RPR 550IS Series
Page 3 - 12 TM1188 Issue 1
AC Feedback
31. The AC Automatic Gain Control (AGC) network operates to keep the audio output
constant over a wide range of input levels. The audio output from the loudspeaker is
rectified and level shifted by C50, D7, R62 and R64. The controlled volume is set by
the ratio of R62 to R64. The rectified peak voltage charges C31 which then causes
TR15a and subsequently TR15b to conduct via R68. The decay time is set by R68
and C31 (1M
6
x 10µF). Transistor TR15b acts as a variable resistance and forms a
potentiometer with R32 thereby controlling the input level back to the power amplifier.
Decoder Circuit
32. The decoder circuit comprises gate array IC2, microprocessor IC3, EEPROM IC4 and
associated components.
33. Gate array IC2 tracks and samples incoming data from the receiver. Multitone Mk7
data then passes to the microprocessor. The interrupt signal, reset, LED drive and
tone waveforms are all generated by the gate array. IC2 also monitors the control
switches and generates an interrupt to IC3 when any switch is made.
34. The function of microprocessor IC3 is to:
a) Decode the received information.
b) Read the control switch registers in IC2 and read and validate the contents of the
EEPROM IC4.
c) Instruct the gate array to generate the audible alert and illuminate the lamp.
d) Control the generation of the 5V supply to the LCD module.
e) Control the LCD module.
35. The pager address and options are programmed in EEPROM IC4, which is read
serially by the microprocessor at switch-on. The EEPROM is programmed via the
microprocessor IC3 with data from the programming unit P648 through the absence
contacts in the base of the pager. Diodes D17, D18, D9, D13, D15 and D16 infallibly
limit the voltage on any pin of IC4, which contains a charge pump.
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