CIRCUIT DESCRIPTION
6-17
August 1993
Part No. 001-9750-001
If optional companding is used, jumper R690 is
removed and the compandor board is installed.
Companding is the compressing and then expanding of
the audio signal to reduce noise. This also increases
the dynamic range of the audio signal which is other-
wise limited by channel spacing. The signal undergoes
a 2:1 compression in the transmitter and then a 1:2
expansion in the receiver to return it to the original
condition. For example, a +2 dB change in audio level
at the input of the compressor produces a +1 dB
change on the output.
Gates U102A and U102B bypass expander
U100B when companding is not used on a selectable
system or when performing testing. U101A provides
buffering, and C101 couples the signal to pin 11 of
U100B which is the input of an internal summing
point. Pin 10 of U100B is the output of an internal
operational amplifier, and pin 12 is the inverting input
of this amplifier.
A portion of the output signal is coupled by C103
to pin 15, and by C104 to pin 14. Pin 15 is the input of
an internal rectifier, and pin 14 is the input of an
internal variable gain stage. The output of the rectifier
is averaged across C106 and then used to control the
gain of the variable gain stage. The value of C106
controls the rate at which the gain changes in response
to changes in the input level. C107 minimizes distor-
tion in the compressor.
The output of the compressor is applied to U604B
which provides limiting and 6 dB per octave pre-
emphasis. Limiting prevents over-modulation caused
by high-level input signals. This stage is an amplifier
which limits by saturating. R692 sets the gain of this
amplifier. R693 and R694 set the input level to the
next stage, and C685 provides DC blocking.
6.7.4 SPLATTER FILTER (U604C, U605D),
SUMMING AMPLIFIER (U605A)
U604C and U605D form a five-pole low-pass
splatter filter which attenuates frequencies over 3 kHz.
This prevents adjacent channel interference caused by
harmonic frequencies produced by amplitude limiting.
Summing amplifier U605A amplifies the sum of
the transmit audio, transmit data, and wideband data
signals. The audio and data modulation levels are
adjusted by R701 and R703. A 2.5-volt bias voltage is
provided by R708 and R709. The output of the summing
amplifier is fed to the synthesizer on the RF board.
The wideband data input from J403, pin 8 of the
universal interface allows data to be injected after the
limiter circuit. If DC coupling is desired, jumper resis-
tors R707 and R705 can be installed and R706
removed. Data injected at this point is gated by U606C
which passes the signal when the control input (pin
12) is a high level.
A DC bias of 2.5 VDC ±2% (50 mV) must be
applied to the wideband data input (J403-8) if DC
coupling is used. This bias keeps the TCXO on
frequency since this input is a direct coupled path to
the TCXO. The TCXO frequency may also need to be
readjusted if DC coupling is used.
NOTE: The use of the wideband data input requires
that the user provide modulation limiting and splatter
filter circuitry and then obtain FCC reapproval of the
transmitter.
6.7.5 TRANSMIT DATA CIRCUIT (U607D, U607A)
The transmit LTR data and Call Guard tone/data
signal is generated by the microprocessor on the 7Q
and 8Q outputs of output latch U409. The four logic
combinations possible on these two lines are applied to
a resistor network formed by R732-R735. This
network produces a four-step approximation of a sine
wave which is then applied to a low-pass filter formed
by U607D and U607A. This filter attenuates
harmonics present in the sine wave which provides
smoothing of the stepped sine wave.
The passband of this filter is controlled by Q612.
When LTR data, digital Call Guard data, or low-
frequency tone Call Guard tones are transmitted, Q612
is turned on and the cut-off frequency is approximately
150 Hz. Then when a high-frequency tone Call Guard
tone is transmitted, Q612 is turned off and the cut-off
frequency is approximately 220 Hz. Turning Q612 on
switches additional capacitance into the circuit.
U610B provides gating of the data signal. When
the control input (pin 5) of this gate is a high level, the
data signal is passed to summing amplifier U605A.
Test gate U610A is used in the test mode to bypass the
data around the data filter so that modulation balance
can be set.
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 ...
