US2832934A - Automatic matching transformer - Google Patents

Description

April 29 1958 G. A. wALTl-:Rs 2,832,934

AUTOMATIC` MATCHING TRANSFORMER Filed June `l, 1954 4 Sl'naebs--Sheetv 1 ilona Pfadi 9 mm# .freq/a uauuaauauauuuuuc A INVENTOR. FIG. 2 Z9 'Glam/. /Ilri irme/vir;

April 29, 1958 G. A. WALTI-:Rs 2,832,934

AUTOMATIC MATCHING TRNSFORMER Filed June 1, 1954 y 4 Sheets-Sheet 2 Abi/:avra

#from/ir April 29, 1958 G. A. WALTERS 2,832,934

AUTOMATIC MATCHING TRANSFORMER Filed June 1, 1954 4 Sheets-Sheet 3 INI/EN TOR.

April 29, 1958 G. A. WALTERS AUTOMATIC MATCHING TRANSFORMER 4 Sheets-Sheet 4 Filed June 1, 1954 IN1/EN TOR. G25/w A. Marie;

Unid .States Patenti *O f AUTOMATIC MATCHING TRANSFORMER Glenn A. Walters, Atherton, Calif., assignor, by mesne assignments, to Textron Inc., Providence, R. I., a corporation of Rhode Island Application June 1, 1954, Serial No. 433,508 6 claims. (C1. ass- 17) This invention relates to self-adjusting transformers or tuners for matching a transmission line to a piece of apparatus to which it is connected. Because it is usually a rather simple matter to match a generator or power source to a transmission line connecting with a load, the equipment to be matched will, in the usualcase, be the load and will be so considered in the description which follows. The methods and instrumentalities comprising the invention are applicable to any type of line; parallel pair, coaxial, or wave guide, and can be applied, in theory at least, to lines carrying electrical waves of any frequency. Because, however, they greatest need for a device `of this character usually arises in connection with ultra-high-frequency and microwave lines, and because other instrumentalities are available for use at lower frequencies, the invention will be considered primarily as applied to the lines carrying ultra-high-frequency or microwave energy, but itis to be recognized that there is no definable limitation as to frequency in applying the principles of the invention.

While the broad principles of tuners or variable impedance transformers of theA type here to be considered are well known, and rnany manually adjustable types have been described in the literature, the general theory will be briefly recapitulated here in order to establish the terminology which will beused hereinafter in describing the invention and to lay the basis for its theory of operation, since there are a number of ways in which such operation can be described. y

It is well 'known that if a transmission line carrying electrical waves is terminated in a load whose impedance matches the characteristic impedance of the line, power fed into the line bythe generator (less that absorbed by attenuation in the line itself) will be wholly absorbed by the load, and maximum transfer of energy from the genr erator to the load occurs. If, however, the load impedance does not match the characteristic impedance of the line, energy will be reflected from the point at which the mismatch occurs. Since the reflected energy is in the form of electrical waves of the same frequency as those incident to the mismatch, and these waves are traveling in the opposite direction, there will be points, spaced one-half wavelength a part, where the opposite traveling waves are in phase and reinforce, developing voltage maxima or loops, and intermediate points where the incident and reflected waves are 180 degrees out of phase and their voltage subtracts, causing'voltage minima or nodes. The ratio between the voltage developed at the maxima and that developed at the minima is the voltage standing wave ratio, or VSWRQ and is one measure of the degree of mismatch. A complete mismatch, such as a short or an open circuit, will result in a complete reection of the incident wave. At points close to the point of mismatch, where the' reflected wave is substantially unattenuated, the voltage at the maxima will be twice that which would be measured 'if the line were feeding a matched load and the minima will ap- 2,832,934 Patented Apr., 29, 195s ICC proach zero, resulting in a VSWR of infinity. v It may be noted that a load of either infinite or zero impedance can absorb no power, and hence accomplish no useful work, and hence, although it is ltheoretically impossible to match such a load the necessity for doing so never arises in practice. A matching transformer usually has to compensate for changes in load or in frequency which fall between definite limits. Most tuners of the manually adjustable type are designed to tune only within the limits of variation in load impedance which is to be expected of the system, and as long as they will do this they are entirely adequate. e

A sudden irregularity or discontinuity in the impedance of the transmission line also causes a reflection; for example, a lumped capacity bridged across the line or in series therewith or an inductance similarly connected. The amount of the reflection from such an impedance discontinuity depends upon the magnitude of the discontinuity. Thus a capacitance bridged across the line, causing a sudden discontinuity in its susceptance will result in a greater reflection as the susceptance is increased.

