The CM8870 integrates the band-split filter and digital decoder capabilities into a single 18-pin DIP, SOIC, or 20-pin PLCC device to give complete DTMF receiver functionality. The CM8870 is made with cutting-edge CMOS process technology to ensure low power consumption (maximum of 35mW) and precise data processing. For both high and low group filters, as well as dial tone rejection, the filter section employs a switched capacitor approach. The CM8870 decoder detects and decodes all 16 DTMF tone pairs into a 4-bit code using digital counting algorithms. An on-chip differential input amplifier, clock generator, and latched three-state interface bus reduce the number of external components in this DTMF receiver. The on-chip clock generator, on the other hand, just requires a low-cost TV crystal or ceramic resonator.
The dual-tone signal is applied to the inputs of two 9th-order switching capacitor bandpass filters to separate the low-group and high-group tones. The bands containing the low-group and high-group tones match to the bandwidths of these filters. Notches at 350Hz and 440Hz are included in the filter portion, which provide good dial tone rejection. A single order switched capacitor section follows each filter output, smoothing the signals before limiting. High-gain comparators are used to restrict the signal. Hysteresis is included in these comparators to avoid the detection of undesired low-level signals and noise.
The CM8870 decoder uses a digital counting technique to determine the frequencies of the limited tones and to verify that these tones correspond to standard DTMF frequencies. A sophisticated averaging method that protects against tone simulation by external signals (such as speech) while allowing for tiny frequency changes
- Central office
- Mobile radio
- Remote data entry
- Call limiting
- Paging systems