EDN Access -- 07.07.94 Sawtooth generator spans a 70-dB rang
-- EDN, July 7, 1994
Design Ideas:July 7, 1994
Sawtooth generator spans a 70-dB range
Paul Hendricks,
Analog Devices, Wilmington, MA
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 CT. 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 RS and CT, the sawtooth waveform output begins to degrade above 50 kHz due to the bandwidth limitations of IC3. (DI #1557)
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