GB2258106A - Impedance matching network - Google Patents

Abstract

The network has a pair of connections with an impedance Z1 and a pair of connections with a different impedance Z2. One end of an inductor is connected to a first of the Z1 connections, and a capacitor is connected in series between the opposite end of the inductor and the second of the Z1 connections. A first of the Z2 connections is connected to a tap part-way along the inductor and the second of the Z2 connections is connected between the capacitor and the second Z1 connection. The network may include a variable resistor (RV, Fig 3) and a voltage dependent resistor (VDR). The network finds particular application in the matching of an unbalanced line or port to balanced line such as STP or UTP for transmission of colour video signals. The network can also be used with data signals, where the network is connected between a host computer and a slave computer or computer terminal. <IMAGE>

Description

IMPEDANCE MATCHING NETWORK TECHNICAL FIELD OF THE INVENTION This invention relates to impedance matching networks.

BACKGROUND The input and output ports of most colour video equipment usually, by convention, have a nominal impedance of 75 ohms to match shielded 75 ohm co-axial cable.

Relatively recently there has been a move to standardise voice, data and other communication links on a common form of cable. Some systems currently use a form of cable known as unshielded twisted pair (UTP) whilst others are attempting to standardise on another form known as shielded twisted pair (STP), but both are essentially balanced lines having an impedance of around 100 ohms. Whilst it is a relatively simple matter, by using suitable matching networks, to pass voice and data signals along such cable for considerable distances without deterioration of the signal, the passage of video signals, and particularly colour video signals, has presented an obstacle which hitherto has proved insurmountable.

In order to pass colour video signals along, say, UTP cable without degradation, an impedance matching device is required at each end of the cable in order to match the 75 ohm ports or co-axial line to the 100 ohm UTP.

This does not present a particular problem in the case of some components of the video signal, most commonly the red and blue components, since a simple wide band impedance matching transformer can be used with reasonable results. On the other hand, major difficulties have been encountered in matching one component of the colour signal (often the green component or, in some systems, a fourth component which carries timing information) because of its extremely wide bandwidth. By way of example, in one commonly used system the green component may include 55 Hz square wave vertical synchronisation pulses, 26 KHz horizontal synchronisation pulses, and picture information which is centred around 8MHz and 16 MHz.

Other current systems may require an even greater bandwidth, sometimes up to 100 MHz or more.

There are also other applications which currently present similar matching difficulties. For example, in some computer systems it is necessary to match 150 ohm shielded cable to 100 ohm UTP linking a host computer and a slave computer or a dummy computer which comprises a keyboard and screen. Such matching networks may be required to pass data from, say, 1 to 4 MHz, which does not present a matching problem in itself, but in some circumstances it is also necessary to pass a dc switching signal down the same line.

There is thus a requirement for a low cost impedance matching network which, in addition to providing the necessary input and output impedances, can be made to operate over a wide bandwidth (e.g. below 55 Hz to around 20 MHz or even higher), is preferably able to pass dc, and has a low insertion loss.

SUMMARY OF THE INVENTION The present invention proposes an impedance matching network having a pair of connections with an impedance Z1 and a pair of connections with a different impedance Z2, the network comprising: (i) transformer means including an inductive winding, one end of which is connected to a first of said Z1 connections, and (ii) capacitance means connected in series between the opposite end of said inductive winding and the second of said Z1 connections, a first of said Z2 connections being connected to a tap part-way along said inductive winding and the second of said Z2 connections being connected between said capacitance means and said second Z1 connection.

Such a matching network can pass signals having both an ac and a dc component. The inductive winding may be viewed as an autotransformer which passes ac signals and provides an impedance transformation. However, athough there is a dc path between the first Z1 and the first Z2 connections through part of the winding the capacitance means prevents dc from being shunted through the remainder of the winding to the second Z1 and Z2 connections.

The transformer means preferably includes a ferrite core or iron dust core which carries the inductive winding. The characteristics of the core largely determine the ac bandwidth of the network. The AL value of the core will usually be in the range of 1 to 1,000 inclusive. Below an AL value of about 1 the insertion loss of the network will rise to an unacceptable level whereas above about 1,000 it becomes difficult to arrive at an accurate value of inductance.

The capacitance means will typically have a value of between 2.5 pf and 500 pf inclusive. Below the lower end of this range problems with stray capacitance may be encountered whereas capacitance values above the range will result in resonance in the pass band. It will be appreciated that although the capacitance means will normally comprise a single capacitor for minimum cost the same value could be achieved by a combination of capacitors connected in series and/or parallel.

The impedance matching network finds particular application in the problem of matching video signals which is outlined above. One of said pair of Z1 and Z2 connections is connected to an unbalanced line or port and the other pair is connected to a balanced line.

The balanced line may be shielded (e.g. STP) or unshielded (e.g. UTP).

