Below is the schematic of an automatic level control circuit ... which I
am using between the output of a dc-coupled rf envelope detector ("rf pickup") and the input
to an REA modulation monitor.
The circuit takes in a signal whose DC level is assumed to have a value of between
3 and 15 volts. The 17 kohm input impedance of the circuit was selected to approximately match the input impedance
of an REA modulation monitor.
With the input signal applied, the 2kohm input potentiometer is adjusted to produce a DC level
on its wiper that is roughly 0.4 volts relative to ground.
The first two operational amplifiers constitute a DC-coupled variable gain amplifier. This is
used to produce an output signal, which is a scaled version of the input signal... but whose DC level
remains close to 3.5 volts as the DC level of the input signal varies over a +/- 6dB range.
The gain control time constant is around 1 second.
The gain-controlled amplifier employs a VTL5C3 "Vactrol", which consists of an opto-resistor
and an LED, co-packaged as a single, integrated component with four leads. With a few milliamperes of applied current,
the LED produces enough light to reduce the opto-resistor's value from above 1M ohm to below 10k ohms.
The resistive element of the Vactol is specified to produce low distortion (i.e., to act as
a linear resistor) if the voltage across it is kept below a few volts. To satisfy this constraint, and to produce
an output signal at nominally 3.5 volts DC level, the circuit includes a fixed gain of 3 in the second op-amp of
the variable gain amplifier.
Under operating conditions, the objective is to have the first op-amp of the variable gain amplifier
operating at a nominal gain of approximately 3 (i.e., +10dB)... when the input to the variable gain amplifier has
a DC level of approximately 0.4 volts. [I.e., 0.4 volts x ~3 x 3 = ~3.5 volts].
With an input signal applied, the DC output level of the second op-amp is set to approximately
3.5 volts by adjusting the 1.5 kohm potentiometer. The combination of the voltage gain of the third op-amp,
and the conversion gain (change in current =>change in LED light output power => resistance change) of the Vactrol results
in a tight control of the output level when the input level varies.
The fourth op-amp is used to produce an isolated, low impedance, DC-coupled, scaled replica
of the input signal... which is labeled as the "auxiliary output" in the schematic. The auxiliary output is not
affected by the AGC circuitry. The auxiliary output, which represents the envelope of the rf signal passing through the
pickup, can be used as the input to an oscilloscope and/or (with an appropriate DC blocking capacitor) as the input
to a headphone monitor.
I have built several copies of this circuit using a fixed 1 kohm resistor in place of the 2 kohm
input potentiometer; and using a fixed 1.5 kohm resistor in place of the 1.5 kohm DC-level adjusting potentiometer. These
circuits (which have nothing to adjust) work fine in maintaining the DC level of the controlled output fixed (so that
the carrier level indicator on the modulation monitor stays perfectly centered) as the rf power passing through the pickup
varies over a +/- 5dB range (e.g., from 35 watts to 375 watts of carrier).