By adjusting the susceptance it is possible to make the reflection from it equal in magnitude to the reflected wave from the mismatched load. If the point at which the discontinuity is introduced into the line is varied, a point can be found where the wave reflected from the discontinuity is 180 degrees out of phase with the wave as viewed from the generator end of the line, the standing wave disappears and the VSWR becomes unity. When this occurs it follows, from energy considerations, that since the discontinuity has been assumed to be a susceptance, and therefore not an absorber of energy, the energy from the generator must therefore all go into the load. Actually energy reflected from the load is re-reflected back to it in such phase as to apply a voltage across the load which will cause it to absorb allof the available power.

It will be'noted that two independent adjustments arek necessary to give this result; an adjustment in the magnitude of the reflecting discontinuity and an adjustment in its position. Either of these adjustments will cause a change in amplitude of the reflected wave, as viewed from the generator, and hence in the VSWR. f

Taking as a point of reference any position between the tuner and the generator, e. g., a voltage minimum resulting from the reflection from the load, it will be seen that if the discontinuity is moved away from the reference point, the length of the path from that point to the discontinuity and back again is increased by double the distancek through which the discontinuity is moved. A movement of one-half wavelength within the guide will therefore vary the phase of the wave reflected from the discontinuity through 360 electrical idegrees. It follows that one-half guide wavelength of the longest wave or lowest frequency, which the line carries is themaximum latitude of adjustment which need be provided.

A second type of discontinuity in impedance which is frequently used in matching transformer is a stub line, bridged across the transmission line between generator and load, the stub being adjustable in effective length and either open-circuited or short-circuited at its end. The

energy from the generator reaching the point of connection of the stub divides between the stub and the connection to the load. That portion entering the stub is totaln ly reflected back to the junction, where it divides between the load and the line back toward the generator. By adjusting the junction point of the stub through a range of not over one-half wavelength, and adjusting the effective length of the stub, a point can again be found where the reflections cancel out and a match is achieved. Again it will be noted that there are two adjustments.

3 which must be made independently and that each adjustment varies the amplitude of the resultant reected wave. With this particular type of transformer any degree of mismatch canbe compensated for, although where the mismatch is complete the adjustment becomes extremely critical.` Y

The third general type o f matching transformers of thejcharacterhere consideredemploys two stubs in xed positions, preferably an oddnumber of eighths of a line wavelength apart. By adjusting the vlengths of the two stubs a combination of adjustment can be found which will reect a resultant wave of any phase back toward the generator. There is a limit to the amplitude of the reflected wave, but this arrangement still provides means of balancing wide variations in load impedance and is capable of handling large powers. The point to be noted, however, is that to complete a balance, within the range of which this device iscapable, there are two independent adjustments, each of whichl changes the amplitude of the resultant` reiiectedK wave as viewed from the'generator. Considering the resultant of the waves reflected by the two stubs, simultaneous variation of the stub length can be thought of as varying the efrective point of reflection with respect to the line, while equal and opposite variations in stub length changes the magnitudeA of the reflection but not its effective position, or, therefore, its phase. Each vof the devices described eiectively introduces an impedance discontinuity in the line andcauses a reflection of the incident wave, and each, through two adjustments, can vary the amplitude and phase of the reflected wave until it is equal in magnitude and opposite in phase to the reflected wave caused by the impedance on the balance of the load. Numerous other matching transformers are described and analyzed in vol. 19 of the Radiation Laboratory Series, Microwave Transmission Circuits, v McGraw-Hill, 1948. Nearly all are adjustable in two independent modes, and as will be shown hereinafter the present invention may be applied to any matching transformer which can balance by two adjustments.

