Wave transmission lines and networks – Coupling networks – With impedance matching
Reexamination Certificate
1999-05-18
2001-05-29
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
With impedance matching
C333S156000, C333S236000
Reexamination Certificate
active
06239671
ABSTRACT:
The present invention relates to transmission or delay line transformers.
Although transmission line transformers are known and have been used in the past in a limited range of applications, there has not in the past been any real understanding of the manner in which these devices operate. Furthermore, no method of modelling the behaviour of the devices so as to be able to accurately predict the performance of a particular design of transformer or how the design should be altered to improve its performance characteristics has previously been described.
The present invention seeks to provide a family of transmission or delay line transformers which may be adapted for use in a wide range of applications so as to have good performance characteristics at lower frequencies than has hereto been possible, and a method of designing the same.
According to a first aspect of the present invention there is provided a transformer comprising a plurality of transmission or delay lines so arranged as to provide an input to, and an output from the transformer, wherein isolation between the input and the output is achieved principally by way of inductive impedance.
Preferably, the inductive impedances of the lines are not all equal, but differ from one another in a predetermined manner.
Preferably, the isolation between the input and the output is enhanced by mutual inductive impedance between the lines.
Preferably, capacitance reducing means is provided for reducing stray capacitance between the transmission or delay lines of the transformer. In this way, the high frequency response of the transformer may be enhanced. In some embodiments of the present invention, the capacitance reducing means may take the form of a substantially air filled material, such as expanded polystyrene, located between coils of adjacent, or nearly adjacent, transmission or delay lines.
According to one preferred embodiment of the present invention, the transformer comprises a plurality of balanced delay lines. By balanced delay line, it is meant that both conductors (e.g. inner and outer conductors in the case of a co-axial cable) have an associated inductance. Preferably, the inductance is shared equally between both conductors of each delay line.
Each delay line may take the form of a lumped element delay line comprising a plurality of inductors and capacitors. In this form, the high frequency response of the transformer may be enhanced by increasing the number of LC sections of each delay line without increasing the total desired inductance or capacitance of the delay line. Thus, in certain preferred embodiments of the present invention, there is provided a transformer comprising a large number of lumped element LC sections each having a relatively low impedance and capacitance compared with the total inductance and capacitance of the delay line. Preferably, there are provided at least 10 LC sections.
Alternatively, each delay line could simply comprise a continuous transmission line adapted or modified to have slow signal transmission characteristics. This may be achieved for example by increasing the permittivity or permeability of the insulating material between the first and second conductors of each delay line, or by increasing the length of one or both of the conductors (e.g. by helically winding the outer conductor), or by a combination of such known techniques.
According to a further preferred embodiment of the present invention, there is provided a multi-stage transformer comprising a plurality of transmission or delay lines, the transformer having a lower stage and a higher stage and optionally having intermediate stages in between, each stage of the transformer being formed from one of said plurality of lines such that there are as many lines as stages of the transformer, each line comprising a first and a second conductor which may conveniently be arranged as a co-axial cable Where the transmission or delay lines take the form of a lumped element delay line, the first and second conductors are formed by a number of inductors connected together in series with a plurality of capacitors connected between the first and second conductors of each line.
At a first or input end of each of the lines, an input to the transformer is formed by electrically connecting the first conductors of the lines to a first input terminal and separately electrically connecting the second conductors of the lines to a second input terminal, whereby an input signal across the first and second input terminals is applied, in parallel, across the first and second conductors of each line At the second or output end of each of the lines, an output to the transformer is formed by connecting the lines together in series (i.e. by connecting the first conductor of one line to the second conductor of an adjacent line) such that the output voltage is equal to the sum of the voltages across the first and second conductors of each line at its second or output end.
Of course, the transformer could be used as a current gain transformer simply by reversing the transformer such that the input terminals become the output terminals and the output terminals become input terminals, in just the same way as one would with a conventional transformer. Similarly, the transformer could be used as an inverting transformer by reversing the polarity of the output terminals (again, this is entirely analagous to how one would proceed to form a conventional inverting transformer).
In some preferred embodiments of the present invention, the transformer further comprises at least one magnetic core, which may, for example, be made of ferrite material, about which at least one of the transmission lines is wound to provide said inductive impedance between the input to and output from the transformer. The advantage of this is that it enables the transformer to be used at relatively low frequencies and reduces pulse droop if used in a pulsed mode.
Preferably the inductive isolation between the input and the output is greatest at the highest stage of the transformer. Conveniently this may be achieved by increasing the inductive isolation at each stage of the transformer relative to each stage's nearest lower neighbour. In some applications, it is also preferable to maintain stray capacitance associated with the inductive windings, especially at the top of the transformer, as low as possible by for instance introducing an air gap between the transmission lines. It is also convenient, when constructing an n-stage transformer according to the present invention, for the transformer to comprise n transmission or delay lines and (n−1) ferrite cores, wherein each line forms a stage of the transformer and wherein successive lines are wound around an increasing number of the ferrite cores, the first line not being wound around any of the ferrite cores and the last line being wound successively around all of the ferrite cores.
According to a second aspect of the present invention, there is provided a method of constructing a transmission or delay line transformer comprising the steps of:
providing a basic design of a transformer comprising a plurality of transmission or delay lines arranged so as to provide inductive isolation between an input and an output of the transformer;
expressing the design in terms of an unsimplified equivalent circuit in which elements of the design and stray capacitances, inductances and resistances are represented by idealised components;
reexpressing said equivalent circuit in a simplified form by “reflecting” the idealised components of a primary circuit (referred to the input of the transformer) to a secondary circuit (referred to the output of the transformer), or vice versa, by multiplying, or dividing, the value of their impedances by a factor equal to the square of the stage number of the transformer to which the components are connected so as to produce a simplified equivalent circuit, which also includes idealised components, from the point of view of the output, or the input of the transformer;
analysing the theoretical performan
Glenn Kimberly
Isis Innovation Limited
Lee Benny
Volpe and Koenig P.C.
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