Chapter 4 136-162 Mhz Vhf Theory Of Operation; Introduction; Vhf Receiver; Receiver Front-End - Motorola CP150 Service Manual

Chapter 4 136-162 MHz VHF Theory Of Operation

4.1

Introduction

This chapter provides a detailed theory of operation for the radio components. Schematic diagrams
for the circuits described in the following paragraphs are located in
4.2

VHF Receiver

The VHF receiver covers the range of 136-162 MHz and provides switchable IF bandwidth for use
with 12.5 kHz or 20/25 kHz channel spacing systems. The receiver is divided into two major blocks as
shown in Figure
• Front End
• Back End
RX from
Antenna Switch
4.2.1

Receiver Front-End

Incoming RF signals from the antenna are first routed through the harmonic filter and antenna switch,
part of the transmitter circuitry, before being applied to the receiver front end. The receiver front end
consists of a preselector filter, RF amplifier, an interstage filter, and a double-balanced first mixer.
The preselector filter is a fixed-tuned 4-pole design using discrete elements (L1-L4 and C1-C9) in a
series/shunt resonator configuration. It has a 3 dB bandwidth of 43 MHz, an insertion loss of 2 dB and
image attenuation of 37 dB at 226 MHz, with increasing attenuation at higher frequencies. Diode CR1
protects the RF amplifier by limiting excessive RF levels.
The output of the filter is matched to the base of RF amplifier Q21, which provides 18 dB of gain and
a noise figure of 2 dB. Operating voltage is obtained from the 5R source, which is turned off during
transmit to reduce dissipation in Q21. Current mirror Q22 maintains the operating current of Q21
4-1.
Preselector RF
Filter Amp
First LO
from Synthesizer
Recovered Audio
RSSI

Figure 4-1. VHF Receiver Block Diagram

Chapter 7
Interstage
Filter 1st Mixer
Inj Filter
6G
Ceramic
Resonator
Demodulator
of this manual.
IF
Crystal
Amp
Filter
BW_SEL
6E
Cer Fltr
Switching 4E