Motorola cm200 Commercial Series, Service Manual

Page: 161 / 352

Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the UHF 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 UHF (438-470 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 (3-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 (4-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. Low-side injection at
+10 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.
Figure 2-1 UHF Receiver Block Diagram
Mixer
Xtal Filter
Controller
Front Filter
Antenna
First LO
2nd LO Xtal Osc
IF Amp
Second Filter
4- Pole
25 kHzFilter
12.5 kHzFilter
Phase Shift
Element
IFIC
LNA
25 kHzFilter
12.5 kHzFilter