Nautel XR12, Troubleshooting Manual, Page: 84 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Page 2-16 Issue 3.0 2009-07-28
Audio chopper
The audio chopper circuit consists of U33D, Q1, and their associated components. It inhibits some
portion of positive audio half-cycles that cause modulation peaks/durations that exceed the RF stress
current threshold of the transmitter's RF power amplifiers. It accomplishes this by clamping the
modulating audio to ground whenever the charge voltage on C137 switches from a positive value to a
negative value and retains this state until C137's charge voltage returns to a positive value at the end
of the positive audio half-cycle.
When C137 has a positive voltage charge, U33D's non-inverting input is more positive than its
inverting input. U33D's output is an open collector. Q1 is reverse biased and has no influence on the
modulating audio. The
Mod Protection output (J13-31) is open collector (logic negative state). When the
modulation level detector circuit provides C137 a discharge path to -15 V (because one or more RF
stress current thresholds have been exceeded) for a long enough period of time to achieve a negative
voltage charge, U33D's non-inverting input will go less positive than its inverting input. U33D's
output is a current-sink-to -15V and causes Q1 to be forward biased (turned on). The modulating
audio is clamped to ground by Q1 until the start of the next negative audio half-cycle (for the balance
of the current positive audio half-cycle), when the zero crossing detector circuit provides a low
impedance source to +15 V and charges C137 to a positive value. The
Mod Protection output is a
current-sink-to-ground (logic true state) for each offending positive audio half-cycle. The system
diagram's
Exciter
lamp will flash on and off when this circuit is active.
Audio gain
U29A is connected as a unity gain, buffer amplifier.
AUDIO GAIN
potentiometer R160 is adjusted for
the desired modulation depth when the interphase PDM driver PWB is installed in its host
transmitter. It has sufficient range to set the modulation depth at 100% when the audio input is
between 0 dBm and +12 dBm.
Audio filter
The audio filter circuit is a low pass filter with a -1.0 dB, high frequency roll-off at one of four user-
selected frequencies. The
AUDIO FILTER
switch S1 has settings to select one of four roll-off
frequencies (16.0 kHz, 13.5 kHz, 10.5 kHz or 7.5 kHz). The high frequency roll-off selection is
normally dictated by complex factors, such as an antenna with sideband limitations, square wave
overshoot, or the equipment that pre-processes the audio. The
AUDIO FILTER
switch and audio filter
are bypassed when IBOC program input is used.
Magnitude gain
MAG GAIN
potentiometer R127 is adjusted for the correct magnitude signal when connected to an
NE IBOC exciter for IBOC operation.