User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-13
reference voltage is then connected to
diode CR5 through choke L11. Chokes
L11 and L12 form a high impedance for
the 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.
4.3.7.8 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 bandpass filter is identical in
both form and function to the one
described at IF input jack J1. In this
case, the filter is intended to make up
for the small errors in frequency
response that are incurred by the signal
while it is 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, which control whether a
6-dB pad is included in the circuit by
the positioning of jumpers W9 and
W10. The 6-dB pad is in when jumpers
W9 and W10 are connected between
pins 2 and 3 on J27 and J28. The 6-dB
pad is out when jumpers W9 and W10
are connected between pins 1 and 2 on
J27 and J28. Normally, the pad is out.
The IF signal is then applied to a two-
stage frequency response corrector
circuit that is adjusted as necessary.
Variable resistors R103 and R106
adjust the depth, or gain, of the
notches and variable caps C71 and C72
adjust the frequency, or position, of the
notches. The IF signal is amplified by
U13 and U14 before it is connected to
J12, the IF output jack of the board.
R99 is an output level adjustment that
is set to provide approximately 0 dBm
of IF output at J12. A sample of the IF
is fed to J11, which provides an IF
sample point that can be monitored
without breaking the signal path and
gives an indication of the IF signal after
the linearity and frequency response
correction takes place.
4.3.7.9 ALC Circuit
The other path of the corrected IF
signal is used in the ALC circuit. The IF
is wired out of the splitter through L25,
which is connected to op-amp U12. The
output of U12 is wired to jacks J8 and
J9; jumpers W4 and W8 control the
normal, or encoded, operation of the
ALC circuitry. For normal operation,
jumper W4 on J8 is between pins 1 and
2 and jumper W8 on J9 is between pins
1 and 2.
The IF signal is applied to transformer
T5, which doubles the voltage swing by
means of a 1:4 impedance
transformation before it is connected to
the ALC detector circuit on the board
and amplified by U10B. For normal
operation, jumper W7 on J26 is
between pins 1 and 2 and jumper W5
on J21 is between pins 1 and 2. The
detected ALC voltage is wired to U10A,
pin 2, where it is summed with the
front panel power control setting. The
output power adjustment for the