Ifa load havingV an unknown degree of imbalance or impedance mismatchwith respect to the line is connected to such a line the amount of mismatch is usually determined by nding the positions of the voltage maxima and minima and thus determining the voltage standing wave ratio. The two adjustments are then made alternately until the VSWR approaches unity. Plfhis may be a tedious'job, sincev either adjustment may change the position of the maxima and the minima as well as their relative magnitude. AThere are formulas and rules for making the adjustment, but it is usually accomplished by cut-and-try.l There are methods of measuring the amplitude of the reflected wave, but this measurement alone is not sufficienttodevelop an error signal which will control a servo-mechanism to make the adjustments automatically. i

The broad purpose of the present invention is, accordingly, to provide means and methods for automatically matching a line to the equipment to which it is connected. To this end, among the objects of the invention are to provide a method of producing an error signal which may beused for the control of a servo-matching mechanism; to provide a method of separating simultaneouslyproduced error signals of this character, so that each error signal is effective to control the adjustment necessary to reduce the error to a minimum; to provide methods and mechanisms whereby the. two adjustments necessary to produce a match may be effected simultaneously, so that the adjustment necessary to effect a balance can be approached by the most direct path and hence in the shortest possible time; to provide a method of servocontrolled matching which is applicable to parallelV wire, coaxial, or wave-guide lines; Vto provide means and methods for balancing mismatches of any magnitude; to

provide mechanisms capable of producing automatic matching which are adapted for various types of services, depending upon the voltage requirement, the power requirement, or both; and to provide automatic matching transformers through the use of well known elements, the characteristics of which have been thoroughly explored and well known, so that transformers can be designed by known formulas to meet any specific requirement.

Broadly considered the self .adjusting matching transformer of this invention comprises means for introducing into a transmission line which is adapted to connect a generator to a load, an impedance irregularity or irregularities, adjustable in two modes to provide a reection of an incident wave which is variable in phase and amplitude throughout the entire range required to balance any mismatch normally to be expected to occur as the result of a connection of theload to a line. Means are provided for cyclically varying each of the two adjustments through an amplitude which is very small in comparisonwith the total range of adjustment required to effect the desired accuracy of balance, to produce a small modulation'in phase, amplitude or both (depending on the adjustment cyclicallyvaried) which when combined with a wave reflected by the load will produce an amplitude modulation in thevresultant wave.

rfhere is also provided, between the generator and the impedance irregularity, a directional coupler, adapted to accept substantially only the reilected wave from the energy transmitted by the line. The modulated wave is detected, to derive therefrom the modulating component; i. e., the component representing the envelop of the wave as modulated by the cyclic variation in adjustment.

, Means are provided for generating a comparison wave,

ofthe same frequency and in phase with the variation in adjustment which causes the modulation of the reected wave. The envelop wave and the comparison wave are supplied to a phase-sensitive detector `or intermodulator, to provide a signal dependingein polarity and magnitude upon the phasev of the envelop wave with respect to the comparison wave. This signal is amplified and supplied to a servo-motor of known type, which is connected to make the major adjustment in the mode which is being cyclically varied to produce the errorsignal. Preferably the cyclic variation of the two lmodes is accomplished in quadrature, the variation in one mode reaching its maximum value as that of the other is passing through its mean value. Two reference waves are, in this case, supplied in quadrature to two phase-sensitive detectors, each detector supplying its own servo-mechanism. The result is that there is no interference between the two sources of modulation, and the two adjustments are effected simultaneously and continuously by the respective servo-mechanisms until the system is brought into balance. It should be noted, however, that while this quadrature arrangement is preferable and leads tobest results, it is not an essential feature of the invention; the sameadjustments can bev effected by a'switching mechanism which makesl the cyclicvariation of the two adjustments alternate through short periods, and switches the error signal alternately to the two servos. y Generally, however, the quadrature cyclic variation of the two adjustments is greatly -to be preferred as'being not only quicker but simpler. j

