On 1/13/2020 7:10 AM, Tox wrote:
I've been using a kx3 to play with ft8. Finding intermittent severe
QRM is rendering it deaf to due desense. (Signals that are coming in
at -5 disappear from the waterfall when the source fires.)
There is some bad advice in the setup instructions for WSJT-X to set the
RX audio drive to the computer at 30 dB indicated on the green level bar
at the bottom right of the display. This WILL cause weak signals to be
lost to the decoder in the presence of a very strong one. I've learned
to set that audio drive level so that, with RX signals present, that
green bar is around 70 dB but never turns red (a bit over 80 dB).
For those reading the mail -- the logic is simple. The A/D converter has
a dynamic range of about 90 dB. The green bar shows where the top of
that 90 dB is set. Let's say a very strong signal, 50 dB over S9, takes
over the AGC and hits the computer at 40 dB. Another signal in the
waterfall would have to be S9 +10 dB to be decoded -- any weaker signal
falls below the range of the A/D! But if I set the green bar to 75 dB,
an S9 signal would be 25 dB above the bottom of the decoder. Each S-unit
is 5 or 6 dB (depending on each mfr's definition of an S-unit); if 5 dB,
S8 would be 20 dB above the bottom, S7 15 dB, S6 10 dB, S5 5 dB. Now, a
signal between S4 and S5 can be decoded.
Another point. The appearance of the spectrum display can be deceiving,
making signals appear wider than they are. WSJT-X can decode signals
that appear to be covered by strong adjacent signals. The apparent
covering is an artifact of how the waterfall is able to display
amplitude of the received signal. The decoder sees "beneath" what the
display shows.
73, Jim K9YC
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