Spectrum DSP M2 User Manual page 51

The ALC (Automatic Level Control)
(Set-up dependent)
Prior to the addition of the ALC the POWER adjustment on the Spectrum DSP was somewhat
irrelevant when in a voice mode as it only added effective attenuation in the audio path. If
one adjusted the audio to 5 watts PEP when in the 5 watt mode, it was possible to switch to
the 1 watt mode and readjust the audio gain to again achieve 5 watts as there was nothing
within the code to set levels!
What is more significant is that there was nothing in the code to prevent the overdriving of the
final amplifier stage, even if it had been set up properly for a "clean" 5 watts as there was no
way to be sure, without using an external RF power meter, that the transmitter audio drive
was properly set.
This was been changed in version code 0.0.207: It is no longer possible to obtain a
higher PEP power at a given power setting than a steady carrier in CW or TUNE mode!
Unless you have a true peak-reading RF power meter, you will read a lower RF output
power in SSB mode than in CW mode.
Please re-read the above paragraph at least once to be sure that you
understand it!
How the ALC works:
All modern SSB transceivers have a form of ALC which monitors the transmit power level and
if it exceeds the set power level, it is cut back to prevent overdriving of the finals. In this way
the maximum output power may be set for a mode that has intrinsically varying power levels.
With the ALC the PEP power from the transmitter should not exceed the carrier level
observed in TUNE mode, no matter the audio drive level.
In order for the ALC to work there must be at least a minimum audio level to drive it and to
provide for this clicking the SWR meter will change to one of three modes:
The SWR meter. This dynamically measures the forward and reflected RF power,
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calculates the VSWR and displays it.
The AUDio meter. This shows the audio level from -20dB to +12dB, with 0 dB being
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"nominal". It is acceptable for audio to occasionally peak at +6 to +10dB.
The ALC meter. This shows the amount of ALC action, from 0 to 34 dB – more on this
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below.
Adjusting for the proper audio level when in SSB transmit mode:
(Setup dependent)
Spectrum DSP M2 User Manual V1.1
P 51