In order to effect accurate balancey with any load the adjustment in phase must be,V capable of variation through a complete cycle of 360 Aelectrical degrees of the longest wavelength which is to be handled by the system. Unless the limits yofload variation or frequency variation which are to be expected are accurately known, there is always the possibilityl that when one of the adjustments is near the limit of its range the error signal produced would be .of such polarity as to Vtend Vto carryfthe adjustment beyond that limit, to a point adjacent to the opposite end of the' range'. If the application of the device is such that this eventually can occur there is "also provided, at each end of the adjustment range, a limit switch, connected to actuate a scheduling device com prising means for overriding the servo-signal and moving the adjustment to the other end of its range, where the arrangement is such that the actuation of the second limit switch removes the overriding signal and permits the error signal again to take control, in which case it is in the proper direction to effect the desired adjustment.

The accompanying drawings illustrate several embodiments of the invention which will next be described in detail. In the drawings:

Fig. 1 is a block diagram illustrative of the invention as applied to a wave guide supplied by a microwave oscillator and feeding a variable load;

Fig. 2 is an isometric view, largely schematic, illus trating the essential features of one form of the matching transformers proper, as illustrated in Fig. 1;

Fig. 3 is a fragmentary cross-sectional View, taken in the plane indicated by the lines 3-3 of Fig. 2, illustrating the scanning or sensing mechanism controlling the adjustments of the transformer;

Fig. 4 is a group of curves illustrating the variation in amplitude of resultant wave by variation in the two adjustments of the transformer shown in the rst three iigures;

Fig. 5 is a schematic diagram of a self-adjusting matching transformer embodying the present invention and adapted to handle higher powers `and voltages than the form illustrated in Fig. l;

Fig. 6 is a schematic circuit diagram of a characteris tic phase-detection and servo-system, adapted for use with either form of transformer illustrated;

Fig. 7 is a circuit diagram of the limit switch and scheduling device as used with either form of transformer.

Considering iirst the embodiment of the invention illustrated in Figs. l, 2 and 3, there is shown a microwave system comprising a generator 1, supplying a rectangular wave guide 3, which, in turn, is connected to a load 5. The load is indicated as a variable resistance, but it is to be understood that the variation may be either resistive or reactive, that the load may be another line, and that the variation may be caused either by the load itself or by a coupling device (such as a rotary joint) through which the load is connected.

Connected as one section of the line, and preferably as near to the variable load as is mechanically feasible, is a line section 7 of a length which is longer than onehalf guide wavelength at the lowest frequency to be transmitted by the device. Mounted `on the line section 7 is a carriage 9 which is movable over a one-half guide wavelength range by a servo-motor 11. In Fig. 1 the servo-motor is shown in the upper right-hand corner of thergure, completely disassociated from the carriage itself, since Fig. l is intended to illustrate primarily the circuit arrangements.

One possible physical arrangement of the motor 11 is illustrated in Fig. 2 in its actual relationship to the carriage and the wave guide. As can be seen from the second figure, the servo-motor 11 is fixed to one side of the guide section 7, with its axis parallel to the axis of the section. A lead screw 13 is continuous with the shaft of the servo-motor, and its distal end is journaled in a bearing 15 on the opposite end of the guide section 7 from that to which the servo-motor itself is attached. The servo-motor may be one of several known types; it may, for example, be a D. C. motor having a permanent-magnet field and an armature supplied by the error signal which controls the servo-action. The direction and speed of rotation of sucha motor depend upon the polarity at which the control currents through the armature is supplied. That illustrated in the device shown is 'a split-phase induction motor wherein the excitation of the'split-phase winding isfreversible in phase and variable inA amplitude.

The carriage 9 is of box-like form, tted around and sliding upon the guide section 7. On its side is a boss 17, carrying a nut (not shown) engaging the lead screw 13. Rotation of the motor shaft will therefore move the carriage `in one direction or the other along the guide, depending upon the direction of rotation of the servomotor 11.

