US2584068A - Impedance transformer - Google Patents

Description

Jam 29 1952 J. M. VAN HoFwEEGElN Erm. 2,584,068

IMPEDANCE TRANSFORMER Filed May 9, 194e A T TORNEY Patented Jan. 29, 1952 UNITED STATES- PATENT OFFICE IMPEDAN CE TRANSFORMER Application May 9, 1946, Serial No. 668,544 In the Netherlands June 3, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires June 3, 1961 4 Claims. (Cl. 178-44) The 'invention relates to impedance transformers and, more particularly, to such transformers wherein the impedance to be transformed is connected to the input terminals of a transmission line and the transformed impedance appears at the output terminals of this transmission line. Impedance transformers are frequently used in ultra-high frequency applications, for example where the impedance of a consuming apparatus of ultra-high frequency electrical oscillations has to be adapted to the source of energy. It is also often desired, for example for measuring purposes or in neutralisation systems, to have at ones disposal very small adjustable capacities and inductances of exactly known values. In such cases use may also advantageously be made of an impedance transformer, in which event an impedance whose value is larger or smaller is connected to the one side of the transformer whereas the desired smaller or higher impedance occurs on the other side of the impedance transformer. In this case it is frequently desirable that the ratio of transformation be determinable beforehead as exactly as possible While, in addition, it should be independent of the value of the impedance to be transformed.

The invention has for its object to provide an impedance transformer whose ratio of transformation can be adjusted beforehand at any desired real value. According to the invention, this is achieved by providing in series with and parallel to the transmission line either on the input side or on the output side such impedances that, whenthe impedance to be transformed is shortcircuited, the impedance measured between the output terminals for the operating frequency is equalto zero or. at least is a minimum and, when the impedance to be transformed is switched out of circuit, is infinitely high or at least is a maximum, In this case an impedance is measured across the output terminals which is higher than vthat'which is connected to the input terminals if the series impedance is located, with respect to the parallel impedance, on the side of the input terminals of the transmission line whereas at the output terminals of the transmission line an impedance is measured which is smaller than that which is connected to the input terminals, if the series impedance is located, with respect to the parallel impedance, on the side of the output terminals of the transmission line. The series and/or parallel impedance themselves are preferably constituted by tunable transmission lines.

' The invention will be explained more fully with transmission line reference to the following description and drawing wherein Figs. 1 and 2 diagrammatically represent impedance transformers according to the invention whilst Figs. 3 and 4 serve to elucidate the calculation and Fig. 5 shows another embodiment of an impedance transformer according to the invention.

Fig. 1 represents a circuit-arrangement according to the invention wherein the impedance Ze which is to be transformed is connected to the input terminals I, 2 of a transmission line with the characteristic impedance Zu. Impedances Z5 and Zp are provided, respectively, in series with and parallel to the transmission line on the output side. IIf the impedances ZS and Zp are chosen,

in accordance with the invention, in such manner that, when Z1 is short-circuited, the impedance measured between the output terminals i, 2 for the operating frequency is equal to zero and, when Z1 is switched out of circuit, is innitely high, the impedance Zt which occurs at the output terminals 3, 4 of the transmission line amounts in this circuit-arrangement, if Z1 is connected in the ordinary manner, to

L cos2 l (l is the wavelength in the conductor of the electric oscillations of operating frequency).

1f it is desired to have at ones disposal an adjustable capacity whose maximum value is 0.1 pf., this capacity may be obtained, for example, by taking for Ze a variable capacity with a maximum value of 20 pf. and to adjust the length of the in such manner that cos2 l=0.01 or cos [31:01. With a wavelength of 50 cms. a length of the transmission line of 11.7 cms is necessary for this purpose, which length may be adjusted beforehand. As may be seen from Fig. 1, the conductor of 10 pf. is connected between the terminals I and 2 on the input side of this transmission line. Then the regulable impedance ZS is provided, for example, in the form of a transmission line, and is adjusted in such manner, that, when the condenser Ze is short-circuited, an impedance zero is measured between the terminals 3 and 4. Subsequently, the impedance Zp is connected between the terminals 3 and 4 and, when the condenser Ze is switched 'Y is equal to i500. that' higher and consequently a capacity which is a hundred times smaller, is observed between the terminals 3-.and 4. Y

With the use of the circuitfarrangement ac-y l cording to Fig. 2 there is measured Yif the imped ances Z'p and Zs are adjusted in a manner;

' at the place of Z1 in Fig. 2, to a fraction of that value.

If in the circuit-arrangement according to.

Fig. 1 the impedance to be transformed lZtis connected to the terminals 3, 4, it is possible to de'- duce that with a correspondingv adjustment of'V the impedances ZS and Zp an impedance is` measured between terminals I and 2 Zt which, is

V equal to Z1 cos2 l wherein consequently the value o ft the` impedance Z1.isstepped.down. If with the circuit-.arrangement according to Fig. 2 the places of the impedance to be transformed and'` V of the transformed impedanceare interchanged in a similar manner, there is measured betweeny the terminalsA I .and 2, animpedanceZ'r which 1. Y lcosl, and wherein consequently. the value of the im:-

pedanceis stepped up.

VAccording to the circuit-arangement ofFig.Y 1

it is shown, on the basisv of the following calcula.

impedancev-Zi, isrepresented by :ze HZ@ tg s1..

