi Domain Debugging web page.
Testing emissions with real-time debugging and time correlation for root cause analysis in a repeatable TEM Cell setup is a cost effective system configuration for many of the common EMI design challenges.
Unprecedented Value
EMC Compliance testing is mandatory for the majority of electronic products that are slated for sale throughout the world. Select your instruments and design your test setup to get the most out of your EMI budget for any Pre-Compliance use case. With the right spectrum analyser, visualize emissions with near field and RF probes. Then, validate EMI pre-compliance measurements against known emissions data. Finally, debug and time correlate signals of interest to improve the design. These pre-compliance test setups will help speed product development and save time and money in your design process.

Products Mentioned In This Article:

• MSO7054 please see HERE
• RSA3000 Series please see HERE
• RSA5000 Series please see HERE
• TEM Cells please see HERE 

EMC Precompliance Testing: Conducted Emissions

Electromagnetic Interference (EMI) can cause undesirable effects on electronic products. These effects can range from annoying glitches to rendering a product unusable.
In an effort to minimise these issues, countries have established standards and limits to products that are being sold within that market. This Electromagnetic Compatibility (EMC) testing is an integral part of product design and qualification for any electronics intended for sale within those markets.
In almost every case, these specifications contain limits on conducted and emitted radiation testing. Conducted emissions are those that propagate through the power line connecting the instrument (Equipment Under Test, or EUT here). Radiated emissions are those that are emitted into the area surrounding the EUT.
This note will briefly cover some common practices for conducted emission testing early in the design phase and we will cover the radiated emissions in another note.

A Word about Precompliance
For full qualification testing, a CISPR 16 qualified EMI Receiver and the proper setup must be used. This generally requires using a certified testing lab and special equipment that can be cost prohibitive for development and design tweaking.
This note covers pre-compliance measurements using a Rigol DSA-815 Spectrum Analyser with the optional EMI Measurement Kit (Part Number DSA800-EMI).
While not fully providing fully compliant measurement data, pre-compliance testing can give you critical visibility into the design limitations of your design. You can find the sources of your EMI and try to limit their contributions before the fully compliant testing even begins.
Perhaps the best methodology is to perform a number of pre-compliance tests on a product using a number of physical configurations. Then, compare that data to the data collected in a fully compliant setup.
Using this “Golden Standard” comparison can give you confidence in your pre-compliance measurements and also give you insight into your testing deficiencies. Many of the differences may be systematic and accounted for by allowing for a bigger error cushion on the limits you are testing against.
Physical measurement setup
The more closely you can match a full compliance setup, the more closely your data will match with the lab. But, this isn’t always practical.
The following diagrams show the standard suggested electrical and physical setups for testing conducted emissions:

Figure 1: Electrical connections for Conducted EMC Testing.

Figure 2: Physical connections for Conducted EMC Testing.

The key points:

• The horizontal and vertical ground planes are typically sheets of metal with surface areas twice the dimensions of the Equipment-Under-Test.
• The horizontal and vertical ground planes should be electrically bonded to each other.
• Equipment placed on insulated table over the horizontal ground plane. No equipment or cabling should run below the equipment.
• LISN electrically bonded to the horizontal ground plane. LISN is short for Line Impedance Stabilization Network. Its job is to separate the AC Mains noise from the conducted noise being generated by the Equipment-Under- Test. Please select a LISN that has the proper voltage, current, and frequency ranges for your equipment-under-test.
• Do not coil cables. You want to minimise inductive loops by laying cabling out smoothly.
• The spectrum analyser should be placed some distance away from the horizontal ground plane. Typically, it is a few feet away.

