FX651 Datasheet, PDF (8/16 Pages) CML Microcircuits – SPM and Security Subsystem

SPM and Security Subsystem
FX651
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 low.
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 V_DD.
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.
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.
Band Pass Filter (Tx Only)
This is a 4th order band pass filter with centre 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 acknowledgement security tone. It is transmitted by the subscriber equipment, via the
telephone link, to the local exchange.
© 1997 Consumer Microcircuits Limited
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D/651/4