Motorola Gm Series Low Band Info Manual

Low Band Transmitter Power Amplifier (PA) 25-60 W 2-5
3.0 Low Band Transmitter Power Amplifier (PA) 25-60 W
The radio’s 60 W PA is a three-stage amplifier used to amplify the output from the VCO to the radio 
transmit level. The line-up consists of three stages which utilize LDMOS technology. The first stage 
is pre-driver (U1401) that is controlled by pin 4 of PCIC (U1503) via Q1504 and Q1505 
(CNTLVLTG). It is followed by driver stage Q1401, and final stage utilizing two devices (Q1402 and 
Q1403)...

2-6THEORY OF OPERATION
antenna of the radio. The variation in the driver’s output power is limited by controlling its DC 
current. The driver’s DC current is monitored by measuring the voltage drop across current-sense 
resistors R1473-6, and this voltage is compared to a reference voltage on pin 6 of U1402-2. If the 
current through the sense resistors decreases, the circuit increases the bias voltage on the gate of 
Q1401 via Q1404. If the current increases, then the bias voltage decreases in order to...

Low Band Frequency Synthesis2-7
current until equilibrium of the two compared voltages is reached. The current through Q1503 
develops a voltage across R1513 which is exactly propor tional to the DC current of the final stages. 
This voltage is applied to the RF IN port of the PCIC (pin 1). 
The PCIC has internal digital to analog converters (DACs) which provide a reference voltage of the 
control loop. The reference voltage level is programmable through the SPI line of the PCIC. This 
reference voltage...

2-8THEORY OF OPERATION
for low frequency digital modulation, a balance attenuator to balance the high frequency analog 
modulation and low frequency digital modulation, a 13V positive voltage multiplier, a serial interface 
for control, and finally a super filter for the regulated 9.3 volt supply.
Regulated 9.3 volts DC applied to the super filter input (U1201 pin 30) delivers a very low noise 
output voltage of 8.3 volts DC (VSF) at pin 28. External device Q1201 allows greater current 
sourcing...

Low Band Frequency Synthesis2-9
4.2 Voltage Controlled Oscillator (VCO)
Separate VCO and buffer circuits are used for receiver injection and transmitter carrier frequency 
generation. Since the receiver uses high-side injection, the receiver VCO frequency range is 10.7 
MHz above the transmit VCO range. The VCO/buffers are bandsplit into three ranges depending on 
radio model, covering radio operating frequencies of 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to 
50.0 MHz. The corresponding three...

2-10THEORY OF OPERATION
The VCO output is taken from the source and applied to the first buffer transistor (Q1304 receive, 
Q1307 transmit). The first buffer output is further amplified by the second buffer transistor (Q1305 
Rx, Q1308 Tx) before being applied to the receiver first mixer or transmitter first stage input.
In TX mode the modulation signal coming from the LVFRAC-N synthesizer IC (MODOUT, U1201 pin 
41) is superimposed on the DC steering line voltage by capacitive divider C1215, C1208 and...

Chapter 3
LOW BAND TROUBLESHOOTING CHARTS
1.0 Troubleshooting Flow Chart for Transmitter
No
 DC
@ Gate of
Q1401 Voltage
?
No
Ye s
              START
        No or Low 
  
  TX
 DC
      @Drains of 
Q1402 &
Q1403 Voltage 
?
@ Cathode of 
D1401 &
D1402DC
Voltage
?
Drains of Q1402 &AC
Q1403 both sine or
both distortedVoltages @
?
Check 9T1 and
Diode Bias Circuit
Verify RF Continuity
   to gates of
Q1402 & Q1403
 
Check circuitry
between Q1402 &
Q1403 and 
Antenna Port
CheckMOSBIAS_1
Supply and
Feed...

3-2Low Band TROUBLESHOOTING CHARTS
2.0 Troubleshooting Flow Chart for Receiver
(Sheet 1 of 2)
Audio
at pin 8 of 
U1103
?
Audio 
heard
?
 Check
voltages on 
U1103.
OK? Bad SINAD
Bad 20dB Quieting
No Recovered AudioSTARTCheck Controller (in the case of no audio).
Or else go to “B” Ye s
No
Spray or inject 10.7MHz 
into XTAL Filter FL1102.
BYe s
No
Check 2nd LO 
(10.245MHz) at C1129. 
LO
present
BYe s
Biasing 
OK
No
No
A
Ye sCheck Q1106 bias 
for faults.
Replace Q1106.
Go to B
Ye s
No
Check 
circuitry...

Troubleshooting Flow Chart for Receiver3-3
Troubleshooting Flow Chart for Receiver (Sheet 2 of 2)
Check for detailed mixer.
Is
9V3 present
? RF
Signal at 
pin 3 of 
U1051
?
RF
Signal at 
C1002
?
RF
Signal at
C1013
?
IF 
Signal at 
pin 2 of 
U1051
?
No
RF
Signal at 
C1017
?
No
No
No or 
Check harmonic filters J1401
and ant.switch 
Check preselector 
and RF amp.
Inject RF into J1401
No
Ye s
Check RF amp (Q1001) 
Stage.
Check filter between 
C1017 & mixer U1051
Ye s
Ye s
1st
LO level OK?
Locked
?Ye s
Check...

3-4Low Band TROUBLESHOOTING CHARTS
3.0 Troubleshooting Flow Chart for Synthesizer
Is
U1201 pin 2
>4.5 VDC in Tx &