[time-nuts] Upper limit on phase noise from two oscillators.

Mike Feher mfeher at eozinc.com
Wed Apr 27 12:26:01 EDT 2005

Thanks for the info but I have not forgotten any of what you said. I do not
believe I ever suggested using a noiseless oscillator as I also am not aware
of such a thing, and if it existed this group would not exist either. I have
been making phase noise measurements and trying to achieve very low phase
noise at millimeter waves requiring the use of ultra clean fundamental
sources for over 20 years. I have had the best minds on this matter working
for me or with me on this issue. An audio or a 10 MHz spectrum analyzer
today is probably one and the same and the noise floor at DC or at 10 MHz is
also the same. Yes, at DC you have to contend with the DC component just
like at 10 MHz you have to contend with the 10 MHz carrier component. I keep
on saying that the only way to really do it is to multiply your source up to
a reasonable frequency like 1 GHz which would raise the phase noise by 40 dB
allowing you to make the measurement at DC using the regular phase shift and
mixing scheme. - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731

-----Original Message-----
From: Jeffrey Pawlan [mailto:jpawlan at pawlan.com] 
Sent: Wednesday, April 27, 2005 12:06 PM
To: time-nuts at febo.com
Cc: Mike Feher; David Kirkby
Subject: Re: [time-nuts] Upper limit on phase noise from two oscillators.

On Wed, 27 Apr 2005, David Kirkby wrote:
>if one oscillator was perfect (no phase noise at all)
also wrote:
> If one oscillator was noiseless

If you or any one on this list has a lead about where to find a noiseless
oscillator, PLEASE let me know immediately.  I want one!

On Wed, 27 Apr 2005, Mike Feher wrote:
> This does not seem to make sense. No matter what you are measuring it is
> assumed that the phase noise of the item being measured is greater than
> piece of test equipment doing the measurement, else you would not see it.

That is the correct description for using a Spectrum analyser to measure
noise. I have the HP phase noise measurement utility built into my spectrum
analyser and as long as the signal being measured is at least 10dB worse
the noise floor of the spectrum analyser noise floor, the readings are
reasonably accurate.  But there is no spectrum analyser made that is good
to measure a precision oscillator like the 10811. Therefore the measurement
done by mixing two of them together and one must phaselock one oscillator to
other and drive the mixer ports at 90 degrees from each other. The resulting
output is zero frequency (DC) and an audio spectrum analyzer reads the
which is the summed phase noise of the two.
  Mike,  you forgot that the audio spectrum analyzer does not need to have
dynamic range of the phase noise unless you are measuring quite close to the
carrier simultaneously with measuring far from the carrier. If you add high
filters you can make your measurements in segments so that a modest audio
spectrum analyser will work. You may have also forgotten that the carrier
becomes a DC voltage since the two oscillators are phase locked to each
You must use a blocking cap (minimal HPF) to not overload the inout of the
soundcard if it is DC  coupled.

I hope this helps but I need to get back to work now.


Jeffrey Pawlan

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