350-Watt Digital VHF LB Transmitter
Chapter 4, Circuit Descriptions
DT325B, Rev. 0
4-6
A2-A3
3-way splitter board (mounted in
[A2] an RF enclosure assembly)
1198-1608
A3-A1, A3-A2 and A3-A3
Three low band VHF amplifier pallets
(mounted in [A3] an RF enclosure
assembly)
P400-VHF-L-18
1304348
A4-A1
3-way combiner board (mounted on
[A4] the 3 way combiner enclosure)
1198-1626
A4-A2 & A4-A3
Low pass filter board (mounted on
[A4] the 3 way combiner enclosure)
1198-1628
A5
AGC control board
1306482
A8
Current metering board
1304362
A10
+30 VDC switching power supply
assembly
PM3329B-5-1-R-2-E
1301504
4.2.1 (A1-A1) Phase Shifter Board
(1198-1602; Appendix D)
The On Channel RF signal (+4dBm),
enters the rear of the Tray at the "BNC"
Jack J1 and is fed to J1 of the (A1)
Enclosure Assembly which contains the
phase shifter board and the filter
amplifier board. The input is cabled to
to J1 of (A1-A1) the Phase Shifter Board
(1198-1602).
The Board provides a Phase Shift
adjustment of the RF Signal that is
needed to provide maximum output
during the combining of multiple
Amplifier Trays in an Amplifier Array. A
front panel mounted Phase Shift
Potentiometer, R2 (A7), which connects
to J3 of the Board, controls the Phase of
the RF Signal.
If the Input Signal level to the Phase
Shifter Board falls below a preset level, a
High, which is an Input Fault, connects
from J5 of the board to J14 on the AGC
Control Board. When an Input Fault
occurs, the AGC Control Board generates
a Fault Output at J1, which is connected
to J4 on the Filter/Amplifier Board. The
Fault cuts back the RF Signal level using
the Pin Diode Attenuator Circuit located
on the Filter/Amplifier Board.
The output at J2 of the Phase Shifter
Board is directed to J7 the input jack on
(A1-A2) on the filter/amplifier board.
4.2.2 (A1-A2) Filter/Amplifier Board
(1198-1606; Appendix D)
The phase-controlled output (+4 dBm) at
J2 of the phase shifter board is directed
to J7, the input jack of the (A1—A2) filter
amplifier board (1198-1606) that is made
up of two circuits. The first circuit is a
channel filter that is adjusted for the
desired channel frequency and
bandwidth. The filtered output (+2 dBm)
is connected to the second circuit that
contains two amplifiers. The RF connects
through a pin-diode circuit to the
amplifier IC U1. The voltage applied to
J4, which is the external control jack of
the board, controls the amplitude of the
RF signal through the pin-diode
attenuator circuit. Jumper W1 on J5
should be between pins 2 and 3, which
provide external control, through J4, of
the gain of the board, as well as the
output level of the tray. R9 is the
manual gain pot that is in the circuit
when the jumper W1 is between pins 1 &
2.
The front panel mounted gain pot R3
(A6) connects to the AGC control board
and is used to adjust the AGC pin-
attenuator bias voltage that connects to
J4 on the filter/amplifier board. The RF
signal, after the pin-attenuator circuit, is
amplified by the second amplifier stage
Q1 to about +16.5 dBm; this signal is
connected to the output of the board at
J2.
