CIRCUIT DESCRIPTION
6-8
August 1993
Part No. 001-9750-001
biased so that the voltage drop across R297 is equal to
the drop across R294 and the base- emitter of Q291.
This arrangement provides a bias that remains stable
as changes occur in temperature.
Impedance matching with filter Z201 on the RF
board is provided by a section of microstrip and C296.
R295 lowers the gain of Q290 so that it remains stable.
C293, C294, C291, C292, and C298 decouple various
RF signals.
6.3.2 FIRST MIXER (Q205)
Bandpass filter Z201 on the RF board is similar to
Z290 and provides additional filtering of the receive
signal. The output of Z201 is applied to a 50-ohm, 3-
dB pad formed by R203-R204. This pad is used to set
the input level to mixer Q206. Impedance matching
between the pad and Q206 is provided by a section of
microstrip and C204. Q206 is biased using the same
method as used with Q290. The injection signal is
applied to the emitter of Q206 and is at a level of
approximately 10 dBm. It is 52.950 MHz below the
receive frequency in order to produce a first IF of
52.950 MHz. Filtering of the injection signal is
provided by Z202. This is a two-pole bandpass filter
with a center frequency of 807 MHz and a bandwidth
of 18 MHz (800 MHz models) or a center frequency of
885 MHz and a bandwidth of 6 MHz (900 MHz
models).
The 52.950 MHz output of Q205 is matched by
L203, C201, and C202 to a 50-ohm, 3-dB pad formed
by R215, R217, and R218. R210 is used to lower the
Q of L203. Matching between the pad and crystal filter
Z205 is provided by C221, C207, and L202.
6.3.3 CRYSTAL FILTERS (Z205, Z206), IF
AMPLIFIER (Q203)
Z205 is a four-pole crystal filter with a –3 dB
bandwidth of 15 kHz (800 MHz) or 8 kHz (900 MHz).
This filter attenuates wideband noise, adjacent chan-
nels, frequencies resulting from inter modulation, and
other undesired frequencies.
The output of this filter is applied to IF amplifier
Q203. Matching between this stage and filter Z205 is
provided by C220, L205, and C217. A constant bias
current is provided by Q202 similar to Q291 described
in Section 6.3.1. The output of the IF amplifier is
applied to a 6 dB pad and then to crystal filter Z206.
This is a two-pole filter with a bandwidth of 18 kHz
(800 MHz) or 10 kHz (900 MHz) at the –3 dB points.
6.3.4 SECOND MIXER/DETECTOR (U200)
U200 contains second mixer and oscillator,
limiter, detector, audio preamplifier, squelch, and
RSSI stages. The second injection signal is 450 kHz
below the 52.950 MHz IF signal or 52.500 MHz.
Coarse frequency adjustment for the internal oscil-
lator in U200 is provided by L206 and C239. Fine
adjust is provided by the third harmonic of the TCXO
frequency (17.500 MHz x 3 = 52.500 MHz). A two-
pole filter formed by C240-C244, L208, and L200 is
tuned to that frequency, and the output locks the oscil-
lator to 52.500 MHz.
The 450 kHz output of the internal double-
balanced mixer is fed out on pin 19 and back in again
on pin 17. It is then applied to another mixer which is
used in this application for amplification only. The
output of this mixer is fed out on pin 6 and applied to a
four-pole ceramic filter which is used to attenuate
wideband noise. The 450 kHz signal is then fed back
into U200 on pin 7 and applied to a limiter/amplifier
stage. This limiter amplifies the 450 kHz signal and
then limits it to a specific value. This eliminates ampli-
tude variations present in the signal.
From the limiter the signal is fed to the quadra-
ture detector. An external phase shift network
connected to pin 12 shifts the phase of one of the
detector inputs 90 degrees at 450 kHz (the other inputs
are unshifted in phase). When modulation occurs, the
frequency of the IF signal changes at an audio rate as
does the phase of the shifted signal. The detector,
which has no output with a 90-degree phase shift,
converts this phase shift into an audio signal. T204 is
tuned to provide maximum undistorted output from
the detector.
The detected audio signal is fed out of U200 on
pin 13 and amplified by U201A which has a gain of
approximately three at 800 MHz or six at 900 MHz.
R207 and R213 provide a bias voltage of approxi-
mately 3.0 volts. The output of this stage is fed to the
receive audio processing circuitry on the
audio/logic board.
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 ...
Страница 179: ...Revised January 1995 Part No 001 9750 003 10 22 PA BOARD SCHEMATIC 900 MHZ 15W ...
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 ...
