[time-nuts] Re: Low Phase Noise 10 MHz bench signal source sought

[Bob kb8tq]

Hi

Crystal filtering for far removed phase noise does work. It’s far from a “perfect”
solution. The crystal it’s self contributes noise and it does AM to PM conversion. 
You pretty much always get a “noise hump” as a result. 

Like any filter, a single crystal filter has an attenuation vs frequency curve. You
can sweep it with a networks analyzer to see this. A simple series crystal will be
a very “lopsided” filter. There are well known added bits and pieces that can 
impact this. 

Any filter will have an input/output impedance vs frequency as well as an attenuation 
vs frequency characteristic. This change likely will have an impact on the noise
of whatever amplifier that follows the filter. 

Coming up with “sub thermal” noise this way assumes that the post amplifier 
has some pretty unusual characteristics. As noted in a lot of places, the -174 dbc/Hz
we typically play with assumes a 50 ohm system. If you have a 1 ohm system …
thats’s a different thing.

Bob



> On Apr 8, 2022, at 8:06 AM, Mike Monett <mike at binsamp.e4ward.com> wrote:
> 
> Dear Fans,
> 
> While I was researching cross-correlation, I came across an interesting
> paper that described different methods of reducing OCXO phase noise. They
> involved putting a 10 MHz crystal between the buffer amplifier and the
> output load. One method promised "sub-thermal" phase noise. The idea is the
> crystal impedance rose sharply off resonance, so the energy at those
> frequencies was heavily attenuated.
> 
> I don't know how much additional noise the crystal ESR would introduce, but
> it is an intriguing idea and might be worth pursuing.
> 
> MRM
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