[time-nuts] How do time-nuts measure phase noise?

[Bob Camp]

Hi

An overly simplistic way to look at it:

When locked, the "beat note" between the two oscillators is DC and easy to
block. When unlocked the "beat note" is at an audio frequency and you need
to knock it down a lot before your sound card can handle it.

One assumption we are making - it's close in noise that you are concerned
with rather than noise that's > 10% off of carrier. If you are only
concerned with far removed noise, there are notch filter approaches that do
work on a single oscillator.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Monday, August 17, 2009 11:24 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] How do time-nuts measure phase noise?

The mixer output is only sensitive to phase differences for small
deviations about 90 degrees phase difference between the 2 sources.
With unlocked sources AM noise also affects mixer output.
In order to measure phase noise without phase locking you need something
like a COSTAS receiver or equivalent.
For detail on one approach to this download and read the Symmetricom
5120A manaual.

Bruce

iovane at inwind.it wrote:
> In order to simplify the project, and given that I wouldn't need to
distinguish 
> what source the noise comes from, I could use only S1 and X sources. And
here is 
> my last question. If I don't lock the sources to one another (say two HP
10811), 
> what is the unconvenience? An additional signal in the spectrum, or
something else 
> that I don't figure now?
>
> Thanks and kind regards,
>
> Antonio I8IOV
>
>   
>> Some have commercial phase noise measurement systems like Symmetricoms
>> 5115A, 5120A. 5125A.
>> Some have older HP phase noise measurement systems like the 3048A etc.
>> The rest of us have to build our own systems based around sound cards or
>> surplus spectrum analysers together with the necessary mixers, low noise
>> amplifiers etc.
>>
>> Unless your sources are very noisy you wont be able to measure their
>> phase noise with just a selective voltmeter or even an RF spectrum
analyser.
>> If you cant build it, then you need to acquire something like the HP3048
>> or the NIST phase noise measurement box currently on ebay.(Item:
>> 260423038423)
>>
>> Bruce
>>     
>
>
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