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This is a variation of a circuit published by K2CU (Rob) that employs
a precision rectifier and an averaging circuit to create a precision detector for AM demodulation. The limitations from
ideal performance are imposed by: the maximum slew rate of the first operational amplifier; the amount of peak output voltage,
from the first operational amplifier, that is required to drive the peak current that must flow through each diode to
develop the necessary feedback signal; and the peak output voltage that each operational amplifier can produce.
The version below uses an op-amp with a faster slew rate... which is helpful for rectifying higher
frequency carriers. The objective is to move the voltage at the output of the op-amp by ~ twice the forward bias voltage of
the diodes being employed... within a small fraction of one cycle of the carrier. For example, if we assume that the
forward bias voltage of the diode is (nominally) 0.5 volts, and if we assume that the carrier frequency is 455 kHz... then
we want to move the output of the op-amp by ~ 1 volt in a small fraction of 2 usec. If we define a small fraction as 1/20
(5%), then we need a slew rate of ~10 volts/usec. The 833 op amp has a rated slew rate of ~5 volts/usec... so it might be
a little slow for use with a 455 kHz carrier. The NTE918M has a rated slew rate of 50 volts / usec.
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The above circuit is a combination of a precision AM detector, and an AGC (a.k.a. AVC) controller for a
75A-4. The 75A-4 uses an AGC control voltage of approximately minus 6 volts. The circuit above incorporates an inverter
/ 0.16 Hz low pass filter to extract the short term average level of the 455 kHz i.f. signal from the 75A-4 ... and feeds
this back into the AGC bus of the 75A-4. The AVC Test Point is a convenient place to insert this AGC control signal. Set the
75A-4's AVC switch to "off", so that this new control signal will not compete with the existing AVC control signal from the
75A-4's peak-detecting AVC. [Note: there is a lot of gain around the AGC loop, and the 7Hz
low pass filter, just prior to the 1N4007 diode, was added to further reduce residual audio frequency components in the
control signal, that get past the 0.16 Hz filter]
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