RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-5
01 November 2003
applied through connector J1-5, resistor R43 and
passed to switches U1B-5 and U1C-6. The two
analog switches will turn on.
The audio information is applied through the two
analog switches and applied to the inverting gate of
operational amplifier U6A-2. A pre-set bias level,
established by voltage divider resistors R5 through
R7 is applied to the non-inverting input of U6A-3.
This level determines the maximum carrier power
output of the transmitter. The output of operational
amplifier U6A-1 is applied through resistor R8 and
passed to dynamic analog divider U3. The output of
U6A-1 can be monitored at test point TP1.
2.2.4.2
+15V to -15V Converter
: The +15V to
−
15V converter circuit is comprised of inverter gates
U2A through U2F, transistors Q8/Q9 and associated
components. Inverter gates U2A, U2B, resistors
R41/R42 and capacitor C3 form an oscillator that
produces a +15 volt DC square wave at a frequency
of approximately 36kHz. The output of U2B is
applied to parallel inverters U2C through U2F which
provide buffering for the 36kHz input from U2B and
produce a low impedance, high current drive for
transistors Q8/Q9. Balance drive transistors Q8/Q9
are gated on and off at the 36kHz switching
frequency. The output at the emitter junction of
transistors Q8/Q9 is applied through capacitor C6
and across diode CR4. Diode CR4 allows only the
negative-going portion of the square wave to remain.
Diode CR5 and capacitor C25 form a peak detector
and smoothing circuit for the unloaded -15V. Test
point TP5 provides a convenient location to monitor
the -15V (approximately -12.0V will be measured
due to the load presented by the modulator driver
circuits).
2.2.4.3
PWM Square Wave Generator
: The
pulse-width modulator square wave generator
consists of programmable timer U4 and associated
components. The oscillator frequency is adjusted to a
nominal frequency of 70kHz by
FREQ
variable
capacitor C11. The output of the generator will be a
15-volt square wave at the oscillator frequency and
will be applied to the ramp integrator circuit.
2.2.4.4
PWM Ramp Integrator
: The PWM ramp
integrator circuit consists of operational amplifier
U7A and associated components. The 15-volt square
wave from the PWM square wave generator is
applied to the inverting input of U7A. Capacitor C16
and resistor R32, which are located in the feedback
circuit of U7A, result in a linear sawtooth waveform
being produced at the output of U7A. Test point TP3
provides a convenient location to measure the
waveform.
RAMP ADJUST
potentiometer R26 is
adjusted to set the negative going peaks of the linear
waveform to a DC reference potential of zero volts.
2.2.4.5
Variable Pulse-Width Generator
: The
variable pulse-width generator circuit is a differential
amplifier that compares the linear sawtooth
waveform from the ramp integrator with the audio
signal and produces a nominal 70kHz rectangular
waveform as the pulse-width modulation signal. The
circuit comprises operational amplifier U7B,
transistors Q4, Q5 and associated components. They
are configured to form an emitter coupled differential
amplifier. A portion of the audio signal is applied to
the non-inverting input of U7B from the wiper of
O/P
POWER
potentiometer R31. Unity gain buffer
amplifier U7B applies this voltage to the base of Q5.
The linear sawtooth waveform from U7A is applied
to the base of Q4. Refer to figure 2-1 for a simplified
schematic of the differential amplifier.
For initial explanation purposes, assume the audio
signal is a DC reference voltage that does not contain
the audio component. When the linear sawtooth
waveform voltage is greater than the DC reference
voltage, Q4 will be forwarded biased and Q5 will be
reverse biased. When the linear sawtooth waveform
voltage is less than the DC reference voltage, Q5 will
be forward biased and Q4 will be reverse biased.
The output at the collector of Q5 will be
approxi15V when Q4 is forward biased and
open circuit when it is reverse biased. The
forward/reverse bias ratio of Q5 is determined by the
level of the audio signal.
When audio is superimposed on the DC reference
voltage, the audio input applied to the base of Q5 will
increase or decrease at the audio rate. The magnitude
of the change will be determined by the amplitude of
the audio component. When it is less, Q5 will be
reverse biased for a longer portion of the linear
sawtooth waveform period. When it is greater, Q5