The matching network is preferably included within a housing at the interface between the two lines. The housing will normally be of plastics material so that the physical volume of the housing can be made as small as possible without interfering with the electical field of the transformer means. The housing preferably comprises one part of a plug and socket connector, the other part of the connector being coupled to the balanced line. In a preferred arrangement the connector includes at least three such matching networks for matching the red, blue and green components of a colour video signal with the balanced line. The or each unbalanced line may enter the housing via a co-axial lead which is conveniently terminated in a BNC or other co-axial connector.

The matching network also finds application with data signals. One of said pairs of Z1 and Z2 connections are connected to a host computer and the other pair are connected to a slave computer or computer terminal.

The matching network is preferably connected to the slave computer or computer terminal via a balanced line. The matching network is preferably included within a housing at the interface with the balanced line. The housing will normally be of plastics material for the reasons stated above. The housing preferably comprises one part of a plug and socket connector, the other part of the connector being coupled to the balanced line. The housing may include two or more such matching networks connected with the balanced line via the same plug and socket. The or each unbalanced line may enter the housing via a coaxial lead which is conveniently terminated in a coaxial connector.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a graphic illustration of the green component of a colour video signal, with signal strength on the y axis plotted against frequency on the x axis, Figure 2 is a circuit diagram of a basic matching network of the invention, Figure 3 is a circuit diagram of a matching network in the form in which it can be utilised to pass the green component of a three colour video signal, and Figures 4 and 5 are plan and end views respectively of a connector incorporating the matching network.

DETAILED DESCRIPTION OF THE DRAWINGS By way of example, Fig. 1 shows diagrammatically the frequency spectrum of a typical green component of a three component colour video signal. The main components of the green signal include 55 KHz square wave frame synchronisation pulses, 26 KHz square wave line synchronisation pulses, and the pixel (picture) information at 8 and 16 MHz.

In order to pass such a signal from standard 75 ohm unbalanced co-axial cable to 100 ohm UTP (or vice versa) a suitable matching network must be capable of meeting the following requirements: 1. To provide an impedance transformation of 75 100 ohms.

2. To pass ac signals from below 55 Hz to at least 20 MHz, and often up to about 100 MHz.

3. The shape of the square wave timing (synchronisation) pulses should be maintained.

4. There must no significant attenuation of the signal (i.e. low insertion loss).

5. The network must not resonate at any of the signal or timing frequencies.

When the matching network is required for use in certain computer applications it must meet additional requirements, namely: 6. The network must be able to pass dc.

7. The dc signal must not cause significant saturation of any transformer core which is employed.

A basic matching network which meets the above requirements is shown in Fig. 2. The network has a pair of high impedance connections Al and A2 and a pair of lower impedance connections Bl and B2. The first high impedance connection Al is connected to one end of an inductance L which is wound on a suitable ferrite core. The other end of the inductance is connected via a capacitor C to the second high impedance connection A2 and to the second low impedance connection B2. The first of the low impedance connections B1 is connected to a tap T on the inductance L.

The inductance L acts as an autotransformer which passes ac signals and gives the required impedance transformation and allows dc to flow from input to output. The capacitor C acts to block dc from the majority of the winding and prevents saturation of the core.

When the matching network is used to pass a green signal of the kind described above, the ferrite core is selected to have a nominal working bandwidth of about 50 Hz to 100 MHz. The ratio of the total number of turns N1 comprised in the inductor L to the number of turns between tap T and capacitor C (N2) is arranged to be about 6:5, giving an impedance transformation of about 100:75. A variable resistor RV is connected between the first low impedance connection Bl and the tap T, as shown in Fig. 3, or alternatively, between the first high impedance connection Al and the inductor L. In addition, a voltage dependent resistor VDR may be connected across the high impedance connections Al and A2, or across the low impedance connections Bl and B2, for line protection.

The design considerations which must be taken into account at arriving at the values for C and L are as follows. The ferrite core can be of any physical form, such as toroidal, tubular, double apertured or in the form of a rod or slab. The core should be chosen to have a suitable ac bandwidth. The AL value of the core should be chosen to allow the number of turns to be sufficiently small to keep self, mutual and leakage inductances low enough to avoid high insertion losses.

The value of the capacitor C is chosen such that the inductor and capacitor resonate at a different frequency to any of the signal or timing pulse frequencies which the network is required to pass.

Subject to this requirement, the resonant frequency may be inside or outside the passband of the network. The value of the capacitor may also affect the shape of the square wave timing pulses.

