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:   
¥  
Receiver  
¥  
Transmitter  
¥  
Controller  
¥  
Dc Power Control and Regulation  
¥  
Operator Interface (Control Head)
The receiver, transmitter, controller, and dc power control and 
regulation functions are all located on a single circuit card assembly 
(CCA) in the main body of the radio. The CCA is called the transceiver 
board. The operator interface function consists of the control head, 
which plugs into the main body of the radio. There are three different 
control head types: the Model I for the Model I Radio; the Model II for 
the Model II Radio; and the Model III for the Model III Radio. The three 
control heads are covered in their entirety in Volume 1 of this service 
manual.
The transceiver board in the main body of the radio is physically 
separated into six functional sections as follows:  
¥  
Receiver Front End  
¥  
Receiver Intermediate Frequency (IF)  
¥  
Receiver Back End  
¥  
Power Amplifier (PA)  
¥  
Synthesizer  
¥  
Controller
The controller section is further divided into two sub-sections: main 
controller; and power control.
The mechanical layout of the transceiver board is illustrated in 
Chapter 3.   
						

							 
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 
for the other five physical sections of the transceiver board are located 
in this volume. 
Block Diagram 
Level Theory of 
Operation 
The following discussion refers to the functional block diagram for the 
radio, Figure 1.
The receiver function of the radio detects, demodulates, amplifies, and 
outputs via the loudspeaker, radio signals picked up by the vehicle or 
fixed-station antenna. The radio signal input reaches the receiver from 
the antenna via the antenna switch, which is located in the transmit-
ter function of the radio. The radio signals picked up by the antenna 
are signals that have been re-broadcast by trunked or conventional 
repeaters, or that have been broadcast directly by other mobile or fixed 
station radios.
The receiver function of the radio consists of: the receiver front end 
section; the receiver intermediate frequency (IF) section; the receiver 
back end section; and the audio signal filter (ASFIC) and receiver audio 
power amplifier circuits in the controller section.
The receiver function of the radio uses the double conversion super-
heterodyne design to optimize image rejection and selectivity. The 
receiver front end section converts the receiver input signal to a first 
IF of 44.85 MHz. The frequency upon which the receiver operates is 
determined by a first local oscillator signal generated by the synthesiz-
er section. For the purpose of this discussion, the synthesizer section is 
considered to be part of the transmitter function of the radio.
The 44.85 MHz IF output signal from the receiver front end section 
passes through the receiver IF section where it is filtered and amplified. 
The output of the receiver IF section goes to the receiver back end 
section. In the receiver back end section, which contains the zero 
intermediate frequency (ZIF) integrated circuit (IC), the receiver IF 
signal is demodulated to produce receiver audio and squelch signals.
The receiver audio and squelch signal outputs from the receiver back 
end section are processed by the audio signal filter integrated circuit 
(ASFIC) in the controller section of the radio to generate receiver audio 
(filtered) and squelch detect signals. The filtering characteristics and 
other processes of the ASFIC are controlled by the central processor 
unit in the controller section.
The receiver audio signal (filtered) from the output of the ASFIC goes 
to the input of the receiver audio power amplifier circuit, which is 
located in the controller section of the radio. 
						

							 
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
CIRCUITSREGULATED
DC POWER P/O CONTROLLER SECTION
(NOTE 1)
TO TRANSCEIVER BOARD AND
CONTROL HEAD CIRCUITS
P/O CONTROLLER
SECTION
(NOTE 1)
AUDIO
SIGNAL
FILTER
(ASFIC)AUDIO
MODULATION
SIGNAL
SYNTHESIZER
SECTION
MEMORY
RF
POWER
CONTROL FEEDBACK
CONTROL
SB9600
DATA BUS ACCESSORY
INPUTS AND
OUTPUTS2.1 MHz
REFERENCE
OSCILLATORCENTRAL
PROCESSOR
UNIT (CPU)
SUPPORT
LOGIC
(SLIC) RF POWER
AMPLIFIERTRANSMIT
RF SIGNALPOWER
DETECTOR
AND
ANTENNA
SWITCHANTENNATRANSMITTER POWER AMPLIFIER SECTION
FIRST RECEIVER
LOCAL OSCILLATOR,
2.1 MHz
REFERENCE OSCILLATOR
TRANSMITTER
INJECTION
SIGNAL
SERIAL
PERIPHERAL
INTERFACE
(SPI) BUS
AUDIO PA
ENABLE
TRANSMITTER
MICROPHONE
PUSH TO
TALK (PTT)OPERATOR
KEYBOARD,
PUSHBUTTONS,
AND OTHER
CONTROLS MICROPHONE
AUDIO
CONTROL HEAD
PROCESSOROPERATOR
DISPLAY
DISPLAY DATA
   1.   REFER TO VOLUME 1 OF THIS
   SERVICE MANUAL FOR INFORMATION
   ABOUT CONTROLLER SECTION. NOTES:
   2.   REFER TO VOLUME 1 OF THIS
   SERVICE MANUAL FOR INFORMATION
   ABOUT CONTROL HEAD.
OPERATOR INTERFACE (CONTROL HEAD  -  NOTE 2)
MAEPF-26567-B   
P/O CONTROLLER
SECTION (NOTE 1)
HARMONIC
FILTER
 
