Motorola Gm328 Gm338 Gm398 Detailed 6804112j18 E Manual

VHF (136-174 MHz) Transmitter Power Amplifier (PA) 25 W 2-3
2.4 Low Intermediate Frequency (IF) and Receiver Back End
The 44.85 MHz high-IF signal from the second IF amplifier feeds the IF IC (U3101) at pin 1. 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...

2-4 VHF (136-174 MHz) Transmitter Power Amplifier (PA) 25 W
3.1 First Power Controlled Stage
The first stage (U3401) 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 U3401 is 
controlled by a DC voltage applied to pin 1 from the op-amp U3402-1, pin 1. The control voltage 
simultaneously varies the bias of two FET stages within U3401. This biasing point determines the 
overall gain of U3401 and...

VHF (136-174 MHz) Transmitter Power Amplifier (PA) 25 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 Q3441. The coupled signal is rectified by D3451. The DC voltage is proportional 
to the RF output power and feeds the RFIN port of the PCIC (U3501 pin 1). The PCIC controls the 
gain of stage U3401 and Q3421 as necessary to hold this voltage constant, thus ensuring the 
forward power out of the radio to...

2-6VHF (136-174 MHz) Frequency Synthesis
4.0 VHF (136-174 MHz) Frequency Synthesis
The frequency synthesizer subsystem consists of the reference oscillator (Y3261 or Y3263), the 
Low Voltage Fractional-N synthesizer (LVFRAC-N, U3201), and the voltage-controlled oscillators 
and buffer amplifiers (U3301, Q3301-2 and associated circuitry).
4.1 Reference Oscillator
The reference oscillator (Y3263) contains a temperature compensated crystal oscillator with a 
frequency of 16.8 MHz. An analog to digital (A/D)...

VHF (136-174 MHz) Frequency Synthesis2-7
A voltage of 5V applied to the super filter input (U3201 pin 30) supplies an output voltage of 4.5 VDC 
(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R3363) and the synthesizer 
charge pump resistor network (R3251, R3252). The synthesizer supply voltage is provided by the 
5V regulator U3211.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at 
pin 47 VCP (U3201-47), a voltage of 13 VDC is being...

2-8VHF (136-174 MHz) Frequency Synthesis
The VCOBIC together with the Fractional-N synthesizer (U3201) generates the required frequencies 
in both the transmit and receive modes. The TRB line (U3301 pin 19) determines which tank circuits 
and internal buffers are to be enabled. A high level on TRB enables the TX tank and TX output (pin 
10), and a low enables the RX tank and RX output (pin 8). A sample of the signal from the enabled 
RF output is routed from U3301 pin 12 (PRESC_OUT), via a low pass...

VHF (136-174 MHz) Frequency Synthesis2-9
In order to modulate the PLL, the two spot modulation method is utilized. Via U3201 pin 10 (MODIN), 
the audio signal is applied to both the A/D converter (low frequency path) as well as the balance 
attenuator (high frequency path). The A/D converter changes the low frequency analog 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...

2-10VHF (136-174 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 
C3035?
No
RF Signal at 
T3001?
RF Signal at 
C3012?
No
RF Signal at 
C3008?
No
RF Signal at 
C3474?
No or 
Check Harmonic Filter 
J3401 and
Antenna Switch 
D3471,D3472,L3472
Check filter between C3474 
& C3008. Check tuning 
voltage at R3019
Inject RF into J3401
Is tuning 
voltage OK?
No
Ye s
Check RF amp 
(Q3001) Stage
Check filter between 
C3012 & T3001
Ye s
Check T3001, T3002, 
D3031,...