Low Intermediate Frequency (If) And Receiver Back End; Vhf (136-174Mhz) Transmitter Power Amplifier (Pa) 25 W - Motorola 6864115B62-C Detailed Service Manual

VHF (136-174MHz) Transmitter Power Amplifier (PA) 25 W

2.4

Low Intermediate Frequency (IF) and Receiver Back End

The 44.85 MHz high-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 (2nd LO)
to produce the low-IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The
low IF signal is amplified and filtered by an external pair of 455 kHz ceramic filters FL3112, FL3114
for 20/25 kHz channel spacing or FL3111, 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 U3101 pin 8 (DISCAUDIO) which is fed to the ASFIC_CMP (U0221) pin 2 (part of
the Controller circuitry).
A received signal strength indicator (RSSI) signal is available at U3101 pin 5, having a dynamic
range of 70 dB. The RSSI signal is interpreted by the microprocessor (U0101 pin 63) and in addition
is available at accessory connector J0501-15.
3.0 VHF (136-174MHz) Transmitter Power Amplifier (PA) 25 W
The radio's 25 W 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 (U3401)
and the second stage (Q3421) is adjustable, controlled by pin 4 of PCIC (U3501) via U3402-1 and
U3402-2. It is followed by an LDMOS final stage (Q3441).
From VCO
Controlled
Stage
To Microprocessor
Devices U3401, Q3421 and Q3441 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
A S F I C _ C M P
SPI BUS
Controlvoltage
Figure 2-1 VHF Transmitter Block Diagram
Pin Diode
Antenna
Switch
Power
Sense
PA
PWR
SET
P C I C
Antenna
Harmonic
Filter
RF Jack
Temperature
Sense
To Microprocessor
2-3