Phase Noise Doubler - RF Cafe Forums
Post subject: phase noise Posted: Wed Apr 05, 2006 11:52
Joined: Wed Apr 05, 2006 11:42 am
Who can explain why phase noise after doubler increase
and not 10log2=3dB ?
Post subject: Phase noisePosted: Wed Apr 05, 2006 7:18 pm
Joined: Wed Feb 22, 2006 3:51 pm
After a doubler, there are intermodulation products
added, as well as harmonic products. See Steven Maas' book Nonlinear
Microwave & RF Circuits for more details.
Post subject: Posted: Thu Apr
06, 2006 10:02 am
Joined: Mon Jun
27, 2005 2:02 pm
Without reading the book so far, intermodulations and harmonics
are considered as signals and not noise, so how can they increase
the phase noise?
Post subject: Phase NoisePosted:
Thu Apr 06, 2006 4:55 pm
Feb 22, 2006 3:51 pm
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.
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.
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.