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<p class=MsoNormal><span style='color:black'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:black'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:black'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:black'>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</span><span
style='color:#1F497D'> </span><span style='color:black'>was problematic.
In short, as I said previously, to avoid this problem, DON’T connect the
microphone ground to anything but the microphone.</span><span style='color:
#1F497D'> </span><span style='color:black'>This applies whether you’re
running Penelope QRP or running Penelope with amplifiers.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:black'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:black'>73,<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'>Warren NR0V<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p><span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>No virus
found in this incoming message.<br>
Checked by AVG - www.avg.com<br>
Version: 9.0.851 / Virus Database: 271.1.1/3047 - Release Date: 08/02/10
23:35:00</span><o:p></o:p></p>
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