WSJT-X開発者チームメンバーからのALCに関するコメント ― 2020年10月07日 22時30分25秒
A lot of the information given about transmitter ALC indication is worse than useless. You must first understand what the ALC indication on your rig means. It is normally a measure of how much gain reduction is being applied before the PA to limit the final output within required levels. Without ALC the PA could be driven beyond its intended design parameters. As such ALC indications are not harmful but there are circumstances when high values might indicate a problem. Note that on some rigs the POWER or DRIVE control is implemented with ALC so the indication may be high simply because you have reduced the transmitter output, other rigs avoid that and ALC is not skewed by power controls. So here are some things to watch for with ALC:
● when using a mode that requires linear amplification, like AM, voice SSB, or PSK31, high levels of ALC may indicate compression of the output, which in turn may distort the transmitted signal. That distortion may include unwanted widening, e.g. the "splatter" often heard close in around signals using too much drive. Note that for constant envelope modes like FM, and the modes used in WSJT-X, except maybe MSK144, linear amplification is not a requirement and high ALC indication may not be any problem at all, but see below.
● with narrow bandwidth constant envelope modes generated using AFSK with an SSB transmitter high ALC *may* be a sign that too much audio drive is being applied to the transmitter modulator. This is only the case when the applied audio frequencies are attenuated because they are not within the passband of the transmitters SSB Tx filter. To explain, imagine an audio tone of 3000 Hz being applied to a typical SSB transmitter with an audio passband of 200 to 2800 Hz. To get any ALC indication the audio level would have to be huge because it would be attenuated by ~40 dB (maybe more) by the Tx filter. Applying an audio signal to the rig's modulator that is 40 dB higher than necessary will inevitably cause distortion by clipping, which will result in a signal rich in unwanted audio harmonics. With the case above the audio harmonics would be above 3000 Hz so they are relatively harmless, being attenuated even more than the wanted 3000 Hz signal, but imagine the case where the tone is below 200 Hz!
So in summary, ALC is desirable so do not equate ALC indication with incorrect set up, but be aware that it may indicate a problem in some circumstances. What is essential is understanding how your transmitter works and the consequences of using narrow band AFSK modes with an SSB transmitter. High ALC indications may be an indication that audio drive levels are too high, OTOH audio drive levels can also be excessive and causing distortion with little or no ALC indication. Little or no ALC indication is not a panacea leading to correct setup of your transmitter.
A good rule of thumb, that works with almost every transmitter when using WSJT-X, is to set the audio level using a 1500 Hz tone such that the transmitter ALC indication is within the manufacturer's "safe" zone (usually indicated by a red marking over the lower 1/3 of the meter scale). Do this with the transmitter power at maximum to avoid extra ALC indication from the transmitter power control on some rigs, also use a well matched dummy load as some rigs compensate for high VSWR by increasing ALC. With the correct setup you should achieve about 90% of the transmitter's design output at he point ALC indication starts, maximum power will be achieved as the ALC indication approaches the top of the "safe" zone. Most of the time power levels above 90% of your transmitter output are not needed so restricting ALC indication to zero is OK, but be aware that alone does not mean that you have correctly set up your system.