[Elecraft] K3 on JT-65, ALC varies widely

[Jim Brown]

Hi Dave,

What you're seeing is fairly normal for your setup. The 2.7 kHz 5-pole 
filter is not as flat as the 2.8 kHz 7-pole filter, which I noticed on 
RTTY. I subsequently switched to 7-pole filters (but not in the Sub-RX). 
A JT65 signal is about 220 Hz wide, so when you get near the top of the 
audio bandpass, you also run into the slope of that filter. As with all 
other data modes transmitted by audio, 4-5 bars flashing is optimum.

There's another trick that the latest WSJT-X plays to minimize audio 
harmonics that result from overdriving in the audio chain. If you allow 
WSJT-X to do so, they will automatically put you in split mode, with an 
offset that puts the second harmonic outside the passband of the TX filter.

My recommendations for setting audio levels is here. it follows the 
discussion of USB interfaces. http://k9yc.com/USB_Interfaces.pdf  The 
reason I recommend setting computer sound cards 6-10 dB below audio clip 
is that their percent distortion often rises by 10 dB in that last 6 dB 
of output level. On a vintage Thinkpad, I measured -30 dB second 
harmonic relative to fundamental just below clip and  and -40 dB below 
fundamental with the output reduced by 6 dB.

One update to my "rule of thumb" about whether you need a better 
interface is that, thanks to decoding improvements in the latest version 
of WSJT-X, you should occasionally see good decodes in the range of -26 
to -28 for JT65. And a caution -- while second pass decoding often finds 
a second station on nearly the same frequency as one decoded on the 
first pass, roughly 5% of the time a second pass decode may be bogus.

73, Jim K9YC


On Sun,4/16/2017 2:29 PM, Dave Hachadorian wrote:
> Transmitting on JT-65, I see a lot of variation in ALC and power as the various tones change.  I’m using Data A mode with the stock 2.7 filter.  I just had a QSO with tone frequencies of about 1800 – 1950 Hz, and ALC varied from zero to five bars on the different tones.  Sound card output is about 50%, and Line In level is at 12.  Is this variation normal, or am I doing something wrong,  and where should I set the ALC?