Design Ideas: July 7, 1994

The circuit in Fig 1 demonstrates a simple method for generating a voltage-programmable sawtooth waveform having a dynamic range greater than 70 dB. In addition to the sawtooth waveform, the circuit also produces corresponding triangle- and square-wave outputs. The three ICs and associated components cost a total of about $13. IC₁'s astable-multivibrator-type V/F converter produces a square wave whose frequency is accurately proportional to the converter's analog input and produces a differential 1.8 V p-p triangle wave of equal frequency across its timing capacitor C_T. IC₂'s video-difference amplifier level shifts and amplifies this square wave to produce a 20 V p-p triangle wave. A high-pass filter differentiates this signal to generate an in-phase square wave. The triangle wave and differentiated signal drive the analog input and reference-comparator inputs of IC₃'s gain-of-one balanced modulator.

IC₃'s output under these circumstance is a 20 V p-p sawtooth waveform having a positive slope and twice the original frequency of the triangle wave. IC₁'s output is a square wave with dc offset; ac coupling level-shifts the signal and produces a symmetrical square wave having no dc level shifts with a change in frequency. Although the full-scale frequency of IC₁ can be set as high as 500 kHz by the proper selection of R_S and C_T, the sawtooth waveform output begins to degrade above 50 kHz due to the bandwidth limitations of IC₃. (DI #1557)