Service manual

CIRCUIT DESCRIPTION
6-8
Month 2000
Part No. 001-2004-601
6.1.15 FIRST AND SECOND INJECTION AM-
PLIFIERS
U303 provides the +12V source for these ampli-
fiers. First injection amplifier Q133 is biased by
CR131, R143, R144, R145 and R146. C148, C151,
C149 and C150 provide RF bypass from the DC line.
L134 on the collector is an RF choke. Q133 is
matched to the 50 ohm signal pad R147, R148 and
R149 by lowpass filter C152/L135/C153, C154.
C155, L136, L156, L137, C157 and a section of
microstrip match Q134 to the 50 ohm signal pad.
Second injection amplifier/buffer Q134 is simi-
lar in design to Q133. The output of Q134 is matched
to 50 ohms by L134/C162/C163 and C164 provides
DC blocking. L140/L141 are tuned to the receive fre
-
quency plus 52.95 MHz and passed to Mixer U101.
This injection frequency is also coupled through C165
to the injection test voltage circuit U102A. CR133,
R158, R159 provide DC input to U102A,
pin 3. The output of U102A, pin 1 is connected to
J201, pin 13 for a receive injection test point and to
the RF Interface Board on J103, pin 13.
6.2 EXCITER
6.2.1 VCO (A007)
The Voltage-Controlled Oscillator (VCO) is
formed by Q802, associated circuitry and a resonator
consisting of Z801. The VCO oscillates in a 40 MHz
frequency band somewhere between 380-520 MHz
depending on the model of the repeater. Biasing of
Q802 is provided by R805, R806 and R807. An AC
voltage divider formed by C807 and C808 initiates
and maintains oscillation. C806 couples Q802 to res
-
onator Z801. Resonator Z801 provides the shunt
inductance of the tank circuit. The shunt capacitance
of the tank circuit is made primarily of C803/C804 in
series with CR801/CR802. RF choke L804 completes
the DC bias path to ground.
The VCO frequency is controlled in part by DC
voltage across varactor diode CR802. As voltage
across a reverse-biased varactor diode increases, its
capacitance decreases. Therefore, VCO frequency
increases as the control voltage increases. The control
line is RF isolated from tank circuit by choke L802.
The amount of frequency change produced by CR802
is controlled by series capacitor C804.
The frequency is modulated in a similar manner.
The transmit audio/data signal is applied across varac
-
tor diode CR803 to vary the VCO frequency at an
audio rate. C815/C816 in series with CR803 deter
-
mine the amount of modulation produced by the audio
signal.
6.2.2 ACTIVE FILTER
Q801 functions as a capacitance multiplier to
provide filtering of the 12V supply to Q802. R803
and R804 provide transistor bias, and C812 provides
capacitance that is effectively multiplied by the gain
of Q801. If a noise pulse or other quick voltage
change appears on the collector, base voltage does not
change significantly because of C812. Therefore, the
base current does not change and transistor current
remains constant. R805 decouples the VCO output
from AC ground. L803 is an RF choke and C810,
C811, C813, C814 and C809 provide RF bypass.
6.2.3 VCO/TCXO FREQUENCY MODULATION
Both the VCO and TCXO are modulated in order
to achieve the required frequency response. If only
the VCO was modulated, the phase detector in U403
would sense the frequency change and increase or
decrease the VCO control voltage to counteract the
change (at the lower audio frequencies inside the
closed loop bandwidth of the synthesizer). If only the
TCXO frequency was modulated, the VCO would not
track the higher audio frequencies (those beyond the
closed loop bandwidth of the synthesizer). However,
by modulating both the VCO and TCXO a flat audio
response is achieved. Potentiometers R425 and R446
balance the modulating signals.
There are two 3.5V sources on the Exciter board;
one is a reference for the modulation amplifier to the
VCO, the other is for the modulation amplifier to the
TCXO.
The reference voltage on U402B, pin 5 is also on
buffer U407B, pin 5 to J401, pin 9 and RFIB connec
-
tor J102, pin 9. The voltage leaves the RFIB on J101,
pin 14 to J2, pin 27 on the backplane, to the bottom
connectors via pin 7 and finally to the MAC on P100,
pin 7.