[hpsdr] Echo on Transmitted Audio - Problem Resolution

[Warren C. Pratt]

I believe that I have now totally resolved the problem  (described in
previous posts) I was having with an "echo" on my transmitted audio and I'm
posting what I found as I think there are others experiencing the same
problem.  I am now receiving reports of (and hearing in the monitor)
excellent audio on all bands at all power levels.

 

The short version of the solution is:  Don't connect the shield/ground of
the Penelope microphone input to any other system grounds.  Connect it ONLY
to the microphone.

 

For those enquiring minds who want more detail . after considering the
risks, I had decided to connect the microphone ground to the metal case in
which my hpsdr is fully enclosed at the point where I have a microphone
connector on the front of the case.  The Atlas backplane is also grounded to
the case.  As I'm using my hpsdr at the 1500W power level, my design intent
was to connect all cable grounds to the case at the point they enter - the
objective being to prevent unwanted ground currents from external sources
from flowing through the hpsdr boards.

 

The surprise in diagnosing the problem is that I was able to replicate the
"echo" problem using ONLY the Penelope output level and even with nothing
connected to the output BNC of Penelope!  The problem was barely perceptible
with nothing connected to Penelope's output, quite noticeable with a 50 ohm
resistive termination connected to Penelope's output, and yet worse with a
25 ohm termination connected to Penelope.  In this test, the only
connections to Penelope were the microphone connection, the resistive
termination, and the Atlas backplane.  I did not find it necessary to dig
too much deeper into the Penelope grounding design; however, it became clear
that disturbing the Penelope ground levels by this external microphone
ground connection was problematic.  In short, as I said previously, to avoid
this problem, DON'T connect the microphone ground to anything but the
microphone.  This applies whether you're running Penelope QRP or running
Penelope with amplifiers.

 

Finally, as far as other precautions to prevent RF currents from entering
the case through the microphone port, I have done two things which appear to
be sufficient:  (1) for differential mode currents, I've connected a 1000pf
capacitor from the microphone lead to the microphone ground at my microphone
connector and a 1000pf capacitor from the PTT lead to the microphone ground
at the same location, and (2) for the common-mode currents, I've installed a
31-mix snap-on ferrite on the microphone cable just inside the case.  These
appear to be sufficient on all bands at all power levels.

 

73,

Warren  NR0V

 

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