WO1999040716A2

WO1999040716A2 - Active splitter

Info

Publication number: WO1999040716A2
Application number: PCT/SE1999/000089
Authority: WO
Inventors: Stig Andersson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1998-02-03
Filing date: 1999-01-22
Publication date: 1999-08-12
Also published as: SE9800299D0; AU2647099A; WO1999040716A3; SE9800299L; SE514378C2

Abstract

The invention relates to an active impedance converter circuit for interconnecting a high pass filter and a low pass filter. The high pass filter comprises a transmission line bridge with three co-operating transformer inductor coils. A first coil (17) is connected to a high frequency modem (11) via a first and a second capacitor (20). A second coil (18) is connected in series with a third coil (19) via a third capacitor (20). The subscriber line (10) is connected over the second and third coils (18, 19). The invention also relates to a splitter for splitting a subscriber line into a first transmission branch and a second transmission branch. The invention also comprises a termination of a twisted pair telephone transmission line for delivering two-way service including low frequency band telephone signals and high frequency band digital subscriber line signals.

Description

Title:

ACTIVE SPLITTER

TECHNICAL FIELD

The present invention relates to an active impedance converter circuit for interconnecting a high pass filter and a low pass filter which is provided with means for active conversion of its characteristic resistance within the passband to a complex impedance, wherein the high pass filter comprises a transmission line bridge with three co-operating transformer inductor coils. The present invention also relates to a splitter for splitting a subscriber line into a first transmission branch including a low pass filter for providing telephone service signal transmissions along said subscriber line and a second transmission branch including a high pass filter for attenuating said telephone service signals and for providing high rate digital data transmissions along said subscriber line. The invention also relates to a termination of a twisted pair telephone transmission line for delivering two-way service including low frequency band telephone signals and high frequency band digital subscriber line signals.

STATE OF THE ART

In the field of conventional twisted pair telephone transmission lines for delivering two-way service, there is a growing worldwide interest in providing high rate digital transmission over the copper access network (POTS) . This is due in part to the desire to take advantage of the large installation base that already exists, without the need to wait for the deployment of fibre lines m the access network. Examples of such systems include Asymmetric Digital Subscriber Line (ADSL) and a very high speed version (VADSL) .

A vital part of this technology lies in its ability to simultaneously support secure telephony while delivering the broadband service. This facility obviates any need for extra copper pairs which would otherwise destroy the big advantage over fibre installation. To enable this, the telephony signals are separated from the ADSL signals by means of a pair of splitters, one in the central office and one at the customer's premises. Thus, the splitters provides the filtering required to separate the POTS and the ADSL bands before being input to their respective transceivers. Normally there is a low pass filter between the telephone and the line, and a high pass filter between the ADSL transceiver and the line. The insulation generated by the splitter is important for power limiting, for noise attenuation and for the removal of transients.

In many countries, POTS uses complex, i.e. capacitive impedances for termination and balance in the transmission bridges at telephone and central office. The introduction of the ADSL technique into POTS will not be successful if it involves a reduced quality in the transmission of speech or if the digital signals are influenced by transients in the POTS. For example, the speech quality may be impaired by echo, sidetone or transmission loss. On the other hand, the digital signals may be influenced by the low frequency POTS signalling voltages.

A passive inductance-capacitance filter within each respective passband has an almost entirely resistive impedance. This necessitates some form of impedance conversion, to meet demands from the operator that insertion loss and return loss should be measured against a complex impedance . The impedance for POTS signalling frequencies must be high for an unterminated filter. Usually, the impedance converter function of the impedance converters is 1 for this frequency, and therefore the impedance of the low pass filter must be high.

A splitter having an active impedance converter is described in "ADSL and VADSL Splitter Design and Telephony Performance" , by

John Cook and Phil Sheppard, IEEE JOURNAL ON SELECTED AREAS IN

COMMUNICATIONS, VOL. 13, NO. 9, DECEMBER 1995. This impedance converter is provided with an amplifier which has more gain for low frequencies than for high frequencies. As a consequence of this, a high inductance transformer is needed which causes problems with a high leakage inductance for high frequencies. The impedance for the filters will be too low so that the parallel capacitances will load ringing signals and impulsing for POTS excessively. This prior art splitter is provided with a capacitor which is located outside the impedance converter and thereby creates an impedance differing from the reference.

