EP0927471A1

EP0927471A1 - Hybrid circuit

Info

Publication number: EP0927471A1
Application number: EP98902928A
Authority: EP
Inventors: Stefan RÜEGSEGGER
Original assignee: Siemens Schweiz AG
Current assignee: Siemens Schweiz AG
Priority date: 1997-03-25
Filing date: 1998-02-26
Publication date: 1999-07-07
Also published as: AU5979898A; NO985487L; AU730313B2; WO1998043385A1; NO985487D0

Abstract

The present invention concerns a hybrid circuit which serves to connect a transmitting unit (TX) and a receiving unit (RX) to a double-conductor transmission line and has a bridge connection with four bridge branches each with two resistors (R1t, R2t: R1r, R2r; R3r, Rqr; R3t, Rqt) which are so designed and so connect the two outputs (txa1, txa2) of the transmitting unit (TX) and the two inputs (rxe1, rxe2) of the receiving unit (RX), that the components of the transmission signal which arise at the input of the receiving unit (RX) cancel each other out; the transmission line is connected under optional cut-in of a transmitter (TF) via the first resistors (Rqt and Rqr) of the third and fourth bridge branches (R3r, Rqr; R3t, Rqt) with the transmitting unit (TX) and via the second resistors (R3t and R3r) of the third and fourth bridge branches (R3r, Rqr; R3t, Rqt) with the receiving unit (RX). According to the invention, the resistors (R1t, R2t, R3r, Rqr and R1r, R2r, R3t, respectively) of the four bridge branches are selected so that impedance (Zb) provided to compensate impedance (ZI) of the transmission line takes on a value high enough that the error tolerance provided for the hybrid circuit can be adhered to, even without building in this impedance (Zb). Preferably, the hybrid circuit is connected to the input of the receiving unit (RX) via a filter (FR) which serves to equalize the dynamic range (dyn1, dyn2) of the signals which are transmitted over cables of differing diameters.

Description

Hybrid

The present invention relates to a hybrid circuit according to the preamble of claim 1.

To transfer two-wire lines, on which signals are transmitted in duplex mode using the frequency equality method, to four-wire lines, on which the signals are transmitted in simplex mode, switches or hybrid circuits are used, such as are used e.g. in Steinbuch / Rupprecht, communications engineering, Springer-Verlag, Heidelberg 1982, 3rd edition, volume 2, pages 46 - 52 or Hertert örcher, communications engineering, Hanser Verlag, Munich 1994, 7th edition, pages 72 and 329. Fork circuits are e.g. used in amplifier circuits, which are inserted between sections of two-wire lines for the necessary raising of the signal level of the bidirectionally transmitted signals. In addition, hybrid circuits are provided on the subscriber or exchange side in order to route the transmitted signals from a two-wire transmission line to a receiving amplifier and the signals to be transmitted from a transmission amplifier to the transmission line (see, for example, LW Couch, DIGITAL AND ANALOG COMMUNICATION SYSTEMS, Prentice- Hall Inc. 1997, pages 536-543 or). An important requirement for the hybrid circuit is that no portion of the output signal of the transmit amplifier may reach the input of the receive amplifier.

1 shows a passive hybrid circuit known from Siemens, IC's for Communications, ISDN Echocancellation Circuit, IEC-Q, PEB 2091 - Version 4.3, User's Manual 02.95, Figure 83 (Hybrid Circuit), in which the outputs of a transmitter unit or a transmitting amplifier TX each via a resistor Rqt or Rqr and the inputs of a receiving unit or a receiving amplifier RX, which has an internal resistance Rr, are each connected via a resistor R3t or R3r to the connections of a transformer TF which connects the hybrid circuit with connects a transmission line which has an impedance ZI. In order to compensate for the portion (interference portion) of the transmission signal which acts on the reception amplifier RX from the transmitter TF, the reception amplifier RX is supplied with an inverted portion (correction portion) of the transmission signal from the outputs of the transmission amplifier TX via resistors R1t and R2t or R1 r and R2r. For complete compensation of the interference signal, a point-symmetrical structure of the resistance bridge is provided with respect to the input of the receive amplifier RX, the resistors of the compensation path being weighted in accordance with the correction component to be provided. It must be taken into account that the interference component is influenced by the line impedance ZI. To compensate for the line impedance ZI, a corresponding balance impedance Zb is therefore provided in the correction branch of the hybrid circuit. The resistances of the known circuit are chosen as follows: Rqr and Rqt = 24 ohms; R1t and R1r = 619 ohms, R2t, R2r, R3t and R3r = 10 kOhm. The balance impedance Zb consists of a 681 ohm resistor, which is connected in series to connect a 3.01 kOhm resistor and a 6.8 nF capacitor in parallel. The balancing condition of this bridge circuit with a transmission ratio of the transformer TF of 1: 1 is: (R1t + R1 r): Zb = (Rqt + Rqr): ZI where R2t = R2r and R3t = R3r or preferably R2t = R2r = R3t = R3r