A slot 19 is formed in the center of the wider face of the guide section 7, this slot having a length of at least one-half guide wavelength at the lowest input frequency. A similar slot 19 (shown only in Fig. 3) is in this case formed in the opposite face of the guide section. Through these slots project the elements used to produce the impedance discontinuity in the guide. The body of the carriage is long enough to cover the slots at the extreme limits of its motion, shielding the slots and preventing radiation therefrom. Although the slots themselves proav duce a slight discontinuity in the line impedance this can `be tuned out by the transformer adjustments, together with the load mismatch.

In the form of invention here shown the major ad justment of the impedance discontinuity and the arrangement for varying its magnitude cyclically are accomplished by mechanically separate mechanisms, the major adjustment beinU effected through the upper slot 19 and the cyclic variation through the lower slot 19'. With the type of matching transformer shown in this embodi-y with the guide at the position of maximum electric iield.`

The probe increases the capacity of the guide at this point by an amount depending upon the distance which the probe projects toward the opposite face of the guide. In this embodiment of the invention, therefore, the mag# nitude of the relieetion produced by the probe is dependent upon the degree 'into which it extends into the guide, and the phase of the reflection as viewed from the generator end of the line, is dependent upon the position of the carriage along the guide section.

In this case the main probe comprises a small threaded rod 21 which is splined within the hollow shaft 23 of a servo-motor 25, the servo-motor being fixed to the carriage with its axis vertical. The threaded rod 21 engages a fixed nut 27, so that when rotated by the servoshaft the rod moves up or down, to be projected a greater or less distance through the slot 19, into `the guide section. character as the servo-motor 11.

The cyclically adjustable impedance element is mounted in a housing 29 which hangs from the underside of the carriage 9. This element is a small sensing or scan ning probe 31 which projects through the slot 19', in the center of the underface of the guide section 7, through a small sleeve bearing 33, pivoted for oscillation around an axis transverse to the axis of the guide section 7. The probe 31 is mounted pivotally to a very small crank 35, driven by a constant-speed motor 37. Rotation of the motor shaft, which is centered immediately below theY pivotal mounting of the sleeve bearing 33, slides and oscillates the probe 31 through the bearing. The parts are so proportioned that when the crank 35 is at the bottom of its travel the tip of the probe barely projects in the guide section, while when at the top of its travel it projects a short distance (perhaps a millimeter) therein. The tip of the shaft therefore executes a substantially circular orbital motion. Such motion can be resolved into sine and cosine components, the sine component resulting in a substantially sinusoidal variation in the effectivecapacity introduced by the probe while the cosine The servo-motor 25 can be of the same general i? component introduces a variation in the point of reflectionof the wave, and therefore its phase as viewed from the termination of the line.

Means are provided for supplying two comparison waves, in phase, respectively, with the sine and cosine components of the motion of the end of the probe 31. If the motor 37 is driven by alternating current from an external source, sine and cosine components may be derived from `suitable phase-splitting circuits connected to such source. In the case shown, however, a two phase spin generator 39 is fixed to the shaft of the motor 37, which projects out through the housing 29 to where the spin generator 39 is connected. The spin generator can be very simple in form, the rotating element being a small permanent magnet iixed to the yshaft vand coupling with two pairs of pole pieces arranged in quadrature. These pole pieces carry windings, those on op posite pole pieces being connected and each pair of pole pieces supplying one phase; i. e., one of the two carrier waves. The actual power required for either the oscillation of the sensing probe or the generation of the cornparison waves is very small. Both the motor 37 and the spin generator 39 can be very minute.

The reflected waves set up by the impedance discontinuity introduced into the line by the probes 2. and 31,

respectively, are picked up by a directional .coupler 41, coupled into the wave guide 3 on the generator side of the transformer. Any one ot the known types of directional coupler may be used for this purpose, several being described in Technique of Microwave Measure mer1ts,7 vol. 1l, Radiation Laboratory Series, McGraw- Hill, 1947. The coupler shown comprises an auxiliary guide lying parallel to the guide 3 and coupled thereto by two small holes, one-quarter guide wavelength apart at the mid-frequency which the system is designed to handle. The reflected wave, traveling from right to left as shown in Fig. l, is coupled into the guide in such fashion that the waves induced through the two holes are in phase to excite in the guide a wave traveling toward the right, whereas the power wave, traveling in the opposite direction, is out of phase with respect to right-traveling waves at the two couplings. Hence waves traveling in the auxiliary guide from right 'to left are cancelled out in the direction toward the left, but travel only toward the right. To the right the auxiliary guide is terminated in its characteristic impedance by an absorber 43, so that the power waves induced in the guide are completely absorbed and not reflected toward the right. The coupling is very loose, so that the wave transmitted through the directional coupler is from l0 to 20 db in level below the reiiected wave as it exists in the line 3.

