500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-7
15 January 2005
level and audio information. Logic circuits monitor
RF drive level and modulation pulse width output.
They turn on the appropriate
MOD DRIVE
and/or
RF
DRIVE ALARM
lamp if a fault threshold is exceeded.
The
AUDIO LIMITER
lamp will flash when audio
peak/trough thresholds are exceeded.
2.5.9.1
Voice Audio Filter:
The
voice audio
input
on J1-4 is applied across varistor RV1 and
VOICE
LEVEL
potentiometer R2 and passed to the non-
inverting input of operational amplifier U1A.
Varistor RV1 provides protection against high-level
interference transients (e.g., lightning). Op amps
U1A/U1B and their associated components form low
and high pass four-pole filter circuits. The filters
define the lower and upper limits of the audio
bandwidth of the subject transmitter at 300 to 3000
Hz respectively.
2.5.9.2
Voice Audio Amplifier/Limiter:
Resistor
R14 in series with the voice signal forms an attenuator
with JFET Q1 which acts as an audio limiter and is
turned on by unusually high peaks of the audio signal.
This improves the voice power of the transmitter by
compressing high voice peaks and preventing over-
modulation on high peaks. Limiting of both positive
and negative peaks of voice is provided since after
amplification by operational amplifier U2A, the total
signal is applied to inverting comparator U3B and
non-inverting comparator U3A which turn on JFET
Q1 when comparator thresholds are exceeded by
voice signal peaks. Operation of either comparator
also switches U3D which will turn on the
AUDIO
LIMITER
lamp, indicating the occurrence of a voice
peak and limiter action. The voice signal is then
applied through
VOICE MOD %
potentiometer R21 and
passed to emission mode multiplexer U4. Test point
TP2 provides a monitoring point for the voice signal
after filtering and limiting.
2.5.9.3 Emission
Mode
Multiplexer:
The
voice
audio
and/or the
keyed tone
are selected as the
modulating audio output, for each emission mode, by
emission mode multiplexer U4. The binary sum of
the
mod enable
and
press-to-talk
inputs select the
appropriate gate of U4 (0/1-no audio, 2-keyed tone
and voice, 3-keyed tone only). The level of the keyed
tone audio is adjusted by the setting of
TONE LEVEL
potentiometer R19 determines the keyed tone level
when keyed tone only is selected. The setting of
TONE MOD %
potentiometer R20 determines its level
when keyed tone and voice are selected.
VOICE MOD
%
potentiometer R21 is adjusted for the voice
modulation level. A ground potential
press-to-talk
command will apply a logic 0 to U4-A.
2.5.9.4 Line Volts Compensation/Power Trim
Control:
Line volts compensation and power trim
control circuits contain wideband monolithic analog
multiplier U6, operational amplifiers U5A, U5B and
U2B and their associated components. The audio
output of the emission mode multi-plexer, the
B-VDC
and
power trim
inputs are converted to current inputs
and applied to U6-8/5/1 respectively. The output on
U6-4 is applied to U5B and converted to a voltage on
U5B's output. If the B- voltage decreases, the output
voltage at U5B-7 will increase proportionally,
similarly if the B- voltage increases, the voltage on
U5B-7 will decrease proportionally. When the
power
trim
control signal on J2-2 decreases, the output
voltage on U5B-7 will decrease and visa-versa.
Resultant modulation depth will remain constant
regardless of B- voltage variations and power trim
variations to -3dB.
2.5.9.5 PWM Square-Wave Generator:
The
pulse-width modulator square wave generator
contains programmable timer U8 and its associated
components. The oscillator frequency is adjusted to a
nominal frequency of 70 kHz by
FREQ
variable
capacitor C22. 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.
2.5.9.6 Ramp
Integrator:
The ramp integrator
circuit contains operational amplifier U9B plus its
associated components. The 15 volt square wave
from the PWM square wave generator is applied to
the inverting input of U9B. Capacitor C26 and
resistor R63, which are located in the feedback circuit
of U9B, result in a linear sawtooth waveform being
produced at the output of U9B. TP5 provides a
convenient location to measure the waveform.
RAMP
ADJUST
potentiometer R58 is adjusted to set the
negative going peaks of the linear waveform to a DC
reference potential of zero volts.
