[time-nuts] Phase noise measurement (was - no subject)
grant at ghengineering.co.uk
Fri Aug 20 11:56:05 UTC 2010
You've come to the right place - well, that is if you want to devote a
significant amount of your life in the pursuit of ever-more accurate
time and frequency measurements....
If you've only got one source then you need to use the frequency
discriminator method (aka delay line method) of phase noise measurement.
Basically you take the output of the source, split it in two, delay
one of the signals, re-combine the two and then measure the resultant
signal on a base-band spectrum analyser.
There are loads of references to this on the web, which describe the
method in more detail, including :-
The Art of phase noise measurement - Dieter Scherer
HP Application Note AN270-2
both available from John Miles web site
The references at the end of these articles, especially the HP ones, are
particularly useful. The operating manual for the HP 11729B or 11729C
Carrier Noise Test Set is also highly recommended.
Yes, there's some maths, you need to understand the relationship between
phase and frequency measurements, but you don't necessarily need ALL the
theory that most of the papers give - don't give up just because of a
few differential equations :)
The limitation of the frequency discriminator method is that the noise
floor of the measurement system is often worse than the DUT, especially
if your DUT is very good, and it's even worse if you're trying to
measure close-in noise. The Sherer article gives a good graph
illustrating this. If you're trying to measure the phase noise of the
oscillator inside a Tbolt then I don't think that a frequency
discriminator will be sensitive enough, although I might be wrong.
Despite what you said, you might want to consider buying an HP 10811
oscillator or similar which you could use in a phase detector
measurement system which is likely to give superior results.
Hope that helps
Mark wrote :-
My new GPSDO leaves me with the question of "how do I measure the phase
noise of what is by far the best oscillator I own... without buying a
better one to compare it to". That question is what brought me to
time-nuts. I'm starting to read some papers on oscillator
characterization that are collected together in a technical note from
NIST that a co-worker pointed me towards, but some of them are giving me
a math-induced headache.
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