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.
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/F1-13-300x225.png)
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.
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/F2-13.png)
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.
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/F3-13-300x225.png)
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.
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/F4-13-300x180.png)
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
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/f5-8-300x180.png)
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.
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/f6-8-300x225.png)
Figure 6: Cable-under-test connected.
The screen displays the cable-under-test losses plus the error of the cable-to-cable adapter.
![](https://telonic.co.uk/jg/wp-content/uploads/2021/06/f7-7-300x180.png)
Figure 7: Cable-under-test loss.
Products Mentioned In This Article:
- DSA800 Series please see HERE