keysight spectrum analizer accurate spur measurement using attenuation reduction

[yefj]

Hello,I am trying to see the spurs arround the signal by lowering the attenuation of the spectrum analizer.
but i notice that my noise floor rises for some reason when i lower the attenuation.
also there is the offset parmeter that plays a role in controlling the attenuation.
my spectrum analizer stops attenuation lowering if my offset is too high for some reason??
Why lowering attenuation gives us a more accurate result?
Thanks.

 

[biff44]

well, generally, the thermal noise floor at the input of the spectrum analyzer will get LOWER if you LOWER the step attenuator at the front end.

So if you are seeing the reverse of that, then something is odd about your signal, or your spurious outputs, that are overloading the spectrum analyzer somewhat, or some other artifact of the FFT that the baseband of the analyzer uses. this sort of thing happens a lot when measuring modulated signals, as the specific attenuator, rbw, and vbw settings determine how the signal is displayed

And as a separate comment, a spectrum analyzer front end is for all intents and purposes a mixer...so lowering the input step attenuator drives that mixer harder, and generates new spurs INSIDE of the spectrum analyzer itself. in other words, you can fool yourself to think there are more spurs, or higher spurs in your device if you have too low a step attenuator setting,

 

[yefj]

Very interesting answer.
What is the meaning of driving the inside mixer harder ?
You mean more input power?

Another question is , so you think that the mixer will be compressed?
Thanks .

And as a separate comment, a spectrum analyzer front end is for all intents and purposes a mixer...so lowering the input step attenuator drives that mixer harder, and generates new spurs INSIDE of the spectrum analyzer itself. in other words, you can fool yourself to think there are more spurs, or higher spurs in your device if you have too low a step attenuator setting,

 

[biff44]

if you look up the spur table for any typical microwave mixer, you will see that the level of internally generated spurs typically depends on the RF input power. This would be with a fixed LO input power.

So lets say you have a -20 dBm signal you are trying to measure, and then increase that power to -10 dBm.
you will find some spurs go up 10 dB, some go up 20 dB, and some stay the same. you are just driving the input of that mixer to be operating in a more non-linear region.