SUMMARY OF THE INVENTION

One object of the invention is therefore to provide a simple and efficient active splitter which reduces the above described problems .

According to the invention, this is accomplished by means of an impedance converter circuit which is characterized in that a first coil is connected to a high frequency modem via a first and a second capacitor, that a second coil is connected in series with a third coil via a third capacitor, and that the subscriber line is connected over the second and third coils.

In a particular embodiment of the invention, the third capacitor is common to both the low pass filter and the high pass filter.

Preferably, the series capacitance of the two capacitors of the modem branch is about twice as high as the capacitance of the third capacitor which is common to both the high pass filter and the low pass filter.

According to still another embodiment of the invention, the circuit is provided with an amplifier circuit comprising a capacitor, a resistor and an operational amplifier. Alternatively, the amplifier circuit may comprise a capacitor, a first resistor, an operational amplifier and a second resistor.

According to another embodiment of the invention, the three transformer inductor coils co-operate to form a 2:1:1 transformer.

A preferred embodiment of the invention is used for separating ADSL signals from mixed ADSL and POTS signal transmissions along a twisted pair telephone transmission line.

The splitter according to the invention comprises an active impedance converter circuit for interconnecting a high pass filter and a low pass filter which is provided with means for active conversion of its characteristic resistance within the passband to a complex impedance, wherein the high pass filter comprises a transmission line bridge with three co-operating transformer inductor coils, and is characterized in that a first coil is connected to a high frequency modem via a first and a second capacitor, that a second coil is connected in series with a third coil via a third capacitor, and that the subscriber line is connected over the second and third coils.

The termination comprises a splitter for splitting a subscriber line into a first transmission branch including a low pass filter for providing telephone service signal transmissions along said subscriber line and a second transmission branch including a high pass filter for attenuating said telephone service signals and for providing high rate digital data transmissions along said subscriber line, said splitter comprising an active impedance converter circuit for interconnecting the high pass filter and the low pass filter which is provided with means for active conversion of its characteristic resistance within the passband to a complex impedance, wherein the high pass filter comprises a transmission line bridge with three co-operating transformer inductor coils, and is characterized in that a first coil is connected to a high frequency modem via a first and a second capacitor, that a second coil is connected in series with a third coil via a third capacitor, and that the subscriber line is connected over the second and third coils.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will in the following be further described in a non- limiting way with reference to the accompanying drawings in which:

Fig. 1 is a block diagram schematically showing the termination in accordance with the invention, Fig. 2 is a circuit diagram showing the function of the active impedance converter in accordance with the invention, Fig. 3 is a circuit diagram showing details of a first embodiment of the impedance converter circuit according to Fig. 2, Fig. 4 is a circuit diagram showing details of a second embodiment of the impedance converter according to Fig .

2 , and Fig. 5 schematically illustrates the arrangement of the high pass and the low pass filters in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The active impedance converter circuit according to the present invention is included in the termination of a telephone subscriber line which presents both complex and resistive impedance.

The circuit may for example be used in an ADSL splitter where the line is specified as complex for low frequencies like POTS (0.3 - 3.4 kHz) and complex or resistive for ADSL frequencies (30 - 1100 kHz) as well as the associated high pass and low pass filters. The circuit is designed for so called twisted pair telephone transmission lines for delivering two-way service.

The block diagram in Fig. 1 shows a subscriber line 10 which is connected to a modem 11 via a high pass filter 12. A telephone or a switchboard is also connected by means of the lines 13 to the high pass filter 12 via a first impedance converter 14, a low pass filter 15 and a second impedance converter 16. The modem 11 is for example the ADSL modem.

Fig. 2 shows the impedance converter including a transmission line bridge comprising a set of three co-operating transformer inductor coils 17, 18 and 19. The three transformer inductor coils cooperate to form a 2:1:1 transformer, i.e. nl=n2=n3/2. An amplifier circuit comprises capacitors 20, resistors 21 and operational amplifiers 22.