Figure 1 . Overall Radio Functional Block Diagram 
						

							 
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 signal, the 
frequency channel, and the operating mode of the radio. When the 
audio PA enable signal is generated, the audio power amplifier (PA) is 
activated and passes the receiver audio signal to the loudspeaker. 
The transmitter function of the radio produces a nominal 25-Watt 
radio frequency output signal. The radio frequency output signal is 
frequency modulated by an audio signal from the microphone or from 
another source such as a telephone keypad or handset.
The transmitter function of the radio consists of: the audio signalling 
filter integrated circuit (ASFIC) in the controller section; the 
synthesizer section; and the transmitter power amplifier (PA) section. 
The ASFIC develops a modulation signal by amplifying an audio signal 
from the microphone, keypad, or handset. The synthesizer section 
generates a radio frequency carrier signal upon which the transmitter 
portion of the radio operates.The radio frequency carrier signal 
generated by the synthesizer section is frequency modulated in the 
synthesizer section by the modulation signal output from the ASFIC.
The frequency modulated output signal from the synthesizer section 
is amplified to the required nominal 25-Watt power level by the power 
amplifier (PA) section.The output of the PA section passes through the 
antenna switch and is radiated by the vehicle antenna or fixed-station 
antenna.
The controller section of the radio contains a microprocessor that 
controls the radio in accordance with its built in programming as well 
as commands input manually by the radio operator. The radio 
operator inputs manual commands to the controller section using the 
pushbuttons and other controls located on the control head. In 
addition to its controlling functions, the controller section provides 
audio amplification of the audio output signal in the receiver 
function. It also contains squelch detect circuitry based on a buffered 
discriminator signal from the Zero Intermediate Frequency Integrated 
Circuit (ZIF IC).
The operator interface function of the radio consists of: a microphone 
or the microphone portion of a telephone handset; a telephone 
keypad if used; the pushbuttons and other controls on the control 
head; and the digital and graphics displays on the control head. The 
pushbuttons and other controls on the control head provide digital 
commands to the controller section, and in some instances, hardwired 
commands to controlled circuits. The digital and graphics displays 
receive display data from the controller section. The control head 
contains its own microprocessor, which communicates with the 
controller section of the radio via an SB9600 serial digital data bus.
The DC power control and regulation function regulates and 
distributes to the various sections of the radio, DC power from the 
vehicle battery or fixed station power supply. 
						

							 
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 filter tuned by dual varactor diode CR3302 preselects the 
incoming receive signal at terminal IF10-3-1 to reduce spurious effects 
to the stages that follow. The filter tuning voltage at terminal IF1-3-6, 
which ranges from 2.5 VDC to 7.5 VDC, is controlled by a digital to 
analog (D/A) converter integrated circuit in the controller section of 
the radio. The filter has three bandstops: at approximately at 45 MHz 
for the first IF; at 95 MHz for broadcast frequencies; and at the receive 
frequency plus 50 MHz for the receiver local oscillator frequency. The 
three bandstops improve receiver performance at those three specific 
spurious signal frequencies.
In transmit mode, the input signal to the receiver front end is reduced 
by PIN diode CR3330, which is part of the RX/TX switch in the power 
amplifier section of the radio. A dual hot carrier diode (CR3303) limits 
any in-band signal to 10 dBm to prevent damage to the receiver 
preamplifier. 
Preamplifier 
The receiver preamplifier (Q3302) is a surface mount device (SMD), 
which has emitter and collector base feedback to stabilize gain and 
impedance, and minimize intermodulation distortion. To provide 
optimum transistor current drain, Q3302 is actively biased by a 
collector current sense circuit (Q3301 and associated components), 
which in receive mode is also used to bias the receive series PIN diode 
CR3641 (in the power amplifier section) via the decoupled emitter 
path of Q3302.
The Q3302 collector current is set to approximately 22 mA at an 
emitter voltage of about 2 volts. The combined preamplifier and PIN 
diode bias circuit is driven by the keyed 9-Volt line (K9V1) from the 
controller section of the radio.
To stabilize its output impedance, the preamplifier is followed by a 
3 dB pad. 
Fixed Tuned
Band-pass Filter 
A five pole fixed tuned band-pass filter, with two bandstops (both at 
approximately 220 MHz to suppress image frequencies), is included 
after the preamplifier. The filter provides signal transmission over the 
VHF range, wideband spurious signal suppression, and preamplifier to 
mixer matching. 
Mixer 
Q3303 is a double balanced active Gallium Arsenide (GaAs) mixer. Its 
three ports are matched for incoming VHF signal conversion to the 
44.85  MHz IF using high-side injection. The mixer bias is set by R3341 
to approximately 24 mA with P 
inj 
 (RX INJ) = 5 dBm.
The mixer IF output signal is fed through transformer T3303, where its 
impedance is transformed to match the input impedance of the 
receiver IF section. 
						

