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. Â
Required:Â
–   Two N-type to BNC Adapters. Select adapters that convert N-type (in/out connectors on most spectrum analyzers) to the cable type you are testing. Also note that higher quality connectors (Silver plated, Beryllium Copper pins, etc..) equal better longevity and repeatability.
Â
    Figure 1: N-type to BNC adapter
–   A short reference cable with terminations that match your adapters and cableunder-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, Beryllium Copper pins, etc..) equal better longevity and repeatability.
Â
      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. Â
–   Spectrum analyser with Tracking Generator (TG)
Steps:
1)Â Â Â Turn on Spec An and attach adapters to the tracking generator (TG) output and RF Input.
2)Â Â Â Connect the reference cable to the TG out and RF In.
Â
    Figure 3: Measuring reference cable
3)Â Â Â Adjust Span of scan for frequency range of interest.
4)Â Â Â Adjust TG output amplitude and spectrum analyser display to view the entire trace.
5)Â Â Â Enable TG.
Â
     Figure 4: Reference cable insertion loss before normalization.
6)Â Â Â Normalize the reference insertion loss. This mathematically subtracts a reference signal (stored automatically) from the input signal.Â
– With the Rigol DSA815 Press TG > NORMALIZE > STOR REF and then Enable  Â
Normalize
Â
   Figure 5: Reference cable insertion loss after normalization.
7)Â Â Â Disconnect the reference cable from the RF input.
8)Â Â Â 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.
Â
    Figure 6: Cable-under-test connected.
The screen displays the cable-under-test losses plus the error of the cable-to-cable adapter.Â
Â
      Figure 7: Cable-under-test loss.
Measuring Cable Loss with a Spectrum Analyser
