Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-15
level pot R71, which is set at the
factory to cut back the output to
approximately 25%, is applied to U9B,
pin 5. The output of U9B at pin 7 goes
low and is applied through the power
adjust pot to U10A, pin 2, whose
output goes low. This low is applied to
the pin-diode attenuator to cut back
the level of the output to approximately
25%.
4.2.5.5 Pin-Diode Attenuator Circuit
The input IF signal is fed to a pin-diode
attenuator circuit that consists of CR1
to CR3. Each of the pin diodes contains
a wide intrinsic region; this makes the
diodes function as voltage-variable
resistors at this intermediate
frequency. The value of the resistance
is controlled by the DC bias supplied to
the diode. The pin diodes are
configured in a pi-type attenuator
configuration where CR1 is the first
shunt element, CR3 is the series
element, and CR2 is the second shunt
element. The control voltage, which can
be measured at TP1, originates either
from the ALC circuit when jumper W3
on J6 is in the ALC Auto position,
between pins 1 and 2, or from pot R87
when the jumper is in the Manual Gain
position.
On the pin-diode attenuator circuit, a
current path exists from J6 through R6
and then through the diodes of the pin
attenuator. Changing the amount of
current through the diodes by forward
biasing them changes the IF output
level of the board. There are two
extremes of attenuation ranges for the
pin-diode attenuators. In the minimum
attenuation case, the voltage,
measured at TP1, approaches the +12
VDC line. There is a current path
created through R6, through 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. In addition, the larger
current flow increases the voltage drop
across R9 that tends to turn off diodes
CR1 and CR2 and causes them to act
as high-value resistors. In this case,
the shunt elements act as a high
resistance and the series element acts
as a low resistance to represent the
minimum loss condition of the
attenuator (maximum signal output).
The other extreme case occurs as the
voltage at TP1 is reduced and goes
towards ground or even slightly
negative. This tends to turn off (reverse
bias) diode CR3, the series element,
causing it to act as a high-value
resistor. An existing fixed current path
from the +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.
4.2.5.6 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 contains the biasing and
impedance-matching circuits that
makes it operate as a wide-band IF
amplifier. The output of U1 is available,
as a sample of the pre-correction IF for
troubleshooting purposes and system
setup, at jack J2. The IF signal is then
connected to the linearity corrector
portion of the board.
4.2.5.7 Linearity Corrector Circuits
The linearity corrector circuits use three
stages of correction to adjust for any
amplitude non-linearities of 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