[time-nuts] Low noise frequency multiplication

Ulrich Bangert df6jb at ulrich-bangert.de
Fri Mar 2 04:27:37 EST 2007


Bruce,

thank you for your help! Since it is not known a priori what kind of
source will serve for the 10 MHz input I must take into account that it
is not the absolute state of art. Since the VCXO solution is not far
away from state of the art I consider it the better general choice.

I know Rick's papers about synthesizers since a few years and I have
been impressed by them a lot. Until now I have been thinking that the
complexity with the additional mixers, buffers and filters is to high
but perhaps I am going to re-think it. The private lessons that I
received from you concerning low noise amplification make at least the
buffer part a handable task.

Best regards
Ulrich Bangert


> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Dr Bruce Griffiths
> Gesendet: Freitag, 2. März 2007 00:23
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Low noise frequency multiplication
> 
> 
> Ulrich Bangert wrote:
> > 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
> >
> >   
> Ulrich
> 
> Whilst in general the answer does depend on the application the 
> following observations concerning the phase noise floor of the ~50MHz 
> signal may be useful.
> 
> With a state of the art OCXO with a phase noise floor of less than 
> -170dBc/Hz multiplying by 5 with a low phase noise multiplier 
> will raise 
> the phase noise floor to around -156dBc/Hz somewhat less than that of 
> your proposed VCXO. However if your 10MHz standard has a phase noise 
> floor higher than -160dBc/Hz the 54MHz VCXO will have a lower phase 
> noise floor.
> 
> The phase noise at offsets closer to the carrier will usually be less 
> when multiplying a low noise 10MHz reference than for a higher VCXO.
> 
> If you only need to adjust the frequency by a few ppm then one can 
> cleanup the spurs and phase noise of a DDS reducing them to very low 
> levels by using a cascaded mix and divide technique like that in:
> 
http://www.karlquist.com/FCS95.pdf

With such a circuit you can achieve a phase noise floor (if you use 
appropriate dividers especially in the last stage) approaching that of a

good OCXO.
With this technique there is no need to use a ~ 50MHz reference for the 
DDS if all you want is a corrected 10MHz signal.

Bruce

_______________________________________________
time-nuts mailing list
time-nuts at febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts




More information about the time-nuts mailing list