Motorola P100 Series, Detailed Service Manual, Page: 151 / 336

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
Crysta l
F i l ter
Crysta l
44.64 5M Hz
1st Mi x er
RF
A mp
Prese l ector
F i l ter
Postse l ector
F i l ter
RX f rom
A ntenna S w itc h
I n j F i l ter
F irst LO
f rom Synt h esi z er
IF
A mp
IF
A mp
L P F
Ceramic
F i l ter
D escriminator
4 55 FW
4 55 HW
Qu a d rat u re
D etector
R SS I
N /S _ S W
R eco v ere d Aud io
4 55C2 4