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<p class=MsoNormal><span style='color:#1F497D'>Although it’s clearly not
the topic of the day, perhaps some might find this useful:</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>PROBLEM: In a high field strength RF environment,
Mercury emits “spurious audio” on the headphone and line-out ports.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>DETAILED PROBLEM DESCRIPTION: When operating Mercury
near a legal-limit station (e.g., 1500W), there is audio output from headphones
connected to the headphone jack or amplified speakers connected to the line-out
jack. This audio is of the form of rectified, amplified RF – for
example, of the type of the “Donald Duck” sound many of us have
heard from telephones, PC speakers, and audio amplifiers when near a high-power
SSB transmitter. This problem exists with NO ANTENNA CONNECTED to
Mercury, i.e., Mercury is connected to ONLY the Atlas backplane and headphones or
speakers. The problem is much more pronounced on the higher frequency
bands covered.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>ANALYSIS: <span style='color:black'>The first</span>
question is HOW does the RF get into Mercury. <span style='color:black'>The
somewhat surprising</span><span style='color:#1F497D'> </span>answer: The
headphones or speakers connected are the ANTENNA. Try earbuds rather than
larger headphones and the problem is significantly diminished.
Apparently, the TLV320AIC23 CODEC and headphone amp chip is quite sensitive to
RF injected on its headphone and line outputs. The chip rectifies the RF
and then amplifies and outputs the audio on these same ports.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='color:black'>SOLUTION: Fortunately the
headphone amplifier provides a very low impedance output (at audio frequencies)
making it possible to add bypass capacitors from the L and R outputs of the
jack to the GND output of the jack. There is ample space and easy
accessibility to add these capacitors on the bottom of the Mercury board
directly below the headphone jack. I have used 0.01uF Surface Mount
devices. This value (in my case) is sufficient to “bypass”
the RF to a level that it is below the rectification threshold AND is small
enough so as not to load the low impedance outputs or create any amplifier
instabilities. Unfortunately, the line-output is NOT of such low
impedance and would require more complex filtering. In my case, I have a
front-panel jack for headphones and a rear-panel jack for amplified
speakers. I connect both by simply using a “Y” adapter at the
Mercury headphone jack. The headphone amplifier has more than ample
drive. For me, this has completely resolved the problem. Should the
problem return in a future higher field intensity environment, I’ll
perhaps consider a simple buffer amplifier on the output of Mercury</span><span
style='color:black'>. Note that, due to the nature of this susceptibility,
ferrites on the headphone cord will have little or no effect.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><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>
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