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Design >> RF Design >> why phase noise, why not frequency noise??
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Message started by raja.cedt on May 12^th, 2012, 10:56am

Title: why phase noise, why not frequency noise??
Post by raja.cedt on May 12^th, 2012, 10:56am

dear all,
can any one please tell me why people are so interested in phase noise, why not frequency noise? I guess 2nd order or higher order plls lock the phase so frequency will be constant and so it wont fluctuate but phase may still fluctuate. am i correct...any kind of view will be appreciated..

Thanks,
Raj.

Title: Re: why phase noise, why not frequency noise??
Post by aaron_do on May 13^th, 2012, 5:51pm

If the phase is shifting, then I think the instantaneous frequency must be shifting too...probably in answer to your question, phase noise just sounds more cool.

Title: Re: why phase noise, why not frequency noise??
Post by raja.cedt on May 14^th, 2012, 12:27am

hello,
it didn't understand why phase shift means frequency shift? For example assume pll locked and just phase got shifted....

Title: Re: why phase noise, why not frequency noise??
Post by Forum Administrator on May 14^th, 2012, 1:01am

Frequency is the instantaneous derivative of phase, so if there is noise in the phase then there is noise in the frequency. Since they are so closely related, you are free to use either to describe your situation. As to why the term phase noise is more common that frequency noise, I am not sure. It is likely historical. It is clearly an electrical engineering convention. I have mentioned it to physicists occasionally, and they are largely unfamiliar with the term but very familiar with the phenomenon. They generally refer to phase noise in oscillators as linewidth.

-Ken

Title: Re: why phase noise, why not frequency noise??
Post by loose-electron on May 20^th, 2012, 10:34am

Forum Administrator wrote on May 14^th, 2012, 1:01am:

The "phase noise" term got glued to this based upon the use of PLL's in the timing recovery process.
(disk drives, modem's and other comm signal processing stuff) - the "phase relationship" between data and PLL clock and eye diagrams caused that .

The "line width" term (have not heard that one in a long time) I believe comes from looking at spectral spreading of a single tone on a spectrum analyzer.

Title: Re: why phase noise, why not frequency noise??
Post by biff44 on Jun 10^th, 2012, 10:54am

There actually are both types of noise. You use the one that is most pertinent to your system design. If you have a QPSK radio system, then phase noise most effectively describes the amount of angular noise you would have in the QPSK decision plane.

If you have an FM radio, frequency noise in hz rms might be a more meaningful measurement.

The measurement methods are different.

In general, high phase noise also means high frequency noise, so they are similar. And since frequency is the integral of phase, they are related closely.

Title: Re: why phase noise, why not frequency noise??
Post by Kevin Aylward on Jul 7^th, 2013, 2:59am

Its called phase noise because for V(t)=A(t).Sin(B(t)), “phase noise” are those real voltage/power measured signals in V(t) entirely generated from the B(t) bit, regardless as to whether its frequency or phrase originating in the B(t) bit. Everything in the Sin() bracket is phase in radians, i.e. unitless. Its relevant because any system can, ideally, be set up to ignore either the A(t) bit or the B(t) bit. Hence the value of errors in A(t) or B(t) are of interest, but not, usually, both. The fundamental bit to understanding “phase noise” is that it can be shown that, if A(t) is ignorable, the RATIO of AMPLITUDES of the relevant frequency signals in V(t), in VOLTS or WATTS, is EQUAL to the phase error bit of B(t), in RADIANS. Technically, there is no such thing as phase power (PSD), because phase is unitless!. http://www.kevinaylward.co.uk/ee/phasenoise/PhaseNoiseTutorial.xht