µP-Controlled Oscillator Delivers Rock-Bottom Distortion

Function generators often play a critical role in the design, testing, and operation of encoders, modulators, demodulators, and measurement instruments. Here`s an inexpensive way to build a bus-controlled sinewave oscillator that has downright low distortion. The circuit generates a sinusoidal output with typical second and third harmonics down fr
µP-Controlled Oscillator Delivers Rock-Bottom Distortion - schematic

om the fundamental by -76. 1 and -74. 2 dB, respectively, across its full output range of 10 Hz to 10 kHz. That performance represents better than a 40-dB improvement over common diode-shaped sinewave generators, which employ a diode-shaping technique to transform a square wave into a sinewave. Typically, their second-and third-order harmonics are down from the fundamental by -35 and -25. 5 dB, respectively. The circuit consists of four sections ( see the figure ). At the heart of the design, the first one is the oscillator comprising an IC dual-filter building block (U1), a second-order clocked filter (whose bandpass-filter section sets the oscillator`s frequency), and a comparator (U2A). The bandpass filter determines the oscillator`s frequency by allowing signals only around its center frequency to pass. Equation 1 shows the oscillator frequency, and Equation 2 the filter The second circuit section is the tracking notch filter, which is set to and tracks the oscillator`s third harmonic, which is the higher-amplitude harmonic. The tracking filter is synchronously clocked with the oscillator`s frequency-setting filter to provide lock-step oscillator-tracking filter-response characteristics. The third section has a buffer amplifier (U3A) with a gain of -1. This section includes a 13. 3-kHz low-pass filter to reduce the high-frequency component generated by the clocking steps in the output waveform. The fourth section is a...

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