Re: simple frequecny multiplier


"Frank Raffaeli" <SNIPrf_man_frTHIS@xxxxxxxxx> wrote in message
news:1112843892.580277.247660@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> The advice Mark gave to use a PLL is pretty sound. What is the phase
> noise requirement?
> You could end up with lower phase noise using a PLL than with a
> multiplier. If you do use
> a multiplier, consider a step recovery diode if you're trying to do it
> in one go. Otherwise,
> consider a two or three stage multiplier.
>
> The best you can do with a 25x multipler is 25x the phase noise offset
> of the source. The phase
> noise of the PLL will be dominated by the VCO if you do it right, and
> that is determined
> mostly by the loaded Q.
>
> see the following text:
> Microwave Circuit Design Using Linear and Nonlinear Techniques
> by Vendelin, Pavio, and Rohde.

thanks il look into that, arnt SRD hard to get now? i was not trying to do
it in one go as such the harmonics from the xtal oscilator contain suficent
5th harmonic to drive the tuned input of a mosfet quite well.

subsequent tuned filter would extract the next 5th harmonic of that asuming
it it was distorting enough. so it is a 5x 5x rather then 25x but i just
wont be able to see the results with my scope. the frequency is fairly
flexible i could use more stages with less multiplication. say 3x 3x 2x

i had asumed a good crystal oscilator would outperform a LC type of vco, and
i know PLL can wander slightly as they can have dead spots in the edge
detector, i hadnt considered the efect of multiplying on the phase noise, i
asumed the Q of the tank circuits will have the same efect
as the Q of the PLL VCO ? the jitter/phase noise/stability of the oscilator
im using is extremly good.

basicaly its one of those projects where it needs to be as good as i can get
without going to extremes (atomic clocks are too expensive) and the phase
noise is integrated over 100ms or so. but nevertheless needs to be
ridiculously small. the reason for multiplying the frequency is to increase
the sensitivity of the phase detector to phase changes in the 40mhz signal
to detect phase change coresponding to changes in delays of less than a
picosecond.

my albeit limited experience of PLL is that it can sometimes take quite a
while to get them to work as well as you hope if you are trying for such
small jitter.

im not sure exactly of the nature of phase noise and what hapens when you
integrate it over such a long period, or how to compare the integreated
noise from a multiplied XO compared to a PLL VCO of a higher frequency.

idealy of course i would like a 1ghz crystal oscilator but ...

Colin =^.^=


.

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