AN1500A

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18.1 Quiescent current path

time. The absolute time span depends on the quality (internal discharge) of C9 and the leakage resistance of D1.

In on-hook state the circuit is supplied by a very small current to maintain the retention of stored numbers and the

ringing melody

Frequency discrimination assures that the tone ringer is activated only when a valid ring signal is applied and not

when pulse dialing from a parallel telephone (false “bell-tinkle”).

19.1 Ringing frequency comparator

The ring signal is checked at pin FCI (#21) for a valid ringing frequency. As soon as a signal is applied to the line

the internal “ring frequency detector” will start, provided that the signal level at FCI is above the trigger threshold (

≈

charging the piezo ringer via Q4 and discharging it via D5 and an internal high voltage transistor. As soon as a non-

valid or missing ring signal is detected, the bitstream is stopped and the circuit returns to standby.

Ringing signal path

another path exists from C1 -- D2 -- R3 to FCI // C2 // R4 for the ringing frequency detector input.

20 Oscillator input

A 3.58MHz ceramic resonator (recommended type MuRata CSA 3.58MHz ) must be connected at OSC (#11). The

parallel capacitor C10 is to trim the oscillation frequency (not required with the recommended resonator type).

The exact resonant frequency should not be measured at the OSC input directly, because the capacitive load of the

Oscilloscope probe will shift the oscillation frequency.

DTMF frequencies are derived from the resonant frequency, if these frequencies (see DTMF frequency standards

or data sheet) are not centered, the oscillator must be trimmed.

EMC (electromagnetic compatibility) and RFI (radio frequency interference) is a major concern in most PTT

approvals. And due to the upcoming digital networks (GSM, CDMA, DECT) also a feature which can be “heard” by

the user. Therefore it is a major headache for telephone designers, since EMC testing is generally done with

finished designs and failing EMC tests may result in adding expensive components, like coils, chokes etc.

Much can be done by considering EMC from the very beginning ! The most important factors are IC technology,

layout and placement of EMC blocking components:

21.1 Technology:

Due to SAMES unique CMOS technology the circuits show far less sensitivity to RFI than bipolar circuits which

makes EMC a much easier task.

21.2 Layout hints:

19 Ringing mode

21 EMC & RFI issues