FT8 on a Yaesu FTDX101MP and ALC, does it really matter? - KJ7E

[@SparkyRadio999]

Thanks I’ve been switching between 2 radios and wondering about this. I thought I was clipping the audio when I was looking at 7300 audio, but then I realized I needed to change the display attenuation. Also noticed that not running with any ALC showing, my tx power was way too low. The whole run without triggering ALC thing is a myth. Thanks for your video!

[@pietleroux5169]

Thanks for showing us the graphical results, seeing is believing! I think the idea that there should be no ALC when sending FT8/FT4 comes from poorly designed equipment where the ALC level worked like a distortion indicator. My TS570D is 25 years old and were tested by the ARRL in 1989, the tests showed that if the ALC level is kept in the ALC zone the distortion is negligible, the goal of ALC is to prevent distortion not indicate it!

[@g0fvt]

Well done for addressing this myth, of course it is still possible to overload the analogue audio path in the radio but that is a separate issue. My FT8 activity was a nuisance to a local (he lives a few hundred feet away) but that was more down to the phase noise of my rig (he too was using FT8).

[@timsul3092]

Would the ALC have any effect on FT8 while using the FTDX-10? I would think not since they're basically the same radio in not all, but many aspects.

[@mdblack98]

The ALC level indication varies by rig model. Some rigs need no ALC showing....some need a fair bit of ALC to achieve full power without clipping. Elecraft rigs expect 4 bars with 5 bar flickering as the proper level. The way you tell is set your rig for 40W and adjust the Pwr slider on WSJT-X to get 30W -- there should be no ALC at all on any rig that I've tested. But start increasing the power towards 40W and some rigs will start showing ALC before getting there. In any case I haven't found a rig yet that can't put 90% of the full power rating without creating harmonics (from clipping). So 45W into a 50W rig setting or 90W into a 100W setting. Most rigs can get full power without any ALC showing. Read the FT8Noise paper on my QRZ page that explains a lot of this.

[@damonkj7e]

Staying in the ALC range does not = clipping (in almost any radio). Clipping is hard limiting which will cause distortion and increased IMD. The ALC range is typically an audio gain stage compression (usually 3 to 6 dB) before you run into saturation and hard limiting. Since the amplitude FT8 tones are constant, non linear compression of the ALC circuit does not add to distortion or increased PA IMD. That was the point behind these videos, it was to refuted claims made by another video I saw. If your seeing audio tone harmonics, then that is being generated in the audio gain stage somewhere other than the PA of the radio.

[@mdblack98]

As I said...the ALC varies between rigs. There are some rigs where any ALC at all will start showing clipping. If you can get full power without any ALC that is desirable. If you start seeing ALC as you increase your percentage (e.g. 40W in to 50W setting) then you need to understand what your ALC is telling you and that is what is rig dependent. There are a few rigs that start clipping at 90% of the rig's power where ALC is not doing what it should be doing. I've tested this on hundreds of rigs.

[@mdblack98]

That's my point "almost any radio" means there are radios where any ALC is indicative of clipping. That's why I said it is rig dependent.

[@Aussie_Ham_Radio]

Hi there thanks for your video - could you do another video where your oscilloscope flat tops and then do a signal report comparison with something like PSK reporter and compare the difference between some ALC and a lot of ALC

[@damonkj7e]

Even with the ALC well above scale, the AFSK audio tone will not clip and distort. See my second video here: kzbin.info/www/bejne/amHKnHh_hbeCf9k

[@M0JHN]

With the changes in digital modulation with the ALC compensating up and down fluttering would that not introduce unexpected changes in the receive signal thereby increasing the decode error.

[@damonkj7e]

With single tone FSK type of modulation (FT8, FT4, PSK31...) the transmitter ALC will have no effect as the audio levels do not change, they are a constant level and shift in frequency only. It sounds like you could be referring to the receiver AGC controls which is an another subject not covered here.