AN97 Datasheet, PDF (2/5 Pages) Maxim Integrated Products – Telephone Tone Generator Requires No trimming

(a µP can control the cadence). This generator suits applications such as the tone-
generation portion of a test stimulus for CPM circuits.
Figure 1. In this tone generator, the uncommitted 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 component 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
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