Motorola Uhf Manual

							Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the UHF circuits in the radio. For details of 
the theory of operation and trouble shooting for the the associated Controller circuits refer to the 
Controller Section of this manual.
2.0 UHF (403-470MHz) Receiver
2.1 Receiver Front-End
The receiver is able to cover the UHF range from 403 to 470 MHz. It consists of four major blocks: 
front-end bandpass filters and pre-amplifier, first mixer, high-IF, low-IF and receiver back-end . Two 
varactor-tuned bandpass filters perform antenna signal pre-selection.  A cross over quad diode 
mixer converts the signal to the first IF of 44.85 MHz. Low-side first injection is used.
Figure 2-1 UHF Receiver Block Diagram
Demodulator
1. Crystal 
Filter Mixer Varactor 
Tuned Filter RF Amp Varactor 
Tuned Filter Pin Diode 
Antenna 
Switch
RF Jack Antenna
Control Voltage
from  PCICFirst LO
from FGU
Recovered Audio
RSSI
Second LO
2. Crystal 
Filter
455kHz Filter
(25kHz)455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)455kHz Filter
(12.5kHz)SwitchSwitchSwitchSwitch
Limiter
1. IF Amp
2. IF Amp
Filter Bank Selection
from  Synthesizer IC
   Harmonic 
   Filter
BWSELECT 
						

							2-2THEORY OF OPERATION
There are two 2-pole 44.85 MHz crystal filters in the high-IF section and 2 pairs of 455 kHz ceramic 
filters in the low-IF section to provide the required adjacent channel selectivity .The correct pair of 
ceramic filters for 12.5 or 25KHz channel spacing is selected via control line BWSELECT. The 
second IF at 455 kHz is mixed, amplified and demodulated in the IF IC. The processing of the 
demodulated audio signal is performed by an audio processing IC located in the controller section.
2.2 Front-End Band-Pass Filters & Pre-Amplifier 
The received signal from the radio’s antenna connector is first routed through the harmonic filter and 
antenna switch, which are part of the RF power amplifier circuitry, before being applied to the 
receiver pre-selector filter (C4001, C4002, D4001 and associated components). The 2-pole pre-
selector filter tuned by the varactor diodes D4001 and D4002 pre-selects the incoming signal 
(RXIN) from the antenna switch to reduce spurious effects to following stages. The tuning voltage 
(FECTRL_1) ranging from 2 volts to 8 volts is controlled by pin 20 of PCIC (U4501) in the 
Transmitter section. A dual hot carrier diode (D4003) limits any inband signal to 0 dBm to prevent 
damage to the pre-amplifier.
The RF pre-amplifier is an SMD device (Q4003) with collector base feedback to stabilize gain, 
impedance, and intermodulation. The collector current of approximately 11-16 mA is drawn from the 
voltage 9V3 via L4003 and R4002. A switchable 3dB pad (R4066,R4007,R4063, R4064 and 
R4070), controlled via line FECTRL_2 and Q4004 stabilizes the output impedance and 
intermodulation performance. 
A second 2-pole varactor tuned bandpass filter provides additional filtering of the amplified signal. 
The varactor diodes D4004 and D4005 are controlled by the same signal FECTRL_1, which 
controls the pre-selector filter. A following 1 dB pad (R4013 - R4015) stabilizes the output 
impedance and intermodulation performance.
2.3 First Mixer and High Intermediate Frequency (IF)
The signal coming from the front-end is converted to the first IF (44.85 MHz) using a cross over 
quad diode mixer (D4051). Its ports are matched for incoming RF signal conversion to the 44.85 
MHz IF using low side injection via matching transformers T4051 and T4052. The injection signal 
(RXINJ) coming from the RX VCO buffer (Q4332) is filtered by the lowpass filter consisting of 
(L4053, L4054, C4053 - C4055) followed by a matching transformer T4052 and has a level of 
approximately 15dBm.
