HP 8753E manual SWR / Return Loss

RF Feedthru

RF feedthru, or RF-to-IF isolation, is the amount the RF power that is attenuated when it reaches the IF port. This value is low in double balanced mixers. RF feedthru is usually less of a problem than the LO isolation terms because the LO power level is significantly higher than the RF power drive.

You can make an RF feedthru measurement using the same instruments and setup that you use to measure conversion loss. Because the source and receiver frequencies are the same, the analyzer can use narrowband (tuned receiver) detection to make the measurement. The only difference that you need in the hardware configuration is that the IF filter needs to be removed so the RF feedthru will not be filtered out.

The RF signal is applied to the RF port of the mixer and the feedthru is measured at the IF port.

The RF feedthru level is very dependent on the LO signal that is applied. For this reason, you should make the measurement with the LO signal present at its normal operating level.

You should perform a frequency response calibration to improve accuracy.

SWR / Return Loss

Reflection coeflkient (I’) is defined as the ratio between the reflected voltage (Vr) and incident voltage (Vi). Standing wave ratio (SWR) is deilned as the ratio of maximum standing wave voltage to the minimum standing wave voltage and can be derived from the reflection coeflicient (r) using the equation shown below. Return loss can be derived from the reflection coefficient as well.

r=!z

V i

Return loss = -20 log IFI

Note Mixers are three-port devices, and the reflection from any one port depends on the conditions of the other two ports. You should replicate the operating conditions the mixer will experience as closely as possible for the measurement.

When you measure the RF port SWR, you should have the LO drive level present and set to the expected frequency and power levels The IF port should be terminated in a condition as close to its operating state as possible.

The measurements of LO port SWR and IF port SWR are very similar For IF port SWR, you should terminate the RF port in a matched condition and apply the LO signal at its normal operating level. For the LO port SWR, the RF and IF ports should both be terminated in conditions similar to what will be present during normal operation.

Application and Operation Concepts 6-165