1
\$\begingroup\$

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)?
\$\endgroup\$
  • \$\begingroup\$ What sort of equipment gives enough resolution to read 4.761904761904762 MHz with that level of precision? \$\endgroup\$ – Transistor Jul 10 '18 at 22:32
  • \$\begingroup\$ As an engineer, numbers like 4.761904761904762 make me chuckle. \$\endgroup\$ – evildemonic Jul 10 '18 at 22:34
  • \$\begingroup\$ It just does not matter. Then no numbers. Numbers removed. \$\endgroup\$ – divB Jul 10 '18 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\$ – KingDuken Jul 10 '18 at 23:08
  • \$\begingroup\$ If you have random noise, doesn’t an average of 2 = -3dB and 1000= -30dB in noise? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 11 '18 at 0:30
0
\$\begingroup\$

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?

\$\endgroup\$
  • \$\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\$ – divB Aug 7 '18 at 7:53
  • \$\begingroup\$ If you show the noise as a centroid around the tip this would be clearer. \$\endgroup\$ – Scott Seidman Jun 22 at 16:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.