User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-12
attenuator (minimum signal output). By
controlling the value of the voltage
applied to the pin diodes, the IF signal
level is maintained at the set level.
4.1.5.8 Main IF Signal Path (Part 2 of 3)
When the IF signal passes out of the pin-
diode attenuator through C11, it is
applied to the modular amplifier U1. This
device includes within it the biasing and
impedance matching circuits that makes
it operate as a wide-band IF amplifier.
The output of U1, at jack J2, is available,
as a sample of the pre-correction IF for
troubleshooting purposes and system
setup. The IF signal is then connected to
the linearity corrector portion of the
board.
4.1.5.9 Linearity Corrector Circuits
The linearity corrector circuits use three
stages of correction to correct for any
amplitude non-linearities in the IF signal.
Each stage has a variable threshold
control adjustment, R34, R37, or R40,
and a variable magnitude control
adjustment, R13, R18, or R23. The
threshold control determines the point at
which the gain is changed and the
magnitude control determines the
amount of gain change that occurs once
the breakpoint is reached. Two reference
voltages are needed for the operation of
the corrector stages. The Zener diode
VR1 using R33 and R135 provides a +6.8
VDC reference from +12 VDC. The
diodes CR11 and CR12 provide a .9 VDC
reference that temperature compensates
for the two diodes in each corrector
stage.
The linearity corrector stages begin
operation when an IF signal is applied to
transformer T1, which doubles the
voltage swing by means of a 1:4
impedance transformation. Resistors
R14, R15, and R16 form an L-pad that
lowers the level of the signal. The
amount that the level is lowered is
adjusted by adding more or less
resistance, using R13, in parallel with the
L-pad resistors. R13 is only in parallel
when the signal reaches a level large
enough to turn on the diodes CR4 and
CR5. When the diodes turn on, current
flows through R13, putting it in parallel
with the L-pad. When R13 is put in
parallel with the resistors, the
attenuation through the L-pad is lowered,
causing signal stretch. The amount of
stretch is determined by the adjustment
of R13. The signal is next applied to the
amplifier U2 that compensates for any
loss through the L-pad. The breakpoint,
or cut-in point, for the first corrector
stage is set by controlling where CR4 and
CR5 turn on. This is accomplished by
adjusting the cut-in resistor R34, which
forms a voltage-divider network from
+6.8 VDC to ground. The voltage at the
wiper arm of R34 is buffered by the
unity-gain amplifier U5D. This reference
voltage output of U5D is then applied to
R35, R36, and C39 through L12 to the
CR4 diode. C39 keeps the reference
from sagging during the vertical interval.
The .9 VDC reference created by CR11
and CR12 is applied to the unity-gain
amplifier U5B. The reference voltage is
then connected to the diode CR5 through
the choke L11. The two chokes L11 and
L12 form a high impedance for IF that
serves to isolate the op-amp ICs from the
IF.
After the signal is amplified by U2, it is
applied to the second corrector stage
through T2. The second and the third
corrector stages operate in the same
fashion as the first. All three corrector
stages are independent and do not
interact with each other.
The corrector stages are disabled by
moving the jumper W1 on J4 to the
Disable position, which is between pins 2
and 3. This moves all of the breakpoints
of the stages past the tip of sync so that
if adjusted they will have no affect on the
IF signal.
The IF signal exits the board at the IF
output jack J3 (0 dBm) after passing
through the three corrector stages. A