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Zero span spectrum analyzer .FFT, sweep, filters and RBW.

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xugmu:
I don't actually need to demodulate the signal. I'm just going to look at it in the time domain, the question is whether it would be better to do it in FFT analyzer mode or in sweep analyzer mode.

xugmu:

--- Quote from: ebastler on August 06, 2024, 02:13:43 pm ---
So, prey tell, what did you mean by "spam"? Or do you want to tell us that you mis-spelled it six times in a row, could not be bothered to correct it after it was pointed out to you, and instead chose to write a snide comment?

--- End quote ---




It's not that complicated to understand either. I don't speak English perfectly and I have confused spam with span.

radiolistener:
You can use digital oscilloscope or digital analyzer with analog input with high enough input bandwidth and capability to record long enough continuous sample. Then you can analyze it in Octave or Matlab.

For example my SDS1102X allows to record 14 Mpts record at 1 GHz, 500 MHz, 50 MHz, 10 MHz, 5 MHz, 2 MHz, 1 MHz, 500 kHz, 200 kHz, 100 kHz, 50 kHz, 20 kHz sample rate (sample rate below 50 MHz needs to manually disable Roll mode).

RBW = SampelRate / N

where N is point count

As you can see at 10 MHz sampel rate it has RBW = 10M / 14M = 0.71 Hz.

But this is for rectangular window (or no window) function.

If you're applying some window function before FFT, it affects RBW. So it depends on your window function choice, which depends on what is more critical for you - frequency resolution or amplitude resolution.

With window function RBW = RBWrectangular * ENBWwindow.

For Kaiser window it can be estimated as: ENBWKaiser ≈ 1 + β​/2

where β​ is β​ parameter for Kaiser window, which depends on your requirements for frequency or amplitude resolution.

For example you can estimate Kaiser β parameter with this Octave function:

--- Code: ---% Oppenheim, Schafer, "Discrete-Time Signal Processing", p.475-476.
% A - attenuation in dB
function beta = kaiserBeta(A)
  if (A >= 50)
    beta = 0.1102 * (A - 8.7);
  elseif (A < 50 && A > 21)
    beta = 0.5842 * (A - 21)^0.4 + 0.07886 * (A - 21);
  else
    beta = 0;
  end
end

--- End code ---


PS: as you can see, there is no universal solution and it all depends on your requirements.

ebastler:

--- Quote from: xugmu on August 06, 2024, 06:20:20 pm ---It's not that complicated to understand either. I don't speak English perfectly and I have confused spam with span.

--- End quote ---

Yes, I figured that out. My point was that (a) you can and should correct your original post, and in particular its thread title. And (b) there was no need for the snide reply to joeqsmith. "Oops, sorry, I have corrected the typos" would have done nicely.

gf:

--- Quote from: xugmu on August 06, 2024, 06:17:46 pm ---I don't actually need to demodulate the signal. I'm just going to look at it in the time domain, the question is whether it would be better to do it in FFT analyzer mode or in sweep analyzer mode.

--- End quote ---

Neither with FFT nor with a swept SA you'll look at the signal in the time domain. They provide a frequency domain view.

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