With an input resistance Rr of the receiving unit, the condition must also:

Rr »(R2t, R2r, R3t, R3r)» R1t, R1r, Rqt, Rq3)

be fulfilled. The disadvantage of this circuit is that a relatively large number of resistors is required, the values of which must be selected precisely. Furthermore, the balance impedance Zb is to be selected in each case in accordance with the transmission line used or with the characteristic impedance ZI of the transmission line.

The present invention is therefore based on the object of providing a simplified switching circuit which does not require a balance impedance Zb.

This object is achieved by the measures specified in the characterizing part of patent claim 1. Advantageous embodiments of the invention are specified in further claims.

The hybrid circuit according to the invention, which does not require a balance impedance Zb, has a simplified circuit structure and can therefore be manufactured inexpensively. Since no balance impedance Zb is required, the control circuit can be connected to different transmission lines without adjustments. The elimination of any interference that may occur in the low-frequency range is carried out with a simple filter for all normally used transmission lines. Calibrations or adaptations of the hybrid circuit according to the invention due to the tolerances of the components or when the diameter of the transmission line changes are therefore eliminated. Furthermore, the hybrid circuit according to the invention is particularly suitable for receiving units which have a relatively low input resistance.

The invention is explained in more detail below with reference to a drawing, for example. It shows:

1 shows a hybrid circuit with a balance impedance,

2 shows the typical resistance curve of a transmission line as a function of frequency,

3 shows the typical phase profile of a transmission line as a function of frequency, FIG. 4 shows a hybrid circuit according to the invention,

5 curves of the signal attenuation of transmission lines with different line diameters depending on the frequency,

6 shows a hybrid circuit according to the invention with the filter switched on,

7 shows a possible embodiment of the filter according to FIG. 6, FIG. 8 shows a known hybrid circuit with active echo cancellation and FIG. 9 the hybrid circuit according to FIG. 8 with passive echo cancellation according to the invention.

Fig. 1 shows the hybrid circuit described above, which has a balance impedance Zb. It can be seen from FIG. 2 that the magnitude of the resistance IZII of a transmission line from 10,000 Hz to 20,000 Hz is typically in the range of 135 ohms. The phase curve also moves in this area by 0 °. For the frequency range mentioned, compensation of the resistance and phase characteristics of the transmission line can therefore be dispensed with, or an ohmic resistance can be used for Zb, which according to the invention is eliminated by scaling the resistance bridge, but the symmetry of the resistance bridge must be preserved. This is done by scaling the resistance bridge in such a way that the value to be set for the balance resistor Zb moves towards infinity until the balance resistor Zb loses its influence on the correction component and is therefore no longer required.

This is achieved in the hybrid circuit according to the invention shown in FIG. 4 by selecting the resistors R1t, R2t and R3t and, in the case of symmetry, R1r, R2r and R3r, at least approximately the same value R. The values of the resistors Rqt and Rqr (in the case of symmetry, Rqt = Rq3) are selected taking into account the transmission ratio of the transformer TF and the resistors R1t, R1r, R2t, R2r, R3t, R3r and the input resistance Rr of the receive amplifier RX connected in parallel such that the hybrid circuit is adapted to the characteristic impedance ZI. The values of the resistors R1t, R1r, R2t, R2r, R3t, R3r are also selected such that the transmitted signal from the corresponding voltage dividers reaches the input of the receive amplifier RX without significant reduction. The value R of the resistors R1t, R1r, R2t, R2r, R3t, R3r is to be selected around 10 to 100 times larger than the value of the resistors Rqr and Rqt, which is approximately 1 when the transformer ratio is 1: 1 / 2 IZII corresponds.