The mixer IF output signal (IF) from transformer T4501pin 2 is fed to the first two pole crystal filter 
FL3101. The filter output in turn is matched to the following IF amplifier.
The IF amplifier Q3101 is actively biased by a collector base feedback (R3101, R3106) to a current 
drain of approximately 5 mA drawn from the voltage 5V. Its output impedance is matched to the 
second two pole crystal filter FL3102. The signal is further amplified by a preamplifier (Q3102) 
before going into pin 1 of IFIC (U3101).
A dual hot carrier diode (D3101) limits the filter output voltage swing to reduce overdrive effects at 
RF input levels above -27 dBm.
2.4 Low Intermediate Frequency (IF) and Receiver Back End
The 44.85 high IF signal from the second IF amplifier feeds the IF IC (U3101) at pin1. Within the IF 
IC the 44.85 MHz high IF signal mixes with the 44.395 MHz second local oscillator (2nd LO) to  
						

							UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W 2-3
produce the low IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The 
low IF signal is amplified and filtered by an external pair of 455 kHz ceramic filters FL3112, FL3114 
for 20/25 kHz channel spacing or FL3111,FL3113/F3115 for 12.5 kHz channel spacing. These pairs 
are selectable via BWSELECT. The filtered output from the ceramic filters is applied to the limiter 
input pin of the IF IC (pin 14).
The IF IC contains a quadrature detector using a ceramic phase-shift element (Y3102) to provide 
audio detection. Internal amplification provides an audio output level of 120 mV rms (at 60% 
deviation) from U3103 pin 8 (DISCAUDIO) which is fed to the ASFIC_CMP (U0221) pin 2 (part of 
the Controller circuitry). 
A received signal strength indicator (RSSI) signal is available at U3101 pin 5, having a dynamic 
range of 70 dB. The RSSI signal is interpreted by the microprocessor (U0101 pin 63) and in addition 
is available at accessory connector J0501-15.
3.0 UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W
The radio’s 25W PA is a three stage amplifier used to amplify the output from the VCOBIC to the 
radio transmit level. All three stages utilize LDMOS technology. The gain of the first stage (U4401) is 
adjustable, controlled by pin 4 of PCIC (U4501) via U4402-1.  It is followed by an LDMOS  stage 
(Q4421) and LDMOS final stage (Q4441).
Figure 2-2 UHF Transmitter Block Diagram 
Devices U4401, Q4421 and Q4441 are surface mounted. A pressure pad between board and the 
radios cover provides good thermal contact between the devices and the chassis.
3.1First Power Controlled Stage
The first stage (U4401) is a 20dB gain integrated circuit containing two LDMOS FET amplifier 
stages. It amplifies the RF signal from the VCO (TXINJ). The output power of stage U4401 is PCIC
Pin Diode 
Antenna 
Switch
RF JackAntenna
Harmonic 
Filter
PowerSensePA-FinalStage From VCOControlledStage
VcontrolBias 1Bias 2
To Microprocessor
Temperature
Sense SPI BUS
ASFIC_CMP
PA
PWR
SET
To Microprocessor
PADriver 
						

							2-4THEORY OF OPERATION
controlled by a DC voltage applied to pin 1 from the op-amp U4402-1, pin 1. The control voltage 
simultaneously varies the bias of two FET stages within U4401. This biasing point determines the 
overall gain of U4401 and therefore its output drive level to Q4421, which in turn controls the output 
power of the PA.
Op-amp U4402-1 monitors the drain current of U4401 via resistor R4444 and adjusts the bias 
voltage of U4401 so that the current remains constant. The PCIC (U4501) provides a DC output 
voltage at pin 4 (INT) which sets the reference voltage of the current control loop. A raising power 
output causes the DC voltage from the PCIC to fall, and U4402-1 adjusts the bias voltage for a lower 
drain current to lower the gain of the stage.
In receive mode the DC voltage from PCIC pin 23 (RX) turns on Q4442, which in turn switches off 
the biasing voltage to U4401.
