UHF Analog Driver/Transmitter/
Chapter 4, Circuit Descriptions
Translator
LX Series, Rev. 3
4-10
level increases, until normal output
power is reached.
The input level at TP3 is also fed to a
pulse detector circuit, consisting of IC U8,
CR17, Q3, and associated components,
and then to a comparator circuit made up
of U9C and U9D. The reference voltage
for the comparators is determined by a
voltage divider consisting of R243, R65,
R66, and R130, off the -12 VDC line.
When the input signal level to the
detector at TP3 falls below this reference
threshold, which acts as a loss-of-signal
peak detector circuit, the output of U9C
and U9D goes towards the -12 VDC rail
and is split, with one part biasing on
transistor Q5. A current path is then
established from the +12 VDC line
through Q5, the resistors R69 and R137,
and the red LED DS3, input loss
indicator, which is illuminated. When Q5
is on, it applies a high to the gate of Q6.
This causes it to conduct and apply a
modulation loss pull-down output to
J42C, pin 7C, which is applied to the
front panel display on the
Control/Monitor module.
The other low output of U9C and U9D is
connected through CR18, CR19 & CR20
to jack J5. Jumper W2 on J5, in the
Cutback Enable position, which is
between pins 2 and 3, connects the low
to the base of Q4 that is now forward-
biased.
NOTE:
If jumper W2 is in the
Disable position, between pins 1 and 2,
the auto cutback will not operate. With
Q4 biased on, a negative level
determined by the setting of cutback
level pot R71 is applied to U24D, pin 12.
The level is set at the factory to cut back
the output to approximately 25%. The
output of U24D at pin 14 goes low and is
applied through the power adjust pot to
U24C, pin 9, whose output goes low.
The low connects to U24B, pin 5, whose
output goes low. The low then connects
to U24A, pin 2, whose output goes high.
The high is applied to U10A, pin 2, whose
output goes low. The low connects
through the switch SW1, if it is in the
auto gain position, to the pin-diode
attenuator circuit, CR1, CR2 & CR3. The
low reverse biases them and cuts back the
level of the output to approximately 25%.
4.3.1.5 Pin-Diode Attenuator Circuit
The input IF signal is fed to a pin-diode
attenuator circuit that consists of CR1, CR2
& 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 the switch SW1 is in the ALC Auto
position, between pins 2 and 3, or from pot
R87 when SW1 is in the Manual Gain
position, between pins 1 and 2.
In the pin diode attenuator circuit,
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
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Страница 99: ...APPENDIX A LX SERIES ANALOG SYSTEM SPECIFICATIONS...
Страница 102: ...APPENDIX B DRAWINGS AND PARTS LISTS...
Страница 105: ...APPENDIX C TRANSMITTER LOG SHEET...