Test Procedure
Once you have setup the EUT and bonded the LISN and ground planes, power on the DSA815 for at least 30 minutes to ensure stability and accuracy.
Configure Spectrum Analyser

• Enable the EMI filter by pressing BW/DET > Filter Type > EMI
• Set Resolution Bandwidth by pressing BW > RBW

NOTE: The resolution bandwidth is determined by the standard and specific device type you are testing. As an example, FCC subpart-15 specifies an RBW of 9kHz when testing from 150kHz to 30MHz.
You should consult the standards you are testing to for more information on the specifications governing your testing.
NOTE: Many specifications give limits and values in dBuV or V.
Optional: Set scale for volts by pressing AMPT > Units > V

NOTE: The DSA-815 has a pass/fail feature that will allow you to configure an upper limit line. This can be useful when evaluating the frequency scan respect to the limits set forth by the EMC standard you are testing to.
You can also save any limit lines to the internal storage, once it has been created. Simply configure the limit line on the instrument, press Storage > change File Type to Limit, and save the file.
Optional: Add an upper limit line by pressing Trace/ P/F > Pass/Fail > Switch ON > Setup > Upper > Edit and configure point 1 to have an X Axis start at 150kHz with an amplitude of 1mV.
Next, add a point 2 with start at 30MHz and an amplitude of 1mV. Set Connected to Yes for point 2.
This will connect point 1 and 2 with a purple line. This denotes the upper Pass/Fail line.

• Set detector type to Peak by pressing BW/Det > Type > Pos Peak
• Set the attenuator to 10dB by pressing AMPT > Input Atten > 10dB
• If the signal is unknown, adding significant external attenuation will minimise the likelihood of damage to the sensitive front end of the spectrum analyser.

NOTE: There are two reasons to add attenuation. The attenuator protects the input circuit from any unknown signals that could damage the input. It also serves as a convenient check on overloading after we check the background readings.
The DSA has protection circuitry, but there are transients that are too fast to protect against.

• Set frequency start, stop values set forth in the EMC Specifications that apply to the product. In this example, we are going to configure the instrument to sweep from 150kHz to 30MHz by pressing FREQ > Start and set to 150kHz. Then, press FREQ > Stop and set to 30MHz.
• Set the RBW to the value set forth in the EMC Specifications that apply to the product by pressing BW/Det > RBW > 9kHz

Check background readings

• Power up LISN
• Connect Spectrum Analyser to the LISN output
• Scan over the frequency band of interest using the detector set to Peak and with the attenuator set to 10dB.
• Optional: If you are not using the Pass/Fail line, you can freeze the background trace for reference by pressing Trace/ P/F > Trace Type > Freeze or you can store the trace data by using a USB drive and the storage menu for offline analysis. Either way, noting the peak values and frequencies of the base electrical environment is important.

Peak Test

• Disconnect the Spectrum Analyser from LISN
• Connect EUT
• Reconnect Spectrum Analyser to LISN. This process helps to minimise damage to the Spectrum Analyser due to transients on the input
• On the Spectrum analyser, set up a new trace as Clear by pressing Trace/P/F > Select trace 1 > Trace Type > Clear
  NOTE: This setting will overwrite the trace with new data as the scan continues through its frequency range.
• Observe the conducted emissions scan.. and adjust the attenuation value to 20dB. If the line does not change for different attenuation values, then it is likely that you are not overloading the input and the measurement quality is high. You can proceed with the pre-compliance testing.

If the scan changes value with different attenuation settings, then it is likely that the input is being overloaded with broadband power and additional attenuation is recommended. You can try comparing scans of 20dB and 30dB, etc.. until a range is found without variation.
You want to select the smallest attenuation value that does not show errors due to the overloading effects of the input signal.
In the worst case, the EUT may not be able to be successfully tested with a Spectrum Analyser. You may need to test using a true EMI receiver with pre-selection filters.

