[time-nuts] Low noise frequency multiplication

Stephan Sandenbergh stephan at rrsg.ee.uct.ac.za
Thu Mar 1 08:32:16 UTC 2007


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.



> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Dr Bruce Griffiths
> Sent: 01 March 2007 03:52 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Low noise frequency multiplication
> 
> 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
> 
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