16-10-2012, 04:09 PM
ISO2-CMOS Integrated DTMF Receiver
CMOS Integrated DTMF Receiver.pdf (Size: 466.87 KB / Downloads: 40)
Applications
• Receiver system for British Telecom (BT) or
CEPT Spec (MT8870D-1)
• Paging systems
• Repeater systems/mobile radio
• Credit card systems
• Remote control
• Personal computers
• Telephone answering machine
Description
The MT8870D/MT8870D-1 is a complete DTMF
receiver integrating both the bandsplit filter and digital
decoder functions. The filter section uses switched
capacitor techniques for high and low group filters;
the decoder uses digital counting techniques to detect
and decode all 16 DTMF tone-pairs into a 4-bit code.
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 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”).