Ericsson Monogram Series LBI-39132, Руководство по обслуживанию

Страница: 20 / 110

UHF RECEIVER
The receiver uses dual conversion superheterodyning techniques and is comprised of:
1. RF Amplifier
2. First Mixer And First If Amplifier
3. Second Mixer, Second If Amplifier And Fm Detector
4. Receiver Audio Circuit
5. Mute (squelch) Circuit
RF Amplifier
The receiver RF amplifier contains helical resonators FL1 to FL3 and Q13. FL1 and FL2 are configured as
2-pole band-pass filters. The RF signal passes through the tuned circuit FL1, RF amplifier Q13, FL2,
enabling the RF signal at the operating frequency to pass to the first mixer.
First Mixer and First IF Amplifier
FET Q12, transistor Q16, crystal filter XF1, helical resonators FL3 and coils T4 form the First Mixer and
First IF Amplifier. The vco local oscillator signal is filtered by FL3. Q12 produces a difference frequency
of 21.4 MHz at the drain connection, from the filtered RF signal at the gate connection and the filtered
VCO local oscillator signal at the source connection. The 21.4 MHz difference frequency is filtered by the
2-pole crystal filter XF1.The tuned circuit T4 and associated components provide matching of the crystal
filter to insure good pass-band response and selectivity. The IF signal is amplified by Q16 and passed to the
second mixer, second IF, and FM detector.
Second Mixer, Second IF, FM Detector
A single conversion FM receiver integrated chip, IC3 contains the second mixer, second IF, and FM
detector functions. The second local oscillator frequency is determined by the crystal X2 connected to Pin 1
of IC3. The IF signal is received at Pin 16 of difference frequency is applied to the mixer via Pin 16. The
output of the second mixer via Pin 3 is applied to a 455KHz band-pass filter, CF1. The output of CF1 is
passed to a high gain IF coupled to the adjustable quadrature detector T5. Any detected signal is produced
at Pin 9 of IC3 and applied to the Receiver Audio Circuit and the Mute (Squelch) Circuit.
Receiver Audio Circuit
The receiver audio circuit comprises a low pass filter, high pass filter and an audio amplifier on the RF PCB.
Low Pass Filter
The low pass filter is configured from coil L12, capacitor C64 and resistor R39. AF signals from Pin 9 of
IC3 are filtered by the low pass filter to remove any components of the 455KHz IF signal. The filtered
signal at flexible PCB 7 Pin 14 is passed to the high pass filter on the digital PCB via CON 402 Pin 15.
High Pass Filter
The de-emphasized audio signal from the high pass filter on the digital PCB is fed to the audio amplifier
(IC4) on the RF PCB via VR3, R156 and flexible PCB 7 Pin 12.
Audio Amplifier
IC4 is the audio amplifier. The audio signal at flexible PCB 7 Pin 12 is passed to IC4, Pin 3 via variable
resistor VR3 (located on the top of the radio). The gain of the amplifier is set by resistor R124 and C145.
The amplified audio signal at Pin 5 of IC4 is applied to the internal speaker (SPKR) by flexible PCB 7 Pin
MONOGRAM SERIES HANDHELD
CIRCUIT ANALYSIS
Page-16-
Dec.94