Observe conducted emissions and look for frequency lines that are above the limit line you have set. Make note of the frequencies failing the limit lines.
Quasi-peak Scans

• Using the failed frequencies above, adjust the spectrum analyser to center the failed peak.
• Note the RBW setting for your scan, and make the frequency span 2x the RBW setting used for the peak scan by pressing FREQ > Start and FREQ > Stop
  NOTE: If there is an over limit peak at 10MHz, and an RBW of 120kHz, then you would center your QP Scan at 10MHz, and scan from 9.88MHz to 10.12 MHz.
• Change the detector type to Quasi-peak ( )
  NOTE: The Quasi-peak detector is based on charge and discharge times of a standardised resonant circuit. This detector type can take greater than 3x the scan time of a peak measurement. That is why it is best to only use Quasi-peak over short spans. The DSA-815 digitally replicates this response.
• Compare the quasi-peak data to the pass/fail limit line for that frequency.
• It is advisable to keep the conducted emissions at least 10dB below the
  specified limit line. This margin of error will increase the likelihood of passing a full compliance test.
• It is also advisable to compare your pre-compliance data and setup to that of the full compliance lab that will perform your EMC certification testing. This will allow you to identify any problems with your pre-compliance testing. With more comparisons, you will be able to hone your pre- compliance error budget and have much more confidence in the results you obtain.

Products Mentioned In This Article:

• DSA800 Series please see HERE
• LISN’s please see HERE

EMC Pre-Compliance Testing: Near Field Probing
Solution: Electronic products can emit unwanted electromagnetic radiation, or electromagnetic interference (EMI). Regulatory agencies, such as the FCC in North America, create standards that define the allowable limits of EMI over specific frequency ranges.
Testing designs and products for compliance to these standards can be difficult and expensive. But, there are tools and techniques that can help to minimize the cost of testing and help to enable designs to pass compliance testing quickly.
One of the most often used techniques for EMI testing is near field probing. In this technique, a spectrum analyser is used to measure electromagnetic radiation from a device-under-test using magnetic (H) field and electric (E) field probes.
In this application note, we are going to describe some common techniques used to identify problem areas using near field probes.
A Word about Pre-Compliance
Most governments have regulations in place that specify the amount of electromagnetic interference (EMI) a product can emit into the environment (radiated emissions) and conduct down the power cord (conducted emissions).
Products being sold within the areas covered by these regulations must comply with the defined test limits. Compliance tests use these regulations to define the proper instrumentation, physical setup, and experimental techniques and experience to correctly record and report properly. This testing is very important and required for legal sale of the product within the covered area. Unfortunately, compliance testing can be expensive and difficult to execute due to the specialised equipment and knowledge required to properly conduct the tests.
Pre-compliance testing simulates the major details of a compliant test setup at a lower investment in time and money. Before you go to a compliance lab for testing, you can use pre-compliance tests to gather information about the performance of a design, make changes (if needed), and retest.. all in an effort to minimize the return trips to the compliance lab.
A word of caution, however. Pre-compliance data can be useful in hunting down many, if not all, of the non-compliant areas of a design but it is not a substitute for testing at a fully accredited compliance lab. Ultimately, the company (you) is responsible for proof of compliance to the full regulations for your product.
Setup
Board level emission testing can be performed using a spectrum analyser, like the Rigol DSA-815 (9kHz to 1.5GHz), near field electric (E) and magnetic (H) probes, and the appropriate connecting cable.

Figure 1: The Rigol DSA815-TG Spectrum Analyser.

A commercial example of near field probes are the Rigol NFP-3 probes shown below:

Figure 2: Rigol NFP -3 EMC probes.

You can also build your own probes by removing a few cm of outer shield and insulator from a semi-rigid RF cable, bending it into a loop, and dipping in plastic tool dip or other insulating material. Larger diameter loops will pick up smaller signals, but do not have as much spatial resolution as smaller diameter loops.
For the first pass, configure the spectrum analyser to use the peak detector. This setting ensures that the instrument is capturing the “worst case” peak RF. It also provides a fast scan rate to minimize the time spent at one position as you scan over your DUT. Larger probes will give you a faster scanning rate, albeit with less spatial resolution. Smaller probes, like the E Field probe, provide fine spatial resolution and can be used to detect RF on single pins of circuit elements.
Probe orientation (rotation, distance) is also important to consider. The probes act as an antenna, picking up radiated emissions. Exposing the loop to the largest perpendicular field possible will maximize the signal strength. You can also use a fixture to hold both the Device-Under-Test (DUT) and the probe. This will help create repeatable measurements and minimize differences in measurements due to probe orientation.

