Motorola Radius Cm200 Cm300 Pm400 Detailed 6881098c00 A Manual

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...

2-2THEORY OF OPERATION
2.2 Receiver Back End
The IF signal from the crystal filter enters the IF amplifier which provides 20 dB of gain and feeds 
the IF IC at pin 1. The first IF signal at 44.85 MHz mixes with the second local oscillator (LO) at 
44.395 MHz to produce the second IF at 455 kHz. The second LO uses the external crystal Y301. 
The second IF signal is amplified and filtered by two external ceramic filters (FL303/FL302 for 
12.5 kHz channel spacing and FL304/FL301 for 25 kHz channel spacing)....

UHF Transmitter Power Amplifier (438-470 MHz) 2-3
power of the PA.Op-amp U103-3 monitors the drain current of U101 via resistor R122 and adjusts 
the bias voltage of U101.
In receive mode, the DC voltage from RX_EN line turns on Q101, which in turn switches off the 
biasing voltage to U101. 
3.2 Power Controlled Driver Stage
The next stage is an LDMOS device (Q105) which provides a gain of 12 dB. This device requires a 
positive gate bias and a quiescent current flow for proper operation.The voltage of...

2-4THEORY OF OPERATION
3.6 Harmonic Filter
Inductors L111, L113 and L128 along with capacitors C1011, C1023, C1020, C1016, C1025 and 
C1026 form a low-pass filter to attenuate harmonic energy coming from the transmitter. Resistor 
R150 along with L130 drains any electrostatic charges that might otherwise build up on the antenna. 
The harmonic filter also prevents high level RF signals above the receiver passband from reaching 
the receiver circuits to improve spurious response rejection.
3.7 Power...

UHF (438-470 MHz) Frequency Synthesizer2-5
parameters (four characterize the inverse Bechmann voltage curve and one for frequency accuracy 
of the reference oscillator at 25 degrees C). This algorithm is implemented by the LVFRAC-N (U200) 
at the power up of the radio.
TCXO Y200, along with its corresponding circuitry R204, R205, R210, and C2053, are not placed as 
the temperature compensated crystal proved to be reliable.
4.2 Fractional-N Synthesizer
The LVFRAC-N U200 consists of a pre-scaler,...

2-6THEORY OF OPERATION
4.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U201), the TX and 
RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Figure 2-4 UHF VCO Block Diagram
The VCOBIC together with the LVFRAC-N (U200) generate the required frequencies in both 
transmit and receive modes. The TRB line (U201, pin 19) determines which VCO and buffer is 
enabled (high being TX output at pin 10, low being RX...

UHF (438-470 MHz) Frequency Synthesizer2-7
4.4 Synthesizer Operation
The synthesizer consists of a low voltage FRAC-N IC (LVFRAC-N), reference oscillator, charge 
pump circuits, loop filter circuit, and DC supply. The output signal (PRESC_OUT) of the VCOBIC 
(U201, pin 12) is fed to the PREIN, pin 32 of U200 via a low pass filter which attenuates harmonics 
and provides a correct input level to the LVFRAC-N in order to close the synthesizer loop.
The pre-scaler in the synthesizer (U200) is a dual modulus...

2-8THEORY OF OPERATION
5.0 Controller Theory of Operation
This section provides a detailed theory of operation for the radio and its components. The main 
radio is a single-board design, consisting of the transmitter, receiver, and controller circuits. A 
control head is connected by an extension cable. The control head contains LED indicators, a 
microphone connector, buttons, and speaker. 
In addition to the power cable and antenna cable, an accessory cable can be attached to a 
connector on the rear...

Controller Theory of Operation2-9
The DC voltage applied to connector P2 supplies power directly to the following circuitry:
• Electronic on/off control
• RF power amplifier
• 12 volts P-cH FET -U514
• 9.3 volt regulator
•Audio PA
Figure 2-6 DC Power Distribution Block Diagram
Regulator U501 is used to generate the 9.3 volts required by some audio circuits, the RF circuitry 
and power control circuitry. Input and output capacitors are used to reduce high frequency noise. 
Resistors R5001 / R5081 set the...

2-10THEORY OF OPERATION
Voltage regulator U503 provides 5 V for the RF circuits. Input and output capacitors are used to 
reduce the high frequency noise and provide proper operation during battery transients.
5.2 Protection Devices
Diode VR500 acts as protection against ESD, wrong polarity of the supply voltage, and load dump.
VR692 - VR699 are for ESD protection.
5.3 Automatic On/Off
The radio can be switched ON in any one of the following three ways:
• On/Off switch. (No Ignition Mode)
• Ignition and...