C can be considered as being large. Zf is the impedance of the low pass filter. The transformed impedance Zin may be calculated in accordance with the following:

Zin = (Zf * Zr * R3)/(R * (R3 + R4) ) + (Zf * R3)/(R3 + R4)

For telephony frequency, Zf may be considered as equal to Rf which is the characteristic impedance of the low pass filter 15.

Zr may be designed to have the same configuration as the complex reference impedance Zo, often in accordance with Fig. 3.

Thus: Zin = (Zf * R3)/(R3 + R4) * (Zr/R + 1) = Zo

When R3/ (R3 + R4) is equal to Ra/Rf, Zr may be designed in accordance with Fig. 4.

According to this, Rf may be selected larger than Ra (a common value is 270 ohms) , which makes it possible to dimension the low pass filter so that the total capacitance between the branches 11 and 13 will be sufficiently small in order for the impedance for the telephony signals to meet the demands of the operator. For frequencies higher than the switching frequency of the low pass filter, where Zr<<R (Zr will have the same switching frequency as Rb and Cb) , it is possible to write the in-impedance as: Zin = ( Zf * R3 ) / (R3 + R4 )

If the low pass filter begins and ends with a capacitor Cf, it is possible for frequencies considerably higher than the switching frequency Zf to write:

Zf = l/(j * ω * Cf)

The capacitance which is reduced over to the line side becomes

Ch = Cf * Rf/Ra

this capacitance is a part of the high pass filter and also functions as a low impedance for high frequencies. The high frequencies are switched by means of the low inductance transformer 17-19 and forwarded to the modem branch 11. The transformer inductances in the line branch 10 constitutes a serial impedance for telephony signals, but since the resulting impedance is low, the attenuation will be so small that it is well within the requirements. The inductance of the transformer in the modem branch 10 provides the inductance in the high pass filter 12.

The high pass filter of Fig. 5 constitutes a third order high pass filter. The series capacitance of the two capacitors 20 of the modem branch 11 is about twice as high as the capacitance of the capacitor 20 which is common to both the high pass filter 12 and the low pass filter 15.

The above described splitter filter fulfils the following requirements. Conditions: both filters are interconnected without any line. The generator is connected either on the telephone side or the switch side with the other end terminated with the complex reference impedance.

Insertion loss: < 1 dB within 300 - 3400 Hz with a maximum of 0.5 dB difference between the maximum and the minimum value. Return loss: > 20 dB within 300 - 3400 Hz.

Impedance at 25 Hz: > 20 kohm.

Return loss from modem connection towards 100 ohm: > 10 dB within the range from about 30 kHz to 1.1 MHz.

The invention is not limited to the above described embodiments, instead several modifications may be made within the scope of the invention. For example, the amplifier may be realized differently. Also, it is possible to limit the effect of transient pulses as well as dampen the amplitude of ringing signals in the impedance converter, by means of a high pass filter.

Claims

1. An active impedance converter circuit for interconnecting a high pass filter (12) and a low pass filter (15) which is provided with means for active conversion of its characteristic resistance within the passband to a complex impedance, wherein the high pass filter comprises a transmission line bridge with three cooperating transformer inductor coils (17, 18, 19), wherein a first coil (17) is connected to a high frequency modem (11) via a first and a second capacitor (20) , a second coil (18) is connected m series with a third coil (19) via a third capacitor (20) , and the subscriber line (10) is connected over the second and third coils (18, 19), c h a r a c t e r i z e d in that the third capacitor (20) is common to both the low pass filter (15) and the high pass filter (12) .

2. A circuit according to claim 1, c h a r a c t e r i z e d in that the series capacitance of the two capacitors (20) of the modem branch (11) is about twice as high as the capacitance of the third capacitor (20) which is common to both the high pass filter (12) and the low pass filter

(15) .

3. A circuit according to claim 1 or 2 , c h a r a c t e r i z e d m an amplifier circuit comprising a capacitor (20), a resistor (21) and an operational amplifier (22) .

4. A circuit according to claim 1 or 2 , c h a r a c t e r i z e d in an amplifier circuit comprising a capacitor (20) , a first resistor (21) , an operational amplifier (22) and a second resistor (21) .

