Nautel XR6, Troubleshooting Manual, Page: 78 / 291

XR6/XR3 Troubleshooting Manual Detailed Circuit Descriptions
Page 2-10 Issue 3.0 2009-07-15
Linear integrator
The linear integrator circuit converts the f
PDM
square wave on its input to a triangular waveform at its
output. The triangular waveform has negative and positive voltage excursions of equal amplitude and
duration. The long R/C time constant formed by C27/R11 ensures a linear rise and fall time.
Integrator peak detector
The integrator peak detector circuit detects the positive going excursions of the linear integrator's
triangular waveform. A portion of the resulting positive dc voltage (nominally 1.8 V) at U8A's output,
is applied to the inverting input of differential amplifier U3A. The
carrier ref + audio output of U3A is
offset by this voltage which is proportional to the triangular waveform voltage peaks. This offset
effectively sets the
carrier ref + audio 's zero power reference to the triangular waveform's peak voltage.
This technique ensures no RF output is produced when the carrier ref input (J1-2) is 0 V. It also
ensures the RF output is reduced to near 0 W at the 100% modulation envelope valleys when
operating at the transmitter's rated carrier level.
Interphase pdm generator
The interphase PDM generator consists of two variable pulse duration generators (PDM1 and
PDM2). Both produce identical 0 V to +15 V rectangular waveforms at the PDM repetition rate, but
they are 180
0
out of phase with each other. The waveform on/off ratio (duty cycle) is directly
proportional to the carrier level and amplitude of the modulating audio.
PDM1 generator
The PDM1 generator consists of U10B, Q4, Q5, and their associated components. The compensated
carrier ref + audio input from the integrator peak detector circuit is applied to differential amplifier
U10B's non-inverting input, where it is compared to the triangular waveform from the linear
integrator circuit. When the compensated
carrier ref + audio is more positive than the triangular
waveform, U10B's output is an open collector. +15 V is applied through R60 and R62 to the
balanced drive formed by Q4/Q5. Q4 turns on and a15 V to K1-3 as the
PDM1 output (Q5
is off). For the remainder of the triangular waveform's period, the compensated carrier ref + audio is
less positive than the triangular waveform. U10B's output is a current-sink-to -15 V and Q4 turns off.
Q5 turns on and clamps the
PDM1 output to ground. The minimum 'on time' (zero RF power)
occurs at the negative peaks of the triangular waveform. See Figure 2.1 on page 2-11 for a timing
diagram.