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MX651P Datasheet PDF : 13 Pages
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Telephone SPM and Anti Fraud Tone Processor
7
MX651 - PRELIMINARY INFORMATION
4.2.6 Clock Oscillator and Dividers
These circuits generate the internal clocks by division of the oscillator reference frequency. The crystal is connected as
shown in Figure 2 without any need for other external components. When a crystal is used, the CLKIN pin should be left
open circuit.
Alternatively, when a frequency of 3.579545MHz is externally available, the use of a crystal is unnecessary. The external
frequency is applied to the CLKIN pin. In this case, the XTAL pin should be tied to VDD.
In Tx, the dividers also generate a pulse train, frequency 18kHz, which is passed to the Tx circuitry (Gate ON - OFF and
Band Pass Filter) for digital modulation and filtering.
4.2.7 (Gate ON-OFF) (Tx Only)
This circuit is a digital block. It performs ON - OFF modulation of the 18kHz pulse train generated from the crystal
frequency by the digital dividers. It is controlled by the NRZ DATA pin and is used to encode NRZ data packets as pulsed
18kHz (ASK data). The modulated digital signal is passed to the Tx filter for wave shaping.
4.2.8 Band Pass Filter (Tx Only)
This is a 4th order band pass filter with center frequency 18kHz. It shapes the modulated digital pulse train from the
previous block into a low distortion pulsed sinusoidal waveform. The resulting signal is the system acknowledgment
security tone. It is transmitted by the subscriber equipment, via the telephone link, to the local exchange.
5. Application
5.1 Device Sensitivity versus VDD - Input Amplifier Gain
The device sensitivity is proportional to VDD because internal voltage references are derived from the power supply.
Accordingly, input amplifier signal gain should be adjusted via external components. Figure 2 shows components
recommended for 5.0V and 3.3V operation.
5.2 Alias Responses - False Decodes
The MX651 is designed to work in telephone systems where certain ranges of input frequencies are expected and other
ranges are not. The 12kHz SPM channel works in the presence of speech and a tone in the range 16kHz - 20kHz. The
security tone detector functions in the presence of an interfering tone in the range 11kHz - 13kHz. A significant tone at
other frequencies may interfere with correct decoding of valid transmissions and should be avoided.
A small range of input frequencies could cause false decoding when no valid tone has been received. This is because of
the frequency shifting property of the SPM channel and the sampled data filters in both channels.
The tone detectors in both the 12kHz and 18kHz channels will respond to ‘other than normal’ tones. Accordingly, such
tones should be avoided.
5.2.1 12kHz Channel
The 12kHz tone detector response frequencies are described by the following two equations:
sign
f 12kHz plus = [(n x 17.898kHz)
f 12kHz minus = [(n x 17.898kHz)
+ 5.893kHz] ± 0.185kHz
− 5.893kHz] ± 0.185kHz
nominal nominal
plus
minus
detect detect
frequency frequency
n
(kHz)
(kHz)
0
5.893
N/A
1
23.791 12.005
2
41.689 29.903
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© 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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