GE LBI-31482 Maintenance Manual Download Page 4

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tains both tone and voice circuits, and utilizes both discrete
components and Thick Film Integrated Circuit Modules (IC’s).

The receiver has intermediate frequencies of 10.7 MHz and

455 kHz. Adjacent channel selectivity is provided by using
two, 2-pole ceramic filters. References to symbol numbers
mentioned in the following text are found in the Outline Dia-
gram, Schematic Diagram and Parts List (See Table of Con-
tents). A block diagram of the receiver is shown in Figure 3.

Receiver Front End

An RF signal from the antenna is coupled through the an-

tenna circuit to the base of RF amplifier Q1. The antenna cir-
cuit consists of L1, CV1 and C2. The circuit is tuned by CV1.

The output of Q1 is coupled through three tuned circuits

that provide most of the front end selectivity. The tuned cir-
cuits are L2, L3, L4 and associated circuitry. The output of L4
is coupled through C10 to the base of the first mixer.

1st Oscillator

Q2, X1, L5 and associated circuitry make up a Colpitts os-

cillator. The frequency is controlled by a third mode crystal op-
erated at one third of the required injection frequency. L5 is
used to set the oscillator on frequency. R3 is in parallel with
X1 to ensure operation on the third overtone of the crystal. The
injection frequency is the operating frequency (-) 10.7 MHz,
and is coupled through C16 to the first mixer. L6 is tuned to
three times, the crystal frequency. The output to the 1st mixer
is approximately 60-90 millivolts rms.

1st Mixer and IF Filter

RF from the Pager front end is applied to the base of first

mixer Q3. Injection voltage from the first oscillator is also ap-
plied to the base of Q3. The 10.7 MHz first IF frequency is
coupled through L7 to 10.7 MHz filter FL1. L7 is used to
match the mixer output to the input of FL1.

The highly-selective filter provides the first portion of the

receiver IF selectivity. The 10.7 MHz output of FL1 is applied
to the second mixer IC (Al).

2nd Oscillator, Mixer and IF Filter

Al and associated circuitry make up the 2nd oscillator and

mixer. The crystal for the oscillator is X2. The oscillator oper-
ates at 10.245 MHz for low side injection of the 2nd IF (stand-
ard), or 11.155 MHz for high side injection for those radios
determined to be operating on a tweet frequency. This fre-
quency is mixed with the 10.7 MHz input to provide the 455
kHz 2nd IF frequency.

The output of Al is coupled through ceramic filter FL2

which provides the 455 kHz selectivity. The filter output is ap-
plied to IF amplifier A2.

IF Amplifier and Detector

A2 and associated circuitry make up the IF amplifier. The

amplifier IC also provides the 455 kHz limiting. The output of
A2 is applied to the discriminator.

The discriminator demodulates the 455 kHz signal. This

type of detector provides a high degree of AM rejection. The
recovered audio, tone and noise is applied to audio amplifier
IC A51.

Audio Stages

The discriminator output is applied to buffer/pre-amp A51.

One output at A51-4 is coupled through high-pass filter A56
which filters out frequencies under 200 Hz to eliminate any
Channel Guard (CTCSS) tone. The filter output is applied to
squelch circuit A55, and to the decoder circuitry.

Applying the proper sequential tones to the receiver acti-

vates the decoder circuitry and audio stages, causing the sec-
ond alert tone to be heard at the speaker.

After the alert tone is heard, the output of A51 is applied to

the push-pull audio amplifiers Q51 and Q52, and then to the
speaker.

Squelch

Squelch control IC A55 monitors noise in the 30-40 kHz

range on A55-1. When there is no carrier present, the noise is
above a squelch threshold set by RV52. This condition makes
A55-7 high, shutting off the audio amp at A51-9. When a car-
rier is present, the noise level drops below the threshold driv-
ing A55-7 and A51-9 low. This turns the audio amp on
allowing transmissions on the channel to be monitored.

SEQUENTIAL TONE DECODER

The decoder is a two-reed, sequential tone decoder for op-

eration with any two-tone sequential encoder in individual call
applications.

The two reeds mount at the bottom of the circuit board, and

are available for operation on tone frequencies in the 288.5 to
2000 Hz range.

Figure 3 - Pager Block Diagram

LBI-31482