Mixer measurement: Conversion gain too low

Question

The result of the conversion gain changes with the bandwidth settings:

With 500 kHz RBW, the result is correct.

With 100 kHz and 10 kHz, the conversion gain is too low.

What is the reason for the difference?

Answer

This effect occurs only if an external signal generator is used (i.e. for the LO frequency).

To solve the problem it is necessary that VNA and signal generator use the same reference

frequency source. Otherwise the frequency variation will be larger than the bandwidth and the

receiver of the VNA cannot detect the signal correctly.