As picked up by the directional coupler, the reilected wave is detected by a crystal detector 45, to develop a direct current modulated by a wave which is proportional only to the modulating component introduced by the oscillation of the sensing probe 31. The envelop wave thus developed is amplified in an A. C. amplifier 47 ot conventional type, and thence is conducted in parallel to two phase-sensitive detectors 491 and 492, where it is intermodulated with the comparison waves developed by the spin generator 39. The resultant waves from the two phase-detectors are supplied to servo-amplifiers 511 and 512, to develop the actual control or error signals for the two servos 11 and 25.

One such circuit arrangement as has just been generally described is shown schematically in Fig. 6. It may be assumed, for purposes of illustration, that the motor 37 is of the synchronous type, operating at 3600 R. P. M., and that therefore the spin generator 39 will deliver, from its two windings 39' and 3?", 6G cycle, twophase current. This is supplied to primary windings of identical input transformers 531 and 532 of the phasesensitive detectors 491 and 492. Each of these transformers has an accurately balanced, centerk tapped ysecondary, andthe center tap ofeach is fed with the envelop signal 'from amplifier 47. Each end of the secondary of each of transformers 531, 532, connects to a rectifier 55, which may be a diode or a contact rectifier, such as a copper oxide or selenium rectifier, since each is only required to rectify 60 cycle current. The two rectifiers 5:1' of each detector connect across the terminals of a pair of lcondensers 57, 57 in series, their junction being grounded to complete the circuit back to the grounded terminal of amplifiers 47. Bridged across each of condensers is a resistor 59, 59' and it is important that both condensers and resistors be accurately balanced.

In general the excitation of the transformers 53 by the reference signal will be materially greater than their excitation by the envelop signal. When the reference signal has one polarity current can flow through one-half of the secondary only, charging one of the condensers (say, 57) and leaking back to ground through the shunting resistor 59. On the other half-cycle current will flow through the other half of the circuit through the symmetricai path, charging the other condenser 57' equally, so that there will vbe no potential difference across the two condensers, provided the time-constant of condensers 57, 57 and their bridging resistors 59, 59' is long in comparison with the 60 cycle reference frequency rate. The charge on the condensers provides a backvoltage which prevents current iiow through thek associated rectifier circuits except at the crests of the applied voltage waves.

If, however, the 60 cycle signal supplied by amplifier d'7 is in phase with the reference signal or has an in-phase component, it will add to the voltage applied to one of the rectifiers 55 but will subtract from that applied to the other, and as a result produce an unbalance between the charges on the two condensers 57, 57 depending upon whether the positive swing of the envelop signal is in phase with the reference signal as applied to one or the other of the two rectifiers 55.

lf, instead of being in phase with the reference signal as applied to either rectifier of the detector circuit the envelop signal is 90 degrees out of phase, it will add to (and :subtract from) the two halves of the wave equally and the potential dierence developed across the output terminals will therefore become Zero. At intermediate phases the potential diference developed will be substantially proportional in sign and amplitude to the arnplitude of the signal times the cosine of phase angle with respect to the reference or comparison signal.

lt will be seen therefore that the phase-sensitive detectors are in fact balanced modulators; their output contains the sum and difference frequencies of the two signals supplied to them; i. e., D. C. and a double-frequency component, and the latter is largely filtered out by the condensers 57, 57. It follows that practically any type of balanced modulator can be used as a phase-sensitive detector, and, from the general principles of modulation, will separate quadrature components of a message signal with reference to a comparison signal which represents the carrien ln general the envelop will have two components in quadrature, due to the quadrature components of oscillation of the sensing probe 31. The two phase detectors therefore resolve these two components, detector 491 being responsive only to the component of probe motion along the guide while detector 492 responds only to that component of the signal resultant from the vertical component oi motion of the probe, into and out of the guide.

