HP 8753E User Manual page 446

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.
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
coefficient as well.
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.
V i
Return loss = -20 log IFI
Application and Operation Concepts
6-165