Receiver VS Spectrum Analyzer:about 3rd intermodulation test

Hi all,

Assuming that we are going to do two-tone test for 3rd intermodulation. The simplified test diagram is: a signal generator followed by Receiver and Receiver followed by spectrum analyzer.

We will see 3rd intermodulation from spectrum analyzer, but how do you know the 3rd intermodulation is caused by Receiver or by the spectrum analyzer?

Thanks,

Lunren

Spectrum analyzers will show the various frequency content under the rated input power.
So no chance of 3rd inter modulation is caused by the spectrum analyzer.

Added after 57 seconds:

How ever it is possible to measure the 3rd inter modulation with one tone ?

Two-tone receiver intermodulation generally is done using 3 signal generators combined with a 3-way splitter (output of the splitter goes to the input of the receiver).
Two of the signal generators are used to generate the two-tone interferer (f1 and f2), and the third one is generating the useful received signal (f3 placed at 2f1-f2).
The receiver is tuned on f3, which usually has an initial level 3dB higher than sensitivity level.

You have to be careful how you combine the two signal generators, as they sometimes interact. Use as much isolation as you can.

You can check to see if your spectrum analyzer is a part of the intermods generating by adding a 10 db pad right in front of the analyzer. If the displayed spectrum is the same with or without the pad (same dBc for the intermod products), then the SA is not at fault. If the displayed spectrum improves with the addition of a 10 dB pad, then the SA was partly generating the tones seen. Many SAs have a way of adding this pad from the front panel controls.

Thank you very much for your reply. But what is the 10 dB pad? Can you (or anyone) explain its function? Sorry that I don't know it.

http://www.pasternack.com/product-2-...-10-71469.html

If 10dB attenuator was added, then all frequency (including intermodulations) passing the 10dB attenuator will be attenuated by 10dB. In this case, the displayed spectrum must change with or without the 10dB pad, right? If not, where am I wrong?