AN97 Datasheet, PDF (3/5 Pages) Maxim Integrated Products – Telephone Tone Generator Requires No trimming

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.
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