Isolation Measurements of Frequency Converting Devices


See other Frequency Converting Device Measurements

What is Isolation?

Isolation is a measure of the leakage, or feedthrough, from one port to another. The more isolation a mixer provides, the lower the amount of feedthrough. Isolation is measured at the same frequency as the stimulus, not the converted or shifted frequency. Therefore, Frequency Offset capability is not necessary for these measurements.

Three main isolation terms are of interest for mixer measurements:

  

Why Measure Isolation?

Any unwanted signal "leaking" through the device will mix with the desired output signal creating intermodulation products, adding to  intermodulation distortion. These unwanted signals may be difficult to filter out.

How to Measure Isolation

Use the following setups to measure the isolation of a mixer:

Note the following:

 

LO-TO-IN ISOLATION

 

LO-TO-OUT ISOLATION

 

IN-TO-OUT ISOLATION

 

 

Measuring Converters vs. Mixers

Measuring IN-to-OUT feedthrough of a converter is identical to that of a mixer. The IN-to-OUT feedthrough is generally very small for a converter due to the inclusion of an IF filter in the device. Because of this, the measurement may require the VNA to have increased dynamic range.

Measuring LO leakage (LO-to-OUT and LO-to-IN) of a converter requires a different technique because the LO port is typically not accessible: