[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 05:34:00 UTC 2007


Bilal Amin wrote:
> Hi John and Bruce,
>
> Thank you for your ideas. Now I have a much better picture for the 
> experiment using the comparator. Now, my only concern is a noise generator. 
> I have searched and found out that all the commercially available now 
> generators are very expensive and some what out of my price range.
> What would you guys suggest for a noise generator? Do you think generating 
> noise using a zener diode circuit is a good idea? Or any cheap noise 
> generators are available in the market. Any one has any experience? As I am 
> working in 8Ms/s zone, therefore the noise generator should be of high 
> specs.
> I have a signal generator (Agilent Tech. 8648A). I can use the PM option in 
> this signal generator to get the results, but again the only problem is a 
> noise generator which I can use at the modulating input of the signal 
> generator to modulate the RF signal.
>
> Cheers
>
> Bilal
>
>
>   
Bilal

Zener diodes biased at low currents make good wideband noise generators 
but you need to select and calibrate them.
If the shunt capacitance is too high the bandwidth will be restricted.

Noisecom used to make their diodes available in one off quantities.
One of these would be best if you can obtain one. Some of these are 
usable to 3GHz or more.
You may need to bandlimit the noise somewhat and pehaps amplify it to 
the desired level with a linear amplifier with relatively (less than 
that of the diode) low noise.

A few decades ago thermionic diodes were used as noise sources, in the 
microwave region fluorescent lamps installed through the walls of a 
waveguide have been used.

Bruce




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