SM8224B
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.
Table 1. FSK signal
P arameter
Modulation type
Logic “1” data (mark)
Logic “0” data (space)
Signal level (mark)
Signal level (space)
Data transfer rate
Description
Continuous-phase binar y frequency-shift-
keying
1200 ± 12 Hz
2200 ± 22 Hz
−32
to
−1
2 d B m
−36
to
−1
2 d B m
1200 ± 12 baud
Table 2. Function select
S1
LOW
HIGH
LOW
HIGH
LOW
S0
LOW
LOW
HIGH
HIGH
LOW
MODE
HIGH
LOW
LOW
LOW
LOW
Pow er-dow n
Dual-tone detection from
DT1/DT2
1
Function
STR
HIGH
L O W (dual tone confirmation)
L O W (dual tone confirmation)
HIGH
FSK and dual-tone detection from TIP/RING
FSK detection from TIP/RING
Test mode
2
1. DT1, DT2, DTQ are active in this mode only.
2. Test mode should not be used.
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.
NIPPON PRECISION CIRCUITS—9
NPC [ NIPPON PRECISION CIRCUITS INC ]
