AN97 Datasheet | Maxim Integrated

SIGNAL GENERATION CIRCUITS Application Note 97: Jul 09, 1998 Telephone Tone Generator Requires No Trimming Many products that connect to phone lines (modems, for instance) incorporate a "callprogress monitoring" function known as CPM. CPM circuits "listen" to the lines as a human would, and respond according to what they "hear." You shouldn't dial a number unless you first hear a dial tone, for example. Neither should your puter. Tone accuracy is not very important when people monitor the call-progress tones, but the use of puters for this purpose has produced a need for tone-accuracy specifications to prevent errors in interpretation. Accordingly, CCITT has introduced the North American Precise Audible Tone Plan (the following data is from the CCITT Green Book, Volume VI-4): Figure 1 illustrates a simple circuit for generating single or dual tones. They must be ±0.5% accurate in frequency, and they must be gated as shown in the "Cadence" column .maxim-ic./an97 Page 1 of 5 (a µP can control the cadence). This generator suits applications such as the tonegeneration portion of a test stimulus for CPM circuits. Figure 1. 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...