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I am doing a simple $P_{in}$ - $P_{out}$ linearity test for my RF generator/receiver system. The generator provides an IQ modulated CW single tone with $P = - 80dBm$ at 1 GHz.

If we look at the RF front end of the detector;

  • We see that RF input is first shifted to IF band (diagram), then adjusted to cover ADC full range and filtered before ADC.
  • ADC has $f_{sample} = 250\,\text{MHz}$. Also, the device allows $f_{sample}$ down to 10 kHz.

enter image description here

When I check out the PSD window in FFT Analyzer, I observe the presence of the transmitted signal and that's fine.

What I also observe is the noise floor roll-off on both ends of the PSD figure.(below) For several sampling frequencies up to 100 MHz, the onset is at ~ 80% $f_s/2$. Then, for the range covered at $f_s = 250 MHz$, we observe that the flatness is severely degraded.

  • What is this phenomenon? Can you explain the mechanism behind it ?

enter image description here

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  • $\begingroup$ So If I understand this right you are sampling a sinusoid at 1 GHz directly with a 100MhZ or 250Mhz sampling rate? $\endgroup$
    – Dsp guy sam
    Apr 9, 2020 at 17:05
  • $\begingroup$ No. The sinusoid is first shifted to IF band before sampling. The full diagram of the RF Digitizer board can be found here and here $\endgroup$
    – Krlngc
    Apr 9, 2020 at 17:09
  • $\begingroup$ @krlngc, ok, then please add these details regarding the IF frequency being used, because it is not obvious from the question. $\endgroup$
    – Dsp guy sam
    Apr 9, 2020 at 17:11
  • $\begingroup$ Also, what band of RF Frequencies are going into the 3 IF Paths (one of them being directly passed)? $\endgroup$
    – jithin
    Apr 9, 2020 at 17:13
  • $\begingroup$ There is a 62.5, 187.5 MHz IF. If the sinusoid lies in these IFs the two sided or actual signal bandwidth is till 125 and 375 MHz respectively. You mention that sampling rate at maximum is 250MHz which is not good enough for both IFs. Am I missing something here? $\endgroup$
    – Dsp guy sam
    Apr 9, 2020 at 17:34

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