User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-16
The IF input connects to the board at J7
and is fed to mixer Z1 at pin 3 (-3 dBm).
Mixer Z1 takes the LO input at pin 1 and
the IF input at pin 3 to produce an RF
output at pin 8. The RF output at pin 8
(-14 dBm) connects through a pi-type
attenuator, made up of R3, R4, and R5,
before it is connected to RF output jack
J6. Normally, jack J6 is connected by a
coaxial jumper to J1 on the board. J1
connects to the input of a filter circuit,
consisting of C25, C1, C23, C2, and L1,
with C2 adjusted for the best input
loading. 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.
4.1.7.3 (A11-A3) VHF High Band
Filter/Amplifier Board (1064252;
Appendix D)
The VHF high 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. NOTE: 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 applied to the RF
output jack for the board at J2.
The output from Q1 is first fed through
direction coupler Z1 before exiting the
board at J2, the RF output. The RF
sample derived from Z1 has two
functions. The first function is to provide
an RF sample at J8 on the board that is
fed to the front panel of the exciter tray
through a voltage divider consisting of
R19 and R18. 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 C14 before being
buffered and amplified by U2. The peak-
detected voltage that is used for
metering purposes is controlled by pot
R28 on the board.