Ericsson 19D902783G1, Руководство по обслуживанию

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CIRCUIT ANALYSIS
INPUT AMPLIFIER NETWORK
The input amplifier, consisting of Q2 and T1, provides a
50 ohm load for the receiver RF module.
Capacitor C1 provides AC coupling and a DC block on
the input line (J1). This DC block protects the module in the
event of a failure in a preceding module.
C1 and L9 are series-resonant at 21.4 MHz and provide a
low-impedance path from J1 to amplifier Q2. C89 and L8
are parallel-resonant at 21.4 MHz and provide a path to the
50-ohm lead, R105, for mixer products other than 21.4
MHz.
CRYSTAL FILTERS, IF AMPLIFIERS
Y1, Y2, U1, and associated circuitry provide IF filtering
and amplification at 21.4 MHz. Filters Y1 and Y2 are both 4-
pole bandpass filters with a center frequency of 21.4 MHz
and a bandwidth of
±
6.5 kHz. Amplifier UI is an integrated-
circuit amplifier. U1 provides 30 dB of gain. The amplifier
and filters have terminal impedances of 50 ohms. In-circuit
gain measurements can be made using a high impedance
probe.
Inductors L3, L5 and associated resistors and capacitors
provide power supply decoupling. R3 provides a path to the
input of the Fault Detector circuit. This input enables the
Fault Detector circuit to monitor the DC voltage of U1.
The RF level detector consists of transistor Q1 along
with associated resistors and capacitors. This detector plays
no role in the normal operation of the IF Module, but aids in
unit testing and module troubleshooting.
OSCILLATOR/MIXER/DETECTOR
Integrated circuit U3 provides several functions includ-
ing 2nd mixer, if amplifier and limiter, and quadrature detec-
tor.
The 20.945 MHz crystal oscillator provides local oscilla-
tor injection to the mixer in U3. This mixer converts the 21.4
MHz IF signal to 455 kHz. C20 and C21 are oscillator feed-
back capacitors and have been chosen to provide the proper
capacitance for crystal Y3. The proper oscillator output level
is difficult to measure directly without affecting the oscilla-
tion.
A preferable measurement is at TP3 which should read
about 10 mV pk. (Measured using a 10 megohm 11 pF oscil-
loscope probe.)
The mixer is internally connected to the crystal oscillator.
Pins 1 and 20 of U3 are the mixer input and output respec-
tively. Typical mixer conversion loss is about 2 dB.
In the 12.5 kHz mode, the output of the mixer drives the
IF amplifier via analog switch U11-2, filter FL1 and analog
switch U11-3. In the 25 kHz mode, the mixer output is
routed through analog switch U11-1 and C85 to the IF am-
plifier. The analog switches are controlled by the signal at
point ’A’; high for 25 kHz, low for 12.5 kHz.
The IF amplifier output drives the limiter via the 6-pole
ceramic filter FL2.
A received-signal-strength indicator (RSSI) is provided
at U3 Pin 7. This indicator signal is generated within the
limiter circuitry and provides an output current proportional
to the logarithm of the input signal strength. This current de-
velops a voltage across R18. The voltage varies from about 1
Vdc for noise input, to about 1.4 Vdc for a 12 dB SINAD
signal, to a maximum of about 4.8 Vdc for a high signal
level (70 dB stronger than that required for 12 dB SINAD).
The quadrature detector provides a demodulated audio
frequency output. The input to the detector is internally con-
nected to the limiter and is not externally available. The out-
put of the detector is U3 pin 9. C28 provides low-pass
filtering to remove 455 kHz feedthrough. Ceramic resonator
Y4 provides the frequency selective component needed for
FM demodulation. Y4 replaces the typical LC resonant cir-
cuit found in most quadrature detectors. In contrast to the
typical LC network, Y4 requires no adjustment.
The DC supply to U3 is provided through voltage drop-
ping resistor R11 to U3 pin 6. R12 provides a path to the in-
put of the Fault Detection circuit. This enables the Fault
Detector to monitor the DC voltage on U3.
AUDIO AMPLIFIER
Operational amplifier U6.3 provides audio frequency
amplification. Its gain is set by its associated resistors, in-
cluding variable resistor VR1. VR1 allows for adjusting the
AF output level to 1 Vrms with a standard input signal to the
module (1 kHz AF, 3 kHz peak deviation). In the 12.5 kHz
mode, the demodulated audio is at a lower level than in the
25 kHz mode. The gain of amplifier U6.3 is, therefore, in-
creased to give the same 1V rms output with a standard input
signal to the module of 1.5 kHz deviation. This is done by
transistor switch Q6 connecting R1 across R40. U6.2 is
used as a voltage regulator to provide 4 Vdc for biasing the
operational amplifier.
Copyright© November 1994, Ericsson GE Mobile Communications Inc.
TABLE 1 - GENERAL SPECIFICATIONS
ITEM SPECIFICATION
I.F. frequency
Input Impedance
l2 dB SINAD
Adj. CH SEL
Image
3rd order Intercept Pt
Variation of Sensitivity with Signal Frequency
2nd I.F. frequency
2nd L.O. frequency
AF output (J2 pin 31C)
AF output impedance
AF distortion
AF response
10 Hz
300 Hz
1000 Hz
3 kHz
Hum & Noise
RSSI output (J2 pin 20C)
RSSI time constant
SQ Threshold Sensitivity
SQ Maximum Sensitivity
SQ Clipping
SQ Attack
SQ Close
SQ output (J2 pin 26C)
Fault output (J2 pin IIC)
DC Supply
21.4 MHz
50 ohm
-120 dBm (25 kHz); -119 dBm (12.5 kHz)
-90 dB (25 kHz); -80 dB (12.5 kHz)
-100 dB
23 dBm (25 kHz); 11 dBm *(12.5 kHz) *@ 50 kHz offset
2 kHz (25 kHz); 1 kHz (12.5 kHz)
455 kHz
20.945 MHz
1 Vrms adjustable (with standard input signal)
1k ohm
5% (25 kHz); 5% (12.5 kHz)
-3 dB
±
1 dB
0 dB reference
±
1 dB
-55 dB (25 kHz); -50 dB (12.5 kHz)
0.7 to 2.7 Vdc prop to log (sig level)
5 ms
-123 dBm (25 kHz); -122 dBm (12.5 kHz)
-110 dBm (25 kHz); -109 dBm (12.5 kHz)
3 kHz
150 ms
250 ms
5V logic (low = squelched)
5V logic (low = fault)
13.8V, 150 mA max.; 12.0V, 18 mA max.
This manual is published by Ericsson Inc., without any warranty. Improvements and changes to this manual necessitated by typographical errors,
inaccuracies of current information, or improvements to programs and/or equipment, may be made by Ericsson Inc., at any time and without no-
tice. Such changes will be incorporated into new editions of this manual. No part of this manual may be reproduced or transmitted in any form or
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Ericsson Inc.
LBI-39123E
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