mac Post subject: calculating phase noise of pll / sampling
loop Posted: Fri Nov 02, 2007 3:52 am
Captain
Joined: Fri Nov 02, 2007 3:45 am Posts: 8 Location: germany
consider I want to phase lock a 23 ghz VCO to a 20mhz reference
( 120dBc/Hz) and i am thinking about how to get the best phase
noise performance
if i do it with a single loop pll i can
roughly calculate phase noise to be 20log(N) (with n= 150) for
3ghz output frequency at 76dBc/Hz inside the loop bandwith
with a perfect phase detector
found some publications using
mixing techniques including comb generators and pretuning and i
wondered how they are superior to the single loop solution with
that division ratio.
if i use a sampling phase detector
with wide bandwith as a clean up loop in a configuration of
: comb generator driven at 20mhz and mix 3ghz vco output with the
149th harmonic ( 2980mhz) what phase noise can i expect ? 
how strong is the influence of the phase noise of the 149 harmonic
of 20mhz onto overall system noise ?? i calculate phase noise
of 2980mhz mixing signal to be 120dBc/hz (43.5dB)=76.5dBc/Hz
am i right to say inside the loop bandwith i can expect
phase noise of my loop to be average of reference phase noise
and harmonic phase noise , calculating sqrt( 120dbc*76.5dBc)=
95.8dBc/hz thus would result in an improvement of nearly 20dB
comparing to the single loop solution ?
does someone has
got experience with phase locked loops using harmonic locking ?
how do i calculate the phase noise i can expect if i use mixing
/ harmonic locking techniques ?
mny thanks for comments
_________________ regards mac
Top
Tony Kurlovich Post subject: Posted: Thu Nov 08, 2007 5:10 pm
Captain
Joined: Thu Oct 19, 2006 6:02 pm Posts:
7 The answer to your last question is no. For this application,
you can't average dbs. If you could, a large PN could be reduced
by adding a lesser PN.
If the phase noise components to
be added are not correlated, it would be the sum of the noise powers.
If the components are correlated (this case), it is the sum of the
noise voltages. This will yield the same answer as 20log(150). Which
is only for small deviation PM.
If your output is 20MHz*150,
0.1 Degree at 20MHz is worth 15 Degrees at 3000Mhz. You don't get
one without the other.
Posted 11/12/2012
