Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-15
1:4 impedance transformation. Resistors
R42, R59, R60, and R43 form an L-pad to
lower the level of the signal. The signal is
applied to amplifier U7 to compensate for
the loss in level through the L-pad. TP1 is
a test point that gives the operator a
place to measure the level of the in-
phase IF signal that is connected to
mixer stage Z2. The amplitude corrector
can be disabled by moving jumper W3 on
J10 to the Disable position; this will move
the breakpoint past sync tip and will have
no effect on the signal.
3.1.6.3 Output Circuit
The phase-corrected signal from pin 1 on
combiner Z2 exits the board at IF output
jack J4 after passing through a matching
network consisting of six resistors.
3.1.7 (A11) VHF Mixer/Amplifier
Enclosure Assembly (1070902;
Appendix C)
The VHF mixer/amplifier enclosure
assembly is made up of the x2 multiplier
board, the VHF filter/mixer board, and
the low-band VHF filter/amplifier board.
3.1.7.1 (A1) x2 Multiplier Board (1172-
1111; Appendix D)
The x2 multiplier board multiplies the
frequency of an RF input signal by a
factor of two. The board is made up of a
x2 broadband frequency doubler.
The input signal (+5 dBm) at the
fundamental frequency enters through
SMA jack J1 and is fed through a 3-dB
matching pad, consisting of R1, R2, and
R3, to amplifier IC U1. The output of the
amplifier stage is directed through a
bandpass filter, consisting of L1 and C4,
that is tuned to the fundamental
frequency. The voltage measured at TP1
is typically +0.6 VDC. The doubler stage
consists of Z1 with bandpass filter L2 and
C6 tuned to the second harmonic. The
harmonic is amplified by U2 and fed to
the SMA output jack of the board at J2.
The typical LO signal output level is a
nominal +15 dBm.
The +12 VDC for the board enters
through jack J3-3 and is filtered by L3
and C7 before being distributed to the
circuits on the board.
3.1.7.2 (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 board is
mounted inside of (A11) the VHF
mixer/amplifier enclosure assembly an
aluminum enclosure that provides RFI
protection. The filter/amplifier board
(1064251) is also mounted inside the
enclosure.
The LO input (+5 dBm) connects to the
board at J3 and is fed to a filter circuit.
The input to the filter consists of C11,
C12, and L5, with C12 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 LO
output jack J4. Typically, the output at
jack J4 is jumpered by a coaxial jumper
to jack J5 on the board. The LO at J5
connects to mixer Z1 at pin 1 (+14
dBm).
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
connects through a pi-type attenuator,
made up of R3, R4, and R5, before it is
connected to RF output jack J6 (-14
dBm). 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.