[Elecraft] K3 port sharing and FT-8

[Jim Brown]

On 6/11/2019 5:42 PM, Rick WA6NHC wrote:
> A simple test, into a dummy load, set your audio tone for a lower 
> tone, watch the output, repeat with a mid-range tone and a higher 
> pitched tone.  If you don't have to readjust the audio drive to the 
> radio (while keeping NO ALC), you're fine.  If you do have change 
> levels, use the frequency shifting of WSJT-X to stay within the same 
> range.

Rick,

You (or at least your test) are (is) blaming the computer sound card for 
what may also be the response of the TX bandpass filter. I would be VERY 
surprised if the response of a sound card varied more than a few dB from 
200-3,000 Hz. If audio levels between the computer and the radio are 
reasonably set, a few dB is not going push the system into distortion.

It's common for RF, digital, and video engineers, whose signals are at 
generally fixed levels, to fail to understand that analog audio is 
NOTHING like that. Indeed, the level of ordinary voice signals varies 
over a range of at least 30 dB. In analog systems, levels are set so 
that peaks always stay comfortably below clip, where distortion rises 
sharply.

My recommended methods of setting the output levels of sound cards all 
boil down to one principle -- set the output of the sound card at least 
6dB below where it starts to sound distorted. Very easy to do, at least 
three ways.

1) Use your ears -- plug headphones into the computer output, activate 
"Tune," start with the Power slider fairly low and increase it until the 
tone starts to sound harsh. THAT'S distortion. Now, back off the Power 
slider until it sounds half as loud. That will bring it 7-10 dB lower. 
Run the sound card there. (This works because humans perceive 7-10 dB 
"half as loud" or "twice as loud."

2) Use a scope to find clip by seeing flat-topping of the sinewave, then 
back down to at least half the voltage, or maybe slightly more.

3) Use a voltmeter to find where the voltage stops increasing with the 
Power slider, then it down so that the voltage is half or less of the 
max value.

Once you've done this, the sound card should safely drive a line level 
input without overload, and audio transmit level can be set just like 
you would with speech. If the radio has no line level input, you simply 
build a 20 dB voltage divider, (470 ohms in series between computer and 
rig, 47 ohms in parallel with the mic input). or 1K and 100 ohms.

This is all very simple, it's stuff that we demonstrated that we learned 
when we passed our license exams.

73, Jim K9YC