Motorola Radio Mcs2000 Vol 2g 68p81080c48 O Manual

 
Theory of Operation 3 
Theory of
Operation 
2 
This chapter provides theory of operation information for the radio. It 
starts with a block diagram level functional description of the entire 
radio. This is followed by a detailed functional description for each of 
the four major functions of the radio. 
Introduction 
The radio is composed of the following five major functions:   
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Receiver  
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Transmitter  
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Controller  
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Dc Power Control and Regulation  
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Operator Interface (Control...

 
4 Theory of Operation 
Separate component location diagrams, parts lists, and schematic 
diagrams are provided in this service manual for each of the six 
physical sections of the transceiver board and for the control heads. 
The component location diagrams, parts lists, and schematic diagrams 
for the controller section of the transceiver board and for the three 
types of control heads are located in Volume 1 of this service manual. 
The component location diagrams, parts lists, and schematic diagrams...

 
Theory of Operation 5
RECEIVER
RECEIVE
RF SIGNAL
RECEIVE RF SIGNAL RECEIVER
FRONT END
SECTIONRECEIVER IF
SECTIONRECEIVER
AUDIO
AUDIO
SIGNAL
FILTER
(ASFIC)
AUDIO PA
ENABLE
SQUELCH
DETECT RECEIVER
AUDIO
RECEIVER
AUDIO 
POWER 
AMPLIFIER RECEIVER
BACK END
SECTION
(ZERO IF)SQUELCH
FIRST
RECEIVER
LOCAL
OSCILLATOR2.1 MHz
REFERENCE
OSCILLATOR IF
IFLOUDSPEAKER P/O CONTROLLER SECTION (NOTE 1)
DC POWER CONTROL AND REGULATION
13.8VDC
FROM VEHICLE
BATTERY OR
BASE STATION
POWER SUPPLYCONTROL AND
REGULATION...

 
6 Theory of Operation 
The receiver audio power amplifier circuit does not pass the receiver 
audio signal to the loudspeaker until it receives an audio PA enable 
signal from the controller section of the radio. The reason is that the 
receiver portion of the radio includes a squelch function, which 
prevents receiver noise from passing to the loudspeaker during periods 
of no signal reception.The controller generates the audio PA enable 
signal based on such variables as the level of the received...

 
Theory of Operation 7 
Receiver
Detailed Functional 
Description 
The portion of the receiver function that is not part of the controller 
section of the radio is composed of three main sections: receiver front 
end; receiver intermediate frequency (IF), and receiver back end. The 
receiver covers the VHF range from 136 to 174 MHz. 
Receiver Front End 
The following discussion is based on the schematic diagram for the 
receiver front end section on page 19. 
Varactor Tuned Band-
pass Filter 
A two pole...

 
8 Theory of Operation 
Receiver Intermediate 
Frequency (IF) 
The following discussion is based on the schematic diagram for the 
receiver IF section on page 21. 
Intermediate Frequency 
(IF) First Selection 
To optimize half-IF stability, a diplexer shunt circuit (C3401, R3401, 
L3401, C3402) matches the output impedance of the mixer in the 
receiver front end section to the input of two pole crystal filter Y3401 
at the input of the receiver IF section. The output of filter Y3401 is 
matched to the...

 
Theory of Operation 9 
Transmitter 
Detailed Functional 
Description 
The transmitter function of the radio is distributed between the 
controller, synthesizer, and power amplifier (PA) sections of the radio. 
This is shown on the overall functional block diagram for the radio, 
Figure 1.
The portion of the transmitter function physically located in the 
controller section is described in the  
Controller Section Theory of 
Operation 
 located in Volume 1 of this service manual. That portion 
includes...

 
10 Theory of Operation 
The Fractional-N synthesizer Integrated Circuit (U5801) contains the 
following circuits:
- Prescaler
- Programmable loop divider
- Control divider logic
- Phase detector
- Charge pump
- A/D converter for low frequency modulation
- Modulation low-frequency/high-frequency balance attenuator
- Positive voltage multiplier drivers
- Serial interface for control
- Super filter low noise supply
Transistor Q5770 is a current amplifier for the super filter low noise 
supply. The super...

 
Theory of Operation 11  
output signal of the phase detector is a pulsed DC signal, which is 
routed to the charge pump. The charge pump outputs a current at 
U5801-32 (I OUT). The loop filter (which consists of R5760 through 
R5762 and C5775 through C5780) transforms this current into a 
voltage, which is applied to both VCOs to control their output 
frequencies.
The current can be set to a value fixed in the FRACN IC or to a value 
determined by the currents ßowing into CPBIAS 1 (U5801-29) or 
CPBIAS...

 
12 Theory of Operation 
printed circuit board provides thermal contact of the transistor to the 
heat sink on the radio chassis. Transistor Q3561 is attached directly to 
the heat sink.
The RF power output of Q3521 is proportional to the Q3521collector 
current. This current is adjusted by the PA control voltage (PA 
Control). If the PA control voltage raises, the base voltage of Q3601 is 
also raised, which causes more current to ßow to the collector of 
Q3601 and a higher voltage drop across R3606....