Ericsson 9D902780G3, Руководство по обслуживанию, Страница: 2 / 12

DESCRIPTION
The principle function of the Transmitter Synthesizer Mod-
ule is to provide the RF excitation for input to the MASTR III
station power amplifier. The output of the synthesizer is a
frequency modulated signal at the desired frequency. The mod-
ule contains the following functional blocks:
•
A voltage controlled oscillator.
•
Frequency Doubler (Multiplier).
•
A chain of integrated circuit RF Amplifiers.
•
A reference buffer amplifier.
•
Dual modulus prescaler and synthesizer integrated
circuits.
•
Loop amplifiers and passive loop filter.
•
An audio amplifier and a pre-modulation integrator.
•
IC voltage regulators for +5 and -5 Vdc. A discrete
component regulator for +8 Vdc, and an Opera-
tional Amplifier regulator for +4 Vdc.
•
Logic circuitry: address decoder, input signal gates,
and a lock indicator circuit.
CIRCUIT ANALYSIS
VOLTAGE CONTROLLED OSCILLATOR
Transistor Q1 and associated circuitry comprise a low
noise Voltage Controlled Oscillator (VCO). Inductor L1 and
associated capacitors form the oscillator resonant circuit
(tank). The noise characteristic of this oscillator is dependent
on the Q of this resonant circuit. The components used in the
tank are specified to have especially high Q. Diode D1 aids
in setting the bias point for low noise operation. (Any field
replacement of oscillator parts should use identical parts).
Variable Capacitor C10 sets the fixed capacitance in the
tank, and therefore sets the frequency range over which the
oscillator can be voltage tuned.
The oscillator frequency is voltage tuned by the signal
applied through R5 and L5 to the two varicap diodes D2 and
D3. Additionally, audio modulation is applied as an AF
voltage to the two varicap diodes. This RF voltage varies the
oscillator frequency at an audio rate (i.e., it frequency modu-
lates the oscillator). Low frequency audio is applied along
with the varicap control voltage through R5 and L5 while
high frequency audio (MOD) is applied via C16.
Resistors R6 through R9 provide a two volt negative bias
on the varicap diodes.
Transistors Q101 and Q102 and associated circuitry form
the oscillator enable switch. This switch allows the station
control circuitry to turn the VCO ON or OFF via the
ANT_REL line. Setting the ANT_REL line to a logic low
causes Q102 to conduct. The five (5) volt output at Q102
collector (OSCON) enables the fault indicator gates, U705-3
and U705-4, and turns on Q101. Q101 starts to conduct,
providing a ground path for Q1. This turns ON the VCO.
FREQUENCY DOUBLER
Transistors Q801 and Q802 form a buffer stage to drive
transistor multiplier Q803. The buffer isolates VCO Q1 from
loading effects which could degrade oscillator loaded Q and
hence noise performance. Transistor multiplier Q803 is
tuned to pass the second harmonic of the VCO output and
serves as a frequency doubler. Tank elements L802, C812-
C814 and L803 form a resonant circuit and matching net-
work to drive resistive splitter R201-R204.
RF AMPLIFIERS
The RF chain begins with resistive splitter R201-R204
and R216-R218. The output of the splitter at R203 is attenu-
ated by 10 dB and provides impedance matching helical filter
FL201, which is tuned to pass the fundamental while reject-
ing harmonics by approximately 40 dB. The output of FL201
is fed thru resistive pad R205-R207 to MMIC Amplifier
U201 which operates in compression. U201 drives output
amplifier U202 into compression. The output amplifier is
followed by a bandpass filter (C208-C210, L203-L205) and
resistive attenuators (R210-R215). The final output at the
front panel BNC Connector (J2) is nominally 11.5 dBm, and
drives the station Power Amp.
The other output of the resistive splitter at R218 is
attenuated by 20 dB and drives buffer amp U203 into com-
pression. U203 drives the synthesizer prescaler providing a
feedback signal for the synthesizer phase locked loop.
REFERENCE BUFFER AMPLIFIER
Transistor Q401 and associated components comprise a
buffer amplifier for the reference oscillator signal. (The
reference oscillator signal is produced by the receiver syn-
thesizer module of a MASTR III station.) The 0 dBm refer-
ence oscillator signal is fed through the front panel BNC
connector J1. Resistor R405 provides a 50 ohm load to the
reference oscillator. The output of the Reference Buffer
Amplifier is fed directly to the synthesizer integrated circuit.
The output level at TP9 is approximately 3 volts peak to
peak.
Copyright © June 1992, Ericsson GE Mobile Communications, Inc.
TABLE 1 - GENERAL SPECIFICATIONS
ITEM SPECIFICATION
FREQUENCY RANGE 450-470 MHz (G3)
425-450 MHz (G7)
403-430 MHz (G6)
380-400 MHz (G8)
470-494 MHz (G9)
CHANNEL SPACING 6.25 kHz
RF POWER OUT (50 Ohm load) 10 to 13 dBm
(10 to 20 mW)
RF HARMONICS < -30 dBc
NON-HARMONIC SPURS
1 to 200 MHz < - 90 dBc
200 MHz to 1 GHz < - 60 dBc
CARRIER ATTACK TIME <25 mSec
REFERENCE INPUT
input level 0 dBm
±
1.5dB
input impedance 50 Ohm
frequency 5 to 17.925 MHz (must be integer divisible by
channel spacing)
MODULATION SENSITIVITY 5 kHz peak dev/1 Vrms, Adjustable
AF INPUT IMPEDANCE 600 Ohm
AF RESPONSE
10 Hz
±
1.5 dB
1000 Hz 0 dB reference
3 kHz
±
1.5 dB
10 Hz SQUARE WAVE MODULATION <10%
Sq wave droop
HUM & NOISE -55 dB
POWER REQUIREMENTS 13.8 Vdc @ 275 mA
-12.0 Vdc @ 10 mA
LBI-38671
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