MX651P Ver la hoja de datos (PDF) - MX-COM Inc
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MX651P Datasheet PDF : 13 Pages
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Telephone SPM and Anti Fraud Tone Processor
6
4. General Description
MX651 - PRELIMINARY INFORMATION
4.1 Overall Function
The MX651 is a dual channel tone detector for use in European, French, and other payphone systems where 12kHz SPM
and 18kHz security tones are used.
4.1.1 SPM (12kHz) Detector
The SPM detector channel responds to a low level (50mV) 12kHz tone in the presence of a large security tone (16kHz -
20kHz) and speech.
The device responds after a period of continuous valid tone and so recognizes a valid SPM toneburst (minimum
transmission duration, 40 - 50ms).
This function is permanently enabled.
4.1.2 Security Tone (18kHz) Detector (Rx Mode Only)
This demodulates the 18kHz ASK signal in the presence of the SPM signaling.
4.1.3 Security Tone Transmission (Tx Mode Only)
An 18kHz tone modulated ON - OFF by the NRZ data pin. A logic 0 gates the signal ON so that 18kHz is transmitted. A
logic 1 gates the signal OFF so that no tone appears at the TXOUT pin.
4.2 Description of Blocks (Reference Figure 1)
4.2.1 Input Amplifier
This Input Amplifier is connected as a differential amplifier and is used to couple the signal into the device. It also
attenuates the combined speech, SPM tone and security tone to prevent its output saturating. The signal gain should be -
10dB at 5.0V supply and -13dB at 3.3V. (See Figure 2 for recommended component values).
4.2.2 Sample and Hold (12kHz Channel)
This samples the input signal at 17.898kHz and creates images of the incoming frequencies. The 12kHz SPM is
translated to 5.898kHz and the security tone is translated to between 978Hz and 1182Hz. This simplifies the subsequent
signal processing.
4.2.3 Filtering and Frequency Detection (12kHz Channel)
The output of the sample and hold circuit is passed to filter HPF1. This is a switched capacitor high pass filter which
amplifies the frequency shifted SPM tone (about 5.9kHz) but rejects the frequency shifted security tone (about 1kHz). The
filter also rejects any speech signals present. The filter output is passed to the level detect and frequency measurement
circuitry which determines the presence or absence of a valid SPM signal on the line. A valid signal sends the "DETECT
12kHz" pin to logic low.
4.2.4 Sample and Hold (18kHz Channel)
This samples the input signal at 71.592kHz. It is synchronized with the following high pass filter, which is a switched
capacitor circuit with the same sampling rate. The sample and hold circuit stores the value of the input waveform value
between sampling instants so that it is a suitable input for the filter.
Without the sample and hold circuit, speech or SPM tone components which are large compared with the security tone
would interfere with the level discrimination at the filter output.
4.2.5 Filtering and Frequency Detection (18kHz Channel)
The output of the sample and hold circuit is passed to filter HPF2. This is a switched capacitor high pass filter which
amplifies the security tone (about 16.92kHz - 19.08kHz) but rejects the SPM tone.
The filter output signal is passed to the level detect and frequency measurement circuitry which determines the presence
or absence of a valid security tone on the line. A valid signal sends the "DETECT 18kHz" pin to logic 0.
© 1997 MX•COM Inc.
www.mxcom.com Tele: 800 638-5577 910 744-5050 Fax: 910 744-5054
Doc. # 20480137.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.
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