[hpsdr] Hermes

[Lyle Johnson]

> I mentioned this before without any returned comments so here again as a 
> warning/request for correction of my misunderstanding.
> 
> I have one of the QS1R boards which does not have a preamp.  Input 
> voltage for the AD is basically Vdd plus the base breakdown voltage.  I 
> popped the AD by turning an external pre-amp on before turning the QS1R 
> on.  The preamp alone could not put out more voltage than the AD could 
> stand IF IT WERE TURNED ON.  Phil C. left the preamp off so that the 
> QS1R could be used in undersampling mode.  I would prefer to have the 
> preamp chip in line to protect the expensive
> AD, even if it does  reduce the dynamic range a bit.  Or a jumper 
> block/solder link to put it in and out.  Comments welcome.  N0UU

Interesting observation.  Might need some protection (that doesn't 
compromise IMD) at the input to the board with the ADC.  One of the 
problems with building blocks...

Or, a better preamp.  One can be made with discrete parts that could 
outperform the LTC6400.  Or maybe the dynamic range isn't compromised 
enough to worry about. I know we faced this issue with Mercury and 
decided the convenience of the preamp for some applications might 
outweigh the small reduction in dynmaic range it caused.  There was 
lively debate on this topic!

73,

Lyle KK7P


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