Switch S5440 is a pressure pad with a conductive strip which connects two conductive areas on the 
board when the radios cover is properly screwed to the chassis. When the cover is removed, S5440 
opens and the resulting high voltage level at the inverting inputs of the current control op-amps 
U4402-1 & 2 switches off the biasing of U4401 and Q4421. This prevents transmitter key up while 
the devices do not have proper thermal contact to the chassis.
3.2Power Controlled Driver Stage
The next stage is an LDMOS device (Q4421) providing a gain of 12dB. This device requires a 
positive gate bias and a quiescent current flow for proper operation. The bias is set during transmit 
mode by the drain current control op-amp U4402-2, and fed to the gate of Q4421 via the resistive 
network R4429, R4418, R4415 and R4416.
Op-amp U4402-2 monitors the drain current of U4421 via resistors R4424-27 and adjusts the bias 
voltage of Q4421 so that the current remains constant. The PCIC (U4501) provides a DC output 
voltage at pin 4 (INT) which sets the reference voltage of the current control loop. A raising power 
output causes the DC voltage from the PCIC to fall, and U4402-2 adjusts the bias voltage for a lower 
drain current to lower the gain of the stage.
In receive mode the DC voltage from PCIC pin 23 (RX) turns on Q4422, which in turn switches off 
the biasing voltage to Q4421.
3.3 Final Stage
The final stage is an LDMOS device (Q4441) providing a gain of 12dB. This device also requires a 
positive gate bias and a quiescent current flow for proper operation. The voltage of the line 
MOSBIAS_2 is set in transmit mode by the ASFIC and fed to the gate of Q4441 via the resistive 
network R4404, R4406, and R4431-2. This bias voltage is tuned in the factory. If the transistor is 
replaced, the bias voltage must be tuned using the Golbal Tuner.  Care must be taken not to 
damage the device by exceeding the maximum allowed bias voltage. The device’s drain current is 
drawn directly from the radio’s DC supply voltage input, PASUPVLTG, via L4436 and L4437.
A matching network consisting of C4441-49 and striplines transforms the impedance to 50 ohms 
and feeds the directional coupler.
3.4 Directional Coupler
The directional coupler is a microstrip printed circuit, which couples a small amount of the forward 
power delivered by Q4441. The coupled signal is rectified by D4451. The DC voltage is proportional 
to the RF output power and feeds the RFIN port of the PCIC (U4501 pin 1). The PCIC controls the 
gain of stages U4401 and Q4421 as necessary to hold this voltage constant, thus ensuring the 
forward power out of the radio to be held to a constant value. 
						

							UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W 2-5
3.5 Antenna Switch
The antenna switch consists of two PIN diodes, D4471 and D4472. In the receive mode, both diodes 
are off. Signals applied at the antenna jack J4401 are routed, via the harmonic filter, through 
network L4472, C4474 and C4475, to the receiver input. In the transmit mode, K9V1 turns on Q4471 
which enables current sink Q4472, set to 96 mA by R4473 and VR4471. This completes a DC path 
from PASUPVLTG, through L4437, D4471, L4472, D4472, L4471, R4474 and the current sink, to 
ground. Both diodes are forward biased into conduction. The transmitter RF from the directional 
coupler is routed via D4471 to the harmonic filter and antenna jack. D4472 also conducts, shunting 
RF power and preventing it from reaching the receiver port (RXIN). L4472 is selected to appear as a 
broadband lambda/4 wave transmission line, making the short circuit presented by D4472 appear as 
an open circuit at the junction of D4472 and the receiver path.
3.6 Harmonic Filter
Components L4491-L4493 and L4472, C4491, C4496-98 form a Butterworth low-pass filter to 
attenuate harmonic energy of the transmitter to specifications level.  R4491 is used to drain 
electrostatic charge 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, improving 
spurious response rejection.
