SPM and Security Subsystem

FX651

© 1997 Consumer Microcircuits Limited

8

D/651/4

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

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.