Motorola Gm Series Detailed 6864115b62 A Manual

GM640/GM660/GM1280 Model Chart1-3
3.0 GM640/GM660/GM1280 Model Chart 
GM Series  VHF  136 -174  M Hz
Model Description
MDM25KHC9CK1_EGM640, 136-174 MHz, 1-25W, 6 Ch
MDM25KHF9CK5_E GM660, 136-174 MHz, 1-25W, 255 Ch
MDM25KHN9CK8_EGM1280, 136-174 MHz, 1-25W, 255 Ch
M DM 25K HA9C K7_E   Dat ab ox,  13 6-1 74  MHz,  1-2 5W , 16  C h
Item Description
XGCN6112_Control Head GM640
X GCN6120_ Control Head GM660
XGCN6121_Control Head GM1280
X GCN6116_ Databox Radio Blank Head
XIMUD6018_SField Replaceable Unit (Main...

1-4 MODEL CHART AND TECHNICAL SPECIFICATIONS
4.0 Technical Specifications
Data is specified for +25°C unless otherwise stated.
General Specifications
Channel Capacity
GM140
GM160
GM340
GM360
GM380
GM640
GM660
GM1280
Databox
4
128
6
255
255
6 
255
255
16
Power Supply 13.2Vdc (10.8 - 15.6Vdc)
Dimensions:  H x W x D (mm)  Depth excluding knobsGM140/340/640
56mm x 176mm x 177mm (1 - 25W)
56mm x 176mm x 189mm (25 - 45W)
(add 8mm for Volume Knob)
Dimensions:  H x W x D (mm)  Depth excluding knobsGM160/360/660...

Technical Specifications1-5
*Availability subject to the laws and regulations of individual countries.
Transmitter VHF
*Frequencies - Full BandsplitVHF 136-174 MHz
Channel Spacing12.5/20/25 kHz
Frequency Stability
(-30°C to +60°C, +25° Ref.)±2.5 ppm
Power 1-25W / 25-45W
Modulation Limiting±2.5 @ 12.5 kHz 
±4.0 @ 20 kHz
±5.0 @ 25 kHz
FM Hum & Noise-40 dB @ 12.5kHz
-45 dB @ 20/25kHz
Conducted/Radiated Emission (ETS)-36 dBm 1 GHz
Adjacent Channel Power-60 dB @ 12.5 kHz
-70 dB @ 25 kHz
Audio Response (300 -...

1-6 MODEL CHART AND TECHNICAL SPECIFICATIONS 

Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the VHF 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 VHF (136-174MHz) Receiver
2.1 Receiver Front-End
The receiver is able to cover the VHF range from 136 to 174 MHz. It consists of four major blocks: 
front-end bandpass filters and pre-amplifier, first mixer, high-IF,...

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...

VHF (136-174MHz) Transmitter Power Amplifier (PA) 25 W 2-3
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....

2-4THEORY OF OPERATION
Op-amp U3402-1 monitors the drain current of U3401 via resistor R3444 and adjusts the bias 
voltage of U3401 so that the current remains constant. The PCIC (U3501) 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 U3402-1 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...

VHF (136-174MHz) Transmitter Power Amplifier (PA) 25 W 2-5
3.5 Antenna Switch
The antenna switch consists of two PIN diodes, D3471 and D3472. In the receive mode, both diodes 
are off. Signals applied at the antenna jack J3401 are routed, via the harmonic filter, through 
network L3472, C3474 and C3475, to the receiver input. In the transmit mode, K9V1 turns on Q3471 
which enables current sink Q3472, set to 96 mA by R3473 and VR3471. This completes a DC path 
from PASUPVLTG, through L3437, D3471, L3472,...

2-6THEORY OF OPERATION
4.0 VHF (136-174MHz) 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) converter internal...