AN97 MAXIM | Alldatasheet
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www.maxim-ic.com/an97 Page 1 of 5 Telephone Tone Generator Requires No Trimming Many products that connect to phone lines (modems, for instance) incorporate a "call- progress 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 computer. Tone accuracy is not very important when people monitor the call-progress tones, but the use of computers 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
generation portion of a test stimulus for CPM circuits. Figure 1. In this tone generator, the uncommitted op amp of the lowpass filter inputs will not cause clipping at the two-tone output.
www.maxim-ic.com/an97 Page 3 of 5 the stopband divided by the edge of the passband.) The parameters necessary for generating each tone pair (or tone) are summarized in the table below. The switched-capacitor lowpass filter (IC1), with a transition ratio of 1.5 and a clock-to- corner ratio of 100, meets each of these four sets of requirements. Setting the cutoff frequency to 528Hz, for example, allows 440Hz and 480Hz to pass. The resulting 792Hz stopband (528Hz times the 1.5 transition ratio) blocks the critical third harmonic of 440Hz (1320Hz), enabling generation of the ringing signal. To generate low and high tones for the ringing signal, divide 52.8kHz by the divisors 120 and 110. The division scheme (left to the reader) can be implemented with simple logic, a PAL, or an up-counter/timer chip. If you use a programmable divider, you should follow it with a divide-by-two circuit to assure a 50% duty cycle. (The other CPM signals are produced in a similar way.) Dual tones for the ringing signal are illustrated in the time domain (Figure 2) and the frequency domain (Figure 3). The frequency domain shows that the generator is free of spurious tones. In some CPM applications, tone detection involves a combination of frequency verification and guard margin. By verifying that tone energy is sufficiently above the remaining spectral energy, the tone margin ensures that voice and other signals won't fool the system into thinking a CPM tone is present.
Figure 2. These time-domain waveforms show the low and high square-wave
Figure 3. In this frequency spectrum of the top trace in Figure 1, the highest-