Figure 3: An example of using an H field probe and spectrum analyser to find trouble spots on a board. Note the orientation of the H field probe

Take care to test enclosure seams, openings, traces, and other elements that could be emitting RF. A thorough scan of all of the circuit elements, connectors, knobs, openings in the case, and seams is crucial to identifying potential areas where RF can “leak” out of an enclosure.

Figure 4: Measuring a display ribbon cable for emissions using an H field probe.

You can use tinfoil or conductive tape to cover suspected problem areas like vents, covers, doors, seams, and cables coming through an enclosure. Simply test the area without the foil or tape, then cover the suspected area, and rescan with the probe.
Once you have identified the physical locations of the areas that have the highest emissions, you can get more detail by implementing a few common techniques. If possible, select a spectrum analyser that has the standard configuration used in full compliance testing. This includes a Quasi-Peak detector mode, EMI filter, and Resolution Bandwidth (RBW) settings that match the full test requirements specified for your product.
This type of setup will increase testing time but should be used on the problem areas. A full compliance test utilizes these settings.. and so, your pre-compliance testing with this configuration will provide a greater degree of visibility into the EMI profile of your design.

Conclusion

In closing, near field probes and a spectrum analyser can be useful tools in troubleshooting EMI issues.
– With H field probes, try different probe orientations to help isolate problem
areas
– Remember to probe all of the seams around any enclosure surrounding electronic components/boards.. surface contact and finish effect grounding and shielding
– Openings in enclosures radiate just like solid structures. They act like
antennas.
– Ribbon cables and cables/inputs with bad shielding and grounds are common causes of radiated emissions

Products Mentioned In This Article:

• NFP-3 please see HERE
• DSA800 Series please see HERE

Testing Cable Loss with a Spectrum Analyser

Solution: A spectrum analyser with a tracking generator can be a useful piece of test gear. This application note covers making a simple loss measurement on a coaxial cable with BNC connectors.
Required:
– Two N-type to BNC Adapters. Select adapters that convert N-type (in/out connectors on most spectrum analysers) to the cable type you are testing. Also note that higher quality connectors (Silver plated, Beryllium Copper pins, etc..) equal better longevity and repeatability.

Figure 1: N-type to BNC adapter

– A short reference cable with terminations that match your adapters and cable- under-test.

– An adapter to go between the reference cable and the cable-under-test. This experiment will use a BNC “barrel connector”. Note that higher quality connectors (Silver plated, Berylium Copper pins, etc..) equal better longevity and repeatability.

Figure 2: BNC barrel adapter

– Alternately, you can use two adapters a short cable as a reference assembly to normalize the display before making cable measurements. This removes the need to have the cable-to-cable adapter.
– Spectrum analyser with Tracking Generator (TG) Steps:
1) Turn on Spec An and attach adapters to the tracking generator (TG) output and RF Input.
2) Connect the reference cable to the TG out and RF In.

Figure 3: Measuring reference cable

3) Adjust Span of scan for frequency range of interest.
4) Adjust TG output amplitude and spectrum analyser display to view the entire trace.
5) Enable TG.

Figure 4: Reference cable insertion loss before normalisation.

6) Normalize the reference insertion loss. This mathematically subtracts a reference signal (stored automatically) from the input signal.
– With the Rigol DSA815 Press TG > NORMALISE > STOR REF and then Enable Normalise

Figure 5: Reference cable insertion loss after normalization.

7) Disconnect the reference cable from the RF input.
8) Place cable-to-cable adapter (BNC barrel or other) and connect to the cable to test.

9) Connect the cable-under-test to test to RF input and enable the TG.

Figure 6: Cable-under-test connected.

The screen displays the cable-under-test losses plus the error of the cable-to-cable adapter.

Figure 7: Cable-under-test loss.

Products Mentioned In This Article:

• DSA800 Series please see HERE