5. A circuit according to any one of claims 1 to 4 , c h a r a c t e r i z e d in that the three transformer inductor coils (17-19) co-operate to form a 2:1:1 transformer

6. Use of a circuit according to any one of claims 1 to 5 for separating ADSL signals from mixed ADSL and POTS signal transmissions along a twisted pair telephone transmission line (10) .

7. A splitter for splitting a subscriber line (10) into a first transmission branch (13) including a low pass filter (15) for providing telephone service signal transmissions along said subscriber line and a second transmission branch (11) including a high pass filter (12) for attenuating said telephone service signals and for providing high rate digital data transmissions along said subscriber line, said splitter comprising an active impedance converter circuit for interconnecting the high pass filter (12) and the low pass filter (15) which is provided with means for active conversion of its characteristic resistance within the passband to a complex impedance, wherein the high pass filter comprises a transmission line bridge with three co- operating transformer inductor coils (17-19) , wherein a first coil (17) is connected to a high frequency modem (11) via a first and a second capacitor (20) , a second coil (18) is connected in series with a third coil (19) via a third capacitor (20) , and the subscriber line (10) is connected over the second and third coils (18, 19), c h a r a c t e r i z e d in that the third capacitor (20) is common to both the low pass filter (15) and the high pass filter (12) .

8. A splitter according to claim 7, c h a r a c t e r i z e d in that the series capacitance of the two capacitors (20) of the modem branch (11) is about twice as high as the capacitance of the third capacitor (20) which is common to both the high pass filter (12) and the low pass filter (15) .

9. A splitter according to claim 8 or 9, c h a r a c t e r i z e d m an amplifier circuit comprising a capacitor (20) , a resistor (21) and an operational amplifier (22) .

10. A splitter according to claim 8 or 9, c h a r a c t e r i z e d an amplifier circuit comprising a capacitor (20) , a first resistor (21) , an operational amplifier

(22) and a second resistor (21) .

11. A splitter according to any one of claims 7 to 10, c h a r a c t e r i z e d in that the three transformer inductor coils (17-19) co-operate to form a 2:1:1 transformer

12. Use of a splitter according to any one of claims 7 to 11 for separating ADSL signals from mixed ADSL and POTS signal transmissions along a twisted pair telephone transmission line (10) .

13. A termination of a twisted pair telephone transmission line (10) for delivering two-way service including low frequency band telephone signals and high frequency band digital subscriber line signals, said termination comprising a splitter for splitting the subscriber line into a first transmission branch (13) including a low pass filter (15) for providing telephone service signal transmissions along said subscriber line and a second transmission branch (11) including a high pass filter (12) for attenuating said telephone service signals and for providing high rate digital data transmissions along said subscriber line, said splitter comprising an active impedance converter circuit for interconnecting the high pass filter (12) and the low pass filter (15) which is provided with means for active conversion of its characteristic resistance within the passband to a complex impedance, wherein the high pass filter comprises a transmission line bridge with three cooperating transformer inductor coils (17-19) , wherein a first coil (17) is connected to a high frequency modem (11) via a first and a second capacitor (20) , a second coil (18) is connected m series with a third coil (19) via a third capacitor (20) , and the subscriber line (10) is connected over the second and third coils

(18, 19), c h a r a c t e r i z e d in that the third capacitor (20) is common to both the low pass filter (15) and the high pass filter (12) .

14. A termination according to claim 13, c h a r a c t e r i z e d in that the series capacitance of the two capacitors (20) of the modem branch (11) is about twice as high as the capacitance of the third capacitor (20) which is common to both the high pass filter (12) and the low pass filter

(15) .

15. A termination according to claim 13 or 14, c h a r a c t e r i z e d in an amplifier circuit comprising a capacitor (20) , a resistor (21) and an operational amplifier (22) .

16. A termination according to claim 13 or 14, c h a r a c t e r i z e d in an amplifier circuit comprising a capacitor (20) , a first resistor (21) , an operational amplifier (22) and a second resistor (21) .

17. A termination according to any one of claims 13 to 16, c h a r a c t e r i z e d in that the three transformer inductor coils (17-19) co-operate to form a 2:1:1 transformer

18. Use of a termination according to any one of claims 13 to 17 for separating ADSL signals from mixed ADSL and POTS signal transmissions along a twisted pair telephone transmission line (10) .