Specifications
4–8
FM500 User's Manual
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4.5 Metering Circuit Board
The ALC and metering circuitry is on the metering board (see Illustration 4–4).
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.
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.
Illustration 4–5 Metering Board
Metering
FM500
FM BROADCAST TRANSMITTER
®
Power
Carrier
Modulation
Fault
Stereo
Mono RF Output
ProcessingInput Gain
Wide Band
High Band
Audio Input
High
Low
2
-6
-12
-18
2
+6 dB
+12 dB
10
20
Expand
Compress
RF Power
SWR
ALC
PA DC Volts
PA DC Amps
PA Temperature
Supply DC Volts
Voltmeter
SWR
Lock
Input
PA DC
PA Temp
Over
100
90
80
70
60
50
40
30
20
Pilot