Service manual

June 21,2004 1-19
NP88 FLEX GENERAL
Theory of Operation
Receiver
The antenna receives the modulated RF paging signal and couples it to the
amplifier/preselectortoamplifyandfiltertheRF signal. The mixer converts the RF
signal to an intermediate frequency (IF).
The crystal filter attenuates the signal above and below the RF carrier. The
oscillator/multiplier supplies the appropriate injection signal to convert the RF
paging signal to the correct IF.
The IF demodulator amplifies, filters, and mixes with the first IF to produce a 140
kHz second IF and demodulates the signal to recover the audio (FLEX) or digital
(POCSAG) data.
Decoder
The microcomputer controls the receiver circuits, powering them up and down at
the proper time intervals, decodes the binary information received, stores the
received message and generates the alert tones. It also processes user inputs from
the buttons, controls the display and drives the lamp and vibrator motor.
The support module provides boosted dc voltage to all decoder circuits, monitors
battery conditions, and drives the speaker.