SM8224B

NIPPON PRECISION CIRCUITS—9

FUNCTIONAL DESCRIPTION

The SM8224B receiver with caller ID service con-

forms to the Bellcore standards. It incorporates FSK

demodulator and dual-tone detection functions in a

single chip. It has a dedicated dual-tone signal input

so that the FSK signal input amplification and dual-

tone signal input amplification can be set indepen-

dently. This allows systems can be easily constructed

that provide pre- and mid-conversation information

services.

FSK Demodulator

Calling number identification service is sent as an

FSK signal, and the SM8224B FSK demodulator

processes this signal.

The FSK signal conforms to the following Bellcore

standard.

Note: S1, S0, MODE setting should be used from above combination except Test mode.

Dual Tone Detector

When using mid-conversation information services,

2 mixed signals of 2130Hz and 2750Hz are sent on

lines L1 and L2. The SM8224B detects the 2 signals

from the background noise. It uses 2 separate high-

order filters with center frequencies of 2130Hz and

2750Hz to detect the presence of the signal frequen-

cies.

In series with the filters are level detectors. When the

input level exceeds the preset rating, the signal is

detected. When the input level is below the rating,

the signal is not detected.

If both the 2130Hz and 2750Hz signals are simulta-

neously detected, DTDET goes HIGH and starts

charging the time constant circuit comprised by an

external capacitor and external resistor. When the

time constant circuit voltage rises above a fixed volt-

age level, STR signal goes LOW to indicate dual

tone detection.

Table 1. FSK signal

P arameter

Description

Modulation type

Continuous-phase binar y frequency-shift-

keying

Logic “1” data (mark)

1200 ± 12 Hz

Logic “0” data (space)

2200 ± 22 Hz

Signal level (mark)

−32 to −12 dBm

Signal level (space)

−36 to −12 dBm

Data transfer rate

1200 ± 12 baud

Table 2. Function select

S 1

S 0

MODE

Function

S T R

L O W

L O W

HIGH

P ow er-dow n

HIGH

HIGH

L O W

L O W

L O W (dual tone confirmation)

L O W

HIGH

L O W

FSK and dual-tone detection from TIP/RING

L O W (dual tone confirmation)

HIGH

HIGH

L O W

FSK detection from TIP/RING

HIGH

L O W

L O W

L O W

1. DT1, DT2, DTQ are active in this mode only.

2. Test mode should not be used.