STLC3075

8/26

The metering pulse is obtained starting from two logic signals:

the CKTTX pin or at least for all the duration of the TTX pulse (including rising and decay phases).

These two signals are processed by a dedicated circuitry integrated on chip that generate the metering

pulse as an amplitude modulated shaped squarewave (SQTTX) (see fig. 7).

Both the amplitude and the envelope of the squarewave (SQTTX) can be programmed by means of ex-

ternal components. In particular the amplitude is set by the two resistors RLV and the shaping by the ca-

pacitor CS.

Figure 7. Metering pulse generation circuit.

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

The low pass filter can be obtained using the integrated buffer OP1 connected between pin FTTX (OP1

non inverting input) and RTTX (OP1 output) (see fig. 7) and implementing a "Sallen and Key" configura-

tion. Depending on the external components count it is possible to build an optimised application depend-

ing 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 fig. 7).

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 +6dB gain or +12dB 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 re-

sistors value. In the typical application (TTX line impedance =200

Ω , RP = 50Ω, and ideal TTX echo cancel-

lation) the metering pulse level on the line will be 1.33 or 2.66 times the level applied to the RTTX pin.

As already mentioned the metering pulse echo cancellation is obtained by means of two external compo-

nents (RTTX and CTTX) that should match the line impedance at the TTX frequency. This simple network

has a double effect:

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

1

X

13%

2

XXXX

6%

3

XXXXX

3%