FSK Modem and DTMF Codec
CMX865
4.1
Ring Detector Interface
Figure 3 shows how the CMX865 may be used to detect the large amplitude ringing signal voltage
present on the 2-wire line at the start of an incoming telephone call.
The ringing signal is usually applied at the subscriber's exchange as an ac voltage inserted in series with
one of the telephone wires, and will pass through either C20 and R20 or C21 and R21 to appear at the
top end of R22 (point X in Figure 3) in a rectified and attenuated form.
The signal at point X is further attenuated by the potential divider formed by R22 and R23 before being
applied to the CMX865 RD input. If the amplitude of the signal appearing at RD is greater than the input
threshold (Vthi) of Schmitt trigger 'A' then the N transistor connected to RT will be turned on, pulling the
voltage at RT to VSS by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will
then go high, setting b14 (Ring Detect) of the Status Register.
The minimum amplitude ringing signal that is certain to be detected is:
( 0.7 + Vthi x [R20 + R22 + R23] / R23 ) x 0.707 Vrms
where Vthi is the high-going threshold voltage of the Schmitt trigger A (see section 7.1).
With R20-22 all 470kΩ as Figure 3, then setting R23 to 39kΩ will guarantee detection of ringing signals of
44Vrms and above for a nominal VDD of 3.3V.
R20, 21, 22
R23
R24
C20, 21
C22
D1-4
470kΩ
0.1µF
0.33µF
1N4004
See text
470kΩ
Resistors ±5%, capacitors ±20%
Figure 3 Ringing Signal Detector Interface Circuit
© 2005 CML Microsystems Plc
6
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
