User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-10
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.
4.1.7.8 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.
4.1.7.9 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 transmitter is accomplished, if the
(optional) remote power raise/lower kit
(1227-1039) is purchased, by R75, a