UHF Analog Driver/Transmitter/
Chapter 4, Circuit Descriptions
Translator
LX Series, Rev. 3
4-11
+12 VDC line, and through R5, CR1,
CR2, and R9, biases series element CR3
off and 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.
4.3.1.6 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 the modular amplifier U1. This
device contains the biasing and
impedance-matching circuits that makes
it operate as a wide-band IF amplifier.
The output of U1 connects to J40 that is
jumpered to J41. The J40 jack is
available, as a sample of the pre-
correction IF for troubleshooting
purposes and system setup. The IF signal
is connected to a splitter Z1 that has an
In Phase output and a 90° Quadrature
output, which are then connected to the
linearity corrector portion of the board.
4.3.1.7 Amplitude and Phase
Pre-Correction Circuits
The linearity corrector circuits use three
stages of correction, two adjust for any
amplitude non-linearities and one for
phase non-linearities of the output signal.
Two of the stages are in the In Phase
Amplitude pre-correction path and one
stage is in the Quadrature Phase pre-
correction path. Each stage has a
variable threshold control adjustment,
R211 and R216, in the In Phase path,
and R231, in the Quadrature path, that
determines the point at which the gain is
changed for that stage.
Two reference voltages are needed for
the operation of the corrector circuits.
The Zener diode VR3, through R261,
provides the +6.8 VDC reference. The
VREF is produced using the path through
R265 and the diodes CR30 and CR31.
They provide a .9 VDC reference, which
temperature compensates for the two
diodes in each corrector stage.
The first corrector stage in the In Phase
path operates as follows. The In Phase IF
signal is applied to transformer T6, which
doubles the voltage swing by means of a
1:4 impedance transformation. Resistors
R222 and R225 form an L-pad that lowers
the level of the signal. The input signal
level, when it reaches a set level, causes
the diodes CR24 and CR25 to turn on,
generating current flow that puts them in
parallel with the L-pad. When the diodes
are put in parallel with the resistors, the
attenuation through the L-pad is lowered,
causing signal stretch.
The signal is next applied to amplifier U17
to compensate for the loss through the
L-pad. The breakpoint, or cut-in point, for
the first corrector is set by controlling
where CR24 and CR25 turn on. This is
accomplished by adjusting the threshold
cut-in resistor R211. R211 forms a
voltage-divider network from +6.8 VDC to
ground. The voltage at the wiper arm of
R211 is buffered by the unity-gain
amplifier U16B. This reference voltage is
then applied to R215, R216, and C134
through L44 to the CR24 diode. C134
keeps the reference from sagging during
the vertical interval. The .9 VDC reference
voltage is applied to the unity-gain
amplifier U16D. The reference voltage is
then connected to diode CR25 through
choke L45. The two chokes L44 and L45
form a high impedance for RF that serves
to isolate the op-amp ICs from the IF.
After the signal is amplified by U17, it is
applied to the second corrector stage in the
In Phase path through T7. These two
correctors and the third corrector stage in
the Quadrature path operate in the same
fashion as the first. All three corrector
stages are independent and do not interact
with each other.
The correctors can be disabled by moving
the jumper W12 on J30 to the Disable
position, between pins 1 and 2, this moves
