User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-7
select enable/disable jumper W11 on J29
controls whether the Modulator Select
command at J30 controls the operation of
the relays. With jumper W11 on J29, pins
1 and 2, the Modulator Select command
at J30 controls the operation of the
relays but, with jumper W11 on J29, pins
2 and 3, the modulator is selected all of
the time.
4.1.7.1 Modulator Selected
With the modulator selected, J11-10 and
J11-28 on the rear of the UHF exciter
tray are connected together; this makes
J30 low and causes relays K3 and K4 to
de-energize. When K4 is de-energized, it
connects the receiver IF input at J1, if
present, to 50 watts. When K3 is de-
energized, it connects to the modulator
IF input at J32 and from there to the rest
of the board. At this point, the Modulator
Enable LED DS5 will be lit.
4.1.7.2 Receiver Selected
With the receiver selected, J11-10 and
J11-28, which are on the rear of the UHF
exciter tray and connect to J30 on the
board, are not connected together;
relays K3 and K4 are energized. When K4
is energized, it connects the receiver IF
input at J1, if present, to the rest of the
board. When K3 is energized, it connects
to the modulator IF input at J32 to 50
watts. At this point, the Modulator Enable
LED DS5 will not be lit.
4.1.7.3 Main IF Signal Path (Part 1 of 3)
The selected visual + aural IF input (0
dBm) signal is split, with one half
entering a bandpass filter consisting of
L3, L4, C4, L5, and L6. This bandpass
filter, which can be tuned with C4, is
substantially broader than the IF signal
bandwidth. It is used to slightly steer the
frequency response of the IF and make
up for any small discrepancies in the
frequency response in the stages that
precede this point. The filter also serves
the additional function of rejecting
unwanted frequencies that may occur if
the tray cover is off and the tray is in a
high RF environment. This allows for the
servicing of this transmitter with the tray
cover off in spite of being in the presence
of other RF signals. The filtered IF signal
is fed through a pi-type matching pad,
consisting of R2, R3, and R4, to the pin-
diode attenuator circuit consisting of
CR1, CR2, and CR3.
4.1.7.4 Input Level Detector Circuit
The other part of the split IF input is
connected through L2 and C44 to U7, an
IC amplifier, which is the input to the
input level detector circuit. The amplified
IF is fed to T4, a step-up transformer,
that feeds diode detector CR14. The
positive-going detected signal is then
low-pass filtered by C49, L18, and C50;
this allows only the video with positive
sync to be applied through emitter
follower Q1. The signal is then connected
to detector CR15, which produces a
peak-sync voltage that is applied to op-
amp U9A. There is a test point at TP3
that provides a voltage reference check
of the input level. The detector serves
the dual function of providing a reference
that determines the input IF signal level
to the board and acting as an input
threshold detector.
The input threshold detector prevents the
automatic level control from reducing the
attenuation of the pin-diode attenuator to
minimum (maximum signal) if the IF
input to the board is removed. The ALC,
video loss cutback, and the threshold
detector circuits will only operate when
jumper W3 on jack J6 is in the Auto
position, between pins 1 and 2. Without
the threshold detector, with the pin-diode
attenuator at minimum when the signal
is restored, the stages following this
board will be overdriven.
On the threshold detector, the minimum
IF input level at TP3 is fed through
detector CR15 to op-amp IC U9A, pin 2.
The reference voltage for the op-amp is
determined by the voltage divider
consisting of R50 and R51 off of the +12