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
module 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 integra-
tor.
•
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 form a low noise
Voltage Controlled Oscillator (VCO). Inductor L1 and asso-
ciated 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 ap-
plied 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 AF 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 con-
trol 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, provid-
ing 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 attenuated
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 (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 compression.
U203 drives the synthesizer prescaler providing a feedback
signal for the synthesizer phase locked loop.
REFERENCE BUFFER AMPLIFIER
Transistor Q401 and associated components from a buffer
amplifier for the reference oscillator signal. (The reference
oscillator signal is produced by the receiver synthesizer
module of a MASTR III station.) The 0 dBm reference
oscillator signal is fed through the front panel BNC 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 © 1992-2004, M/A-COM, Inc. All rights reserved.
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)
490-512 MHz (G10)
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 Hz0 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
This manual is published by
M/A-COM, Inc.
, without any warranty. Improvements and changes to this manual necessitated by typographical errors, inaccuracies of current information, or improvements to programs
and/or equipment, may be made by
M/A-COM, Inc.
, at any time and without notice. Such changes will be incorporated into new editions of this manual. No part of this manual may be reproduced or transmitted in
any form or by any means, electronic or mechanical, including photocopying and recording, for any purpose, without the express written permission of
M/A-COM, Inc.
CREDITS
MASTR and EDACS are registered trademarks of M/A-COM, Inc.
All other brand and product names are trademarks, registered trademarks, or service marks of their respective holders.
LBI-38671N
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