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Post subject: phase noise Posted: Wed Apr 05, 2006 11:52 am


Joined: Wed Apr 05, 2006 11:42 am

Posts: 1

Who can explain why phase noise after doubler increase by 20log2=6dB

and not 10log2=3dB ?



Post subject: Phase noisePosted: Wed Apr 05, 2006 7:18 pm


Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104


After a doubler, there are intermodulation products added, as well as harmonic products. See Steven Maas' book Nonlinear Microwave & RF Circuits for more details.

Good Luck,




Post subject: Posted: Thu Apr 06, 2006 10:02 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello Fred,

Without reading the book so far, intermodulations and harmonics are considered as signals and not noise, so how can they increase the phase noise?


Best regards,

- IR



Post subject: Phase NoisePosted: Thu Apr 06, 2006 4:55 pm


Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Sorry, yz, I misread your question, and answered a similar but not identical question about why performance is often worse than 20 log n.

The quick answer to yz's question is that phase noise comes from undesired phase modulation of the carrier by noise. The amplitude of the sidebands is determined by the modulation index (approximately a linear dependence at low modulation indexes). The modulation index doubles when the frequency is doubled. Since power is voltage squared, the 10 log for dB becomes 20 log : log v squared = 2 log v.


Signals and noise are ultimately indistinguishable. If you could distinguish them in any way, noise could be removed from a corrupted signal - an outcome we'd all stand up and cheer for! Considering phase noise as resulting from incidental phase modulation by noise (usually device noise) is a standard approach.

Sometimes principles are easier to see when discussed in sine wave terms, which is why the distinction sometimes gets made.

In this case (and again, my apologies for answering the wrong question), sometimes you see worse-than-20 log n performance due to the intermodulation products.



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