AE/LZB 119 1647 R1A
25
standard 3052 Hz busy tone or the alternate 2918 Hz busy
tone.
The output of the notch filter is U602 pin 9 (VOUT)
which is feed to U804 pin 28. In the ASP, the multiplex
audio switch (ISA/ISB) routes the notched audio to the
digital volume control. The output of the volume control is
routed through another switch (RXO) and leaves the ASP on
U804 pin 27 to be volume adjusted by RV104 of the RF
board and then amplified by U603.
GE-MARC Tone Decoding
Bandpass filtered detector audio on U804 pin 45 is
applied to notch filter U602, bandpass filter U301.B &
U601.A, and the high-speed limiter.
For signaling tone decode, busy tone decode switch
Q603 is off since the SW5 output from the ASP (U804 pin
13) is high. Wide band audio passes from U804 pin 45
through R612 to the non-inverting input (U804 pin 32) of
the comparator in the ASP. This comparator forms the high-
speed limiter. The average DC level of the signal is applied
to the comparator’s inverting input (U804 pin 31) as a dc
reference for the comparator. R611 and C605 filter the
signal component to provide the DC reference. The output
of the limiter (U804 pin 21) is sent to micro controller U701
pin 6 for tone decoding.
During a busy tone decode (Q603 is on), bandpass
filtered audio at the busy tone frequency feeds the high-
speed data limiter through U301.B, U601.A, and Q603.
Since the output impedance of U601.A is very low and
Q603 is on, wide-band audio from U804 pin 45 is greatly
attenuated across R612. The 3 kHz low-pass filtered audio
from U804 pin 45 provides some of the high-frequency roll-
off. U301.B provides a notch at 2.3 kHz plus a high -pass
response to reject voice frequencies. U601.A is a bandpass
filter centered at 3 kHz. From this point, the busy tones are
decoded similarly to signaling tone decodes.
Receive Noise Squelch
The squelch circuit monitors the detector's high-
frequency noise level to determine if a carrier is quieting the
receiver. A D/A (Digital-to-Analog) converter in the ASP
sets the squelch threshold level. This level is normally 8 dB
SINAD. When receiver noise falls below the threshold
level, the ASP CAS (Carrier Activity Sensor) output at
U804 pin 23 switches low. The ASP CAS signal feeds the
input on micro controller U701 pin 43.
Buffered and unfiltered detector audio leaves U804 pin
43 and feeds pin 50 which is the input to the squelch high-
pass filter (7.5 KHz). In the ASP, the high-pass filtered
audio is rectified and sent out on U804 pin 52. The rectified
noise is filtered by C612 (and C611 if Q604 is on) to
provide an average dc level proportional to receiver noise
level. This dc level is applied to a non-inverting dc buffer
amplifier at U804 pin 55. The output of the amp is on U804
pin 53. The gain of the dc amp is set by R620, R622, R623,
and thermistor RT601. The thermistor increases in
resistance at colder temperatures therefore causing an
increase in the dc amps gain. This compensates for a
decrease in receiver noise level from the RF Board at colder
temperatures.
The buffered DC level that is tracking receiver noise
level is sent to a comparator’s inverting input at U804 pin
49. This comparator’s non-inverting input is set to a
voltage generated by the D/A converter in the ASP. The
comparator’s output switches high when the DC level falls
below the comparator’s reference level. This output is
inverted and it appears at U804 pin 23. This ASP CAS
output is normally high and switches low when a carrier is
detected.
To tighten the squelch, the D/A reference voltage is
lowered. Hysteresis for the squelch is done with software.
When the ASP CAS output switches to indicate a signal is
detected, the D/A reference value is increased slightly to
loosen the squelch. This action eliminates "bubbling" or
chattering noises in the speaker. "Bubbling" is normally
caused by changes in the dc level around the reference point.
Transistor Q604 is normally turned on via SW2 output
at U804 Pin 16 (SW2 = high). This action places C611 into
the dc level averaging circuit. C611 provides slow squelch
(60 ms) operation to prevent audio chopping with rapid
squelch closings in weak signal areas. When Q604 is turned
off, a 5ms fast squelch is provided by only C612.
Receive Alert Tones
The programmable alert tones are generated in the ASP
using a 66.6 kHz clock divided by a 6-bit divider and then
divided by two. Therefore, the lowest alert tone frequency
that can be generated is 66.6 kHz divided by 64 then divided
by 2 (520 Hz).
The output of the alert tone divider is on U804 pin 76.
This output connects back to U804 pin 30 to feed the audio
multiplex switch (ISA/ISB) in the receive audio path. The
tones then pass through de-emphasis stages. The de-
emphasized audio passes through the digital volume control,
through an audio switch (RXO), RV 103 on the RF Board
via P107 pin 2, and U603 for amplification.
Summary of Contents for EDACS Monogram Series
Page 1: ...ericssonz ericssonz Maintenance Manual MONOGRAM SERIES EDACS Trunking Portable 800 MHz ...
Page 45: ...AE LZB 119 1647 R1A 45 COMPONENT PINOUT 780 202 0009 Sh 1 Rev A ...
Page 46: ...AE LZB119 1647 R1A 46 COMPONENT PINOUT 780 202 0009 Sh 2 Rev A ...
Page 47: ...AE LZB 119 1647 R1A 47 SYSTEM WIRING DIAGRAM 930 010 0002 Rev A ...
Page 48: ...AE LZB119 1647 R1A SERVICE OUTLINE 48 KEYPAD DISPLAY BOARD 930 010 0002 Rev A ...
Page 49: ...SCHEMATIC DIAGRAM AE LZB 119 1647 R1A 49 PTT SWITCH BOARD 650 180 0001 S1800001 Rev A ...
Page 50: ...AE LZB119 1647 R1A SCHEMATIC DIAGRAM 50 KEYPAD DISPLAY BOARD 650 170 0001 S1700001 Rev A ...
Page 51: ...RF BOARD 650 020 0002 770 020 0002 Rev D AE LZB 119 1647 R1A 51 ...
Page 52: ...DIGITAL BOARD 650 010 0003 770 010 0003 Sh 1 Rev E AE LZB 119 1647 R1A 52 ...
Page 53: ...DIGITAL BOARD 650 010 0003 770 010 0003 Sh 2 Rev E AE LZB 119 1647 R1A 53 ...
Page 54: ...BLOCK DIAGRAM 060 010 0001 Rev B AE LZB 119 1647 R1A 54 ...
Page 55: ...This page intentionally left blank AE LZB 119 1647 R1A 55 ...