3.7 Power Control
The transmitter uses the Power Control IC (PCIC, U4501) to control the power output of the radio. A 
portion of the forward RF power from the transmitter is sampled by the directional coupler and 
rectified, to provide a DC voltage to the RFIN port of the PCIC (pin 1) which is proportional to the 
sampled RF power. 
The ASFIC (U0221) has internal digital to analog converters (DACs) which provide a reference 
voltage of the control loop to the PCIC via R4505. The reference voltage level is programmable 
through the SPI line of the PCIC. This reference voltage is proportional to the desired power setting 
of the transmitter, and is factory programmed at several points across the frequency range of the 
transmitter to offset frequency response variations of the transmitter’s power detector circuit.
The PCIC provides a DC output voltage at pin 4 (INT) which sets the drain current of the first 
(U4401) and second (Q4421) transmitter stage via current control op-amps U3402-1 and U3402-2. 
This adjusts the transmitter power output to the intended value. Variations in forward transmitter 
power cause the DC voltage at pin 1 to change, and the PCIC adjusts the control voltage above or 
below its nominal value to raise or lower output power.
Capacitors C4502-4, in conjunction with resistors and integrators within the PCIC, control the 
transmitter power-rise (key-up) and power-decay (de-key) characteristic to minimize splatter into 
adjacent channels.
U4502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity 
of the transmitter driver and final devices, and provides a dc voltage to the PCIC (TEMP, pin 30) 
proportional to temperature. If the DC voltage produced exceeds the set threshold in the PCIC, the 
transmitter output power will be reduced so as to reduce the transmitter temperature. 
						

							2-6THEORY OF OPERATION
4.0 UHF (403-470MHz) Frequency Synthesis
The synthesizer subsystem consists of the reference oscillator (Y4261 or Y4262), the Low Voltage 
Fractional-N synthesizer (LVFRAC-N, U4201), and the Voltage Controlled Oscillator VCO.
4.1 Reference Oscillator
The reference oscillator (Y4262) contains a temperature compensated crystal oscillator with a 
frequency of 16.8 MHz. An Analogue to Digital (A/D) converter internal to U4201 (LVFRAC-N) and 
controlled by the microprocessor via serial interface (SRL) sets the voltage at the warp output of 
U4201 pin 25 to set the frequency of the oscillator. The output of the oscillator (pin 3 of Y4262) is 
applied to pin 23 (XTAL1) of U4201 via a RC series combination.
In applications where less frequency stability is required the oscillator inside U4201 is used along 
with an external crystal Y4261, varactor diode D4261, C4261, C4262 and R4262. In this case, 
Y4262, R4263, C4235 and C4251 are not used. When Y4262 is used, Y4261, D4261, C4261, 
C4262 and R4262 are not used, and C4263 is increased to 0.1 uF.
4.2 Fractional-N Synthesizer
The LVFRAC-N synthesizer IC (U4201) consists of a pre-scaler, a programmable loop divider, 
control divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital 
modulation, a balance attenuator to balance the high frequency analogue modulation and low 
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and 
finally a super filter for the regulated 5 volts.