6, the resistors R1t and R2t or R1r and R2r are combined to form a resistor R4t or R4r, which has the value 2R. This results in a further simplification of the circuit arrangement compared to the hybrid circuit of FIG. 4. 6, the hybrid circuit according to the invention is also advantageously connected to the receiving unit RX via a filter FR.

The utilization of the capacities of copper cables, which are still predominantly available in the connection area of telecom networks, enables the implementation of services with broadband data transmission via the existing cable network. Larger investments in the expansion of existing cable networks are therefore no longer necessary, at least in the medium term. With the HDSL technology (High bit rate Digital Subscriber Line according to ETSI standard RTR / TM-03036), data rates of around 2 Mbit / s are realized for data transmission via copper lines (see e.g. COMTEC, Technical Announcements from CH-Telecom, 2/1997 , Page 29 or Siemens, telecom report CH-Edition 3/95, page 11). In isolated cases, problems are identified when setting up HDSL transmission links, which put the use of newly developed HDSL components into question or make adjustments within the transmission system necessary. In particular, it was found that the establishment of HDSL transmission links with certain copper cables is not possible or only possible using selected components or using complex adjustment procedures. Due to the volume of the expected use of HDSL technology in the connection area of telecom networks, such adjustments, which usually have to be carried out with a great deal of time, must be avoided.

The present invention is therefore based on the object of specifying measures which, without occasional adaptations, allow transmission links for high data transmission rates to be implemented using copper cables, as are present in the connection area of communication networks.

This object is achieved by the measures specified in the characterizing part of claim 1.

Thanks to the measures according to the invention, which can be implemented with little effort, copper cables of different lengths and diameters, such as those present in the connection area of communication networks, can be used for the construction of HDSL transmission links without the need to adapt further components of the transmission system to the cable properties . In addition, the use of commercially available components of the transmission system (e.g. commercially available HDSL components) is possible for practically all copper cables that can be used for HDSL transmission links due to their length and diameter.

1 frequency-dependent curves of the signal attenuation of three copper cables with different lengths and line diameters,

FIG. 2 shows a possible embodiment of a filter provided for dynamic limitation. FIG. 3a shows the basic circuit diagram of a known HDSL transmission path,

3b shows the basic circuit diagram of an HDSL transmission link according to the invention for simplex operation,

3c the basic circuit diagram of an HDSL transmission link according to the invention for duplex operation; and FIG. 4 frequency-dependent curves of the signal attenuation of two copper cables with the same length and the same line diameter.

In the connection area of Telecom networks there are copper cables that differ in length, insulation materials as well as the distance and diameter of the wire pairs, and thus different line properties (resistance, inductance, capacitance, lead cover as well as world resistance). Typically, copper cables with lengths of up to 7 km and diameters of approximately 0.4 mm to 1.4 mm are present in the connection area of communication networks. Problems with the implementation of HDSL transmission links with standard components have so far been solved by occasional adjustments to the copper lines to be used. Under certain circumstances, a suitable one was selected after test trials under several lines.

An analysis of problems that occurred has surprisingly found that the cause of the problem is not generally the attenuation of a transmission line, but rather the attenuation distortion, which surprisingly occurs particularly strongly with copper cables with a larger line diameter (e.g. 1.4 mm), which due to the lower damping coverage can be used for the implementation of longer HDSL transmission links. For example, A copper wire with a larger diameter and less attenuation, unlike a copper wire with a smaller diameter and greater attenuation, can cause problems. By examining copper cables of different diameters and lengths, it was found that copper cables with larger diameters and longer lengths have a greater attenuation distortion than copper cables with smaller diameters and approximately the same average attenuation.

