User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-18
at J2. The typical LO signal output level
is +5 dBm. The voltage measured at TP2
is typically +0.6 VDC.
The +12 VDC for the board enters
through jack J3-3 and is filtered by L3,
C2 and C7 before being distributed to the
rest of the board.
4.1.7.2 (A2) VHF Filter/Mixer Board
(1153-1101; Appendix D)
The VHF filter/mixer board is made up of
three separate circuits: a filter and
amplifier circuit for the LO input, a mixer
stage, and a filter and amplifier for the
RF output of the mixer.
The LO input, +5 dBm, from the x2
multiplier connects to the board at J3 and
is fed to a filter circuit. The input to the
filter consists of C11, C12, L5, and C12
that is adjusted for the best input
loading. C13 and C17 are adjusted for
the best frequency response and C18 is
adjusted for the best output loading of
the LO signal. The filtered LO is
amplified by U2 and connected to the LO
output jack J4, typically+14 dBm.
Normally, the output at jack J4 is
jumpered by a coaxial cable to jack J5 on
the board. The LO Input, +14 dBm, at
J5 connects to the mixer Z1 at pin 1.
The combined IF input, typically,
-3 to 0 dBm, from the ALC board,
connects to the board at J7 and is fed to
mixer Z1 at pin 3.
Mixer Z1 takes the LO input at pin 1 and
the IF input at pin 3 and produces an RF
output at pin 8. The RF output at pin 8
connects through a pi-type attenuator,
made up of R3, R4, and R5, before it is
connected to RF output jack J6, typically
-14 dBm in level. Normally, jack J6 is
connected by a coaxial cable to J1 on the
board. The RF from J1 is wired 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 frequency response,
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
to 0 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 (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,
typically 0 dBm, and is fed through a
channel filter circuit. The input filter
consists of C31, 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, which 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 to them 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 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