Figure 2-3 UHF Synthesizer Block Diagram
DATA
CLK
CEX
MODIN
VCC, DC5V
XTAL1
XTAL2
WARP
PREIN
VCP
REFERENCE
OSCILLATOR
 VOLTAGE
MULTIPLIER
DATA (U0101 PIN 100)
CLOCK (U0101 PIN 1)
CSX (U0101 PIN 2)
MOD IN (U0221 PIN 40)
+5V (U4211 PIN 1)7
8
9
10
13, 30
23
24
25
32
47
VMULT2 VMULT1BIAS1 SFOUTAUX3 AUX4 IADAPTIOUTGND FREFOUTLOCK4
19
6, 22, 33, 44
43
45
3
2
28
       14
        1540FILTERED 5VSTEERING LOCK (U0101 PIN 56)
PRESCALER INFREF (U0221 PIN 34)
39 BIAS2
41
 48 5, 20, 34, 36
+5V (U4211 PIN 1)
AUX1 VDD, DC5VMODOUT
U4201 
LOW VOLTAGEFRACTIONAL-N
SYNTHESIZER
AUX21 (NU)
BWSELECTVCO Bias
TRB
To IF
SectionTX RF INJECTION
(1ST STAGE OF PA)LO RF INJECTION
VOLTAGE 
CONTROLLED 
OSCILLATORLINE
2-POLE
LOOP
FILTER 
						

							UHF (403-470MHz) Frequency Synthesis2-7
A voltage of 5V applied to the super filter input (U4201 pin 30) supplies an output voltage of 4.5 
VDC(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R4322) and the 
synthesizer charge pump resistor network (R4251, R4252). The synthesizer supply voltage is 
provided by the 5V regulator U4211.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin 
VCP (U4201-47), a voltage of 13 VDC is being generated by the positive voltage multiplier circuitry 
(D4201, C4202, C4203). This voltage multiplier is basically a diode capacitor network driven by two 
(1.05MHz) 180 degrees out of phase signals (U4201-14 and -15).
Output LOCK (U4201-4) provides information about the lock status of the synthesizer loop. A high 
level at this output indicates a stable loop. IC U4201 provides the 16.8 MHz reference frequency at 
pin 19.
The serial interface (SRL) is connected to the microprocessor via the data line DATA (U4201-7), 
clock line CLK (U4201-8), and chip enable line CSX (U4201-9).
4.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U4301), the TX 
and RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Figure 2-4 UHF VCO Block Diagram
 
Presc
RX
TXMatching
NetworkLow Pass
    Filter
Attenuator Pin8
Pin14
Pin10(U4201 Pin28)
VCC Buffers
TX RF Injection U4201 Pin 32 AUX3 (U4201 Pin 2)
Prescaler Out
Pin 12 Pin 19 Pin 20
      TX/RX/BS
Switching Network
U4301
VCOBIC
       Rx
Active Bias
      Tx
Active Bias
Pin2
Rx-I adjustPin1
Tx-I adjustPins 9,11,17
Pin18Vsens
Circuit Pin15Pin16 RX VCO
 Circuit
TX VCO
 Circuit RX Tank
TX TankPin7
Vcc-Superfilter
Collector/RF in
Pin4
Pin5
Pin6
RX
TX
(U4201 Pin 28)Rx-SW
Tx-SW
Vcc-Logic
(U4201 Pin 28) Steer Line 
Voltage 
(VCTRL)Pin13
Pin3TRB IN
LO RF INJECTION
Q4301
Q4332 
						

							2-8THEORY OF OPERATION
The VCOBIC together with Fractional-N synthesizer (U4201) generates the required frequencies in 
both  transmit and receive modes. The TRB line (U4301 pin 19) determines which tank circuits and 
internal buffers are to be enabled. A high level on TRB enables TX tank and TX output (pin 10), and  
a low enables RX tank and RX output (pin 8). A sample of the signal from the enabled output is 
routed from U4301 pin 12 (PRESC_OUT), via a low pass filter, to pin 32 of U4201 (PREIN).
A steering line voltage (VCTRL) between 3.0V and 10.0V at varactor diode CR4311 will tune the full 
TX frequency range (TXINJ) from 403 MHz to 470 MHz, and at varactor diodes CR4301, CR4302 
and CR4303 will tune the full RX frequency range (RXINJ) from 358 MHz to 425 MHz. The tank 
circuits uses the Hartley configuration for wider bandwidth. For the RX tank circuit, an external 
transistor Q4301 is used in conjunction with the internal transistor for better side-band noise.
The external RX buffers (Q4332) are enabled by a high at U4201 pin 3 (AUX4) via transistor switch 
Q4333.  In TX mode the modulation signal (VCOMOD) from the LVFRAC-N synthesizer IC (U4201 
pin41) is applied modulation circuitry CR4321, R4321, R4322 and C4324, which modulates the TX 
VCO frequency via coupling capacitor C4321.  Varactor CR4321 is biased for linearity from VSF.
