[hpsdr] Neat front end from Phil

[rwmcgwier at comcast.net]

I believe the naturally balanced view, lower noise, and wider gain bandwidth of 
the OPA1632, which I believed is needed will  make this right.   The LT1128 has 
higher noise at 4.2 nv/Hz than the OPA1632 and it has 1/4 the gain bandwidth of the OPA.

 I believe we should prototype this and work out the kinks.  It is my opinion 
that Phil has figured out all of the relevant features in making this mixer work 
if they are born out in implementation details:

The return loss from the sampler circuit, in band and out of band will present 
the filters and preamp/antenna in front of the circuit with proper termination.   I suspect the  return loss will be good over the entire set of amateur bands below 54 MHz.

The switching approach seems right in that it removes the need for inverted 
pulsing and I am only trying to work out in my head how the grounding or 50 Ohm  termination of these parts and the variants does the right thing.  It is long 
days and late nights here at Central States VHFS meeting so my mind is a bit 
foggy.  It was 100+ degrees on the antenna range this morning.  The measurements  went on for hours!

But in my opinion,  this is the approach to take and we have a prototype heart 
for Horton and all we need are the LO's and a few parts selections to be there.

Saddle up, let's ride.

Bob