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I have an ADC sampling a downconverted RF signal at fs. I obtain N=2^15 samples of the RF input signal "fcarrier + fs/10".

The more I average, the more the entire noise floor shifts down (as expected, 10dB/dec) but also the phase noise skirt. I can average down to 100dB SNR (I didn't wait longer). For 1000 averages, the (zoomed) spectrum looks like this:

enter image description here

The phase noise skirt just moved down by 30dB as compared to the non-averaged version. This suggests I can average the phase noise out. But I would not expect it to be averaged out. According to http://www.bitsofbits.com/2015/07/07/signal-jitter-and-averaging/, the signal should be a cosine convolved with an exponential ("signal-leakage like") after infinite averages.

  • Can phase noise be reduced?
  • If yes, does can it be averaged out same as white noise? (This is what I observe above).
  • How does this fit into the link posted above?
  • Does it depend on the input signal? (sinusoid vs wideband)?
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  • \$\begingroup\$ What sort of equipment gives enough resolution to read 4.761904761904762 MHz with that level of precision? \$\endgroup\$ Commented Jul 10, 2018 at 22:32
  • \$\begingroup\$ As an engineer, numbers like 4.761904761904762 make me chuckle. \$\endgroup\$ Commented Jul 10, 2018 at 22:34
  • \$\begingroup\$ It just does not matter. Then no numbers. Numbers removed. \$\endgroup\$ Commented Jul 10, 2018 at 22:37
  • \$\begingroup\$ @Transistor Answer: An extremely picky filter with the highest frigging Q-factor you've ever seen. Float value needs to be more specific. \$\endgroup\$ Commented Jul 10, 2018 at 23:08
  • \$\begingroup\$ If you have random noise, doesn’t an average of 2 = -3dB and 1000= -30dB in noise? \$\endgroup\$ Commented Jul 11, 2018 at 0:30

1 Answer 1

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Let us model additive noise, atop a sinusoidal carrier, as 2 small noise sources in quadrature.

We have this vector diagram

schematic

simulate this circuit – Schematic created using CircuitLab

The inphase noise is AM: amplitude modulation noise.

The quadrature noise is PM: phase modulation, or phase noise or jitter.

Is there any specific reason why more samples, averaged together, would not produce a lower standard deviation?

By the way, what is the ADC/sampling_clock phase noise spectrum?

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Oct 31, 2020

Ahem. Phase is indeed additive.

Take a clean (clean enough) sinsoid, and sum with random noise.

The Time Jitter (phase noise, by another computation) is

Tj = Vnoise / SlewRate

and you can convert from Time Jitter to Phase noise, by recognizing the

PERIOD OF THE CARRIER versus the 1-sigma (RMS) of the computed Tj.

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    \$\begingroup\$ This answer does not really help me yet and does not answer the question (IMO) but maybe I don't understand it yet. There is oscillator noise (consisting on amp.&phase noise) but phase noise is by definition phase only (and let's not assume I/Q for simplicity but only consider one channel). As I hinted in my question, PN is not additive but multiplicative, hence I don't expect averaging to work. Furthermore, does it make a difference if the signal is purely sinusoidal or a complex modulated signal? \$\endgroup\$ Commented Aug 7, 2018 at 7:53
  • \$\begingroup\$ If you show the noise as a centroid around the tip this would be clearer. \$\endgroup\$ Commented Jun 22, 2019 at 16:34
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    \$\begingroup\$ Your first argument, I find indeed a little off the point, as noone doubts that random noise will average away as expeced. Your second argument however is logical. In fact, this is how I simulate the effect of jitter on noise spectra lol. But also another well known argument is: the phase noise spectrum convolves the signal spectrum. So I have the feeling that these two arguments contradict. Is the resolution here, that the phase noise spectrum itself drops in power the longer the portion of analysed waveform ? \$\endgroup\$ Commented Apr 24, 2021 at 10:47

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