Theory Of Operation; Overview; Receiver; Intermediate Frequency (If) - Motorola GTX Service Manual

Overview

This section provides a detailed theory of operation for
the GTX/GTX LTR/LTS 2000 radios and its compo-
nents: the receiver, transmitter, frequency generation
circuitry, controller and audio & data circuitry.

Receiver

The receiver of the GTX/GTX LTR/LTS 2000 radios
consists of 4 major blocks: the front-end module, dou-
ble balanced mixer, 45.1 MHz IF, and back-end IF IC.
The GTX/GTX LTR/LTS 2000 front-end modules con-
sist of three blocks of circuitry: a ceramic pre-selector
filter, RF amplifier and ceramic post-selector filter.
The ceramic pre- and post-selector filters are 3-pole,
bandpass filters. This topology maximizes the attenua-
tion at the worst case image frequency for this receiver,
which is 90.2 MHz below the filter Passband. The 3 dB
bandwidth is approximately 12/22 MHz for 900 MHz
respectively, centered at 937 MHz for 900 MHz respec-
tively. The center of the band insertion loss is approxi-
mately 2 dB. The 3-pole filters are designed to operate
with a 50 Ohm input and output termination.
The RF amplifier, Q1, is a Motorola MRF9411 NPN
device biased in a common emitter configuration. The
amp is stabilized by the shunt feedback coil L2, and has
approximately 15 dB of gain with a noise figure of
about 2 dB. The amplifier draws 6.5 mA of current and
is supplied by the receiver 5 V supply (indicated as
"5R" on the schematics and block diagrams).
Terminating the RF amplifier is the post-selector filter.
This filter is the same as the pre-filter.
The net gain of the receiver front-end module is about
12 dB in the center of the band and about 11.5 dB at the
band edges. The net center of the band noise figure is
approximately 4.5 dB.
The receiver mixer is a double-balanced mixer which
provides excellent protection against receiver spurs
due to non-linearities, such as IM and half-IF. The
received signal mixes down to the frequency of the first
IF, 45.1 MHz, and enters the IF circuitry. The mixer
operates with a LO level of +3 dBm and a conversion
loss of about 7 dB.
February 1997

Theory of Operation

Intermediate Frequency (IF)

The Intermediate Frequency (IF) section of the portable
radio consists of several sections including the high IF,
the second LO, the second IF, and the IF IC chip. The
first LO signal and the RF signal mix to the IF fre-
quency of 45.1 MHz, and then enters the IF portion of
the radio.
The signal first enters the high IF, passes through a
crystal filter, is amplified by the IF amp, and then
passed through a second crystal filter. The first crystal
filter provides selectivity, second image protection, and
intermodulation protection. The amplifier provides
approximately 16 dB of gain to the signal. The signal
then passes through the second crystal filter which pro-
vides further selectivity and second image protection.
The high IF has an approximate 3 dB bandwidth of
7 KHz.
The filtered and amplified IF signal is mixed with the
second local oscillator at 44.645 MHz. The second LO
uses an amplifier internal to the IF IC, an external crys-
tal and some external chip parts. The oscillator pre-
sents an approximate level of -15 dBm to the second IF
mixer, internal to the IF IC.
The mixed output of the IF signal and the second LO,
produces a 455 KHz signal (second IF). This signal is
then filtered by external ceramic filters and amplified.
It is then passed back to the IF IC, sent to a phase-lock
detector, and demodulated. The resulting detected
audio output is then sent to the ASFIC to recover the
audio.

Transmitter

The transmitter contains five basic circuits: a power
amplifier, an antenna switch, a harmonic filter, an
antenna matching network, and a power control. Refer
to the block diagram and the schematic for more infor-
mation.
The power amplifier is an LD-MOS module. This mod-
ule contains a 3-stage amplification with a supply volt-
age at 7.5 V. The LD-MOS is capable of supplying an
output power of 4.4 W with an input signal of 1 mW.
The power output can be varied by changing the bias-
ing voltage at the first stage.
68P02948C90-O
Section 2
2-1