[time-nuts] Low noise frequency multiplication
Ulrich Bangert
df6jb at ulrich-bangert.de
Fri Mar 2 09:27:37 UTC 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_lists.febo.com
mailing list