Setting ALC Levels for Digital Mode Operation
de Joe W4TV
PSK31, PSK63, PSK125 modes use Phase Shift Keying (PSK), thus the amplitude changes constantly as the phase of the modulating audio changes and, in fact, two tones (offset by a few Hertz depending on the baud rate) can be present at the same time. In order to prevent intermodulation (or "mixing") between the two tones, the RF stages of a PSK transmitter must be linear (no compression or clipping). With most transceivers, this means that a PSK signal must be transmitted with no ALC meter deflection. Note that Elecraft transceivers are different due to the way ALC is displayed and modulation is implemented.
FT8, JT65, JT9, and RTTY modes use Frequency Shift Keying (FSK), transmitting one tone at a time with no change in level. They can run in a class C (saturated) amplifier - in other words, with maximum ALC.
Given that the presence of an ALC meter reading in most Icom, Kenwood, Ten-Tec and Yaesu transceivers indicates that the IF stages, low power driver and/or final amplifier are being driven into compression (at least), power output should be reduce to produce a minimum ALC meter indication for PSK modes, but can be increased to be within the ALC meter's acceptable range for FSK modes including AFSK RTTY, FSK RTTY, FT8, JT9 and JT65.
The ALC display of an Elecraft K3, K3S, KX3, or KX2 transceiver in data mode has a dual function. Below five bars, the display indicates the level of audio drive; above five bars, the display indicates ALC action. If the ALC shows fewer than four bars, an Elecraft transceiver will automatically increase the audio gain, resulting in unwanted power creep.
For a KX2 or KX3 transceiver, adjust the MIC GAIN until the ALC display shows four to five bars
For a K3 or K3S transceiver with the soundcard output connected to its frontpanel or backpanel microphone jack, adjust the MIC GAIN until the ALC display shows four to five bars
For a K3 or K3S transceiver with the soundcard output connected to its Line In jack, adjust the LINE IN gain until the ALC display shows four to five bars
Due to the crest factor (peak to average power ratio) of the PSK waveform, the power output for PSK modes should be restricted to approximately 1/4 of the specified CW/PEP power level (for example, ~25 W for a 100 watt transceiver). FSK modes on the other hand have no crest factor (peak to average power ratio = 1), and the power output can be equal to the specified CW/PEP maximum if the transceiver's cooling is capable of supporting the higher duty cycle mode.