CIRCUIT DESCRIPTION
6-4
August 1993
Part No. 001-9750-001
The DC voltage applied across CR904 provides
compensation to keep modulation relatively flat over
the entire bandwidth of the VCO. This compensation
is required because modulation tends to increase as the
VCO frequency gets higher (capacitance of CR901
gets lower). CR904 also balances the modulation
signals applied to the VCO and TCXO (see next
section).
The DC voltage applied across CR904 comes
from a D/A converter on the audio/logic board. This
D/A converter is formed by a resistor network
controlled by shift register U450 and the micropro-
cessor. As the VCO frequency increases, this DC
voltage also increases which lowers the amount of
capacitance applied to the tank circuit and therefore
the amount of modulation produced. R802 applies a
DC biasing voltage to CR903; C834 provides DC
blocking; and C850 attenuates AC signals applied
through R802. RF isolation is provided by C912,
R902, C846, and R805.
VCO Frequency Shift
The VCO must be capable of producing frequen-
cies from approximately 798-869 MHz (800 MHz
models) or 882-941 (900 MHz models) to produce the
required receive injection, transmit, and repeater talk-
around frequencies. If this large of a shift was
achieved by varying the VCO control voltage, the
VCO gain would be undesirably high. Therefore,
capacitance and inductance is switched in and out of
the tank circuit to provide a coarse shift in frequency.
This switching is controlled by shift register
U802 and PIN diodes CR905 and CR909. When a PIN
diode is forward biased, it presents a very low imped-
ance to RF; and when it is reverse biased, it presents a
very high impedance. The capacitive and inductive
legs of the tank circuit are switched as follows:
Mode
Switching
Transmit
Neither switched in
Receive
C leg switched in
Talk-Around
L leg switched in
NOTE: The C and L legs are never switched in at the
same time.
The capacitive leg is formed by C905, PIN diode
CR905, and C906. When the Q6 output of U802 is a
high level and the Q7 output is a low level, CR905 is
forward biased and C905 and C906 are effectively
connected to the tank circuit. This decreases the reso-
nant frequency of the tank circuit. L905/L906 and
C907/C908 provide isolation. The inductive leg is
formed by C909, a section of microstrip (which
provides series inductance), and by C910. When this
leg is switched into the tank circuit, the resonant
frequency increases.
6.2.3 VCO AND TCXO MODULATION
Both the VCO and TCXO are modulated to
achieve the required frequency response. If only the
VCO was modulated, the phase detector in U801
would sense the frequency change and increase or
decrease the VCO control voltage to counteract the
change, especially at the lower audio frequencies. If
only the TCXO was modulated, the VCO frequency
would not change fast enough, especially at the high
audio frequencies. Modulating both the VCO and
TCXO produces a flat response. One function of the
DC voltage applied to J200, pin 20 is to balance these
signals so that modulation of the VCO is equal to that
of the TCXO. The other is frequency compensation
described in the preceding section.
6.2.4 ACTIVE FILTER (Q800), BUFFER
AMPLIFIER (Q801, Q802)
Q800 functions as a capacitance multiplier to
provide a filtered 8-volt supply to the VCO module
and shift register U802. R801 provides bias and C802
provides the capacitance that is multiplied. If a noise
pulse or other voltage change appears on the collector,
the base voltage does not change significantly because
of C802. Therefore, base current does not change and
the voltage on the emitter remains constant. R800
provides isolation, and C800, C801, and C836
decouple RF.
The output signal on pin 2 of the VCO module is
at a level of approximately 0 dBm and is applied to the
base of Q801. This stage amplifies the signal to
approximately 12 dBm and also provides additional
isolation. Impedance matching between the VCO
module and Q801 is provided by a section of micros-
trip and C808. Q802 provides a Q801 bias current that
Содержание Summit DM 975x
Страница 105: ...MULTI NET SYSTEM OVERVIEW 5 10 Revised February 1997 Part No 001 9750 005 ...
Страница 108: ...CIRCUIT DESCRIPTION 6 3 August 1993 Part No 001 9750 001 Figure 6 1 Transceiver Block Diagram ...
Страница 123: ...CIRCUIT DESCRIPTION 6 18 August 1993 Part No 001 9750 001 ...
Страница 131: ...SERVICING 7 8 Revised January 1995 Part No 001 9750 003 ...
Страница 156: ...9 19 Revised December 2000 Part No 001 9750 007 TRANSCEIVER EXPLODED VIEW PART 1 ...
Страница 157: ...Revised January 1995 Part No 001 9750 003 9 20 TRANSCEIVER EXPLODED VIEW PART 2 Remote Control Parts ...
Страница 168: ...Revised January 1995 Part No 001 9750 003 10 11 INTERCONNECT SCHEMATIC ...
Страница 169: ...Revised January 1995 Part No 001 9750 003 10 12 DISPLAY BOARD SCHEMATIC ...
Страница 170: ...Revised January 1995 Part No 001 9750 003 10 13 DISPLAY BOARD TOP VIEW DISPLAY BOARD BOTTOM VIEW ...
Страница 171: ...Revised January 1995 Part No 001 9750 003 10 14 RF BOARD SCHEMATIC REVISED 800 MHZ ...
Страница 172: ...Revised January 1995 Part No 001 9750 003 10 15 RF BOARD BOARD LAYOUT ALL 800 900 MHZ ...
Страница 173: ...Revised January 1995 Part No 001 9750 003 10 16 RF BOARD BOARD SCHEMATIC 900 MHZ ...
Страница 174: ...Revised January 1995 Part No 001 9750 003 10 17 RF BOARD BOARD SCHEMATIC UNREVISED 800 MHZ ...
Страница 178: ...Revised January 1995 Part No 001 9750 003 10 21 PA BOARD SCHEMATIC 900 MHZ 30W ...
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Страница 180: ...Revised January 1995 Part No 001 9750 003 10 23 PA BOARD BOARD LAYOUT 900 MHZ UNREVISED 800 MHZ 15W 30W 35W 15W MODELS ...
Страница 181: ...Revised January 1995 Part No 001 9750 003 10 24 PA BOARD SCHEMATIC UNREVISED 800 MHZ 35W ...
Страница 182: ...Revised January 1995 Part No 001 9750 003 10 25 PA BOARD SCHEMATIC UNREVISED 800 MHZ 15W ...
Страница 183: ...Revised January 1995 Part No 001 9750 003 10 26 AUDIO LOGIC BOARD SCHEMATIC PART 1 OF 2 ...
Страница 184: ...Revised January 1995 Part No 001 9750 003 10 27 AUDIO LOGIC BOARD SCHEMATIC PART 2 OF 2 ...
Страница 185: ...Revised January 1995 Part No 001 9750 003 10 28 AUDIO LOGIC BOARD LAYOUT TOP VIEW ...
Страница 186: ...Revised January 1995 Part No 001 9750 003 10 29 AUDIO LOGIC BOARD LAYOUT BOTTOM VIEW ...
Страница 188: ...Revised January 1995 Part No 001 9750 003 10 31 REMOTE TRANSCEIVER INTERCONNECT SCHEMATIC ...
Страница 189: ...Revised January 1995 Part No 001 9750 003 10 32 REMOTE CONTROL UNIT INTERFACE BOARD SCHEMATIC ...
Страница 216: ...Part Number 001 9750 007 12 00 hph Printed in U S A ...