The potentials developed across the detector output are supplied respectively to the two servo-amplifiers 511 and 512 and since these amplifiers are identical a description of one will serve for both. Each of these amplifiers comprises two sections, a D. C. vacuum-tube amplifier and a magnetic amplifier. The D. C. amplifiers comprise, in the case illustrated, a pair of triodes 61, 61', connected inV push-pull, each having the cathode grounded through the usual biasing resistor 63, with the detector output terminals connecting respectively to the two grids. The anodes of the two tubes are connected to the biasing windings 65, 65' of the magnetic amplifier transformers, these windings being connected in series.v The junction point ofv the twin windings is connected to a suitable source of anode potential for the triodes 61. The space current for the two tubes flows through the biasing windings and the adjustment of the bias applied to tubes 61, 61 is such that the cores of these transformers are magnetized to the knee of their magnetization curves by the normal space current. When the detector supplying the two push-pull tubes is unbalanced, however, the space current flowing through one-half of the biasing winding willincrease whilethat flowing through the other is decreased. The result is an imbalance; the permeability of one trans former is increased while that of the other is decreased.

The differential bias is applied control of direction of rotation of the servo-motor. This motor obtains its driving power from a suitable source, which may be either the same 6() cycle supply as that used to drive the motor 37 or it may be a higher frequency; in one instance a 400 cycle supply was used as leading to smaller servos, but, as has been stated, it makes no difference theoretically what frequency is used. The supply circuit,

indicated by the reference character 67, has two branches. One of these branches is connected directly to one phase winding 69 of the servo-motor to be controlled. The second parallel branch connects to two series-connected windings 71 and 71 coupled, respectively, to the biasing windings 65 and 65. The windings 71 and 71 are also coupled to tertiary windings 73 and 73', connected in series butin bucking relationship to the split-phase winding .69 of the servo-motor. A phase rotating condenser 75, bridges the motor winding 69.

When the two windings 65 and 65 are equally biased, equal and opposite potentials are developed in the tertiary windings 73 and 73. No current therefore flows in the out-offphase coil of the servo-motor, which therefore does not rotate. When the current flowing in coils 65 and 65 ,is unequal; if, for an example, the current through winding v65 exceeds that flowing in 65', the permeability ofthe cores coupling windings 65, 71, and 73 is decreased while the effective permeability of the cores coupling coils 65', 71 and 73 is increased. Hence higher potentials will be 'developed in coil 73' than in coil 73, and a resultnatcurrent will flow in motor winding 69. Owing tothe phase rotation introduced in the circuit core by thecondens'er 75 the current rflowing will have a quadrature component vwith respect to that flowing in coil 69, and a yresultant torque will therefore be effective on the servo. 11n case the current flowing in coil 65 is greater than .in 65, the potential developed across coil 73 will dominate, the current through coil 69' will reverse in phase and the servo willoperate in the opposite direction.

It is to be understoodthat this is merely one of a num ber' of servocircuits, and various others may be used to produce the effect desired.

lIt has been shown that the two-phase modulation introduced by the scanning probe 31 can be resolved into its two components and that these components will separately control the two servos which control respectively the two modes of adjustment used with this form of the invention, it remains, however, to show that whenseparatelyVr and'simultaneously adjusted in this manner the device kwill produce-a prompt and effective match, and that having achieved such a matchunder a. given condition of frequency` and load that the adjustment will be stable. 1 f

