Theory Of Operation; Overview; Receiver; Intermediate Frequency (If) - Motorola GTX 2000 Service Manual

Overview

This section provides a detailed theory of operation for
the GTX/GTX LTR/LTS 2000 and its components: 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
Þlter, RF ampliÞer and ceramic post-selector Þlter.
The ceramic pre- and post-selector Þlters are 3-pole,
bandpass Þlters. This topology maximizes the attenua-
tion at the worst case image frequency for this receiver,
which is 90.2 MHz below the Þlter passband. The 3 dB
bandwidth is approximately 22 MHz, centered at
861 MHz. The center of the band insertion loss is
approximately 2 dB. The 3-pole Þlters are designed to
operate with a 50 Ohm input and output termination.
The RF ampliÞer, Q1, is a Motorola MRF9411 NPN
device biased in a common emitter conÞguration. The
amp is stabilized by the shunt feedback coil L2, and has
approximately 15 dB of gain with a noise Þgure of
about 2 dB. The ampliÞer draws 6.5 mA of current and
is supplied by the receiver 5 V supply (indicated as
Ò5RÓ on the schematics and block diagrams).
Terminating the RF ampliÞer is the post-selector Þlter.
This Þlter is the same as the pre-Þlter.
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 Þgure 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 Þrst
IF, 45.1 MHz, and enters the IF circuitry. The mixer
operates with a LO level of +3 dBm and conversion loss
is about 7 dB.
June 1999

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
Þrst 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 Þrst enters the high IF, passes through a
crystal Þlter, is then ampliÞed by the IF amp, and then
passed through a second crystal Þlter. The Þrst crystal
Þlter provides selectivity, second image protection, and
intermodulation protection. The ampliÞer provides
approximately 16 dB of gain to the signal. The signal
then passes through the second crystal Þlter which pro-
vides further selectivity and second image protection.
The high IF has an approximate 3 dB bandwidth of
7 KHz.
The Þltered and ampliÞed IF signal then mixes with the
second local oscillator at 44.645 MHz. The second LO
uses an ampliÞer 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 output of the mixing of the IF signal and the sec-
ond LO produces a signal at 455 KHz (second IF). This
signal is then Þltered by external ceramic Þlters and
ampliÞed. 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 Þve basic circuits: a power
ampliÞer, an antenna switch, a harmonic Þlter, an
antenna matching network, and a power control. Refer
to the block diagram and the schematic for more infor-
mation.
The power ampliÞer is an LD-MOS module. This mod-
ule contains a 3-stage ampliÞcation 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 Þrst stage.
68P02945C75-A
Section 2

Theory of Operation

2-1