Chapter 2 Theory Of Operation; Introduction; Vhf (146-174 Mhz) Receiver; Receiver Front-End - Motorola Radius CM200 Service Manual

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 (146–174 MHz) 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 C332 and R337. In Local mode, the signal is routed
through C332 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 Mixer
Second Filter
First LO
Figure 2-1 VHF Receiver Block Diagram
THEORY OF OPERATION
12.5kHzFilter
4- Pole
IF Amp
Xtal Filter
25kHzFilter
2nd LO Xtal Osc
Chapter 2
12.5kHzFilter
25kHzFilter
IFIC
Phase Shift
Element
Controller