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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 "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