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MT8870 Datasheet(PDF) 3 Page - Mitel Networks Corporation |
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MT8870 Datasheet(HTML) 3 Page - Mitel Networks Corporation |
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3 / 12 page  ISO2-CMOS MT8870D/MT8870D-1 4-13 Functional Description The MT8870D/MT8870D-1 monolithic DTMF receiver offers small size, low power consumption and high performance. Its architecture consists of a bandsplit filter section, which separates the high and low group tones, followed by a digital counting section which verifies the frequency and duration of the received tones before passing the corresponding code to the output bus. Filter Section Separation of the low-group and high group tones is achieved by applying the DTMF signal to the inputs of two sixth-order switched capacitor bandpass filters, the bandwidths of which correspond to the low and high group frequencies. The filter section also incorporates notches at 350 and 440 Hz for exceptional dial tone rejection (see Figure 3). Each filter output is followed by a single order switched capacitor filter section which smooths the signals prior to limiting. Limiting is performed by high-gain comparators which are provided with hysteresis to prevent detection of unwanted low-level signals. The outputs of the comparators provide full rail logic swings at the frequencies of the incoming DTMF signals. Decoder Section Following the filter section is a decoder employing digital counting techniques to determine the frequencies of the incoming tones and to verify that they correspond to standard DTMF frequencies. A complex averaging algorithm protects against tone simulation by extraneous signals such as voice while Figure 4 - Basic Steering Circuit providing tolerance to small frequency deviations and variations. This averaging algorithm has been developed to ensure an optimum combination of immunity to talk-off and tolerance to the presence of interfering frequencies (third tones) and noise. When the detector recognizes the presence of two valid tones (this is referred to as the “signal condition” in some industry specifications) the “Early Steering” (ESt) output will go to an active state. Any subsequent loss of signal condition will cause ESt to assume an inactive state (see “Steering Circuit”). Steering Circuit Before registration of a decoded tone pair, the receiver checks for a valid signal duration (referred to as character recognition condition). This check is performed by an external RC time constant driven by ESt. A logic high on ESt causes vc (see Figure 4) to rise as the capacitor discharges. Provided signal VDD C vc VDD St/GT ESt StD MT8870D/ MT8870D-1 R tGTA=(RC)In(VDD/VTSt) tGTP=(RC)In[VDD/(VDD-VTSt)] Figure 3 - Filter Response 0 10 20 30 40 50 ATTENUATION (dB) XY A B C D 1kHz EF G H PRECISE DIAL TONES X=350 Hz Y=440 Hz DTMF TONES A=697 Hz B=770 Hz C=852 Hz D=941 Hz E=1209 Hz F=1336 Hz G=1477 Hz H=1633 Hz FREQUENCY (Hz) |
Similar Part No. - MT8870 |
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Similar Description - MT8870 |
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