Crown Low Power Transmitters - Crown Broadcast

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4.5 Metering Circuit Board
The ALC and metering circuitry is on the metering board (see Illustration 4–5).
This board 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 6–8 and accompanying schematic 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; R405 converts the current
to voltage used for metering and control. A voltage divider from the PAV line is
used for DC voltage metering.
Metering
Board
®
FM250
Illustration 4–5 Metering Board
U406A, U406B, and U407A, 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.
U407B, U407C, Q405, and Q406 are components of a DC squaring circuit. Since
the DC output voltage of U407C is proportional to RF voltage squared, it is also
proportional to RF power.
U404C, U404A, U403A, and U404D are level sensors for RF power, reflected RF
power, PA temperature, and external PA current, respectively. When either of
these parameters exceeds the limits, the output of U404B will be forced low,
reducing the ALC (RF level control) voltage, which, in turn, reduces the PA supply
voltage.
The DC voltage setpoint for U404A (reflected RF voltage) is one-fifth that of U404C
(forward RF voltage). This ratio corresponds to an SWR of 1.5:1 [(1+.2)/(1–
.2)=1.5]. The U405 inverters drive the front panel fault indicators.
4–8 FM30/FM100/FM250 User's Manual