Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1998-11-18
2001-01-23
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S185000
Reexamination Certificate
active
06177849
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of filters, and more particularly to a non-saturating, flux canceling filter for power line communications.
BACKGROUND OF THE INVENTION
As competition in the telecommunications market continues to grow, new ways are being sought to deliver telecommunications services, for example, Internet access to users. One way that is attractive for the power supply utilities is to deliver telecommunications services over power lines at frequencies over 1 MHz. Buried cable and short overhead drops are capable of carrying signals at these frequencies, but a major problem is wiring in the home. Such wiring is neither shielded nor twisted and is mainly above ground. It is thus a major source of interference, particularly at the frequencies used for shortwave broadcast, amateur radio, and airborne navigation. A filter is needed at the point of entry into the home. Given the huge difference in frequency between the power line, 60Hz and the signal, greater than 1 MHz, it would appear at first sight that it should not be difficult to construct such a filter. The problem arises from the fact that the low pass filter, carrying the power supply, has to carry huge currents, up to 200 Amperes, so that to avoid saturation the core would have to be physically enormous. Given the second order relationship between heat and current, any small resistive load in the filter will translate in the generation of a large amount of heat that not only has to be dissipated, but also wastes energy.
SUMMARY OF THE INVENTION
According to the present invention there is provided a power line communications filter, comprising an input terminal, an output terminal, and a common terminal; a transformer having primary and secondary windings, each having one end connected to said respective input and output terminals, and another end connected to a common connection, said windings being wound on a high permeability magnetic core in such a way as to promote flux leakage and with a winding polarity such that flux cancellation occurs when current flows through said windings in series; and a capacitor between said common terminal and said common connection of said primary and secondary windings, said capacitor having a large impedance at low frequencies such that low frequency current flows through both said windings in series and flux cancellation occurs.
Preferably, the capacitor and transformer should resonate at about 1 to 5 KHz. Resonance is an unwanted side effect. Since high frequency roll off begins above this frequency, it should be as low as possible without affecting operation at power line frequencies. There are some rare cases where the third harmonic power (180 Hz) exceeds the fundamental power. For this reason, the filter should be essentially lossles well beyond 180 Hz (and possibly even the 5
th
harmonic at 300 Hz). As the frequency is moved higher, the attenuation at high frequencies is reduced.
At low frequencies the transformer essentially behaves like a single piece of wire because there is effectively an open circuit between the common connection and said common terminal. The current flows in series through the primary and secondary windings. Due to the flux canceling configuration, the inductance of the windings is canceled out and there is near zero loss.
At high frequencies, the capacitor effectively provides a closed circuit and the filter behaves like a signal transformer with good roll off.
Contrary to conventional transformer design, where the object is to reduce leakage inductance by bringing the primary and secondary into close proximity, sometimes with bifilour or interleaved windings and having a short magnetic path, the present invention seeks to achieve the opposite effect. The core geometry preferably ensures maximum physical separation of the primary and secondary coils by winding them at opposite ends of the core, and also ensures a long magnetic path. Leakage inductance increases with the length per turn, so a larger cross section can be used to increase the length of the wire.
The magnetic core should be made of a material having a high permeability up to the operating frequency. A suitable choice is Fair-Rite Corp Type 43, which has a permeability factor of approximately 800 at frequencies up to 5 MHz. The permeability should be as high as possible to achieve higher attenuation at high frequencies, reduce the number of turns to reduce the loss at low frequencies, and reduce the size, weight and cost of the unit.
REFERENCES:
patent: 1481945 (1924-01-01), Weinberger
patent: 2521810 (1976-11-01), None
patent: 2638182 (1978-03-01), None
patent: 3545405 (1987-07-01), None
patent: 0 302 746 (1989-02-01), None
patent: 98 28858 (1998-07-01), None
Barsellotti John
McGinnis Mike
Bettendorf Justin P.
Milbank Tweed Hadley & McCloy LLP
OneLine AG
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