STLC3075 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
STLC3075 Datasheet PDF : 36 Pages
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STLC3075
Functional description
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
CTTX1
Low Pass Filter
C1
BURST
SHAPING CS
GENERATOR
RLV
SQTTX
RLV
CTTX2
-
R1 R2 FTTX OP1
+
RTTX
CFL
C2
Sinusoidal wave
pulse metering
Required external components vs. filter order.
D0
CKTTX
Square wave pulse metering
Order CFL R1 C1 R2 C2 THD
1
X
13%
2
X
X
X
X 6%
3
X
X
X
X
X 3%
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,
13/36
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