							 
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 following IF amplifier (Q3401). 
IF Amplifier 
IF amplifier Q3401 is a surface mount device of the same type as 
preamplifier Q3302 in the receiver front end section. It is actively 
biased by a collector base feedback circuit to a current drain of 
approximately 10 mA. Its output voltage swing is limited by a dual hot 
carrier diode (CR3402) to reduce overdrive effects at RF input signal 
levels above 27 dBm. 
IF second selection 
IF amplifier Q3401 is followed by matching networks into and out of 
the second crystal filter (Y3402). To optimize the filter pass band, a 
certain amount of signal is coupled from the input to the output of the 
filter to achieve a notch at the higher and lower adjacent channel. This 
improves the adjacent channel selectivity.
The output from filter Y3402 is coupled via capacitor C3411 and 
terminal 1F3-2-9 to the input of the receiver back end section. 
Receiver Back End 
The following discussion is based on the schematic diagram for the 
receiver back end section on page 23. 
Zero IF (ZIF) Isolation 
Amplifier 
To obtain proper operation of the ZIF IC (U3201) in the receiver back 
end section, an isolation amplifier (Q3203) combined with an IF 
automatic gain control (AGC) and a low-pass filter follows. The 
collector path of Q3203 is loaded with a PIN diode (CR3203) in shunt 
configuration. Its control voltage is generated by a bias circuit with 
CR3204 in conjunction with the voltage out of U3201 pin 4 
(U3201-4). This configuration provides a maximum AGC dynamic 
range of 40 dB and isolates the sensitive ZIF IC from an overdriving IF 
level. Additionally, the signal is low-pass filtered to suppress non-
linear effects within the ZIF IC. 
ZIF IC (U3201) 
Within the ZIF IC, the 44.85  MHz IF signal is mixed in two quadrature 
paths down to baseband, filtered, processed, and FM demodulated. 
The conversion process to baseband is performed by using a second LO 
circuit consisting of Q3201, inductor L3204 and several capacitors 
that, in combination with varactor diode CR3201 and a loop filtered 
steering line from U3201-18, closes a phase locked loop (PLL).
The demodulated audio signal exits the ZIF IC at U3201-28 and is 
applied to the Audio Signalling Filter IC (ASFIC), which is located in 
the controller section of the radio. 
						

