US2457528A - Electric transforming arrangement - Google Patents

Description

Patented Dec. 28, 1948 Cabot Seato'n Bull, Hillin to Electric & Musical I Middlesex, England,

Britain gdon, England, assignor ndustriesi Limited, Hayes,

a company of Great.

Application February 17, 1944, Serial No. 522,847 In Great Britain February 20, 1943 This invention relates to electric transforming arrangements for operating at high frequencies, and for transforming potentials or impedances. With conventional transformers formed by two windings mutually coupled together, in an inductive manner it is well lmown that the degree of the mutual coupling afiects the ratio of transformation between the windings, and that to obtain the greatest ratio of transformation it is necessary to employ the tightest coupling. At high frequencies, however, increasing the amount of inductive coupling between the windings introduces appreciable capacitative coupling at the same time, which acts as a low impedance, joining the output terminals with the input terminals of the transformer, so reducing and ultimately limiting the effective ratio of the transformation. The object of the present invention is to provide a transforming arrangement for operating at high frequencies in which the difilculty of such limitation can be almost entirely avoided.

According to the invention there is provided a'transforming arrangement comprising an input circuit operative within a predetermined range of high frequencies, an output circuit and a network connected between said input and output circuits comprising in series at least one inductance and two capacities of unequal magnitudes said network being resonant at a frequency in said range and being arranged so that said capacities are respectively in shunt across said input and output circuits, the arrangement being so physically constructed and disposed that the effective stray capacity between said circuits arising from said inductance is substantially reduced.

Said stray capacity is preferably reduced by means of an electrostatic screen located at a point of minimum potential on said inductance or similarly located between a pair of inductances, if it is found convenient to use two inductances. If desired, however, the screen may be omitted if two inductances are employed having a common point of minimum potential and which are so separated that the stray capacity between said inductances is substantially reduced. If desired, said inductance or one of said inductances may be constituted by a coaxial transmission line.

In order that the invention may be clearly understood and readily carried into effect it will now be described with reference to the accompanying drawings, in which:

Figure 1 shows an arrangement according to one embodiment of the invention, and

Figures 2 and 3 show modifications of the arrangement of Figure 1.

4 Claims. (01. ns-44.

;Referring to Figure -1, in the arrangement illustrated therein, terminal :9 forms with the grounded terminal l2 an input pair of terminals to which an input circuit operative within a predetermined ran-ge of high frequencies is connected and represented at 4, while terminal ll forms with terminal i2 an output pair of terminals to Which an output circuit 5 is connected. Terminals l0, l2 are connected across a capacity [5, and terminals ll, [2 are connected across a capacity 16 which is unequal in magnitude to the capacity 15. Terminals I 0 and II are joinedby the series arrangement of inductances l3 and M, which are separated from one another by the earthed conductive screen I! by virtue of which capacitative coupling between the inductances l3 and I4 is reduced to a small value. The capacities and inductances l3, M, l6, I5, form in that order a closed series connection of reactive elements and constitute a network resonant to a frequency lying Within the operating range of frequencies of the input circuit. The pairs l3, l5 and I 4, 16 are each such as to resonate separately at said fre-- quency. That point in the circuit at which the screen I! is located and which is a point common to both said inductances l3 and of minimum potential. In practice the capacities l5 and 16 may be capacities associated with the input or output circuit; for example, they may be the inter-electrode capacities of thermionic valves.

On applying high frequency potentials to terminal l0 similar potentials appear at terminal I l by virtue of the circulating current established in said series connection of elements and the potentials at said terminals are substantially in the inverse ratio of the capacities respectively associated therewith. Thus, by suitably choosing the values of said capacities a desired transformation ratio can be obtained.

In a modification of the arrangement described the screen I! may be dispensed with and the inductances l3, l4 physically separated to such an extent as to substantially reduce said capacitative I4 is thus a point coupling. When the screen I! is employed, however, the inductances I 3, i4 may, if desired, be coalesced into a single inductance, the screen I! still being located at a point of minimum potential.

It will be appreciated that the stray capacity to ground of an inductance of lumped kind as shown in Figure 1 can set a limit upon the ratio, of input and output circuit capacities and so on the transformation ratio of the network. In Figure 2 a length of coaxial line I4 is used to tune the capacity I6 instead of the inductance 3 M. In this way no extra capacity is thrown across the capacity I6. In Figure 3 the line 14' also tunes the capacity l5. There is a further advantage in the use of a coaxial line, namely, that losses due to series resistance are reduced, so increasing the effectiveness of the circuit where high ratios of transformation are desired.' In the arrangements according to Figures 2 and 3 the screen 11 need not, if desired, be used. The

capacity l5 or IE, or both, may however be provided with a separate screen or screens.

Besides the stray capacity to ground of the lumped inductances of the arrangement of Figure 1 it will be further appreciated that these elements may have existing across them appreciable stray capacity. This stray capacitymay require H to be taken into account in the tuning of the two resonant halves of the circuit, which should be such that the two halves of the circuit possess a minimum of impedance at substantially the same frequency. desirable that this minimum should be made'as small as possible by reducing losses, since then the screening will be associated with the circuit at a point where the high frequency potential is very nearly zero and is thus most effective and least likely to disturb the functioning of the circuit. Damping to obtain a desired pass range of action may be applied by connecting resistance across any of the reactive elements of the circuit. 7

Whatlclaim is: I p

1. In a transformer apparatus for transferring alternating electric currents in a band of high frequencies from an input circuit to an output circuit: a transfer network for receiving said currents at an input potential and supplying currents at a difierent potential, said network including input connectors; output connectors; at least one inductance in series between an input and an output connector; a first capacitance in shunt across the input connectors; a second capacitance in shunt across the output connec- It may be mentioned that it is 4 tors; said shunt capacitances and series inductance forming essentially a single closed circuit resonant to said band of high frequencies, and the ratio of the capacitance across the input connectors to the capacitance across the output connectors being inversely proportional to the ratio of the potential received to the potential supplied; and decoupling means for reducing the capacitive coupling across said inductance between input and output.

2. A transformer apparatus as defined by claim 1 in which the decoupling means is in' the form of an electrically. conductive shield held close to that intermediate portion of the inductance exhibiting the minimum resonance potential, for shielding the end portions of the inductance from each other.

3. A transformer apparatus as defined by claim 1 in which at least part of the inductance is formed by a pair of conductors one of which substantially completely surrounds the -other.. i

4. A transformer apparatus as defined by claim 1 in which the inductance isformed of two series-connected spaced portions, said. portions having inductance valueswithrespect to .each other, equivalent to the ratio of the corresponding capacitances for establishing the connection between said inductance portions as the network zone of substantially minimumv resonance potential. u CABOT vSEATO-N BULL:-

REFERENCES CITED .The following references are; of I record in the file or this patent: I

UNITED STATES PATENTS Number Date Name 2,163,775 Conklin June 27,1939 2,221,105 Otto Nov. 12,1940 2,264,718 Rust et a1 Dec. 2, 1941 2,267,371 I Buschbeck Dec. 23,1941

Mason May 26, 1942

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