[hpsdr] Alex interface

[MDS Info]

Hello,

I need to tell anyone out there building their own boards and, particularly,
and Alex or Alex-like
board that I ran into a problem with interference coming from the Hermes
board, and getting back into
the rx.

It took me a long time to find this, because the signals could not be
measured on a scope, they were
too low.  But, the Hermes rx saw them easily as harmonics every 8 kHz on 17M
and above.

Basically, if you recall those 'filters' near the I/O connector where the
FWD and REV power come in AND
they also are tied to the Alex FWD and REV power.  The filters I bought were
a substitute for the part
originally specified on the Hermes BOM.

They are a PITA to install, and rarely does the center ground for the middle
capacitor get soldered well
by hand.  Anyway, they either did not work or not soldered well on EITHER of
my Hermes boards (both did
the same thing).

What I did to solve the problem is to install an smt 100uH and a 0.01 to
ground for F11 and F12.  This completely
removed the harmonics.  BTW, removing the zero ohm resistor from FWD and REV
to the aux ADC chip also removed
the harmonics.

So, the harmonics were coming from Hermes via the FWD and REV signals into
Alex and being picked up
by the filter circuitry on Alex and sent back to Hermes via the rx
connector.

Be aware!

73's

Glen K4KV