The network can be incorporated into a connector within a moulded plastic housing at the interface between 75 ohm cable and 100 ohm UTP. The high impedance connections Al and A2 are connected to the UTP whilst the low impedance connections B1 and B2 are connected to the 75 ohm cable. A suitable form of connector is shown in Fig.s 4 and 5. The connector includes a moulded plastics housing 1 which incorporates, at one end of the housing, an eight way modular jack socket 2 for receiving an eight pin plug connected to one end of the UTP. The connector incorporates three (or more) such networks for matching the three colour signals, red, blue and green, to the UTP. The colour signals pass via three (or more) co-axial flying leads 3, 4 and 5 which pass through the housing at the opposite end to the socket 2 and terminate in co-axial BNC connectors 6, 7 and 8.It has been found that the matching network also has a superior performance to a conventional wideband matching transformer when used to pass red and blue signals which do not have a dc offset.

It will be appreciated that the housing could incorporate a plug connector in place of the socket 2, the UTP being coupled instead to a socket-type connector.

The purpose of the variable resistor RV is to introduce a variable delay which can be adjusted so that the three colour components arrive at the output end of the UTP simultaneously.

Normally, the arrangement will be duplicated at each end of the UTP.

It will be appreciated that the impedance matching network can also be used in other applications where the network is required to pass signals having ac and dc components. For example, the network can be used in ICL, Ethernet, and IBM Token Ring systems in which a host computer is connected via a multiple access unit (MAU) to several slave computers or computer terminals by means of UTP. The MAU usually has a data output via 150 ohm shielded line which must be matched to the balanced 100 ohm UTP line, and in some cases the lines are required to carry dc switching signals. The matching network can be included within a plastics connector at the interface between the two lines, similar to that described above. The connector may include two or more such matching networks connected with the UTP via a common plug and socket connection.

Claims (19)

1. An impedance matching network having a pair of connections with an impedance Z1 and a pair of connections with a different impedance Z2, the network comprising: (i) transformer means including an inductive winding, one end of which is connected to a first of said Zl connections, and (ii) capacitance means connected in series between the opposite end of said inductive winding and the second of said Z1 connections, a first of said Z2 connections being connected to a tap part-way along said inductive winding and the second of said Z2 connections being connected between said capacitance means and said second Z1 connection.

2. An impedance matching network according to Claim 1, in which the transformer means includes a ferrite core or iron dust core which carries the inductive winding.

3. An impedance matching network according to Claim 2, in which the AL value of the core is in the range of 1 to 1,000 inclusive.

4. An impedance matching network according to Claim 1, 2 or 3, in which the capacitance means has a value of between 2.5 pf and 500 pf inclusive.

5. An impedance matching network for video signals, the network having a pair of connections with an impedance Zl and a pair of connections with a different impedance Z2, the network comprising: (i) transformer means including an inductive winding, one end of which is connected to a first of said Z1 connections, and (ii) capacitance means connected in series between the opposite end of said inductive winding and the second of said Z1 connections, a first of said Z2 connections being connected to a tap part-way along said inductive winding and the second of said Z2 connections being connected between said capacitance means and said second Z1 connection, one of said pairs of connections being connected to an unbalanced line or port and the other pair being connected to a balanced line.

6. An impedance matching network according to Claim 5, in which the respective conections are connected to a balanced line in the form of shielded or unshielded twisted pair.

7. An impedance matching network according to Claim 5 or 6 which is included within a plastics housing.

8. An impedance matching network according to Claim 7, in which the housing comprises one part of a plug and socket connector, the other part of the connector being coupled to the balanced line.

9. An impedance matching network according to Claim 7 or 8, in which the connector includes at least three such matching networks for matching the red, blue and green components of a colour video signal with balanced line/s.

10. An impedance matching network according to Claim 7, 8 or 9, in which the or each unbalanced line enters the housing via a co-axial lead.

11. An impedance matching network for data signals, the network having a pair of connections with an impedance Z1 and a pair of connections with a different impedance Z2, and the network comprising: (i) transformer means including an inductive winding, one end of which is connected to a first of said Z1 connections, and (ii) capacitance means connected in series between the opposite end of said inductive winding and the second of said Z1 connections, a first of said Z2 connections being connected to a tap part-way along said inductive winding and the second of said Z2 connections being connected between said capacitance means and said second Z1 connection, one of said pairs of connections being connected to a host computer and the other pair being connected to a slave computer or computer terminal.

12. An impedance matching network according to Claim 11, which is connected to the slave computer or computer terminal via a balanced line.

13. An impedance matching network according to Claim 12, in which the matching network is included within a housing at the interface with the balanced line.

14. An impedance matching network according to Claim 13, in which the housing comprises one part of a plug and socket connector, the other part of the connector being coupled to the balanced line.

15. An impedance matching network according to Claim 14, in which the housing includes two or more such matching networks connected with the balanced line/s via the same plug and socket.

16. An impedance matching network according to Claim 14 or 15, in which the or each unbalanced line enters the housing via a co-axial lead.

17. An impedance matching network according to Claim 16, in which the or each co-axial lead is terminated in a co-axial connector.

18. An impedance matching network which is substantially as described with reference to Figure 2 of the drawings.

19. An impedance matching network which is substantially as described with reference to Figures 4 and 5 of the drawings.