DE2913135A1 - LOW-PASS FILTER FOR TELEPHONE SIGNALS - Google Patents

Description

LOW-PASS FILTERB FOR TELEPHONE SIGNALS

The invention relates to a low pass filter for telephony signals and, more particularly, to such a filter which is used in a subscriber line terminal equipment of a central office which processes speech signals in PCM code and conveyed.

Upon reception, the low-frequency signal is restored in a low-pass filter, the role of which is to control the frequencies cut off above 3400 Hz and that must have a very specific frequency response of the damping. aside from that it is required that the frequency response changes with temperature only within given limits. Such filters consist of a certain number of components and compliance with the above conditions with regard to temperature stability is only possible under great effort.

The known low-pass filters usually have a low input impedance. For correct operation of the sample and hold capacitor, it must be on a higher circuit Impedance look. It is therefore necessary to insert an intermediate stage between the capacitor and known low-pass filters generally includes an operational amplifier.

The object of the invention is to provide a low-pass filter with a simple structure in which fewer components are required are than in known filters and in which the intermediate stage is omitted. In addition, the inventive low-pass filter should be a

90 98 4-2 / 072 8

have improved temperature stability and a significantly higher input impedance than known filters.

This object is achieved by the filter defined in the claim. With regard to features of preferred embodiments of the invention, reference is made to claim 2.

The invention is explained in more detail below using an exemplary embodiment with the aid of the drawings.

Fig. 1 shows a low-pass filter according to the invention and Fig. 2 contains characteristics of the second cell.

In Fig. 1, a key switch I and a key and holding capacitor C are schematically between an input E and the Input of the first cell 1 of the filter indicated. In series with the first cell 1 is a second cell 2, which is an output terminal S owns.

The push button switch I is during every 125 microseconds closed for a period of two microseconds.

A resistor Rl connects the capacitor C to the positive input of an operational amplifier A1. This entrance is also connected to ground via a resistor R2 and a capacitor C1 connected in parallel to it. A negative one The input of the operational amplifier is on the one hand via a resistor R3 to ground and on the other hand via the parallel connection of a resistor R4 and a capacitor C2 to the output connected to this amplifier. The operational amplifier A1 together with its circuitry forms the first cell of the low-pass filter.

The second cell 2 # is located between the output of the operational amplifier A1 and the output S of the filter

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consists essentially of an operational amplifier A 2 and a double T-bridge. This second cell forms a second order disymmetrical blocking element with a Cauer characteristic.

Between the output of the operational amplifier Al and the positive input of the operational amplifier A2 are on the one hand two capacitors C3 and C4 in series and on the other hand, two resistors R5, R6 in series. The intermediate tap between the two capacitors is via a resistor R7 at the output of the operational amplifier A2 ^ and the An intermediate tap between the two resistors is connected to the same output via a capacitor C5. The positive input of the operational amplifier A2 is also connected to ground via a resistor R8 and, in parallel therewith, a capacitor C6. The negative input of this amplifier is grounded on the one hand via a resistor RIO and on the other hand via a resistor R9 at the output of this amplifier. The resistors R5, R6, R7 and the capacitors C3, C4, C5 form the Doppe1-T-bridge.

The key capacitor C, which is located as usual at the input of the filter, and the resistor Rl at the positive input of the operational amplifier Al are dimensioned so that is applicable

RlC £> 125.10 " ⁶ j

In other words, the input time constant should be large compared to be the multiplex period (125 microseconds), i.e. the frequency, with which the push button switch I is closed. In the following, the voltages between ground and the

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Input terminal E, the common point between the capacitor C and the resistor Rl, the positive input of the amplifier Al, the output of this amplifier or the output terminal S denotes the second cell.

The first cell consists of two filter units, namely a passive filter unit with resistors R1, R2 and the capacitor Cl, and a second filter unit with the operational amplifier Al, the resistors R3 and R4 and the Capacitor C2.

The first filter unit has the following transfer function ¹

^V3 = G- ^λ -

V2 11+ a _x p )

R2 in this function means G, = j as well as

Rl-R 2Cl .
1 Rl + R2 J

The transfer function of the second filter unit has the following form:

V4 _{= G} ^{1 + a} 2 ^P.
V3 ^ 2 1+ b ₂ p 1

In this equation is

The global transfer function of the first filter cell results from the product of the transfer functions of the two units

V4 _ V3 V4 _ 1 + a ρ . V2 V2 * V3 ^fa 1 + bp + cp ² >

with G = G; G _n ; a = a ^ yb = a, + b "; c = a

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From equation (1) can be derived

(2)

1 * 1 - ο I

| V2 J ^ü H '(1-c U)) +

b ² W ²

This equation is the expression for the gain or the attenuation as a function of the frequency. the corresponding curve has an asymptote with a slope of 6 dB per octave.

The second filter cell has the following transfer function :

3

this results in ι

V4 ^G 3 ' _Ί / "'iTT"".. Ü»)

fv "" ³ ul ² y ^{+ & 3} υ ²

O »O

In the equation (4) means

G is the gain or damping factor at frequency 0,

a _{3 is} the coefficient inversely proportional to the overvoltage coefficient, on which the frequency behavior depends. _u «. „„ / _ DC amplification ο aas ratio go / g «q - ,, j.« ·, ·. ■■ _ · ι * 3 'Gain at very

high frequencies

The curve for equation (4) has an asymptote given by the straight line Go / to, -.

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w is the angular frequency for which the locking effect is infinite.

Figure 2 shows two given curves for equation (4). Curves a and b correspond to two different values of the Coefficients a, where curve a has a larger overvoltage coefficient than curve b. The capacitor C am The input of the first cell 1 yields the following ratio, assuming that RIO is very large compared to 125.10

V2 ^{Sln (}

Vl U> (5)

2 fo

where fo equal to 8000 Hz is the sampling frequency of the received signal.

In the case of a receive filter that consists of cells 1 and 2, the second cell is given a transmission behavior according to curve a from FIG Keying introduced value

2 fo
is corrected.

The number of required for the filter according to the invention Components is much smaller than in known low-pass filters for telephone signals, in which at least four operational amplifiers are provided. This results in addition to the price advantage, the fact that the filter according to the invention is less than known filter responds to temperature changes, which is an advantage of the invention that should not be underestimated.

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As already mentioned, the attenuation of the first cell can be chosen to be greater than 6 dB per octave.

In the context of the invention, the second cell can also be constructed differently from that shown in FIG. 1, provided that then the same transfer function results.

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Claims (2)

FO 11 087 D April 2, J979 COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL S.A. 12, rue de la Baume, 75008 PARIS, France LOW-PASS FILTER FOR TELEPHONE SIGNALS PATENT CLAIMS 1 ή low-pass filter for telephone signals, characterized in that it consists of two cells (1, 2) connected in series, each with an operational amplifier, of which the first cell (1) is of the low-pass type and has a high input impedance and a characteristic curve asymptotic to a straight line has, this straight line having a slope of at least 6 dB per octave, while the second cell (2) is of the second-order disymmetrical block type and has a Cauer characteristic. 2 - low-pass filter according to claim 1, characterized in that the first cell (1) has two Has filter units (3), the first of which consists of a resistor divider of two resistors (Rl, R2) between one There is input terminal and ground, a capacitor (Cl) being connected in parallel to the resistor (R2), which is closer to ground, while the second filter unit consists of the operational amplifier (A1), which has a resistor (R4) in parallel in the feedback branch to a capacitor (C2), the positive input of which 909842/0728 is at the divider point of the resistance divider and its negative input with <3em feedback branch and with a leading to ground Resistor (R3) is connected. 909842/072 '"