Filter arrangement

Details Filter arrangement Filter arrangement

2000-09-18

2003-06-03

Malzahn, David H. (Department: 2124)

Electrical computers: arithmetic processing and calculating

Electrical digital calculating computer

Particular function performed

Reexamination Certificate

active

06574647

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a filter arrangement for separating analog or digital baseband signals, for example POTS or ISDN signals, which are transmitted simultaneously on a two-wire line of a telecommunications system, and digital broadband signals.
2. Description of Related Art
Broadband methods such as ADSL, VDSL and xDSL or the like use the frequency band above the baseband which is occupied, for example, by POTS (Plain Old Telephone) or ISDN signals. In a telephone system, these different transmission services use the same subscriber line as transmission medium.
The broadband transmission method ADSL (Asymmetric Digital Subscriber Line), which is becoming increasingly important, thus permits large amounts of digital information to be transmitted over a two-wire subscriber line in one direction from a central office, for example a central switching office, to the subscriber, and permits control functions to be transmitted in both directions. At present, QAM methods, for example DMT (Discrete Multi-tone Modulation) or CAP (Carrierless Phase modulation), are often applied in ADSL telecommunications systems, the signals which are formed being superimposed on the analog or digital telephone signals, for example POTS or ISDN, in terms of frequency. Both at the office end and at the subscriber end, the signal which is to be transmitted or is transmitted on the subscriber line is separated by means of separate filter arrangements (ADSL Splitter Filter) into the two components baseband signals and broadband signals, for reception, and is superimposed for transmission.
For this purpose, for example, ADSL filters are composed of a bidirectional low pass and a bidirectional high pass filter, the POTS and/or the ISDN signal being separated out of the line signal by means of the low pass filter, and the ADSL or DMT signal being separated out of the line signal by means of the high pass filter.
A filter arrangement of this type is used in WO-A1-97/11534 for simultaneously transmitting broadband signals and digital ISDN baseband signals or analog signals over a two-wire line. A bidirectional broadband transmission device and an ISDN switching unit of a central office are connected here by means of an ADSL filter arrangement to one end of the two-wire line over which data can be transmitted and received. The other end of the two-wire line is connected via a further ADSL filter arrangement to a subscriber-end bidirectional broadband transmission device and an ISDN network connection. The transmission limit of the low pass filter of the ADSL filter arrangement is selected in each case in such a way that its bandwidth is sufficient for the transmission of baseband signals.
Depending on the different national input impedances, the low pass filter is provided as a passive or an analog active filter for each respective subscriber.
In addition, EP-A1-677-938 discloses a filter arrangement which is composed of a low pass filter and a high pass filter and via which baseband POTS and broadband ADSL signals can be coupled to a common two-wire line. One portion of the ADSL signal can be lost here through the POTS low pass filter if the POTS signal is not sufficiently damped by the two-wire line. This occurs, for example, if the two-wire line is too short. The reason for this loss of the ADSL signal is that the filter properties of the low pass filter can change as a result of an excessively high POTS signal, because, for example, filter inductors become magnetically saturated. The low pass filter itself has series inductors which transmit d.c. current. An excessively high d.c. signal causes these inductors to become saturated and thus reduces their inductivity, as a result of which part of the ADSL signal is directed via the low pass filter and is lost. In order to avoid the low pass inductors becoming saturated, a variable resistor is connected between the POTS unit and the low pass filter, which resistor is controlled by a control signal which sets the impedance value of the variable resistor as a function of the damping on the two-wire line in such a way that an excessively high POTS received signal is damped, while POTS signals with a normal level are passed on without damping.
One disadvantage of the known broadband filter arrangements is the very large amount of space required to form an analog filter. In addition, the national telephone input impedances must always be taken into account, resulting in different designs of the filter arrangements for each country.
SUMMARY OF THE INVENTION
An object of the invention is therefore to specify a filter arrangement of the type mentioned at the beginning which requires only a small amount of space and which can easily be adapted, at low cost, to the respective requirements in terms of subscriber-specific terminating unit impedances.
In accordance with exemplary embodiments of the invention, this is achieved in that the low pass filter is composed of at least one first, d.c.-transmissive filter with a first passband and at least one second filter with a second passband, so that the first and second passbands together essentially form the passband of the low pass filter, and in that the inputs of the first and second filters are connected in parallel.
The basic idea of the invention consists in dividing the low pass filtering of the baseband signals into at least two filter bands in order thus to permit better adaptation of the filtering to the present requirements, enabling a particularly favorable filtering option to be provided for each filter band. The two filter bands can be selected here in such a way that existing frequency gaps in the transmission signals come to be located between said bands, and an overlapping region of the filter bands is thus not necessary. The first, d.c.-transmissive low pass filter can be of relatively simple, discrete design, but this eliminates the d.c.-transmissivity for the second filter, which therefore has to be capable of being dimensioned specifically for relatively high frequency bands.
According to one further embodiment of the invention, the outputs of the first and of the second filter can be connected in parallel.
In a further embodiment of the invention it is possible to provide for the first filter to be formed from a low-band low pass filter and for the second filter to be formed from a high band bandpass filter, the passband of the low-band low pass filter extending from d.c.-transmissivity of 0 Hz to a first upper limiting frequency, and the passband of the high-band bandpass filter extending from a second lower limiting frequency to a second upper limiting frequency, the first upper limiting frequency preferably being lower than the second lower limiting frequency.
The baseband frequency band of two filters is therefore divided into a lower band and an upper band, the lower band being transmissive to d.c. current, which is a precondition for maintaining the feeding of the individual subscribers. Furthermore, the lower frequency band also covers low-frequency alternating voltages, for example the call voltage. Between the first upper and the second lower limiting frequency it is also possible to form a frequency gap which has no further disruptive effect because in conventional telephone networks there are no signal frequencies to be transmitted between the frequency bands for signaling or feeding and the lower voice frequencies.
The low-band low pass filter can easily be embodied as a passive low pass filter which contains, for example, inductors and capacitors. The requirements for the lower frequency band, which contains d.c. current and low frequencies, are, however, correspondingly small in comparison with the upper frequency band of the low pass filter operation, with the result that it is virtually impossible to achieve further simplification or savings in terms of space.
However, a significant reduction of the dimensions of the filter provided for the upper frequency band of the low pass filtering operation is achieved in that, according to one

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