Theory Of Operation; Introduction; Vhf (146-174Mhz) Receiver; Receiver Front-End - Motorola VHF2 Service Information

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Table Of Contents

Table of Contents

1.0

Introduction

This Chapter provides a detailed theory of operation for the VHF circuits in the radio. Details of the

theory of operation and trouble shooting for the the associated Controller circuits are included in this

Section of the manual.

2.0

VHF (146-174MHz) Receiver

2.1

Receiver Front-End

The received signal is applied to the radio's antenna input connector and routed through the

harmonic filter and antenna switch. The insertion loss of the harmonic filter/antenna switch is less

than 1 dB. The signal is routed to the first filter (4-pole), which has an insertion loss of 2 dB typically.

The output of the filter is matched to the base of the LNA (Q303) that provides a 16 dB gain and a

noise figure of better than 2 dB. Current source Q301 is used to maintain the collector current of

Q303. Diode CR301 protects Q303 by clamping excessive input signals. Q303 output is applied to

the second filter (3-pole) which has an insertion loss of 1.5 dB. In Distance mode, Q304 turns on and

causes D305 to conduct, thus bypassing C322 and R337. In Local mode, the signal is routed

through C322 and R337, thus inserting 7 dB attenuation. Since the attenuator is located after the RF

amplifier, the receiver sensitivity is reduced only by 6 dB, while the overall third order input intercept

is raised.

The first mixer is a passive, double-balanced type, consisting of T300, T301 and U302. This mixer

provides all of the necessary rejection of the half-IF spurious response. High-side injection at +15

dBm is delivered to the first mixer. The mixer output is then connected to a duplex network which

matches its output to the XTAL filter input (FL300) at the IF frequency of 44.85 MHz. The duplex

network terminates into a 50 ohm resistor (R340) at all other frequencies.

Antenna

Front Filter

LNA

Second Filter

First LO

Figure 2-1 VHF Receiver Block Diagram