Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-13
3.1.5.12 ±12 VDC Needed to Operate the
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.
The +12 VDC also connects to U16, a 5-
VDC regulator IC, that produces the +5
VDC needed to operate timing IC U17.
3.1.6 (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 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 they are combined in Z1,
it provides the required adjustable phase
correction to the IF signal.
The IF input signal 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 coaxial cable W4 to
jack J6, the quadrature input, on the
board. The IF output 2 at J3 is jumpered
through coaxial cable W5 to jack J7, the
in-phase input, on the board.
3.1.6.1 Phase Corrector Circuit
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, and either black or white
stretch can be achieved.
In the phase corrector circuit, the IF
signal from J6 is applied to transformer
T1; T1 doubles the voltage swing using a
1:4 impedance transformation. Resistors
R8, R61, R9, and R48 form an L-pad that
attenuates the signal. This attenuation is
adjusted by adding R7, a variable
resistor, in parallel with the L-pad. R7 is
only in parallel when the signal reaches a
level large enough to bias on CR1 and
CR2 and allow current to flow through
R7. When R7 is put in parallel with the L-
pad, the attenuation through the L-pad is
lowered, causing black stretch.
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 from the +12 VDC line. Diodes
CR11 and CR12 provide a .9 VDC
reference to temperature compensate
the corrector circuits from the effects of
the two diodes in each corrector stage.
The threshold for the first corrector stage
is set by controlling where CR1 and CR2
turn on. This is accomplished by
adjusting R3 to form a voltage divider
from +6.8 VDC to ground. The voltage at
the wiper of R3 is buffered by U9C, a
unity-gain amplifier, and applied to CR1.
The .9 VDC reference is connected to
U9D, a unity-gain amplifier, whose
output is wired to CR2. These two
references are connected to diodes CR1
and CR2 through chokes L2 and L3. The
two chokes form a high impedance for RF
to isolate the op-amps from the RF. The
adjusted signal is next applied to
amplifier U2 to compensate for the loss
through the L-pad. U2 is powered
through L4 and R10 from the +12 VDC
line. After the signal is amplified by U2, it