Wave transmission lines and networks – Coupling networks – Balanced to unbalanced circuits
Patent
1994-02-15
1995-12-12
Gensler, Paul
Wave transmission lines and networks
Coupling networks
Balanced to unbalanced circuits
333 32, H03H 742
Patent
active
054753465
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
This invention relates to impedance matching.
BACKGROUND
The input and output ports of most color 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 standardize 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) while others are attempting to standardize on another form known as shielded twisted pair (STP), but both are essentially balanced lines having an impedance of around 100 ohms. While 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 color video signals, has presented an obstacle which hitherto has proved insurmountable.
In order to pass color 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 a multi-component 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 color 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 synchronization pulses, 26 KHz horizontal synchronization pulses, and picture information which is centred around 8 MHz and 16 MHz. Other current systems may require an even greater bandwidth, sometimes up to 100 MHz or more. In addition, it may be desirable that the network should allow the passage of d.c. in order, for example, that simple continuity tests can be carried out.
An aim of the present invention may be viewed as being to provide an arrangement which is capable of matching different input and output impedances, can be made to operate over a wide bandwidth (e.g. below 55 Hz to around 120 MHz for a color video signal), and has a low insertion loss.
SUMMARY OF THE INVENTION
The present invention proposes a matching arrangement for color video signals, comprising a balanced line and an unbalanced line or port having a different impedance to the balanced line, in which the two lines or the line and port are coupled via low pass filter means.
Although the common forms of low pass filter usually have unbalanced input and output connections its connection to the balanced line does not appear to have any detrimental effect.
In order to reduce unwanted high frequency signals on the balanced line, which may result in a deterioration in the wanted video signal, the filter is preferably a third or higher order filter. The optimum is a fifth order filter since these have a sharp upper cut-off frequency. On the other hand, increasing the number of filter elements beyond five increases the insertion loss and cost of the matching arrangement with no significant gain in performance.
Whilst the filter will normally be arranged to pass the complete effective bandwidth of the color video signal it will be appreciated that the same effect could be achieved by a combination of a low pass filter with a relatively low cutoff frequency and a bandpass filter arranged to pass the high frequencies of the signal.
There are many basic types of low pass filter which each have their respective advantages and disadvantages, e.g. crystal filters, bridge filters, linear filters and elliptical filters. A preferred form of filter is the kind known as a Chebychev filter. In general, a Chebychev filter comprises a pair of input connections and a pair of output connections with one or m
REFERENCES:
patent: 1866525 (1932-07-01), Carter
patent: 2258107 (1941-10-01), Buschbeck
patent: 2264718 (1941-12-01), Rust et al.
patent: 3132313 (1964-05-01), Alford
Amulet Electronics Limited
Dorman Ira S.
Gensler Paul
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