User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-15
than just pulling the ALC reference down.
Two different mechanisms are employed.
One is a very fast-acting circuit to
increase the attenuation of the pin-diode
attenuator, CR3, CR1, and CR2, and the
second is the reference voltage being
pulled away from the ALC amplifier
device. An external Mute is a pull-down
applied to J19 pin 6, to provide a current
path from the +12 VDC line through R78
and R139, the LED DS4 (Mute indicator),
and the LED section of opto-isolator U11.
These actions turn on the transistor
section of U11 that applies -12 VDC
through CR21 to U10A, pin 3, and pulls
down the reference voltage. This is a
fairly slow action that is kept at this pace
by the low-pass filter function of R81 and
C61. When the transistor section of U11
is on, -12 VDC is also connected through
CR22 to the pin-diode attenuator circuit.
This establishes a very fast muting
action, by reverse biasing CR3, in the
event of an external VSWR fault.
4.1.5.14 ±12 VDC Needed to Operate the
ALC Board
The ±12 VDC connects to the board at
J14. The +12 VDC connects to J14-3 and
is filtered by L30, L41, and C80 before it
is applied to the rest of the board. The
-12 VDC connects to J14-5 and is filtered
by L31 and C81 before it is applied to the
rest of the board.
One of the boards +12 VDC connects to
U16, a 5-VDC regulator IC that produces
the +5 VDC needed to operate the timing
IC U17.
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 may
occur in the solid state amplifier devices
located in the external VHF amplifier
trays. Two separate, adjustable IF paths
are found 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 they are combined in
Z2, it provides the required adjustable
phase correction to the IF signal.
The IF input signal (0 dBm) enters at J1
and is AC coupled to U1. U1 amplifies
the IF before it is connected to Z1, a
splitter that creates two equal IF outputs.
IF output 1 is connected to J2 and IF
output 2 is connected to J3. The IF
output 1, at J2, is jumpered through the
coaxial cable W4 to jack J6, the
quadrature input. The IF output 2, at J3,
is jumpered through the coaxial cable W5
to jack J7, the in-phase input.
4.1.6.1 Phase Corrector Circuit,
Quadrature Correction
The phase corrector circuit corrects for
any amplitude nonlinearities of the IF
signal. It is designed to work at IF and
has three stages of correction. Each
stage has a variable threshold and
magnitude control. The threshold control
determines the point at which the gain is
changed and the magnitude control
determines the gain change once the
breakpoint is reached. The second stage
has a jumper that determines the
direction of correction, so that the gain
can increased either above or below the
threshold, therefore either black or white
stretch can be achieved.
Two reference voltages are utilized in the
corrector stages and both are derived
from the +12 VDC line. Zener diode
VR1, with R46 as a dropping resistor,
provides +6.8 VDC. Diodes CR11 and
CR12 provide a .9 VDC reference, V
ref
, to
temperature compensate the corrector
circuits from the effects of the two diodes
in each corrector stage.
In the phase corrector circuit, the
Quadrature IF signal from J6 is applied to
the transformer T1, which doubles the
voltage swing using a 1:4 impedance
transformation. Resistors R8, R61, R9,
and R48 form an L-pad that attenuates