Motorola EP350

Chapter 8 136 – 174 MHz VHF Theory Of Operation
8.1 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 Chapter 10 of this manual.
8.2 VHF Receiver
The VHF receiver design covers the frequency range of 136 – 174 MHz and it is a double
conversion super heterodyne with 1st IF 45.1 MHz and 2nd IF 455 kHz. The receiver is divided into
two major blocks, Front End and Back End as shown in Figure 8-1.
Figure 8-1. VHF Receiver Block Diagram
Crystal
Filter
Crystal
44.645MHz
1st Mixer
RF
Amp
Preselector
Filter
Postselector
Filter
RX from
A
ntenna Switch
Inj Filter
First LO
from Synthesizer
IF
Amp
IF
Amp
LPF
Ceramic
Filter
Descriminator
455FW
455HW
Quadrature
Detector
RSSI
N/S_SW
Recovered Audio
455C24