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If you replace the carrier signal in a phase modulated circuit with noise then presumably the noise will be modulated. What must be considered if we wish to detect this noise modulation? I’ve attempted to phase-modulate various types of noise in Matlab (pmmod), and in the FFT spectra I was expecting to see some sidebanding of the more discrete higher-amplitude components (in some frequency ranges) but none are visible. I assume it depends strongly on noise type.

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    \$\begingroup\$ Not sure what "phase modulated noise signature" is, got a link? Are you just asking about phase noise? \$\endgroup\$ – George Herold Oct 18 '17 at 20:22
  • \$\begingroup\$ Let's say I have 1/f noise and I phase-modulate the noise at 1000 Hz. I am wondering if there is a way to detect the modulation. Is it possible for any type of noise? \$\endgroup\$ – Sansley Oct 18 '17 at 20:49
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    \$\begingroup\$ What is "phase-modulating noise at frequency..." I know all the words pretty well, but your combination of them in a sentence doesn't make any sense to me. Would you please edit your question and add a formula for how one modulates the phase of noise? \$\endgroup\$ – Marcus Müller Oct 18 '17 at 22:03
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    \$\begingroup\$ So the phase of the noise is random. If you modulate it, then the only way I can see to measure the modulation is to do a correlation of the original noise signal and the now modulated noise signal. (Correlation would be done by multiplying the two signals together.. I think that works.) \$\endgroup\$ – George Herold Oct 19 '17 at 13:44
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The classic analog spectrum analyzer cannot detect phase variations of the carrier, unless sidebands are created.

Given the modulation Factor is deltaFreq/Fmodulation, and the modulation factor is expressed in radians, wee can see the simply fact of "some phase modulation" might not cause sidebands to exist.

By the way, on a spectrum-analyzer you cannot tell the difference between PM sidebands and AM sidebands. You need to examine the sidebands as PHASORS; PM have oppositely rotating phases, so the carrier energy remains constant; AM phasors rotate in the same direction and cause variation in the carrier energy.

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