[time-nuts] Photodiodes for high frequency OPLL

Wolfgang DL1SKY timenuts at triplespark.net
Tue Apr 2 02:26:53 UTC 2013


On Monday 01 April 2013, Attila Kinali wrote:
> On Mon, 1 Apr 2013 14:24:58 +0100
>
> Wolfgang <timenuts at triplespark.net> wrote:
> > Anyway, each of these down-mixing approaches needs to resolve the
> > mirror frequency problem. I.e. if you modulate your receiver with
> > 7.1 GHz (or use an EOM to do that optically) and detect at 100 MHz,
> > you do not know whether the input signal has an offset of 7.2 GHz
> > or 7.0 GHz.
> > So you might be locking on the wrong one.
>
> If i'm not mistaken, this shouldn't be a problem
> I'm not demodulating the laser, but using a PLL to fix the difference
> frequency. Being on the wrong side would invert the sign on the loop
> gain, thus the mirror freuquency would be an unstable point.
>
Without too much of thinking it seems to me that there are 4 cases 
that show up as 100 MHz. Only 2 of them without the RF mixing. 
2 will have the same loop sign (only 1 without RF mixing). 

A-B = 7.2 GHz 
A-B = 7.0 GHz
A-B = -7.0 GHz
A-B = -7.2 GHz

BTW, what's the linewidth of your lasers?

-Wolfgang DL1SKY



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