Fig. 1 shows the course of the attenuation distortion (attenuation curve depending on the frequency) of copper cables SL1, SL2 and SL3 with diameters of 0.4 mm, 1, 0 mm and 1, 4 mm, the lengths of which are chosen such that they are Frequency of around 220 kHz have the same attenuation. Of course, the 1.4 mm cable SL1 with the same length in the entire frequency range has a lower attenuation than the 0.4 mm cable SL3, as shown in FIG. 4. It can be seen from the diagram in FIG. 1 that the 1.4 mm cable SL1 has a much lower attenuation in the range of lower frequencies (<10 ⁴ Hz) and a significantly higher attenuation in the range of high frequencies (> 10 ⁵ Hz), than the 0.4 mm cable SL3. In the range around 1000 Hz, the 0.4 mm cable SL3 has an attenuation that is around 10 dB greater than that of the 1.4 mm cable SL1. A signal transmitted via the 1.4 mm cable SL1 therefore has a dynamic range that is around 10 dB higher (see dynl <dyn2) than a signal transmitted via the 0.4 mm cable SL3. Even if the 1.4 mm SL1 cable were a few hundred meters shorter and had less attenuation, this could lead to problems. The different attenuations of the copper lines SL1, SL2, SL3 are mostly compensated in the message sink of a transmission system by a gain control, which, however, does not change the attenuation distortion. As a result, the line equipment or the components provided for digital signal processing must be suitable when using copper cables with a larger diameter for processing signals with greater dynamics. Known digital HDSL reception modules often only marginally fulfill these conditions, so that problems can occasionally arise. The dynamic range of transmitted and processed signals determines the resolution required for the digital part of the receiving circuit (digital / analog converter, equalizer). For the signals transmitted by the 1.4 mm cables SL1, a significantly higher dynamic reserve is required, which corresponds to zone z shown in FIG. 1. According to the invention, a filter FR is therefore provided for HDSL transmission links, through which the

Signal components in the range of lower frequencies (<10 ⁴ Hz) can be reduced by around 10 dB. For this purpose, the filter FR, which corresponds, for example, to the RC filter shown in FIG. 2, which is constructed in a known manner with capacitors Cf and resistors Rf, preferably has an attenuation curve as shown in FIG. 1

(see the course of the line fkl). It can be seen from this that signals in the range up to 1000

Hz can be reduced by around 10 dB through the FR filter. By using the filter FR the

Dynamic range of the signals transmitted via the 1.4 mm cable SL1 adapted to the dynamic range of the signals transmitted via the 0.4 mm cable SL3. However, if a 0.4 mm cable SL3 is connected to the filter FR, the signal components in the lower frequency range (<10 ⁴ Hz) are not reduced below the signal level of the signal components in the higher frequency range (10 ⁴ Hz - 10 ⁵ Hz), which would result in a reduced signal-to-noise ratio. For the copper cables of the cable network with the smallest diameter, it should therefore be noted when choosing the filter characteristic that the attenuation in the low frequency range does not exceed the value that corresponds to the value of the maximum attenuation occurring in the higher frequency range. The circuit arrangement according to the invention can therefore be used for all installed copper cables SL1,... SL3 without inadmissibly high attenuations occurring with copper cables SL3 with a thinner diameter.

3a shows the basic circuit diagram of the HDSL transmission path known from COMTEC, Technical Communications of CH-Telecom, 2/1997, page 28, which shows a line termination HDSL / LT which is connected via subscriber connection cables SL1, ..., SL3 with a network termination HDSL / NT is connected, from which individual base connection lines are led away. 3b shows the basic circuit diagram of an HDSL transmission link according to the invention for simplex operation. From the line termination LTSX, data from transmission units TX are transmitted in simplex mode via the subscriber connection cables SL1, ..., SL3 to the network termination NTSX or a filter FR provided for dynamic compensation and further to receiving units RX. In the basic circuit diagram according to FIG. 3c, the data transmission between line and network termination LTDX and NTDX takes place in a known manner in duplex mode via hybrid circuits GS, a filter FR serving for dynamic compensation being provided between a hybrid circuit GS and the associated receiving unit RX.