1fl theV transmission une isshort-circuited on I the one side, the. impedance at the other end amountsto Zk=.7'Zo tg @1j (2) enduit,'the transmission line, is-open-,at one end; the impedance at the other end is zat-yz cotg s1 3.)

Iii-now the impedance Z .is incorporated in the transmission line which is short circuited'on the output side I, 2` the impedance Z1 between the terminals 3j'and 4 isZ1=Zs+ZH1=Zs+y`Zo tg l. It-follows-therefromhforZS, ifZi must. be equal Zg'=*y`Z0 tgv i (4)l If,- after this impedance .Ze hasbeen providedY at Vthe place indicatedv in Fig. 1, also the impede an'cezZpis. providedv and the vconnection between theterminals I and2 is*` removed, the reciprocal value of: the impedance Z2 whichA occurs between the terminals -3 and -4 is such manner that under these conditions VZz`=v and therefor If,v the values obtained for Z5 and Z according Vto the Formulas 3V and 4 are inserted in the Formula-5 and if is supported toA be, equal to zero, it follows there- 'fromthat 1 sin l cos B1 Inlfigi i' thereprocal value of the impedance Zt which is Vmeasured between the terminals 3 and-14 is, if' Ze is switchediinto. circuit, equal to 1v 'Y 1. Y Zfl'zrzf After, substitution ofthe values. foundvfor Z' (1,), 25(4); and'Zp.(6) and after a fewbperaf tions itis found that:

In. a similar manner,- itisv possible'Y to.- deduce the ratio of transformation` given forfthe; circuit'fV arrangement according to Fig. 2, as-wellaszthe ratios of transformation which occur whenzthe.

places of the impedance toV be transformedand;

of. the transformedimpedance are interchanged;

Y Furthermore, it; appears. from` further.. calculaf tions that, if further-impedancesarelooatediin serieswith or-parallel to thefimpedance tobe transformed, these impedances do notaiectathe.v transformation if; careistaien toz` ensure?Y that upon adjusting the;V impedances- Z5.- and Zp.v exev clusively the impedance to be,;transformedf onli/ y in short-circuitede and switched cutoff-circuit ref.. spectively.

The impedances-Zs andy Zsare formedvin-lprazcrtice; byf transmissionlines of adjustablefeletr-ical length as shown: in Fig; 5, in-Y which Z; andg'Zp are transmissionlines `whose;respective electrical lengths arel made adjustable by `meansioi short:z circuiting bars.t 5 and, respectively;

What we claimis:

1;. An impedance transformer.;` comprising; a

transmission lineY of given; length determined; by;

a givenY transvfqinmationY ratio having; input-,and output terminals. av variab1e;;impedane iiiseries with saidl transmission line, anda variableyimpedancegin: Parallel withasaid line,` saidseriesand parallel impedancesf-each beingadillstablegwthi-n a` given rangeof values at which within; apre determinedrangeof operating frequencies: the. impedance vof` they line-viewed-M from the,A output terminals isV aY minimum whenl theulinetisfshortjf circuiteoll at theginput terminalsland a. maximum when thefline is-,open-.crcui-ted atzthe input ter..- rninalsvwhereby-the line/reiects atthe output terminals when` animpedancejof givenfvalueffis 1 connected thai input terminals.. s, trans# formed value of said impedance across the input terminals.

2. An impedance transformer comprising a transmission line of given length determined by a given transformation ratio having input and output terminals, a variable impedance in series with said transmission line, and a variable irnpedance in parallel with said transmission line, said series impedance being connected in said transmission line between said parallel impedance and said input terminals, said series and parallel impedances each being adjustable within a. given range of values at which within a predetermined range of operating` frequencies the impedance of the line viewed from the output terminals is a minimum when the line is shortcircuited at the input terminals and a maximum when the line is open-circuited at the input terminals whereby the line reflects at the output terminals when an impedance of given value is connected across the input terminals a transformed value of said impedance across the input ter minals.

3. An impedance transformer comprising a transmission line of given length determined by a given transformation ratio having input and output terminals, a variable impedance in parallel with said transmission line, said series impedance being connected in said transmission line between said parallel impedance and said output terminals, said series and parallel impedances each being adjustable within a given range of values at which within a predetermined range of operating frequencies the impedance of the line viewed from the output terminals is a minimum when the line is short-circuited at the input terminals and a maximum when the line is open-circuited at the input terminals whereby the line reflects at the output terminals when an impedance of given value is connected across the input terminals a transformed value of said impedance across the input terminals.

4. An impedance transformer comprising a transmission line of given length determined by a given transformation ratio having input and output terminals, a first tunable transmission line in series with said transmission line, and a second tunable transmission line in parallel with said transmission line, said i'lrst and second tunable transmission lines being each adjustable to provide impedances each having a given range of values at which Within a predetermined range of operating frequencies the impedance of the line viewed from the output terminals is a minimum when the line is short-circuited at the input terminals and a maximum when the line is opencircuited at the input terminals wherebt7 the line reects at the output terminals when an impedance of given value is connected across the input terminals a transformed value of said impedance across the input terminals.

JOHANNES MARINUS VAN HOFWEEGEN. MAXIMILIAAN JULIUS OTTO STRUTT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,909,610 Garter May 16, 1933 2,396,708 Leeds Mar. 19, 1946

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