Motorola Gm328 Gm338 Gm398 Detailed 6804112j18 E Manual

UHF (403-470 MHz) Transmitter Power Amplifier (PA) 40 W 2-3
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 
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,...

2-4 UHF (403-470 MHz) Transmitter Power Amplifier (PA) 40 W
3.1 First 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 
controlled by a DC voltage applied to pin 1 from Q4473. 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...

UHF (403-470 MHz) Transmitter Power Amplifier (PA) 40 W 2-5
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 stage U4401 as necessary to hold this voltage constant, thus ensuring the forward power out 
of the radio to be held...

2-6UHF (403-470 MHz) Frequency Synthesis
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.
4.0 UHF (403-470 MHz) Frequency Synthesis
The synthesizer subsystem consists of...

UHF (403-470 MHz) Frequency Synthesis2-7
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....

2-8UHF (403-470 MHz) Frequency Synthesis
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
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...

UHF (403-470 MHz) Frequency Synthesis2-9
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...

2-10UHF (403-470 MHz) Frequency Synthesis
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3-1
Section 3
TROUBLESHOOTING CHARTS
1.0 Troubleshooting Flow Chart for Receiver (Sheet 1 of 2)
Bad SINAD
Bad 20dB Quieting
No Recovered AudioSTART
Audio at 
pin 8 of 
U3101 ?Check Controller
(in the case of no audio)
OR ELSE go to “B” Ye s
No
Spray or inject 44.85MHz 
into XTAL Filter FL3101
Audio heard ?BYe s
No
Check 2nd LO 
(44.395MHz) at C3135 
LO present ?BYe s
 Check voltages on 
U3101Biasing OK ?
No
No
A
Ye s
Check Q3102  bias 
for faults
Replace Q3102
Go to B
Ye s
No
Check circuitry 
around...

3-2Troubleshooting Flow Chart for Receiver
1.1Troubleshooting Flow Chart for Receiver (Sheet 2 of 2)
IF Signal at 
C3101?
No
RF 
Signal at 
T4051?
RF
Signal at 
C4015?
No
No
RF
Signal at 
C4025?
No or 
Check harmonic filter
L4491-L4493, C4492, J4401
and ant. switch
D4471, D4472, L4472.
Check filter between 
C4025 & C4009. 
Check tuning voltage 
at R4060.
Inject RF into J4401
Is
tuning voltage
OK?
No
Ye s
Check RF amp (Q4003) 
Stage.
Check filter between 
C4015 & T4051.
Ye s
Check T4051, T4052, 
D4051,...