[time-nuts] Low noise frequency multiplication

[Dr Bruce Griffiths]

Stephan Sandenbergh wrote:
> Hi Rick,
>
> You are absolutely right - I should've mentioned the specs first.
>
> It is an Oscilloqaurtz 8788 locked to GPS.
>
>  Phase noise at 10MHz:
> 		Hz		dBc/Hz
> 		1		-100
> 		10		-130
> 		100		-152
> 		1e3		-160
> 		1e4		-165
> 		1e5		-165
> 		1e6  		-165 
>
> Allan dev: < 1.10e-12 (not locked)
>
> I suspect that it shouldn't be too hard to preserve these specs. (that is
> apart from the obvious 20dBc/Hz increase due to the 10x multiplication).
>
> Noise floor is of importance since I'm clocking ADCs and DDSs. These are
> affected by high frequency jitter. I've got more than one of these
> crystals/ADCs/DDSs which I would like to keep reasonably synced (the reason
> for the common-view GPS) so the longer time scales are also important. 
>
> I just noted that the noise level of that diode multiplier in the previously
> mentioned article is way below that of my OCXO. From there my curiosity. 
>
> I agree that phase-locking to 100MHz oscillator is the best way to go, but
> as a first iteration multiplication is a good start. 
>
> Judging by your reply the x2 and x5 approach should probably be avoided? 
>
> Regards,
>
> Stephan. 
>
>
>   
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>> Behalf Of Rick Karlquist
>> Sent: 01 March 2007 01:42 AM
>> To: Discussion of precise time and frequency measurement
>> Cc: 'Discussion of precise time and frequency measurement'
>> Subject: Re: [time-nuts] Low noise frequency multiplication
>>
>> Stephan Sandenbergh wrote:
>>     
>>> How difficult is it to multiply a frequency standard from 10MHz to
>>>       
>> 100MHz?
>>     
>>> The other day I stumbled across the following article on Wenzel's
>>>       
>> website:
>>     
>>>             http://www.wenzel.com/pdffiles/RFDesign2.pdf
>>>
>>>
>>>
>>> It describes a way in which an analogue odd-order frequency multiplier
>>> could
>>> be built cheaply with superior noise characteristics. This circuit that
>>>       
>> is
>>     
>>> described is really simple and quite ingenious. Unfortunately, I would
>>> like
>>>       
>> I remember that article from when it was first published.  It is
>> quite clever, but has no special phase noise advantage compared
>> to any other passive limiter or passive frequency doubler based
>> on a full wave rectifier.
>>
>> You need to be more specific about your multiplier requirements.
>> When I worked for Zeta Labs, we used to get vague RFQ's like this for
>> multipliers, and then have to develop a specification.  That is
>> almost more difficult that actually building the multiplier.
>> Are you after good Allan deviation or low phase noise?  Do you
>> care about phase noise floor?  How clean is the original oscillator?
>> In the HP 8662, they double a 10811 three times to 80 MHz and then
>> strip off the phase noise floor sidebands with a crystal filter.
>>
>> Regarding X10:  I suggest you double to 20 MHz, take that as an
>> intermediate output, and then quadruple the 20 MHz to 80 MHz.
>> Then mix the 80 and 20 to get 100 MHz.  As far as heroically
>> multiplying directly by 5:  been there, done that, got the coffee
>> mug and T-shirt.  Don't do this at home kids.
>>
>> Rick Karlquist N6RK
>>
>>
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>
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>   
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