[time-nuts] Low noise frequency multiplication

[Dr Bruce Griffiths]

Stephan Sandenbergh wrote:
> Hi Bruce,
>
> Thanks for explaining - the picture is starting to become clearer. I knew
> there must be a reason why commercial multipliers are so expensive. 
>
> If I understand you correctly the variation in phase (or group delay) caused
> by a variation in temperature messes with the Allan deviation. I can see
> that a high Q filter will probably have a quite sensitive temperature/phase
> dependency. I guess that crystal filters will also have a large
> temperature/phase dependency. The other side of the coin is that unfiltered
> harmonics ruins the phase noise. 
>
> I now also understand the merit of Rick's suggestion - it avoids odd-order
> multiplication all together. 
>
> Thanks for the doubler circuit you posted - it seems quite nifty.
>
> Maybe my best option (from a design time/cost point of view) is to double to
> 20MHz and then buy the commercial x5 to get a 100MHz? 
>
> Regards,
>
> Stephan.
>
>   
>>>
>>>       
>> Stephan
>>
>> The problem is essentially the difficulty in filtering out all the
>> unwanted harmonics.
>> Using a high Q bandpass filter will increase the phase instability due
>> to temperature variations and drift.
>> Even reactive components contribute phase noise which is exacerbated in
>> a high Q tuned circuit.
>>
>> It is better from the phase noise perspective to use notch filters to
>> attenuate the unwanted harmonics and subharmonics rather than a high Q
>> bandpass filter tuned to the desired frequency.
>>
>> The filtering problem is made worse by the fact that the unwanted lower
>> harmonics all have larger amplitudes than the desired 5th harmonic.
>>
>> Multiplying by 2 in a balanced circuit ensures that the fundamental
>> content of the output is suppressed by 20dB or more with respect to the
>> second harmonic and all higher harmonics have significantly lower
>> amplitudes than the 2nd. A balanced circuit also suppresses the odd
>> harmonics.
>>
>> As far as low phase noise dividers are concerned conjugate regenerative
>> dividers can have significantly lower noise than digital dividers.
>> However these dividers are quite complex as they use a mixer plus at
>> least one amplifier a phase shifter or two and a pair of bandpass
>> filters. Adjusting them for low noise operation isn't easy.
>>
>> Bruce
>>
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>   
Stephan

Its not just the temperature coefficients, real inductors and capacitors 
have inherent phase noise.
Silver mica capacitors can be very bad as are ferrite core inductors.
Mylar capacitors are good as are NP0/C0G ceramics and X7R is acceptable 
for low impedance coupling and decoupling.
Air core and iron powder core inductors are good.

Bruce