User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-4
bias is set by R12. The AFC output at J6
on the board connects to the VCXO on
the UHF generator board. The PLL circuit
will maintain the very accurate VCXO
output because any change in frequency
will be corrected by the AFC error
voltage.
Lock Detector Circuit
IC chip U2 contains an internal lock
detector that indicates the status of the
PLL circuit. When U2 is in a locked state,
pin 3 and pin 7 go low; the low is applied
to Q1, which is biased off. With Q1 off,
pin 1 of J1 goes high and is connected to
the Lock LED on the LED display board
that lights. The low applied to DS1, the
Red Unlock LED, causes it not to light.
If the 50 kHz from the 10-MHz reference
and the 50-kHz from the UHF generator
board become unlocked, out of the
capture range of the PLL, pins 2 and 6 of
U2 go to a logic low; this causes U2, pins
3 and 7, to go high. The high connects to
DS1, the red Unlock LED, which lights,
and to Q1 and Q2, which are biased on.
When Q2 is biased on, it connects a low
remote unlock to jack J1, pin 4. With Q1
biased On, the drain goes low and
removes the high to pin 1 of J1, which is
connected to the Lock LED, on the LED
display board, which is extinguished.
Voltage Requirements
The ±12 VDC needed for the operation of
the board enters through jack J11. The
+12 VDC is connected to J11-3, which is
filtered and isolated by L6 and C38
before it is connected to the rest of the
board. The -12 VDC is connected to J11-
5, which is filtered and isolated by L7 and
C39 before it is connected to the rest of
the board.
The +12 VDC is connected to U7, a 5-
volt regulator IC, that provides the +5
VDC operating voltage to the U2, U4, U5,
and U9 ICs.
4.1.5 (A1) Power Entry Module
Assembly (1227-1206; Appendix D)
The power entry module assembly
provides overvoltage and surge
protection for the input AC lines that
connect to the exciter tray. The AC input
plug connects to J14; J14 is part of the
power entry module on the rear panel of
the exciter tray.
The module assembly contains two 130-
VAC varistors and one 250-VAC varistor
that connect across the AC lines and to
ground. The module also contains two 4-
amp fuses, one in each input line for
overcurrent protection.
4.1.6 (A9) IF Phase Corrector Board
(1227-1250; Appendix D)
The IF phase corrector board has
adjustments that pre-correct for any IF
phase modulation distortion that might
occur in output amplifier devices such as
Klystron power tubes and solid-state
amplifiers. Two separate, adjustable IF
paths are on the board: a quadrature IF
path and an in-phase IF path. The
quadrature IF is 90° out of phase and
much larger in amplitude than the in-
phase IF. When these paths are
combined in Z1, they provide the
required adjustable phase correction to
the IF signal.
The IF input signal enters at J1 and is
capacitively coupled to U1. U1 amplifies
the IF before it is connected to Z1, a
splitter, to create two, equal IF outputs.
IF output #1 is connected to J2 and IF
output #2 is connected to J3. IF output
#1 at J2 is jumpered through coaxial
cable W4 to jack J6, the quadrature
input, on the board. IF output #2 at J3 is
jumpered through coaxial cable W5 to
jack J7, the in-phase input, on the board.
4.1.6.1 Phase Corrector Circuit
The phase corrector circuit adjusts for
any amplitude nonlinearities of the IF
signal. It is designed to work at IF and