[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
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
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