User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-17
The amount of stretch is determined by
the adjustment of R35.
The signal is next applied to the amplifier
U5 to compensate for the loss in level
through the L-pad. The breakpoint, or
cut-in point, for the corrector stage is set
by controlling where CR8 and CR9 turn
on. This is achieved by adjusting the
cut-in resistor R31 to form a voltage
divider from +6.8 VDC to ground. The
voltage at the wiper arm of R31 is
buffered by the unity-gain amplifier U8B.
This voltage is then applied to R34
through L11 to the CR9 diode. The .9
VDC reference created by CR11 and
CR12 is applied to the unity-gain
amplifier U8A. C36 keeps the reference
from sagging during the vertical interval.
The reference voltage is then connected
to the diode CR8 through choke L12.
The two chokes L11 and L12 form a high
impedance for RF to isolate the op-amp
ICs from the RF.
After the signal is amplified by U5, it is
applied to a second stage through T5.
The transformer doubles the voltage
swing by means of a 1:4 impedance
transformation. Resistors R39, R57,
R58, and R40 form an L-pad that delays
the signal to match the Quadrature input
path. The signal is then applied to
amplifier U6 that compensates for the
loss in level through the L-pad and
provides unity gain. 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
that delays the signal to match the
Quadrature input path. The signal is
then applied to amplifier U7 to
compensate for the loss in level through
the L-pad. The in-phase corrected IF
signal is connected to pin 5 on the
combiner stage Z2. TP1 is a test point
that gives the operator a place to
measure the level of the in-phase IF
signal. The amplitude corrector can be
disabled by moving the jumper W3 on
J10 to the Disable position, between pins
2 and 3, which will move the breakpoint
past sync tip and the circuit will then
have no effect on the signal.
4.1.6.3 Output Combiner Circuit, Z2
The Quadrature, phase, corrected input
on pin 6 and the In phase, amplitude,
corrected input on pin 5 are combined in
Z2. The phase-corrected signal from pin
1 on combiner Z2 exits the board at the
IF output jack J4 after passing through a
pad network consisting of six resistors,
R62-R67, that provides a unity gain,
through the board, output level.
4.1.7 (A11) VHF Mixer/Amplifier
Enclosure Assembly, Low Band
(1070902; Appendix C)
The VHF mixer/amplifier enclosure
assembly is an aluminum enclosure that
provides RFI protection for the x2
multiplier board, the VHF filter/mixer
board, and the low-band VHF
filter/amplifier board, which are mounted
inside the enclosure.
4.1.7.1 (A1) x2 Multiplier Board
(1172-1111; Appendix D)
The x2 multiplier board multiplies the
frequency of the RF from the channel
oscillator by a factor of two. The board is
made up of a x2 broadband frequency
doubler.
The input signal, typically +5 dBm, at the
fundamental frequency enters through
the SMA jack J1 and is fed through a 3-
dB matching pad, consisting of R1, R2,
and R3, to the 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 RF is next connected to the doubler
stage that consists of Z1 and the
bandpass filter, L2 and C6, which is
tuned to the second harmonic. The
second harmonic is amplified by U2 and
fed to the SMA output jack of the board