User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-42
amplifier. The output of U1A is split,
with one part going to the AGC circuits,
another sample connected to J1-5 for
remote metering, and the final sample
applied to the meter at position 3 on
S2, the front panel meter switch. R2
can be adjusted to calibrate the %
Forward Power indication on the front
panel meter.
A sample of the reflected power output
of the tray is applied to jack J2-1 and
J2-2 of the board from the dual peak
detector board, single supply. The
reflected power sample is connected
through R21 and R22 to U1B, a buffer
amplifier. The output of U1B is split,
with one part going to the VSWR
threshold circuit, another sample
connected to J2-5 for remote metering,
and the final sample applied to the
meter at position 2 on S2, the front
panel meter switch. R22 can be
adjusted to calibrate the % Reflected
Power indication on the front panel
meter.
A sample of the AGC voltage level at
R20 on the board is connected through
a divider network consisting of R87,
R88, R19, and R18 to the meter at
position 1 of S2, the Front Panel Meter
switch.
4.8.12.5 Operational Voltages
The voltage input to the board is +26.5
VDC from the switching power supply.
The +26.5 VDC connects to the board
at J6-1 and is wired to U4, which is a 3-
terminal regulator IC that takes the
+26 VDC input and produces the +12
VDC needed for the operation of the
board.
The +12 VDC from U4 is connected
through CR6 to U5, a +5-VDC
regulator, that takes the +12 VDC
input from U4, or the transmitter
control board in the UHF exciter tray,
through J6, pin 3, and produces a +5-
VDC output that is applied to the rest of
the board. This +12 VDC is connected
to U5, a regulator IC, which produces a
+5 VDC output that is applied to the
Enable and Overtemperature LEDs that
operate even when the +26.5 VDC
input to the board from the switching
power supply is removed.
4.8.13 (A2) Variable Gain/Phase
Board Enclosure (1265-1426;
Appendix D)
The variable gain/phase enclosure
assembly provides EMI and RFI
protection for the variable gain/phase
board (1265-1425) that is mounted
inside of the assembly.
The RF input to the assembly is at SMA
jack J1 and the RF output is at SMA
jack J2. There are two control inputs
that connect to the assembly: the
attenuator bias input and the phase
control input. The attenuator bias input
from the amplifier control board
connects to FL4 and FL5 on the
assembly and is wired to J5 on the
board. The phase control input from the
amplifier control board connects to FL2
and E1 on the assembly and is wired to
J4 on the board. The input fault to the
amplifier control board connects to FL3
on the assembly and is wired to J6-1 on
the board. E1 on the assembly
connects to J3-4 on the board.
The +26 VDC needed to operate the
board connects to FL1 on the assembly
which is wired to J3-3 on the board.
4.8.14 (A2-A1) Variable Gain/
Phase Board (1265-1425;
Appendix D)
The variable gain/phase board provides
the circuits that adjust the phase and
the gain of the RF signal for the
amplifier tray in which it is mounted.
The RF input signal at J1 is split, with
one output connected to a detector
circuit consisting of C8, CR4, and U3A.
This detected level is then applied to
comparator U3D, which provides a high