Transmitter; Figure 2-2. Transceiver Block Diagram - Motorola PR400 Basic Service Manual

2-2
The IFIC is a low-voltage monolithic FM IF system incorporating a mixer/oscillator, two limiting IF
amplifiers, quadrature detector, logarithmic received signal strength indicator (RSSI), voltage
regulator and audio, and RSSI Op Amps. The second LO frequency, 44.395 MHz, is determined by a
crystal oscillator. The second mixer converts the 44.85 MHz high IF frequency to 455 kHz.
Additional IF selectivity is provided by two ceramic filters. The first ceramic filter is a 4-pole filter used
between the second mixer and IF amp. The second ceramic filter is a 6-pole filter and is used
between the IF amp and the limiter input. For the second ceramic filter, a wider filter is used for 20/25
kHz channel spacing, and a narrower filter is used for 12.5 kHz channels.
A ceramic resonator provides phases vs. frequency characteristic required by the quadrature
detector, with 90 degree phase shift occurring at 455 kHz. The output of the IFIC is the recovered
audio signal which is fed to the audio IC for amplification and signal conditioning. The output of the
audio IC is injecting into the audio PA which drives the 24 Ohm speaker.
2.2.2

Transmitter

• When the radio is transmitting, microphone audio is passed through the audio IC, where pre-
emphasis and low-pass (splatter) filtering are done. The output of the audio IC is used to modu-
late the TX VCO, which creates the modulated carrier. The modulated carrier is then amplified
by the pre-driver and power amplifier circuit, which transmits the signal under dynamic power
control
Ref. Osc.
Frac N
Mod
Audio PA
Spkr
Audio IC
Mic
Recovered Audio
Ceramic
Resonator
Demodulator
RSSI
July, 2004
Tx VCO
Circuit
Loop
Filter
Rx VCO
Circuit
BW_SEL
6G
6E
Cer Fltr
Switching 4E
IF IC

Figure 2-2. Transceiver Block Diagram

Theory Of Operation: Major Assemblies
Tx
Predriver
Tx Out
VCOBIC
Rx Out
Crystal
IF Amp
Filter
Mixer
PA
Module
Power
Cntr
Harmonic
T/R
Switch
Filter
Filter
Preselector
Rx
LNA
Image
Filter
6881096C24-A