AN97
Applications
- In this tone generator, the unmitted op amp of the lowpass filter IC1 acts as a summing amplifier. The amplifier's gain level assures that 5V-logic inputs will not cause clipping at the two-tone output. Generating a sine wave is generally more difficult than generating a square wave of the same frequency. The simplest technique is to filter a square wave of the desired frequency; removing its harmonics leaves you with the fundamental sine wave-the desired signal. For a dual-tone generator you would seem to need two harmonic-removal filters, but a single filter will do if the two square waves are reasonably close in frequency. Square waves contain only odd harmonics, so the lowest frequency ponent to be removed (the critical frequency) is the third harmonic of the lower-frequency square wave. The filter must pass the fundamental of the higher-frequency square wave. To avoid using two filters, each of these square-wave frequencies must be an even-integer divisor of the filter's switched-capacitor clock. (This requirement forces the signal to be square-i.e., with a 50% duty cycle.) As another requirement, the ratio of the lower tone's 3rd harmonic to the filter's corner frequency must be greater than the filter's transition ratio. (Transition ratio is the edge of
