User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-9
series diode CR3, and finally through R9
to ground. This path forward biases CR3
and causes it to act as a relatively low-
value resistor. The larger current flow
increases the voltage drop across R9 and
tends to turn off the diodes CR1 and
CR2, causing them to act as high-value
resistors. In this case, the shunt
elements act as high resistance and the
series element acts as low resistance,
which represents the minimum loss
condition of the attenuator, maximum
signal output.
The other extreme case occurs as the
voltage at TP1 is reduced, going towards
ground or even slightly negative. This
tends to turn off and reverse bias diode
CR3, the series element, which causes it
to act as a high-value resistor. An
existing fixed-current path from the +12
VDC line through R5, CR1, CR2, and R9
biases the series element CR3 off and the
shunt elements, diodes CR1 and CR2, on,
causing them to act as relatively low-
value resistors. This represents the
maximum attenuation case of the pin
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.
4.1.7.6 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 modular amplifier U1. This
device includes the biasing and
impedance-matching circuits that allows
it operate as a wideband IF amplifier. The
output of U1 is available at jack J2, as a
sample of the pre-correction IF, for
troubleshooting purposes and system
set-up. The IF signal is then connected to
the linearity corrector portion of the
board.
4.1.7.7 Linearity Corrector Circuits
The linearity corrector circuits adjust for
any amplitude non-linearities of the IF
signal using three stages of correction.
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 circuits. Zener diode VR1,
with R33 and R135, provides a +6.8-VDC
reference and diodes CR11 and CR12
provide a .9-VDC reference that
temperature compensates for the two
diodes in each corrector stage.
When the linearity correctors are
operating, the 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 the level is lowered can be
adjusted with R13, in parallel with the L-
pad resistors. R13 is only in parallel when
the signal reaches a level large enough to
turn on 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 which is determined by
the adjustment of R13. The signal is next
applied to amplifier U2, which
compensates for the loss through the
L-pad.
The breakpoint, cut-in, for the first
corrector is set by controlling where CR4
and CR5 turn on. This is accomplished by
adjusting 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 unity-
gain amplifier U5D. This reference
voltage 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 unity-gain amplifier U5B. The