FT8 and ALC

Discussion in 'Working Different Modes' started by KB2E, Aug 29, 2018.

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  1. KB2E

    KB2E XML Subscriber QRZ Page

    Can someone explain why "no ALC" is recommended in WSJT-X adjustments? Is it to keep the signal clean, like backing off compression or mic gain on SSB?
     
  2. NK7Z

    NK7Z XML Subscriber QRZ Page

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  3. W2WDX

    W2WDX Premium Subscriber QRZ Page

    To put it simply, ALC action creates problems with phase and attack time, until it settles down. It creates an overshoot condition. Very important parameters to maintain for the correct demodulation of the signal/data on the receive end.

    As they say (whoever "they" are), timing is everything. :D
     
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  4. W4NNF

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    This!

    Oh, if you're seeing a bit of ALC action, maybe a bar or two, that's usually OK. But if you adjust your transceiver's output in the usual way, you will see very little.

    What I do is set the rig to full power and reduce my output power to around 50 watts or so by adjusting the audio level of the sound card. That results in a clean signal...and no ALC action is observed.
     
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  5. KP4SX

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    I started out running my IC-7410 just any ole where as long as it fell into the '50% range' of the ALC scale. I have since backed off to be right on the threshold of no-bars (wish there was a vernier control!) and I swear my rcvd reports have improved.
     
  6. W5DXP

    W5DXP Ham Member QRZ Page

    The quality of a digital signal can be measured by IMD (InterModulation Distortion) which causes the signal to occupy more bandwidth than it is supposed to (splatter). If the ALC reading is high the IMD will be high. FLDigi can measure IMD of PSK31 signals. Here's some information from the FLDigi manual.

    http://www.w1hkj.com/FldigiHelp/spectrum_viewer_page.html

    Here's another good page. Note that the IMD could possibly be coming from receiver overload rather than being present in the received signal. One's attenuator should be used on strong signals to minimize receiver IMD.

    http://n4ser.org/2014/psk-assure-clean-signal/
     
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  7. KB2E

    KB2E XML Subscriber QRZ Page

    Thanks to all! I have some reading to do.
     
  8. N3DT

    N3DT Ham Member QRZ Page

    Just my experience, but I've discovered that if I run my TS2000 at max power and run down the audio to get lower power, my LP100A shows a lot of overshoot and erratic power levels. What I've learned to do is set the TS2000 power level at something slightly above what I want to have the power level (like 10W for 5W out) at and run up the input to the sound card to something at about 80%. Then I don't get the overshoot and the power levels stay solid during transmissions with no ALC. Just what I've seen, YMMV. Maybe someone can educate me on this? But I've never had trouble getting FT8 contacts, until my TS2000 went bezerk and now won't put out a clean digital signal. I'm pretty sure the AUX buffer transistor to the digital input is shot. I can put the signal into the mike input and it's OK. The FT757 works fine also, but that's another radio.
     
  9. W2WDX

    W2WDX Premium Subscriber QRZ Page

    Really it comes down to simply running the digital signal lower than what makes ALC kick in, at all. The few watts lost makes little difference once the signal gets out into the far field and propagated. Even sight amounts of phase change can make big problems and make the signal unreadable. You do not need ANY amount of ALC in digital modes, and in fact should avoid it entirely. Any amount of ALC will cause phase shifts on the outset of sending. It's not really a matter of IMD, it's phase that is needed to be maintained. If you have IMD issues you have other problems more important than just ALC.
     
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  10. K7TRF

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    Basically ALC turns a final amplifier that is very linear into a compression amplifier that is non-linear as the signal level is increased. Non-linear amplification leads to distortion products, mostly Intermodulation Distortion products (IMD) that others have discussed. A bit of IMD is tolerable for SSB waveforms but most digital mode waveforms don't tolerate IMD well and it cause both problems in terms of your signal reception but also splatter to adjacent users neither of which is a good thing. It's best to limit transmit power to below any ALC action to preserve transmit chain linearity.
     
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