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| 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 run with ''NO ALC''. Note that Elecraft transceivers are different due to the way ALC is displayed and modulation is implemented with DSP). | 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'''. Note that Elecraft transceivers are different due to the way ALC is displayed and modulation is implemented with DSP). |
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| 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 esyy 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. | 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. |
Setting AFC Levels for Digital Mode Operation
de Joe W4TV
FT8, JT65, JT9, and RTTY modes use Frequency Shift Keying (FSK), transmitting one tone at at a time and no changing levels. They can run in a class C (saturated) amplifier - in other words, with maximum ALC.
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. Note that Elecraft transceivers are different due to the way ALC is displayed and modulation is implemented with DSP).
Given that the presence of ALC 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), the ALC should be kept to minimum indication for PSK modes but can be allowed to run within the ALC range for FSK modes including AFSK RTTY, FSK RTTY, FT8, JT9 and JT65).
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.