4.4 Synthesizer Operation
The complete synthesizer subsystem comprises mainly of low voltage FRAC-N (LVFRACN) IC, 
Reference Oscillator (crystal oscillator with temperature compensation), charge pump circuitry, loop 
filter circuitry and DC supply. The output signal PRESC_OUT of the VCOBIC (U4301 pin12) is fed to 
pin 32 of U4201 (PREIN) via a low pass filter (C4229, L4225) which attenuates harmonics and 
provides the correct level to close the synthesizer loop.
The pre-scaler in the synthesizer (U4201) is basically a dual modulus pre-scaler with selectable 
divider ratios. This divider ratio of the pre-scaler is controlled by the loop divider, which in turn 
receives its inputs via the SRL. The output of the pre-scaler is applied to the loop divider. The output 
of the loop divider is connected to the phase detector, which compares the loop divider´s output 
signal with the reference signal.The reference signal is generated by dividing down the signal of the 
reference oscillator (Y4261 or Y4262).
The 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 pin 43 of U4201 (IOUT). The loop filter (which consists of 
R4221-R4223, C4221-C4225,L4221) transforms this current into a voltage that is applied to the 
varactor diodes CR4311 for transmit, CR4301, CR4302 & CR4303 for receive and alters the output 
frequency of the VCO .The current can be set to a value fixed in the LVFRAC-N IC or to a value 
determined by the currents flowing into BIAS 1 (U4201-40) or BIAS 2 (U4201-39). The currents are 
set by the value of R4251 or R4252 respectively. The selection of the three different bias sources is 
done by software programming.
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 IADAPT  (U4201-45) for a certain 
software programmable time (Adapt Mode). The adapt mode timer is started by a low to high 
transient of the CSX line. When the synthesizer is within the lock range the current is determined 
only by the resistors connected to BIAS 1, BIAS 2, or the internal current source. A settled 
synthesizer loop is indicated by a high level of signal LOCK (U4201-4). 
The LOCK (U4201-4) signal is routed to one of the µP´s ADCs input U101-56. From the voltage the 
µP determines whether LOCK  is active. In order to modulate the PLL the two spot modulation 
method is utilized. Via pin 10 (MODIN) on U4201 the audio signal is applied to both the A/D 
converter (low freq path) as well as the balance attenuator (high freq path). The A/D converter 
converts the low frequency analogue modulating signal into a digital code that is applied to the loop 
divider, thereby causing the carrier to deviate. The balance attenuator is used to adjust the VCO’s 
deviation sensitivity to high frequency modulating signals. The output of the balance attenuator is 
present at the MODOUT port (U4201-41) and connected to the VCO modulation diode CR4321 via 
R4321, C4325. 
						

							UHF (403-470MHz) Transmitter Power Amplifier (PA) 40W 2-9
5.0 UHF (403-470MHz) Transmitter Power Amplifier (PA) 40W
The radio’s 40 W PA is a four stage amplifier used to amplify the output from the VCOBIC to the radio 
transmit level. It consists of the following four stages in the line-up. The first stage is a LDMOS 
predriver (U4401) that is controlled by pin 4 of PCIC (U4501) via Q4473 (CNTLVLTG). It is followed 
by another LDMOS stage (Q4421), an LDMOS stage (Q4431) and a bipolar final stage (Q4441).
Figure 2-1 UHF Transmitter Block Diagram 
Device Q4401 is surface mounted. Q4421, Q4431 and Q4441 are directly attached to the heat sink.
5.1 Power Controlled Stage
The first stage (U4401) amplifies the RF signal from the VCO (TXINJ) and controls the output power 
of the PA. The output power of the transistor U4401 is controlled by a voltage control line feed from 
the PCIC pin4(U4501). The control voltage simultaneously varies the bias of two FET stages within 
U4401. This biasing point determines the overall gain of U4401 and therefore its output drive level to 
Q4421, which in turn controls the output power of the PA. 
