[time-nuts] Phase noise measurement (was - no subject)

Grant Hodgson 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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