This is the write-up that Doug Smith did on the Orion AM ALC. It
is taken from a paper he did on the Orion.
Sorry I can't include the diagram but I think you will get the
idea.
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A Unique AM ALC
Here is a novel AM ALC system that sports zero carrier shift and
100% maximum modulation, regardless of transmitter gain and
baseband wave shapes. Refer to Fig 6.
A DSP obtains information about peak transmitter output power
from a bridge, as above. Since it already has information about
its peak drive level, it can compute the transmitter's gain. It
is then relatively easy to find the drive level that produces a
carrier of exactly 25% of the set power level.
An audio compressor is employed that sets the maximum baseband
peak level identical to that of the carrier. When modulation is
performed, the result is a 100%-modulated AM wave. The audio
compressor uses a full-wave rectifier. If the baseband voltage
had a higher negative peak than positive, 100% downward
modulation would be reached in compression before 100% upward
modulation could occur.
Carrier amplitude does not change because the transmitter gain
calculation is performed on the peak-detected combination of
Vc+Mt, where Vc is the peak carrier level and Mt is the peak
modulation level. DSP computes the peak drive level based on
carrier plus modulation; it computes the carrier level based on
transmitter gain. Carrier shift, therefore, is avoided entirely--
Doug Smith, KF6DX.
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When you go to a new band on AM, and say the first word or two,
the Orion DSP does the gain calculation talked about above and
sets the carrier to 25% of the power setting on the power knob
(Using the gain calculation of the radio on that band/frequency).
The actual carrier power will then change as the power knob is
changed. I am using the LP-100 Wattmeter to measure the power out
on my Orion running V2.063X. It is easiest to measure the carrier
on the average power setting on the LP-100 and measure the peak
transmit power on the peak settings (Mic gain to zero to measure
carrier). Once the carrier is set for a particular power setting,
it is stable with just slight (Less then 1 watt change) variation
from the main ALC. On 75M AM the output power drives to 103 W
with the carrier at 25.X watts (The power numbers are accurate to
about 3% and were calibrated with NIST traceable calibrated
equipment). I am sure the accuracy of the carrier level varies
some due to the frequency associated measurement error of the
bridge.
Pretty much as the write-up predicts.
73, Merle - W0EWM
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