The filter FR can also advantageously be used together with integrated receiving units RX, which may be part of a transceiver. Suitable are, for example, the SK70704 / SK70707 from LEVEL ONE, which are described in the associated data sheet, "1168 kbps HDSL Data Pump Chip Set", from May 1996. In Fig. 5, the curves of the attenuation as a function of frequency are shown for cables with different diameters (0.4 mm; 1.0 mm and 1.4 mm). These are typically pure copper cables, as they are mostly installed in telephone networks. The diagram shows that cables with a larger diameter have a much lower attenuation in the range of lower frequencies (<10 ⁴ Hz) and a significantly higher attenuation in the range of high frequencies (> 10 ⁵ Hz) than cables with a correspondingly smaller diameter. In the 1000 Hz range, the 0.4 mm cable has a 10 dB greater attenuation than the 1.4 mm diameter cable. In the 10000 Hz range, the attenuation of these two cables is almost identical. A signal transmitted via the 1.4 mm cable therefore has a dynamic range that is around 10 dB higher (see dynl <dyn2) than a signal transmitted via the 0.4 mm cable. However, the resolution required for the analog-to-digital converter is determined by the dynamic range of transmitted and processed signals (see U. Tietze, Ch. Schenk, Semiconductor Circuitry, 8th Edition, Springer Verlag, Berlin 1986, page 765). A much more complex and therefore expensive analog-digital converter would therefore be required for the signals transmitted by the 1.4 mm cables.

According to the invention, a filter FR is therefore provided in the circuit arrangements according to FIGS. 6, 7 and 9, by means of which the signal components in the range of lower frequencies (<10 ⁴ Hz) are reduced by around 10 dB. For this purpose, the filter FR, which corresponds, for example, to the filter shown in FIG. 7, preferably has a damping curve as shown in FIG. 5 (see the curve of line fkl). From this it can be seen that signals in the range up to 1000 Hz are reduced by around 10 dB by the FR filter.

By using the filter FR, the dynamic range of the signals transmitted via the 1.4 mm cable is adapted to the dynamic range of the signals transmitted via the 0.4 mm cable. If, however, a 0.4 mm cable is connected to the hybrid circuit in accordance with FIG. 6, FIG. 7 or FIG. 9, the signal components in the range of lower frequencies (<10 ⁴ Hz) do not become below the signal level of the signal components in the range higher Frequencies (10 ⁴ Hz - 10 ⁵ Hz) lowered, which would result in a reduced signal-to-noise ratio.

The circuit arrangement according to the invention can therefore be used for all installed copper cables, the analog-digital converter not having to have a greater resolution.

The circuit arrangement shown in FIG. 8, which has an active transmission circuit G1, is known from the data sheet from LEVEL ONE in May 1996 for the SK70704 / SK70707 modules, which can be used as "1168 kbps HDSL Data Pump Chip Set" (see in particular Page 8, Fig. 3 and page 26, Fig. 13 of the data sheet). In the circuit shown, the signal to be transmitted is fed to the transmitter TF via a scrambler (scrambler) SCR, an encoder ENC, a filter TXF, an amplifier LD and resistors Rqt and Rqr. The transmitted signals are transmitted from the transformer TF via resistors Rc and Rd, a first addition stage S1, a subsequent differential stage DIFF, an analog-to-digital converter ADC and a third connected to a digital echo canceller

Addition stage S3 fed to an equalizer circuit ETR. In order to compensate for the portion (interference portion) of the transmission signal which acts on the reception amplifier RX from the transformer TF, the differential stage DIFF is supplied with a corresponding inverted portion (correction portion) of the transmission signal from the output of the amplifier LD via resistors Ra and Rb and a second addition stage S2. This compensates for the interference or echo component contained in the input signal. To adapt to the transmission line, complex resistors Za and Zb are provided in the reception path and in the correction path. If the diameter of the transmission line changes, there is insufficient compensation of the interference or echo component by the correction component unless the complex resistors Za or Zb are adjusted accordingly. When selecting transmission lines with a large diameter (e.g. 1.4 mm), signal distortion (clipping) can also occur at the output of the ADC analog-to-digital converter, provided that it does not have a correspondingly high resolution. In Fig. 8, the elements are also designated (see <-IC) which are contained in the above-mentioned integrated circuits.

