[Elecraft] Setting Up For WSJT-X Receive Levels

[Conrad PA5Y]

That makes perfect sense to me Jim, however I have my AGC threshold set quite high to stop contest mush. I will let you know how I go on during this summer's 6m Es season which is a particularly harsh test in this neck of the woods. I may have to adjust the AGC for 50.313 which is the new 'centre of activity'

73

Conrad PA5Y

-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On Behalf Of Jim Brown
Sent: 13 January 2020 19:20
To: elecraft at mailman.qth.net
Subject: [Elecraft] Setting Up For WSJT-X Receive Levels

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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