Motorola GTX Panduan Servis - Halaman 29

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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