User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-12
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.
4.3.7.6 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.
4.3.7.7 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