[time-nuts] External clock for Analog to Digital Converter inGPS Rx front-end

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 3 18:20:18 EST 2007


Arnold Tibus wrote:
> Thanks Bruce, 
> 'it generates poses some interesting challenges' 
> sounds interesting, could you give some little details 
> to get better idea of it?
> many thanks,
>
> Arnold, DK2WT
>   
Arnold

As can be seen from the attached schematic, the 50X frequency multiplier 
simplifies achieving around 1 radian of phase shift at 500 MHz.
The corresponding phase shift at 10MHz is only around 1.15 degrees, 
which as long as the tank Q isn't too high will not produce a 
significant change in amplitude.
A phase shift of more than a few degrees with such a simple circuit will 
introduce significant non linearities and amplitude changes.
Having to use a multiplier chain to overcome these problems is an 
unnecessary complication and expense when one just wishes to phase 
modulate an ADC clock which has a frequency of less than 10MHz.

Calculating the phase noise spectrum requires that the spectrum of the 
varactor modulation noise and the tank component values and their 
associated parasitics be known with reasonable accuracy. This is 
particularly difficult for the varactor, which contributes phase noise. 
It is far easier (in principle) to measure the resultant phase noise 
spectrum, however doing so isn't a trivial exercise.


In contrast its much easier to calculate the phase noise at the output 
of a comparator if the spectral characteristics of the noise signal are 
known.


Bruce
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