Sharing some experiments done with an SDR receiver and the FT-991A to analyze the effects of the digital gain control settings vs ALC and RF power values. Used FLdigi for RTTY and WSJT-X for FT-8 modes.
This basic rule also applies to linear amplifiers. ALC has always caused artefacts. Much better to put low enough power in that the linear that the LAC is hardly used. Really good video.
This is very very helpful. I thought setting the DT again was set and done… but I just learned that I need to set it differently based on the mode. I think I need to check my ALC more often!
Apologize to disappoint you but the reason for the “artifacts” is not the ALC kicking in but is the receiver being overloaded by the higher power level. You’re sending too much of a signal level into it. In fact if you watch carefully at minute 9:08 they appear on the waterfall even before ALC start working, but rather when you have increased the power. Try again this experiment making sure the spectrum analysis is done without overloading. Try to use an attenuator between the radio and the receiver. Go with 30 db at least to start with. Also was your setup for spectrum analysis? I think I missed that. 73
If you set power to 100W , DT = 2 and WSJTX Slider very low then bring up slowly, you should be able to get 30~40W with zero ALC and very clean. Setting power to 100W ensures the PA is biased at it's most linear point, and you are just using audio drive to bring power This procedure was from the PK-31 days, but I find it still valid.
Thanks! High DT Gain means low output from the computer. If you increase the output from the computer, then you have to lower the DT Gain. My computer output settings for audio going to the radio is 80% (to prevent distortion.) For digital (RTTY) my DT Gain is around 4~5 and for phone my DT Gain is around 15. Hope it helps. 73
Thanks for the video ! Please what is the volume level : Speaker audio codec on your computer ? And in the radio menu 73 "data out level" ? Thank you and 73 !
Hi Fernando, audio to radio (playback) is at 80% level, audio from radio (recording) is at 92%. Menu option 73 is at is default value of 50. 73 de VA3HDL
Thanks Pablo for the information and attention! If possible, I will ask more about your radio's menus, such as: Menu : 33, 70 , 71, 72 ? Are these menus above the main ones for the FT8? And I'm sorry but I didn't find the "audio from radio (recording) is at 92%" ? In your video you demonstrate that you work with practically zero ALC and show us that you have done an excellent job. Already my settings until today, I can not proceed a qso with the ALC below 5 -7. Thanks
@@fernandogonzaga1011 33=Enable; 70=REAR; 71=RTS; 72=USB; In the Sound Settings -> Recording Tab -> USB Audio Codec is at 92%, similarly on the Playback tab the USB Audio Codec level is a 80% The coarse adjust can be done with the digital gain setting on the radio, and the fine tuning using the power setting slider on your FT8 program until you obtain the desired peak power without (or minimal) ALC indication. Two things I've seen also impact the ALC is the TX audio frequency selected in the FT8 (I select a TX frequency and lock it in there) and the radio band, when switching bands gain must be tweaked a little again. I noticed the audio gain settings doesn't work exactly same on all bands. 73
@@VA3HDL Good morning, Pablo ! Thanks for your attention, I didn't know that the codec's audio was different from the computer's audio playback. I will makevthe ajustment. The other settings are the sabe. Thank you very much. 73 !
It would be interesting to show a split screen with simultaneous receive station to see what your adjustments do to received quality. Right? Or 5 seconds Rx then 5 seconds Tx would be an option
That is a good idea. One of the reasons I made this video is because a neighbor that is also a ham, was looking at his waterfall when I was transmitting and he was helping me to "tune" my ALC, to remove the harmonics. We never recorded his screen. 73.
Congratulation! I can't complain, I'm very happy with it. I've used all modes, including satellite FM/SSB, FT8, RTTY, C4FM digital voice, etc. The only mode I didn't do is CW, because of lack of practice. There are users groups in FB, groups.io, etc. Lots of information and videos available. 73.
I have had mine for a few months and love it so far, still a lot to learn but have used SSB, FM and FT-8 and WSPR and all seem to work well. I get good reports on the audio.
Hi Pablo, I tried to replicate this experiment today. Do you remember if when doing this, you had any antenna connected to the SDR and what the RF gain of your SDR was? I appreciate this video was posted a year ago and you might not remember. 73
@@VA3HDL Sounds good, thanks for the reply, I did the same. For me, on FT8, DT gain 4 worked better. On lower gain the power my FT991A was putting into the dummy load (as seen on the external cross-needle meter and on the radio display) was lower than my desired power (even with no restriction from WSJTX). On DT gain 5 the ALC started to kick in. Not sure why our results are a bit different... It's very interesting. Could it be because I conducted my tests on the 2m band? (144,174 MHz)
@@ohaya1 Mine is always between 4 and 5. One thing you can try is to set the power around 75% in WSJT-X and the digital gain to a value that starts kicking the ALC. In that way you can fine tune with the WSJT-X power slider only and leave the radio alone. You can set WSJT-X to remember the power setting per band. The ALC will work differently on each band and also on different audio frequencies. This is why I leave my audio TX frequency fix between 1000 and 1500 Hz. So once I set the power level desired I know that it will not change.
