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Simplify 75Ω cable measurement via 50Ω VNA, connector

27 Jan 2014  | Eric Bogatin

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The test and measurement world is almost universally a 50Ω environment. It used to be that, if you wanted to characterise a 75Ω cable for use in a 75Ω environment, you had to buy a very expensive 75Ω VNA (vector network analyser) or an almost as expensive 75Ω calibration kit to recalibrate your VNA for Ω port impedances. Today that process is much simpler and much less expensive.

If you measure return loss or insertion loss, the impedance mismatch between the 75Ω cable and the 50Ω ports of the VNA introduces ringing and reflection artifacts. Such artifacts wouldn't appear if the cable were used in its correct operating environment. Figure 1 shows an example of the measured return and insertion loss of an RG59, 75Ω cable in a 50Ω environment.

A 75Ω cable measured with a 50Ω Teledyne LeCroy SPARQ Signal Integrity Network Analyzer.

Figure 1: A 75Ω cable measured with a 50Ω Teledyne LeCroy SPARQ Signal Integrity Network Analyzer. Scale is 5GHz full scale.

There is a three-step process that lets you avoid these artifacts and use a 50Ω VNA to measure a 75Ω cable, or in any other impedance environment.

The first step is to perform the measurement of the 75Ω cable with the 50Ω VNA, calibrated normally at 50Ω. Of course, the return loss will show reflections and raw data that are difficult to interpret directly from the front screen.

The second step is to renormalise the measured S-parameters to a port impedance of 75Ω. This is a mathematical process that takes the measured S-parameters with the 50Ω VNA and recalculates the S-parameters you would have if the ports were 75Ω. All modern VNAs have this feature, and many free simulation tools can do it, as well.

Figure 2 is an example of the return and insertion loss renormalised to 75Ω. After renormalising, the S-parameters would be the same as if the ports were recalibrated as 75Ω. However, these S-parameters still have a small artifact.

Measured RG59 cable with ports renormalized to 75Ω. Note the improvement in return loss at low frequency.

Figure 2: Measured RG59 cable with ports renormalized to 75Ω. Note the improvement in return loss at low frequency. Scale is 5GHz full scale.

The ports of the VNA are effectively transformed into 75Ω ports, but the connectors on the ends of the VNA (and sometimes the connectors to the cable under test) are 50Ω connectors. This means there is a discontinuity in the signal path that will add artificial reflections in the cable measurement.

The third step is to de-embed the cable from the 50Ω connectors. This is a more complicated process, and it can sometimes be avoided if the connector length is small or the bandwidth of interest is below 1GHz to 2GHz.

This step can be implemented in a few ways, which vary from instrument to instrument. In an Agilent VNA controlled by a physical layer test system, the automatic fixture removal technique is available to de-embed the connector launch. In the Teledyne LeCroy SPARQ, a built-in function called gating can de-embed the connector launch with one mouse click.

Final measurement of a 75Ω RG59 cable showing low return and insertion loss without the launch or port mismatch artifacts.

Figure 3: Final measurement of a 75Ω RG59 cable showing low return and insertion loss without the launch or port mismatch artifacts. Scale is 5GHz full scale.

With these simple steps, the intrinsic S-parameters of a non-50Ω cable can be measured, even though your VNA is 50Ω. Figure 3 shows the final S-parameters of the RG59 cable, measured with the 50Ω SPARQ VNA. The ports and 50Ω launches have been removed to reveal the intrinsic performance of the 75Ω cable, as if it were in a 75Ω environment.




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