CIRCUIT DESCRIPTION
6-15
August 1993
Part No. 001-9750-001
The value of C109 controls the rate at which gain
changes in response to changes in the input level. Pin
5 is the inverting input of an internal operational
amplifier and pin 7 is the output of this amplifier.
R108 sets amplifier gain, and C108 minimizes distor-
tion in the expander.
Mute Gate (U608B), Summing Amplifier (U602B)
Gate U608B mutes the receive audio signal when
no signal is being received or if the message is
intended for someone else. This gate is controlled by
the Q6 output of shift register U451. The micropro-
cessor has total control over receiver squelch because
there is no control input from the squelch circuit. The
line to CR601 closes gates U608A and U608C when
U608B is closed. This improves isolation in the
squelched mode.
Summing amplifier U602B amplifies the
following signals: (1) receive audio, (2) supervisory
tones 1 and 2 from latch U409, (3) receive audio from
the universal interface, and (4) tones from the digital
signal processor circuit. The gain of the amplifier is
controlled by R640 and R639.
Volume Control (U600), Summing Amplifier (U801D)
With front-mount transceivers, volume is
controlled by the front panel volume control in the
standard manner. The output of summing amplifier
U602B is fed to the top of the volume connected to
J401, pin 21 and then back in on J401, pin 19 from the
wiper. This signal is then coupled by C624 to pin 13 of
summing amplifier U601D.
With remote-mount transceivers, the volume is
controlled by integrated circuit U600. The output
signal of summing amplifier U602B is coupled by
C600 to pin 2 which is the input of one of the channels
of U600. The controlled output is fed out on pin 8 and
coupled by C625 to pin 13 of the summing amplifier.
The volume control of the remote control unit
actually controls a DC voltage applied to the micro-
processor on PE4 (pin 44). The microprocessor then
programs U600 for the desired volume level using the
serial bus on the PA3 and PA4 pins.
This channel of U600 is normally not used with
front-mount transceivers and is programmed for
maximum attenuation because the PE4 input is pulled
low by R785. An exception may be when a remote
control unit is used with a front-mount transceiver to
provide dual controls. The volume controls of both
units can then operate in parallel so that the volume is
the sum of both controls (see Section 2.5.1). The
second channel of U600 is not used at this time.
6.6.2 RECEIVE DATA PROCESSING (U606A-D)
The receive audio signal on J300, pin 11 is also
fed to a receive data processing circuit consisting of
U606A-U606D. U606D and U606A form a four-pole
low-pass filter which attenuates voice and harmonic
frequencies above the data band. The passband of this
filter is controlled by Q610. When LTR data, digital
Call Guard data, or low frequency tone Call Guard
tones are received, Q610 is turned on and the cut-off
frequency is approximately 150 Hz. Then when a
high-frequency tone Call Guard tone is received, Q610
is turned off and the cut-off frequency is approxi-
mately 220 Hz. Turning Q610 on switches additional
capacitance into the circuit.
U606B and U606C form a DC restoration circuit
which converts the data from AC floating at half
supply to a digital signal at 0- and 5-volt levels which
can be detected by the microprocessor. U606B is a
standard noninverting amplifier with the gain set by
R726 and R786 (R726 is AC grounded by C710).
Diodes CR610A/B charge and discharge C710 to
establish a DC reference on U606C, pin 9 and U606B,
pin 6. This reference is the average of the positive- and
negative-going alternations of the data signal.
The output signal from U606B is applied to
U606C which functions as a comparator. When pin 10
rises above the reference on pin 9, the output on pin 8
goes to a high level. Conversely, when the level on pin
10 falls below the reference, the output goes to a low
level. R728 and R729 provide 0- and 5-volt output
levels to the logic.
6.6.3 SQUELCH CIRCUIT (U607B, U607C)
The microprocessor uses inputs from this circuit
and the RSSI circuit to determine when to mute or
unmute the receive audio and also to determine when
Summary of Contents for Summit DM 975x
Page 105: ...MULTI NET SYSTEM OVERVIEW 5 10 Revised February 1997 Part No 001 9750 005 ...
Page 108: ...CIRCUIT DESCRIPTION 6 3 August 1993 Part No 001 9750 001 Figure 6 1 Transceiver Block Diagram ...
Page 123: ...CIRCUIT DESCRIPTION 6 18 August 1993 Part No 001 9750 001 ...
Page 131: ...SERVICING 7 8 Revised January 1995 Part No 001 9750 003 ...
Page 156: ...9 19 Revised December 2000 Part No 001 9750 007 TRANSCEIVER EXPLODED VIEW PART 1 ...
Page 168: ...Revised January 1995 Part No 001 9750 003 10 11 INTERCONNECT SCHEMATIC ...
Page 169: ...Revised January 1995 Part No 001 9750 003 10 12 DISPLAY BOARD SCHEMATIC ...
Page 171: ...Revised January 1995 Part No 001 9750 003 10 14 RF BOARD SCHEMATIC REVISED 800 MHZ ...
Page 172: ...Revised January 1995 Part No 001 9750 003 10 15 RF BOARD BOARD LAYOUT ALL 800 900 MHZ ...
Page 173: ...Revised January 1995 Part No 001 9750 003 10 16 RF BOARD BOARD SCHEMATIC 900 MHZ ...
Page 174: ...Revised January 1995 Part No 001 9750 003 10 17 RF BOARD BOARD SCHEMATIC UNREVISED 800 MHZ ...
Page 178: ...Revised January 1995 Part No 001 9750 003 10 21 PA BOARD SCHEMATIC 900 MHZ 30W ...
Page 179: ...Revised January 1995 Part No 001 9750 003 10 22 PA BOARD SCHEMATIC 900 MHZ 15W ...
Page 181: ...Revised January 1995 Part No 001 9750 003 10 24 PA BOARD SCHEMATIC UNREVISED 800 MHZ 35W ...
Page 182: ...Revised January 1995 Part No 001 9750 003 10 25 PA BOARD SCHEMATIC UNREVISED 800 MHZ 15W ...
Page 183: ...Revised January 1995 Part No 001 9750 003 10 26 AUDIO LOGIC BOARD SCHEMATIC PART 1 OF 2 ...
Page 184: ...Revised January 1995 Part No 001 9750 003 10 27 AUDIO LOGIC BOARD SCHEMATIC PART 2 OF 2 ...
Page 185: ...Revised January 1995 Part No 001 9750 003 10 28 AUDIO LOGIC BOARD LAYOUT TOP VIEW ...
Page 186: ...Revised January 1995 Part No 001 9750 003 10 29 AUDIO LOGIC BOARD LAYOUT BOTTOM VIEW ...
Page 188: ...Revised January 1995 Part No 001 9750 003 10 31 REMOTE TRANSCEIVER INTERCONNECT SCHEMATIC ...
Page 189: ...Revised January 1995 Part No 001 9750 003 10 32 REMOTE CONTROL UNIT INTERFACE BOARD SCHEMATIC ...
Page 216: ...Part Number 001 9750 007 12 00 hph Printed in U S A ...
