Telephone Tone Generator Requires No Trimming



Category: Telephone, cellular phone and intercom
Manufacture: Maxim Integrated Products
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APPLICATION NOTE 1904
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): Use Dial Busy Power (per tone, 350 440 480 620 at exchange) Frequencies (Hz) -13dBm0 -24dBm0 Cadence Continuous 0.5sec on; 0.5sec off 0.2sec on; 0.3sec off; or 0.3sec on; 0.2sec off 2sec on; 4sec off 0.3sec on; every 10sec
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 (a ÁP 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 harmonicremoval 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 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.

 
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