RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-6
01 November 2003
will be reverse biased for a shorter portion of the
linear sawtooth waveform period. When an audio
component is present, the resulting pulse-width
modulated output at the collector of Q5 is a varying
width rectangular waveform at the repetition rate of
the square wave generator (nominally 70kHz). This
waveform can be monitored at test point TP4.
2.2.4.6
Linear Attenuator
: The linear attenuator
circuit consists of operational amplifier U5A and
unity gain amplifier U5B, transistors Q1/ Q2 and
their associated components. Operational amplifier
U5B and transistor Q2 form a diode to ground circuit
which results in a bias voltage of approximately 0.6V
being applied to the emitter of transistor Q1.
Normally the control voltage applied to the non-
inverting input of U5A from
VSWR cutback
on J1-4
is zero volts. The zero volt control signal is applied
thru unity gain amplifier U5A and passed to the base
of transistor Q1. Transistor Q1 will be reversed
biased and turn off. The linear attenuator circuit will
present high impedance between the audio signal and
ground.
When a
VSWR cutback
positive control signal is
applied to the non-inverting input of U5A, the output
of U5A will be positive. Transistor Q1 will be
forward biased and turn on. The impedance of the
linear attenuator circuit will begin to decrease in
proportion to the current flow through Q1. The
attenuation factor will vary in proportion to the
VSWR cutback
control signal applied from J1-4.
2.2.4.7
B+ Variation Compensation
: The B+
variation compensation circuit is comprised of
monolithic analog multiplier U3 (connected as a
dynamic analog divider), operational amplifier U6B
and associated components. The compensation
circuit ensures the modulation depth and output
power remain constant if variations on the B+ line
occur and also provides a means of adjusting the
output power through the increase/decrease of the
remote power trim
voltage (+15V maximum).
The keyed tone (audio), B+ voltage and power trim
inputs are applied to analog divider U3. The output
of U3 is passed to the inverting gate of operational
amplifier U6B. The resultant output on U6B-7 can
be monitored at test point TP2. Any increase or
decrease of the B+ voltage on U3-5 will result in a
proportional increase or decrease on the output of
operational amplifier U6B-7. The same results are
applicable with an increase or decrease of the power
trim input on U3-1 as those mentioned for the B+
voltage.
2.2.4.8
Low RF Drive Cutback
: The low RF
drive cutback circuit is comprised of operational
amplifier U9A, inverter/buffers U8A/U8B, transistor
Q3 and their associated components. A DC voltage,
proportional to the RF drive level, is applied through
connector J1-1 and passed to the non-inverting input
of operational amplifier U9A-3. The bias level on the
inverting input of U9A-2 is developed through
voltage divider resistors R33/R47/R34. During
initial turn on, transistor Q3 will be forward biased
and turn on. The RF drive level being applied to
U9A-3, is insufficient to overcome the bias level on
U9A-2. The non-inverting input to U9A-3 will be at
ground level and U9A's output will be low.
The low output from U9A will be applied through
inverter U8A-3/2 and provide a high through diode
CR3 to input of inverter U8A-7 which will inhibit the
mod drive. The high output on U8A-2 will also be
applied through inverter U8B-5/4 and passed to
RF
drive alarm
connector J2-5. The
low
alarm control
signal will be applied to the
RF DRIVE ALARM
lamp
on the front panel. The RF drive alarm lamp will
turn on. After a nominal five seconds, capacitor C14
is charged to +15V (through resistor R23) and
transistor Q3 will turn off. The
RF drive level
voltage on U9A-3 will exceed the bias level on U9A-
2 and the output of U9A-1 will go high The output
of inverter U8B-2 will be low and the high being
applied through diode CR3 to U8A-7 will go low.
The mod drive will no longer be inhibited. The
output of U8B-4 will go high and the RF drive alarm
lamp will turn off.
