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

[Arnold Tibus]

On Sun, 04 Mar 2007 12:20:18 +1300, Dr Bruce Griffiths wrote:

>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


Bruce, 
many thanks for the excurse, one never stops learning. 

Arnold, DK2WT