M8870 Datasheet | Clare

Description M-8870 operating functions (see Figure 2) include a bandsplit filter that separates the high and low tones of the received pair, and a digital decoder that verifies both the frequency and duration of the received tones before passing the resulting 4-bit code to the output bus. Filter The low and high group tones are separated by applying the dual-tone signal to the inputs of two 6th order switched capacitor bandpass filters with bandwidths that correspond to the bands enclosing the low and high group tones. The filter also incorporates notches at 350 and 440 Hz, providing excellent dial tone rejection. Each filter output is followed by a single-order switched capacitor section that smooths the signals prior to limiting. Signal limiting is performed by high-gain parators provided with hysteresis to prevent detection of unwanted low-level signals and noise. The parator outputs provide full-rail logic swings at the frequencies of the ining tones. Decoder The M-8870 decoder uses a digital counting technique to determine the frequencies of the limited tones and to verify that they correspond to standard DTMF frequencies. A plex averaging algorithm is used to protect against tone simulation by extraneous signals (such as voice) while tolerating small frequency variations. The algorithm ensures an optimum bination of immunity to talkoff and tolerance to interfering signals (third tones) and noise. When the detector recognizes the simultaneous presence of two valid tones (known as “signal...