Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-9
current path is then established from the
+12 VDC line through Q5, the resistors
R69, R137, and the red LED DS3 (video
loss indicator), which becomes lit. When
Q5 is on, it applies a high to the gates of
Q6 and Q7. This causes them to conduct
and apply video loss fault pull-down
outputs to J18, pins 5 and 2.
The other low output of U9C and U9D is
connected through CR20 to jack J5.
Jumper W2 on J5, in the Cutback Enable
position (between pins 2 and 3),
connects the low to the base of the
forward-biased Q4. If jumper W2 is in
the Disable position, between pins 1 and
2, the auto cutback will not operate. With
Q4 biased on, a level determined by the
setting of cutback 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%.
3.1.5.5 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 contain 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.
3.1.5.6 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 includes within it 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.