[time-nuts] Low noise frequency multiplication

[Dr Bruce Griffiths]

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