Low distortion controlled oscillator circuit schematic

This article describes a low-cost method for constructing a low-distortion, bus- controlled sine wave oscillator.

This circuit produces a sinusoidal output with typical second and third harmonics -76.1dB and -74.2 dB below the fundamental frequency signal, respectively, over the full output range of 10Hz to 10kHz. The performance of this sine wave oscillator is 40dB higher than that of a common diode-shaped sine generator that uses diode shaping to convert a square wave to a sine wave. Typically, the second or third harmonic of a diode-shaped sine generator is -35dB and -25.5dB below the fundamental frequency, respectively.

This circuit consists of four parts (as shown in the figure). The core of the design, or the first part, is the oscillator, which includes the dual filtering building block IC (U1), the second-order clock filter (the bandpass filter portion of which sets the oscillator's frequency), and the comparator (U2A). The bandpass filter determines the oscillator's frequency by allowing only signals near the oscillator's center frequency to pass. Equation 1 gives the oscillator's frequency, and Equation 2 shows the Q value of the filter.

The second circuit section is a tracking notch filter that is set and tracks the third harmonic of the oscillator, which is a higher amplitude harmonic. The tracking filter is synchronously clocked with the oscillator's frequency setting filter, providing a lock-step oscillator tracking, filter response characteristic.

Equation 3 gives the tracking filter -3dB cutoff point, and Equation 4 represents the notch filter frequency.

The third section has a buffer amplifier (U3A) with a gain of -1. This section includes a 13.3kHz low pass filter that is used to reduce the high frequency components generated by the clock steps in the output waveform.

The fourth section is a bus controlled clock generator which consists essentially of an IC serial port programmable oscillator (U4) which can be either the LTC6903 for the Serial Peripheral Interface (SPI) or the LTC6904 for the Inter-IC (I2C) interface . Some pull-up resistors , decoupling capacitors , and a series resistor at the output are the only external components required.

Furthermore, the circuit can be easily adapted to produce a quadrature, sine/cosine waveform output by simply adding a second output op amp and accepting its input from the bandpass output of U1 (BPB at pin 11).