STLC3075

Functional description

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Metering pulse injection (TTX)

The metering pulses circuit consists of a burst shaping generator that generates a square

shaped wave and a low pass filter to reduce the harmonic distortion of the output signal.

The metering pulse is obtained from two logic signals:

●

CKTTX: is a square wave at the TTX frequency (12 or 16KHz) that must be

permanently applied to the CKTTX pin or at least for all the duration of the TTX pulse

(including rising and decay phases).

●

D0: enables the TTX generation circuit and defines the TTX pulse duration.

These two signals are processed by a dedicated circuitry integrated on chip that generates

the metering pulse as an amplitude modulated shaped square wave (SQTTX) (see

Figure 6).

Both the amplitude and the envelope of the square wave (SQTTX) can be programmed by

means of external components. In particular the amplitude is set by the two RLV resistors

while the shaping is set by the CS capacitor.

Figure 6.

Metering pulse generation circuit

The waveform so generated is then filtered and injected on the line.

The low pass filter is obtained by using the integrated buffer OP1 connected between pin

FTTX (OP1 non inverting input) and RTTX (OP1 output) (see Figure 6) and by implementing

a “Sallen and Key” configuration. Depending on the external components count it is possible

to build an optimized application depending on the distortion level required. In particular

harmonic distortion levels equal to 13 %, 6 % and 3 % can be obtained respectively with

first, second and third order filters (see Figure 6).

The circuit showed in the “Application diagram” is related to the simple first order filter.

Once the shaped and filtered signal is obtained at RTTX buffer output it is injected on the

TIP/RING pins with a +6 dB gain or +12 dB gain.

It should be noted that this is the nominal condition obtained in presence of ideal TTX echo

cancellation (obtained via proper setting of RTTX and CTTX).

In addition the effective level obtained on the line will depend on the line impedance and the

protection resistors value. In typical applications (TTX line impedance =200

Ω, RP = 50 Ω,

CTTX1

CTTX2

CS

RLV

RLV

SQTTX

BURST

D0

CKTTX

SHAPING

GENERATOR

Square wave pulse metering

Sinusoidal wave

pulse metering

RTTX

FTTX

Low Pass Filter

-

+

OP1

CFL

R1

R2

C2

C1

Required external components vs. filter order.

Order

CFL

R1

C1

R2

C2

THD

1X

13%

2

XXX

X

6%

3

X

XXX

X

3%