E.F. Johnson Company 242-20X1-213, Руководство пользователя

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CIRCUIT DESCRIPTION
6-8
August 2000
Part No. 001-2001-200
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 Q101, associated circuitry and High-Q
indicator L102. The VCO oscillates in a frequency
range from 132-178 MHz. Biasing of Q101 is pro-
vided by R102, R103 and R104. An AC voltage
divider formed by C107 and C108 initiates and main-
tains oscillation. C106 couples Q101 to the High-Q
inductor. RF choke L103 completes the DC bias path
to ground.
The VCO frequency is controlled in part by DC
voltage across varactor diode D101. 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 L101.
The amount of frequency change produced by D101 is
controlled by series capacitor C102.
The frequency is modulated in a similar manner.
The transmit audio/data signal is applied across varac-
tor diode D102 to vary the VCO frequency at an audio
rate. C104/C105 in series with D102 determine the
amount of modulation produced by the audio signal.
6.2.2 VCO BUFFER
Q102/Q103 form a cascade-connected buffer cir-
cuitry. DC bias is produced by R107, R108, R109 and
R1212. A signal oscillated at Q101 is DC cut and
adjusted by C107 and fed into the buffer. An output
from RF choke L104 passes through an adjustment
circuit consisting of C114/C119.
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.