[time-nuts] Low noise frequency multiplication

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Mar 1 16:02:32 UTC 2007


Hi foks,

I want to put forward a similar but slightly different question: 

Suppose I need an clock running at around 50 Mhz for an DDS. Because of
the DDS it need not be exactly 50 MHz, can be 52 or 54 MHz too.
Basically this clock shall be derived from a 10 MHz source (OCXO,
Rubidium...) The OUTPUT of the DDS is to be used as an frequency
standard, with the DDS being an complete digital steering circuit. 
If I have the choice to use 

a) an harmonic X5 multiplier for the 10 MHz signal

or 

b) a 54 MHz VCXO with the following specs: 0.8 ps RMS jitter, noise
floor -145 db @ 100 kHz offset phase locked to the 10 MHz

what is the prefered solution? Or is the answer dependent on what I plan
to use the frequency standard for?

TIA
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Dr Bruce Griffiths
> Gesendet: Donnerstag, 1. März 2007 16:03
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Low noise frequency multiplication
> 
> 
> 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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> >> time-nuts at febo.com 
> >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> >>     
> >
> >
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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
> 
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