Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-16
the magnitude control determines the
amount of gain change that occurs
once the breakpoint is reached. Two
reference voltages are needed for the
operation of the corrector circuits.
Zener diode VR1, with R33 and R135,
provides a +6.8 VDC reference and the
diodes CR11 and CR12 provide a .9
VDC reference that temperature
compensates for the two diodes in each
corrector stage.
For the linearity correctors to operate,
an IF signal is applied to transformer
T1, which doubles the voltage swing by
means of a 1:4 impedance
transformation. Resistors R14, R15,
and R16 form an L-pad that lowers the
level of the signal. The amount that the
level is lowered can be adjusted by
adding more or less resistance, using
R13, in parallel with the L-pad
resistors. R13 is only in parallel when
the signal reaches a level large enough
to turn on diodes CR4 and CR5. When
the diodes turn on, current flows
through R13, putting it in parallel with
the L-pad.
When R13 is put in parallel with the
resistors, the attenuation through the
L-pad is lowered, causing signal stretch
(the amount determined by the
adjustment of R13). The signal is next
applied to amplifier U2 to compensate
for the loss through the L-pad. The
breakpoint, or cut-in point, for the first
corrector is set by controlling where
CR4 and CR5 turn on. This is
accomplished by adjusting cut-in
resistor R34; R34 forms a voltage-
divider network from +6.8 VDC to
ground. The voltage at the wiper arm
of R34 is buffered by the unity-gain
amplifier U5D. This reference voltage is
then applied to R35, R36, and C39
through L12 to the CR4 diode. C39
keeps the reference from sagging
during the vertical interval. The .9 VDC
reference created by CR11 and CR12 is
applied to unity-gain amplifier U5B. The
reference voltage is then connected to
diode CR5 through choke L11. The two
chokes L11 and L12 form a high
impedance for RF that serves to isolate
the op-amp ICs from the IF.
After the signal is amplified by U2, it is
applied to the second corrector stage
through T2. This corrector and the third
corrector operate in the same fashion
as the first. All three corrector stages
are independent and do not interact
with each other.
The correctors can be disabled by
moving jumper W1 on J4 to the Disable
position, between pins 2 and 3; this
moves all of the breakpoints past the
tip of sync so that they will have no
affect. The IF signal exits the board at
IF output jack J3 after passing through
the three corrector stages and is
normally connected to an external IF
phase corrector board.
4.2.5.8 Main IF Signal Path (Part 3 of
3)
After the IF signal passes through the
external IF phase corrector board, it
returns to the ALC board at IF input
jack J7. The IF then passes through a
bandpass filter consisting of L20, C97,
C62, L21, C63, L22, L23, C64, and
C99. This filter is intended to make up
for small errors in frequency response
that are incurred by the signal while
being processed through the linearity
and incidental phase correction circuits.
Following the bandpass filter, the signal
is split using L24, L25, and R89. The
signal passing through L24 is the main
IF path through the board. A sample of
the corrected IF signal is split off and
connected to J10, the IF sample jack.
The IF connects to jacks J27 and J28.
These jacks control whether a 6-dB pad
is included in the circuit by the
positioning of jumpers W9 and W10.
The 6-dB pad-in occurs when jumpers
W9 and W10 are connected between
pins 2 and 3 on J27 and J28. The 6-dB
pad-out occurs when jumpers W9 and
W10 are connected between pins 1 and