In FIG. 9 the same integrated circuit is connected to the hybrid circuit g2 according to the invention shown in FIG. 6, the advantages of which are particularly clear in this case too. The hybrid circuit G2 according to the invention in turn allows the connection of transmission lines with different diameters without circuit adjustments. It is also permissible to use an ADC analog-to-digital converter with reduced resolution. Losses resulting from passive echo compensation and filtering can be easily compensated for by means of the integrated circuit IC in that the output signal of the filter FR is supplied to the first addition stage S1 in an inverted manner and to the second addition stage S2 in an inverted manner. In the differential stage, therefore, the in-phase addition of the signals present at the addition stages S1 and S2 takes place.

The invention can also be used for hybrid circuits without a transformer TF.

Claims

PATENT CLAIMS

1. Fork connection for connecting a transmitter unit (TX) and a receiver unit (RX) to a two-wire transmission line with a bridge circuit that has four bridge branches with two resistors each (R1t, R2t; R1r, R2r; R3r, Rqπ R3t, Rqt) , which are dimensioned in such a way and which connect the two outputs (txal, txa2) of the transmitting unit (TX) and the two inputs (rxe1, rxe2) of the receiving unit (RX) in such a way that the portions of the Cancel the transmission signal from one another, the transmission line possibly with the interposition of a transmitter (TF) via the first resistors (Rqt and Rqr) of the third and fourth bridge branches (R3r, Rqπ R3t, Rqt) with the transmitter unit (TX) and via the second resistors (R3t and R3r) of the third and fourth bridge branch (R3r, Rqπ R3t, Rqt) is connected to the receiving unit (RX), characterized in that the resistors (R1t, R2t, R3r, Rqr or R1 r, R2r , R3t, Rqt) of the four bridge branches are selected such that an impedance (Zb) to be provided in principle to compensate for the impedance (ZI) of the transmission line assumes such a high value that the fault tolerance provided for the hybrid circuit even without the installation of this impedance (Zb ) is observed.

2. hybrid circuit according to claim 1 or according to the preamble of claim 1 with a real or complex balance resistor (Zb) provided for compensating the line impedance (ZI), characterized in that all bridge branches with the input of the receiving unit (RX) via a filter (FR), which is selected in such a way that the signal components in the low frequency range are reduced to such an extent that the dynamic ranges dyn1, dyn2 of signals which are transmitted via lines of different diameters are at least approximately the same size.

3. hybrid circuit according to claim 1, characterized in that the resistors (R1t, R2t, R3t, R1r, R2r, R3r) of the bridge circuit, with the exception of the resistors (Rqt, Rqr), which the output of the transmitter unit (TX) with the transmission line connect, have the value R which is at least five times greater than the value of the resistors (Rqt, Rqr).

4. Gabeischaltung according to claim 1 or 3, characterized in that the resistors (R1t, R2t, R3r, Rqr or R1r, R2r, R3t, Rqt) of the four bridge branches are selected such that one to compensate for the impedance (ZI) Transmission line impedance (Zb) to be provided assumes a value that is greater than 10 R.

5. hybrid circuit according to claim 3 or 4, characterized in that the series-connected resistors (R1t, R2t; R1r, R2r) of the first two bridge branches are each combined to form a single resistor.

6. hybrid circuit according to claim 3, 4 or 5, characterized in that the resistors (R1t,

R2t, R3t, R1 r, R2r, R3r) of the bridge circuit, with the exception of the resistors (Rqt, Rqr), which connect the output of the transmitter unit (TX) to the transmission line, are selected such that the signal level of the transmitted signals is reduced as little as possible becomes.

7. hybrid circuit according to claim 3, 4, 5 or 6, characterized in that the resistors (Rqt, Rqr) of the bridge circuit, which connect the output of the transmitter unit (TX) to the transmission line, correspond approximately to half the value of the line impedance (ZI) .

8. hybrid circuit according to one of claims 1-7, characterized in that the filter (FR) with a bandwidth of the transmission channel of a few 100 kHz in the range of the first 10 kHz has an attenuation between 5 dB and 15 dB.

9. hybrid circuit according to one of claims 1-8, characterized in that the receiving unit (RX) has two inputs, possibly connected via input amplifiers with a differential stage (DIFF), the first input inverting the output signal of the filter (FR) and the second input the output signal of the filter (FR) is not supplied inverted.

10. hybrid circuit according to claim 9, characterized in that the hybrid circuit is connected to one of the integrated circuits SK70704 from LEVEL ONE.