User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-15
loss in level through the L-pad. After the
signal is amplified by U6, it is applied to a
third stage through T6. The transformer
doubles the voltage swing by means of a
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 the jumper
W3 on J10, to the Disable position. This
will move the breakpoint past sync tip
and will have no effect on the signal.
4.1.6.3 Output Circuit
The phase-corrected signal from pin 1 on
combiner Z2 exits the board at the IF
output jack J4, after passing through a
matching network consisting of six
resistors.
4.1.7 (A11) VHF Mixer/Amplifier
Enclosure Assembly (1088067;
Appendix C)
The VHF mixer/amplifier enclosure
assembly is made up of the x4 multiplier
board, the VHF filter/mixer board, and
the VHF high band filter/amplifier board.
4.1.7.1 (A11-A1) x4 Multiplier Board
(1174-1112; Appendix D)
The x4 multiplier board multiplies the
frequency of an RF input signal by a
factor of four. The board is made up of
two identical x2 broadband frequency
doublers.
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 L2 and C4,
that is tuned to the fundamental
frequency. The voltage measured at TP1
is typically +0.6 VDC. The first doubler
stage consists of Z1 with bandpass filter
L3 and C6 tuned to the second harmonic.
The harmonic is amplified by U2 and fed
through a bandpass filter, consisting of
C10 and L5, also tuned to the second
harmonic frequency. The voltage
measured at TP2 is typically +1.2 VDC.
The next doubler stage consists of Z2
with bandpass filter C12 and L6 tuned to
the fourth harmonic of the fundamental
frequency. The fourth harmonic is then
amplified by U3 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 L7
and C16 before being distributed to the
circuits on the board.
4.1.7.2 (A11-A2) VHF Filter/Mixer
Board (1150-1102; 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
(1088067), an aluminum enclosure that
provides RFI protection. The
filter/amplifier board (1064252) 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. Capacitor C15 is
adjusted for the best coupling. 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).