System Description Chapter 2

100 Watt High Band VHF Transmitter Chapter 2, System Description
420A, Rev. 0 2-2
audio or to the BNC jack (J3) for
composite audio, which is a loop-through
connected to jack (J13). There are also
connections for the remote monitoring
and operation of the transmitter at the
"D"-connector jacks (J10 and J11).
The baseband audio connects to (A4) the
aural IF synthesizer board (1265-1303),
in the exciter section, which provides the
amplification for each of the three
possible audio inputs and the amplifier
circuit that supplies the single audio
output. The board has the 4.5 MHz VCO
and the aural modulation circuitry that
produces the modulated 4.5 MHz output.
The board also contains a PLL circuit to
maintain the precise 4.5 MHz separation
between the aural (41.25 MHz) and
visual (45.75 MHz) IF frequencies. The
video input or 4.5 MHz composite input
to the exciter section connects to (A5)
the sync tip clamp/modulator board
(1265-1302) that takes the video input
and produces a sync tip clamped and
modulated visual IF + aural IF output at
the output jack J20. If the optional 4.5
MHz composite input is purchased, the
input is directed out of the sync tip
camp/modulator board to the (A24)
composite 4.5 MHz filter board (1227-
1244) and the (A25) 4.5 MHz bandpass
filter board (1265-1307). These two
boards process the 4.5 MHz composite
input and produce the video that
connects back to the sync tip
clamp/modulator board at J27. The
clamp portion of the board maintains a
constant peak of sync level of the video
signal over varying average picture levels
(APL). The modulator portion of the
board contains the circuitry that
generates an amplitude-modulated,
vestigial-sideband IF signal output at
45.75 MHz.
The 45.75 MHz is generated by (A7) the
IF oven oscillator board (1191-1404) and
connected (+5 dBm) to the sync tip
clamp/modulator board. The combined
visual IF + aural IF output (0 dBm)
connects to J32 on (A8) the ALC board
and then through relays to the rest of the
ALC board. The ALC board gives the
operator control over the output power
level of the transmitter by adjusting the
level of the combined IF signal. The IF
connects out of the board at J3 and is
directed to (A9) an IF phase corrector
board (1227-1250). The IF signal is
phase corrected as needed and fed back
to the ALC board.
2.1.2 Upconverter Section
The output of the ALC board (0 dBm)
connects to (A11-A1) the filter/mixer
board (1150-1102), in the (A11)
mixer/amplifier enclosure (1065007),
which takes the LO and heterodynes it
with the combined IF. This signal is then
fed to the high-band VHF filter/amplifier
board, high output (1064150), that
amplifies the RF signal and feeds it to the
(A23) high-band amplifier heatsink
assembly (1065008). The crystal
frequency needed to generate the LO is
produced by (A14-A1) the channel
oscillator board (1145-1201) in (A14) the
channel oscillator assembly (1145-1202).
The crystal frequency (+5 dBm) from the
oscillator is multiplied four times by
(A15-A1) the x4 multiplier board (1174-
1112), in (A15) the multiplier enclosure
(1235-1101), which produces the LO
signal.
2.1.3 Amplifier Section
The RF output of the upconverter section
(+11 dBm) is fed to the amplifier section
of the transmitter. The amplifier section
consists of an (A23) amplifier heatsink
assembly (1065008) and associated
components. The first stage of the
amplifier section amplifies the RF to +31
dBm in (A23-A1) a high-band VHF
amplifier board assembly (1065812) that
consists of a high-band VHF amplifier
board (1218-1201) and associated
components. The RF is divided by the
(A23-A2) 2-way combiner assembly
(1064485) and fed to two parallel (A23-
A3 and A23-A4) high-band VHF amplifier
board assemblies (1065812). The
amplified signals are fed to another (A23-