							 
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 the audio circuits that filter, amplify, and otherwise process 
the audio signal from the microphone and/or telephone handset.
The portion of the transmitter function located in the synthesizer 
section of the radio is described in the  
Synthesizer Detailed Functional 
Description 
, which follows this paragraph.
The remaining part of the transmitter function of the radio is located 
in the power amplifier section, which is described after the description 
of the synthesizer. 
Synthesizer Detailed 
Functional Description 
The synthesizer section of the transmitter receives the amplified and 
processed audio signal from the controller section of the radio and 
produces a frequency-modulated radio frequency carrier signal (the 
transmitter injection signal), which is input to the transmitter power 
amplifier (PA) section.
The synthesizer section of the radio also generates the first conversion 
local oscillator signal (180.85 to 218.85 MHz) and the second 
conversion reference oscillator signal (2.1 Mhz) for the receiver and 
controller sections of the radio.
The following discussion is based on the schematic diagram for the 
synthesizer section on page 25. 
Synthesizer Circuitry 
The synthesizer consists of Pendulum reference oscillator U5800, 
Fractional-N integrated circuit (IC) U5801, transmit and receive 
voltage controlled oscillators (VCOs) Q3803 and Q3804, buffer 
amplifiers Q3805 and Q5801, transmit injection amplifier Q5802, and 
feedback amplifier Q5774.
Pendulum reference oscillator U5800 contains a temperature 
compensated quartz crystal oscillator with a frequency of 16.8 MHz. 
The oscillator is tuned by a temperature referenced 5-bit analog-to-
digital (A/D) converter located in the controller section of the radio. 
The output of the oscillator (U5800-10) is applied to U5801-14 
(XTAL1) via C5754 and R5750.
The transmit and receive VCOs are the grounded drain Colpitts type 
using junction field effect transistors (JFETs) and lumped elements. 
The transmit VCO covers the frequency range of 136 to174 MHz. The 
receive VCO covers the range of 180.85 to 218.85 MHz. The transmit 
VCO is activated by U5801-38 (AUX2) and the action of transistor 
switches Q3806 and Q3802. The receive VCO is activated by U5801-39 
(AUX3) and the action of transistor switches Q3807 and Q3801. The 
frequency of each VCO is proportional to the applied control voltage, 
which is in the range of 2 to11 VDC. The control voltage is applied to 
frequency control varactors CR3803 through CR3806 via a loop filter. 
						

							 
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 filter drops 9.3 VDC (emitter of Q5770) to about 8.6 
VDC (collector of Q5770). The filtered 8.6 VDC supplies the oscillator 
circuit, modulation circuit, VCO switching circuits, and synthesizer 
charge pump resistor network. Feedback amplifier Q5774 provides the 
amplification and isolation necessary to drive the prescaler input 
(U5801-21). Three-terminal regulator U5802 drops 9.3 VDC from the 
controller section of the radio to 5 VDC required by the Fractional-N 
synthesizer IC.
To generate a high voltage needed by the phase detector (charge 
pump) output stage at U5801-36 (VCP), a voltage of 13 VDC at 
CR5750-1 is generated by a positive voltage multiplier (CR5750, 
C5759, C5760). The positive voltage multiplier is basically a diode 
capacitor network driven by two 1.05-MHz 180 degrees out of phase 
signals (U5801-8 and U5801-9).
The serial interface (SRL) of the microprocessor in the controller 
section of the radio is connected to the data line (U5801-2), clock line 
(U5801-3), and chip enable line (U5801-4) of the fractional-N IC. 
Synthesizer Operation 
The complete synthesizer works as follows: Fractional-N IC U5801 is 
programmed for either transmit or receive via the serial data bus. The 
appropriate VCO is activated via the AUX control pins of U5801 and 
the prescaler and loop divider are set for the desired frequency. The 
outputs from the VCOs are amplified by buffer amplifiers Q3805 and 
Q5801. The output of buffer amplifier Q5801 is split three ways. First, 
the feedback amplifier Q5774 is connected via the resistive attenuator 
consisting of R5773, R5771, and R5772. The local oscillator port for 
the mixer in the receiver front end section of the radio is also 
connected at this node (Rx Inj). Finally, transmit injection amplifier 
Q5802 is connected at this node via the resistive attenuator composed 
of R5782, R5783, and R5784. The output of amplifier Q5802 (TX inj) 
is connected to the transmitter power amplifier.
The prescaler in the synthesizer (Fractional-N IC U5801) is basically a 
dual modulus prescaler with selectable divider ratios. The divider ratio 
of the prescaler is controlled by a loop divider, which in turn receives 
its inputs via the SRL. The output of the prescaler is applied to the loop 
divider. The output of the loop divider is connected to the phase 
detector, which compares the loop divider output signal with a 
reference signal. The reference signal is generated by dividing down 
the signal of the reference oscillator (Pendulum oscillator U5800). The  
						

