350-Watt Digital VHF LB Transmitter
Chapter 4, Circuit Descriptions
DT325B, Rev. 0
4-7
The RF output of the filter/amplifier
board connects to J2 of (A2) a RF
enclosure that contains the VHF low band
amplifier board, the overdrive protection
board and the 3-way splitter board.
4.2.3 (A2-A1) VHF Low Band
Amplifier Board
(1198-1605; Appendix D)
The Low Band VHF Amplifier Board takes
the +16.5 dBm input signal and amplifies
it to approxi38.5 dBm Output.
The board contains a MRF151G Dual FET
Amplifier device.
The Input to the board is applied at J1
and is connected through T1 a 9:1 Balun
to the Sources of the parallel FETs, which
make up Q1. C1 is adjustable for
maximum signal to match the impedance
of the input to the Push-Pull Transistor
Stage. The Bias and collector voltage for
the Transistor, +45 VDC, is applied at E1,
which is bypassed by C18. The bias
voltage is regulated to approximately 20
VDC by Q2 and adjusted by R4 before
connecting to Q1. The output of the Q1
push-pull circuit connects to T2 which is a
4:1 Balun that transforms the signal back
to a single 50O impedance Output. The
Collector Voltage is applied at E1, which
is filtered and bypassed by L1, C12 &
C14. The Amplifier Stage has roughly 22
dB of gain and draws 5 to 6 Amps of
current. The RF output of the board is at
J2.
4.2.4 (A2-A2) Overdrive Protection
Board (1198-1601; Appendix D)
The RF output of the low band VHF
amplifier board at J2 (+38.5 dBm)
connects to J4 of (A2-A2) the overdrive
protection board (1198-1601). The RF
signal is through connected directly to J5,
the RF output jack of the board. A
sample of the RF on the board is applied
to a diode-detector circuit that consists of
CR1 and U1A. The gain of amplifier U1D
is controlled by detector gain pot R11,
which is set to +.4 VDC as measured at
TP1. The set output of U1D is connected
to the comparator IC U1B. The trip point
for the comparator is adjusted by R12,
typically set to 110% output power.
When the signal reaches that level, the
overdrive protection board will cut back
the output power of the tray and the red
Overdrive LED DS1 located on the board
and the red Overdrive LED DS1 mounted
on the front panel will be illuminated.
Typically, the output power level will
bounce down and then up and continue
bouncing until the output level is lowered
to the normal operating level (100%).
The red Overdrive LED DS1, the green
Module LED DS3, and the Enable LED
DS2 may blink on and off during the
bouncing of the output level; this is a
normal occurrence. The greater the
output level is above 110%, the larger
the bounce will be. The RF output of the
overdrive protection board (+38.3 dBm)
at J5.
4.2.5 (A2-A3)3 Way Splitter Board
(1198-1608; Appendix D)
The RF output of the overdrive protection
board at J5 connects to J1 on (A2-A3)
the 3-way splitter board (1198-1608).
The splitter board takes the +38.3 dBm
input and provides three +33.3 dBm
outputs at J2, J3 and J4 of the board.
These three outputs connect to J1, J2
and J3 on the (A3) final amplifier
enclosure.
4.2.6 (A3-A1, A2 & A3) VHF LB
Output Amplifier Pallets
(1304349; Appendix D)
The three RF outputs of the splitter board
connect to (A3) the Final Amplifier
Enclosure, which contains three (A3-A1,
A3-A2 & A3-A3) VHF Low Band Amplifier
Pallets (P400-VHF-LB) made by Delta RF
Technology. The RF Signals connect to
J1 on each of the Low Band Output
Amplifier Pallets. Each amplifier pallet
provides approximately 18 dB gain.
The RF signal inputs to the Output
Amplifier Boards, +33.3 dBm, are
amplified to +51.3 dBm outputs at J2,
