[hpsdr] Increased RX Performance

[john_eckert at agilent.com]

Hi All,

I made a simple change to Janus that increases IMDR a few more DB. Janus
has a lower NF than the Delta 44, but to take advantage of it the gain
of the previous stage (sdr1000) needs to be reduced.  Phil, Ahti et. al.
have also suggested this.  

An easy way to see this is to put a load on the SDR1000 and look at the 
DANL (displayed average noise level) then disconnect the RX cable between 
Janus and SRD1000. I turn AVG on and set it to 100ms. I was seeing >15db 
difference. Cascading noise figures suggest a few db difference is all you
need.

My fix is to add two 1K resistors to form a voltage divider at the input 
to Janus.  Phil has a 39K resistor in series with the tip and another with
the ring. Both sides of these resistors are connected to jumpers(labeled JP3
and JP4 in the documentation, but actually JP2 and JP5 on the board). Simply 
connect the 1K resistors from the output side, closer to the op amps, to 
ground.

The MDS is still the same and the overload is about 5db better. I expected 
10 dB. It also flattens the noise floor across the 192 kHz span.  More 
investigation in this area is required.  With the RX patch cable disconnected 
from the SDR1000 the DANL now drops to -172 dBm! Hmmmm.

I tried to measure the IMDR, but my current setup limits me to 95 dbc. 100kHz
BDR is 110 db @ 1.85 MHz.  Image rejection > 110 db and it holds there pretty
well!  Overloads at -1.68 dBm.  Numbers are with preamp at Med.

Regards,
John
k2ox