Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-16
C3 and C6 are adjusted for the best
center frequency, C4 is adjusted for the
best coupling, and C7 is adjusted for the
best output loading of the RF signal. The
filtered RF is amplified by U1 and
connected to the RF output jack for the
board at J2 (-2 dBm).
The +12 VDC needed for the operation of
the board is supplied by an external
power supply in the tray. The +12 VDC
enters the board at J8, pin 3, and is
filtered and isolated from the rest of the
tray by L7 and C22 before being applied
to the board.
3.1.7.3 (A3) Low Band VHF Filter/
Amplifier Board (1064251; Appendix D)
The VHF low band filter/amplifier board is
made up of two separate circuits: a filter
circuit and an amplifier with a gain
control circuit.
The RF input connects to the board at J7
and is fed through a channel filter circuit.
The input to the filter consists of C27,
C28, and C29, with C29 adjusted for the
best input loading. C23 and C26 are
adjusted for center frequency, with C24
adjusted for the best coupling, and C20 is
adjusted for the best output loading of
the RF signal. The filtered RF is
connected to RF output jack J6; J6 is
usually jumpered to jack J1 on the board.
The filtered RF at J1 connects through a
7-dB pi-type attenuator, consisting of R1,
R2, and R3, before it is wired to a pin-
diode attenuator circuit. The pin-diode
attenuator circuit is made up of CR1,
CR2, and CR3 and is controlled by the
bias current applied through R5. The
diodes CR1, CR2, and CR3 are pin-type
diodes with a broad intrinsic region
sandwiched inside the diode. This broad
intrinsic region causes the pin diodes to
act as variable resistors instead of as
detecting devices at the RF frequencies.
The resistance values of the pin diodes
are determined by the relative amount of
forward bias that is applied to the diodes.
Jumper W1 on J5 is set for manual gain
or auto gain by its position on the jack.
Between 1 and 2 is manual gain, which
uses pot R9 to set the output level;
between 2 and 3 is auto gain, which uses
the external control voltage input to jack
J4 as the level control (this arrangement
is not used in this configuration).
The level-controlled RF is pre-amplified
by U1 and connected to Q1, the output
amplifier for the board. C17 is used to
maximize the RF signal. The RF output is
amplified by Q1 and fed through direction
coupler Z1 before exiting the board at J2,
the RF output. Z1 provides a RF sample
that has two functions. The first function
is to provide an RF sample at J8 through
a voltage divider consisting of R19 and
R18 that is fed to the front panel of the
exciter tray. The second function is to
provide a peak-detected voltage that is
used by the exciter tray for metering
purposes. The sample provided by Z1,
pin 3, is first fed through a dB pad
consisting of R20, R21, and R22. The
voltage is stepped up by a 1 to 4
transformer T1. The signal is then peak
detected by C32 and CR4 before being
buffered and amplified by U2A. The level
of the peak-detected voltage at J9-1 and
J9-2, which is used for metering
purposes, is controlled by the pot R29 on
the board.
The +12 VDC needed for the operation of
the board is supplied by an external
power supply in the tray. The +12 VDC
enters the board at J3 pin 3, and is
filtered and isolated from the rest of the
tray by L5 and C19 before being applied
to the entire board. The –12 VDC enters
the board at J3 pin 5, and is filtered and
isolated from the rest of the tray by L6
and C35 before being applied to the
entire board.
3.1.8 (A17) Transmitter Control
Board (1265-1311; Appendix D)
The transmitter control board provides
information on system control functions
and the operational LED indications, that
can be viewed on the front panel of the