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4.5 Metering Circuit
The ALC and metering circuitry is on the motherboard (see Illustration 6–6). This circuit
processes information for the RF and DC metering, and produces ALC (RF level-control)
bias. It also provides reference and input voltages for the digital panel meter, voltages for
remote metering, fan control, and drive for the front-panel fault indicators.
Illustration 6–6 and accompanying schematics complement this discussion.
PA voltage and current come from a metering shunt on the power regulator board. The PAI
input is a current proportional to PA current; R153 converts the current to voltage used for
metering and control. A voltage divider from the PAV line is used for DC voltage metering.
Metering Section
U23A, U23B, and U24A, with their respective diodes, are diode linearity correction circuits.
Their DC inputs come from diode detectors in the RF reflectometer in the RF low-pass filter
compartment.
U24B, U24C, are components of a DC squaring circuit. Since the DC output voltage of
U24C is proportional to RF voltage squared, it is also proportional to RF power.
U22C, U22A, U20A, and U22D are level sensors for RF power, reflected RF power, PA
temperature, and external PA current, respectively. When either of these parameters ex-
ceeds the limits, the output of U22B will be forced low, reducing the ALC (RF level control)
voltage, which, in turn, reduces the PA supply voltage.
The DC voltage set point for U22A (reflected RF voltage) is one-fifth that of U22C (forward
RF voltage). This ratio corresponds to an SWR of 1.5:1 [(1+.2)/(1– .2)=1.5]. The U25 invert-
ers drive the front panel fault indicators.
Principles of Operation
Illustration 4–3 Metering Circuit
4-7
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