STABLE_SINE_WAVE_GENERATOR

Published:2009/7/13 23:11:00 Author:May | From:SeekIC

Semidigital circuits (e.g., crystal oscillators and dividers) can create square waves of very stable amplitude and frequency. Removing the odd harmonics is a reasonable task for a filter. The obvious solution, a narrowband filter, isn't acceptable because analog types are notorious for poor stability.Digital and semidigital types (e.g., switched capacitor) are better in this respect, but they add their own noise and harmonics. The task can be accomplished using the filter shown. Without R1 and R2, it is an active version of a five-pole passive low-pass LC ladder. This type has excellent amplitude stability in the passband, 30 dB/octave slope outside the passband, low component sensitivity, and a capacitor to ground at the output, which ensures continuous high-frequency rolloff and minimizes stray noise pickup. The rejection would be inadequate at the third and fifth harmonics, but notches at these frequencies can be created with just two more resistors, RI and R2. This turns the device into an elliptic-like filter.



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