Low Intermediate Frequency (If) And Receiver Back End; Uhf (450-520 Mhz) Transmitter Power Amplifier (Pa) 40W - Motorola GM328 Service Manual

UHF (450-520 MHz) Transmitter Power Amplifier (PA) 40W

2.4

Low Intermediate Frequency (IF) and Receiver Back End

The 44.85 MHz first IF signal from the second IF amplifier feeds the IF IC (U3101) at pin1. Within the
IF IC the 44.85 MHz high IF signal mixes with the 44.395 MHz second local oscillator to produce a
low IF signal of 455 kHz. The second LO frequency is determined by crystal Y3101. The second IF
signal is amplified and filtered by an external pair of 455 kHz ceramic filters (FL3112 and FL3114) for
20/25 kHz channel spacing, or FL3111 and FL3113/F3115 for 12.5 kHz channel spacing. These
pairs are selectable via BWSELECT. The filtered output from the ceramic filters is applied to the
limiter input pin of the IF IC (pin 14).
The IF IC contains a quadrature detector using a ceramic phase-shift element (Y3102) to provide
audio detection. Internal amplification provides an audio output level of 120 mV rms (at 60%
deviation) from U3103, pin 8 (DISCAUDIO) which is fed to ASFIC_CMP U0221, pin 2 (part of the
controller circuits).
A received signal strength indicator (RSSI) signal is available at U3101, pin 5, which has a dynamic
range of 70 dB. The RSSI signal is interpreted by the µP (U0101, pin 63) and is available at
accessory connector J0501-15.
3.0 UHF (450-520 MHz) Transmitter Power Amplifier (PA) 40W
The radio's 40W PA is a three stage amplifier used to amplify the output from the VCOBIC to the
radio transmit level. All three stages utilize LDMOS technology. The gain of the first stage (U5401) is
adjustable, controlled by pin 4 of PCIC (U5501). It is followed by an LDMOS stage (Q5421) and
LDMOS final stage (Q5441).
From VCO
Controlled
Stage
Vcontrol
To Microprocessor
Devices U5401, Q5421 and Q5441 are surface mounted. A pressure pad between board and the
radio's cover provides good thermal contact between the devices and the chassis.
PA
PA-Final
Driver
Stage
Bias 2
& Bias 3
A S F IC _ C M P
PA
SPI BUS PWR
SET
Bias 1
P C I C
Figure 2-2 UHF Transmitter Block Diagram
Pin Diode
Antenna
Switch
Power
Sense
Temperature
Sense
Antenna
Harmonic
Filter
RF Jack
To Microprocessor
2-3