In receive mode the voltage control line is at ground level and turns off Q4473 which in turn switches 
off the biasing voltage to U4401.
5.2 Pre-Driver Stage
The next stage is a 13dB gain LDMOS device (Q4421) which requires a positive gate bias and a 
quiescent current flow for proper operation. The voltage of the line PCIC_MOSBIAS_1 is set in 
transmit mode by PCIC pin 24 and fed to the gate of Q4421 via the resistive network R4480, R4416 
and R4415. The bias voltage is tuned in the factory.PCIC
Pin Diode 
Antenna 
Switch
RF JackAntenna
Harmonic 
Filter
PowerSensePA-FinalStagePADriver From VCOControlledStage
VcontrolBias 1Bias 2
To Microprocessor
Temperature
Sense SPI BUS
ASFIC_CMP
PA
PWR
SET
To Microprocessor
PreDriver 
						

							2-10THEORY OF OPERATION
5.3 Driver Stage
The following stage is an enhancement-mode N-Channel MOSFET device (Q4431) providing a gain 
of 10dB. This device also requires a positive gate bias and a quiescent current flow for proper 
operation. The voltage of the line Bias_2_UHF_PA_1 is set in transmit mode by the ASFIC and fed to 
the gate of Q4431 via the resistive network R4632, R4631, R4485 and R4486. This bias voltage is 
also tuned in the factory. If the transistor is replaced, the bias voltage must be tuned using the 
Customer Programming Software (CPS). Care must be taken not to damage the device by exceeding 
the maximum allowed bias voltage. The device’s drain current is drawn directly from the radio’s DC 
supply voltage input, A+, via L4421.
5.4 Final Stage
The final stage uses the bipolar device Q4441. The device’s collector current is also drawn from the 
radio’s DC supply voltage input. To maintain class C operation, the base is DC-grounded by a series 
inductor (L4441) and a bead (L4440). A matching network consisting of C4441-C4444, C4491 and 
two striplines transforms the impedance to 50 Ohms and feeds the directional coupler.
5.5 Directional Coupler
The Bi-directional coupler is a microstrip printed circuit, which couples a small amount of the forward 
and reverse power of the RF power from Q4441. The coupled signal is rectified to an output power 
proportional DC voltage by the diodes D4451 & D4452 and sent to the RFIN of PCIC. The PCIC 
controls the gain of stage U4401 as necessary to hold this voltage constant, thus ensuring the 
forward power out of the radio to be held to a constant value.
5.6 Antenna Switch
The antenna switch consists of two PIN diodes, D4471 and D4472. In the receive mode, both 
diodes are off. Signals applied at the antenna jack J4401 are routed, via the harmonic filter, through 
network L4472, C4474 and C4475, to the receiver input. In the transmit mode, K9V1 turns on 
Q4471 which enables current sink Q4472, set to 96 mA by R4511 and VR4471. This completes a 
DC path from PASUPVLTG, through L4437, D4471, L4472, D4472, L4473, R4496 and the current 
sink, to ground. Both diodes are forward biased into conduction. The transmitter RF from the 
directional coupler is routed via D4471 to the harmonic filter and antenna jack. D4472 also 
conducts, shunting RF power and preventing it from reaching the receiver port (RXIN). L4472 is 
selected to appear as a broadband Lambda/4 wave transmission line, making the short circuit 
presented by D4472 appear as an open circuit at the junction of D4472 and the receiver path.
5.7 Harmonic Filter
Inductors L4491, L4492, L4493 and capacitors C4448, C4492,C4494, C4496 and C4498 form a 
low-pass filter to attenuate harmonic energy of the transmitter to specifications level. R4491 is used 
to drain electrostatic charge 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, 
improving spurious response rejection.