Nautel XR12 Troubleshooting Manual

Also see for XR12: Installation manual Operation and maintenance manual Preinstallation manual

Contents

XR12 Troubleshooting Manual Detailed Circuit DescriptionsPage 2-10 Issue 3.0 2009-07-28Linear integratorThe linear integrator circuit converts the f PDM square wave on its input to a triangular waveform at itsoutput. The triangular waveform has negative and positive voltage excursions of equal amplitude andduration. The long R/C time constant formed by C27/R11 ensures a linear rise and fall time.Integrator peak detectorThe integrator peak detector circuit detects the positive going excursions of the linear integrator'striangular 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 isoffset by this voltage which is proportional to the triangular waveform voltage peaks. This offseteffectively 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 alsoensures the RF output is reduced to near 0 W at the 100% modulation envelope valleys whenoperating at the transmitter's rated carrier level.Interphase pdm generatorThe interphase PDM generator consists of two variable pulse duration generators (PDM1 andPDM2). Both produce identical 0 V to +15 V rectangular waveforms at the PDM repetition rate, butthey are 180 0 out of phase with each other. The waveform on/off ratio (duty cycle) is directlyproportional to the carrier level and amplitude of the modulating audio.PDM1 generatorThe PDM1 generator consists of U10B, Q4, Q5, and their associated components. The compensatedcarrier ref + audio input from the integrator peak detector circuit is applied to differential amplifierU10B's non-inverting input, where it is compared to the triangular waveform from the linearintegrator circuit. When the compensated carrier ref + audio is more positive than the triangularwaveform, U10B's output is an open collector. +15 V is applied through R60 and R62 to thebalanced drive formed by Q4/Q5. Q4 turns on and applies +15 V to K1-3 as the PDM1 output (Q5is off). For the remainder of the triangular waveform's period, the compensated carrier ref + audio isless 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 timingdiagram.