							 
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 2 (U5801-28). The currents are set by the values of R5752 and 
R5753 or R5756 and R5757, respectively. Selection of one of the three 
different bias sources is done by the radio software.
To reduce synthesizer lock time when new frequency data has been 
loaded into the synthesizer, the magnitude of the loop current is 
increased by enabling the I ADAPT line (U5801-34) for a certain 
software-controlled amount of time (Adapt Mode). Additionally the 
loop current is increased by bypassing R5752 and R5753 with Q5750, 
and bypassing R5756 and R5757 with Q5751. Bypassing starts when 
the FRACN CE line changes from high to low and ends a certain delay 
time after the transition from low back to high. The adapt mode timer 
and the bypassing delay are both started by the low to high transition 
of the FRACN CE line. The adapt mode time is programmed to be 
somewhat shorter than the bypassing delay time, which is hardware 
dependent. This causes two different current levels during frequency 
acquisition of the loop. When the synthesizer is within the lock range, 
the current is determined only by the resistors connected to CPBIAS 1, 
CPBIAS 2, or the internal current source.
The output of the balance attenuator is MODOUT (U5801-30) and is 
connected to the transmit VCO modulation network. The modulation 
is applied to the transmit VCO via varactor diode CR3807. 
Transmit Injection 
Amplifier 
Transmit injection amplifier Q5802 increases the output of Q5801 to 
about +13 dBm. This is the level required by the first stage of the 
transmitter power amplifier. The transmit injection amplifier collector 
supply of 9.1 volts and originates in the controller section of the radio. 
It is active only during transmit. 
Power Amplifier 
Overall PA 
The power amplifier (PA) is a radio frequency (rf) power amplifier, 
which amplifies the output from the injection string (TX_INJ) to an RF 
output power level of 25 Watts.
The following discussion is based on the schematic diagram for the 
power amplifier (PA), on page 27
The PA is a three stage amplifier, which amplifies the output from the 
transmitter injection string in the Synthesizer Section to the transmit 
level (nominally 25 Watts).
The first stage (Q3521) of the PA is a bipolar stage, which is controlled 
via the PA control line. It is followed by two Metal Oxide 
Semiconductor Field Effect Transistor (MOSFET) devices, Q3541 and 
Q3561.
Devices Q3521 and Q3541 are surface mounted. To remove heat from 
Q3541, the surface mounted heat spreader on the opposite side of the  
						

							 
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. This results in more 
current being drawn by Q3501 so that the base current on Q3521 is 
increased. This in turn causes higher collector current to ßow through 
Q3521. The collector current settles when the voltage drop across the 
parallel combination of resistors R3530 throughR3533 equals the 
voltage drop across R3606 minus the base-to-emitter voltage drop of 
Q3501.
By controlling the output power of Q3521 and in turn the input power 
of the following stages, the automatic level control (ALC) loop is able 
to regulate the output power of the transmitter.
The MOSFET devices Q3541 and Q3561 are enhancement mode N-
Channel MOSFETS. For proper operation, these devices require a 
positive gate bias voltage and a quiescent current ßow with no drive. 
To achieve these requirements, the gates are biased through the 
network consisting of R3582, R3525, and R3526 for Q3541 and 
similarly R3585, R3544, and R3545 for Q3561. The actual value of the 
voltage at this gate is device dependent and determined by trim in the 
factory when the radio is built. 
The output of Q3561 goes through the matching network consisting 
of four transmission lines, inductor L3566, and several capacitors to 
the directional coupler. 
Directional Coupler, RX-TX 
Switch, and 
Harmonic Filter 
The forward power detector (directional coupler) is a microstrip 
printed circuit, which couples a small amount of the forward RF 
energy off and sends it to diode CR3671 where it is rectified. The 
rectified signal forms the V detect voltage, which is proportional to the 
rectified RF energy appearing across the diode. The power control 
circuit holds this voltage constant, thus ensuring that the forward 
power out of the radio is held constant.
The antenna switch is switched synchronously with the keyed 9.1 
voltage (K 9V1). In transmit mode, the K 9V1 line is high and the 
current turns on the PIN diode CR3641. The receiver preamplifier 
transistor Q3302, also turned off by the keyed 9.1 voltage, cuts off the 
current through the PIN diode CR3642 and turns off transistor Q3641.
In receive mode the K 9V1 line is low. This turns off the PIN diode 
CR3641 and turns on the receiver preamp transistor Q3302. The 
current through Q3302 turns on the PIN diode CR3642 and transistor 
Q3641. Q3641 shorts R3644 to increase the current through Q3302.
Harmonics of the transmitter are attenuated by the harmonic filter 
formed by inductors L3661 through L3663, and capacitors C3661 
through C3664. This network forms a low-pass filter to attenuate 
harmonic energy of the transmitter to an acceptable level. L3664 
provides electrostatic protection for the power amplifier. 
Temperature Sense 
In the PA compartment, 100K ohm thermistor R3610 senses the 
temperature of MOSFET devices Q3541 and Q3561. This signal is fed 
back into the power control circuit to protect the power amplifier