Both adjustments result in an amplitude modulation of` ther resultant reflected wave if `a load reflection exists. Preferablythe degree of modulation capable of being soprodnced by the two components of the motion of the scanning probe are equal, so that the resultant wave has atphase angle of 45y degrees withrespectvto either of `the two components. Consider first the situation Where there4 is a reflected wave existing in the system as a result of a mismatched load. superimposed upon this is a reflected Wave from the probe, and this reflected wave has itself two components, one from the main probe, as adjusted by the servo, and the other by the scanning probe. Taking first the effect of the motion of the carriage, and remembering that the character of the irregularity in impedance introduced by the scanning probe is identical in character with that introduced by the main probe, so that at the median horizontal position of the scanning probe the reflected waves from the two probes will be in phase, the horizontal motion of the probe will change only the phase of the wave which it reflects and not its magnitude. When the probe is moving in one direction it will bring the wave reflected from the probe more nearly into phase with the reflected wave from the load and increase the instantaneous amplitude of the reflected wave, while motion in the opposite direction will decrease the amplitude of the resultant reflected wave by carrying that reflected from the probe further out of phase with reflection from the load. With the servos properly connected this will rotate the lead screw 13 in the proper direction still further to decrease the amplitude of the resultant, and such motion will continue as long as motion of the probe in the direction of the motion of the carriage imparted by the servo continues to cause a decrease in the amplitude of the resultant; i. e., until a position of balance has been attained. Once this position has been reached, however, motion of the scanning probe to either side of the mean position will cause an increase of the amplitude of the reflection, and this results in the generation of a double frequency in the resultant period. Consideration of the detector circuit will show that this results in equal flow of current in the two halves of the balanced circuit, with no potential difference appearing across the detector circuit and hence no control valtage applied to the servo-mechanism.

A similar condition obtains with respect to the vertical component of motion of the probe. If the capacity loading provided by the probe is too great, even though the reflected waves from the load and the probe are in reverse phase, further projection of the scanning probe into the guide will cause an increase in the amplitude in the resultant wave, while withdrawal of the scanning probev from the guide will cause a decrease in amplitude, giving a resultant phase to the modulation which causes the servo 25 to withdraw the main probe. If the main probe is not far enough into the guide to effect a balance, the cyclic motion of the probe will reverse the phase of the modulation on the resultant wave, since motion into the guide will decrease the resultant amplitude and cause the probe to be introduced further into the guide. As in this case first discussed, when the probe has reached the proper value, the oscillation of the scanning probe will result in increased amplitude on both sides of its mean position, with the result that a double frequency component will be generated, which, as in the first case discussed, does not result in the development of an effective control voltage.

These relationships will be evident fromv a consideration of the equation for the sum of two sine functions:

sin fbg-sin 3:2 sin 1/2(A+B) cos 1/2(A-B) Calling the phase angle wt of the wave reflected from the load mismatch A and that reflected from the probe B, we can put B=A-(BAB) where B is the average difference in phase angle between. the two reflected waves and AB is the shift in phase angle due to the oscillation of the probe longitudinally of the wave guide. The equation then becomes B' in@ Here the sine term represents the phase of the reflected I1 wave and the cosine term its amplitude. The envelop detector responds only to amplitude and does not recognize phase. When the transformer is in balance, B=l80 and The cosine term therefore becomes equal to sin- L 2 As the electrical angle through which the probe is oscillated is small, the sine varies substantially directly as the angle, and the lamplitude of the resultant wave increases on each side of the position of balance, giving the envelop the form of a rectitied Vsine wave. This is illustrated in Fig. 4, where, in the upper group of curves, the solid line curve n n is a plot of one cycle of the excursion of the sensing probe when so positioned as to balance the transformer and load. As has been shown immediately above, the amplitude of the resultant reected wave varies directly as the displacement of the probe from the central position, and therefore the absolute value of curve a a', also represents the amplitude of modulation of the envelop of the resultant wave, but since the detector recognizes only the amplitude, its output is represented by the lobe a of the solid curve plus the a, dotted lobe, i. e., the detector output has the form of a rectified sine wave. The reference wave is exactly in phase with the displacement a a. Since the phasesensitive detector gives a response of one polarity when the reference and envelop waves are in phase at one secondary terminal of transformer 53 and gives equal response of the opposite polarity when the reference and envelop waves are in phase at the secondary terminal, it will be seen from these curves that the signals which will be developed in the two half-cycles of the reference wave are equal and opposite and the phase-sensitivedetector or intcrmodulator will develop no net control signal.

The second set of curves of Fig. 4 represents in generally similar manner the variation of the resultant wave in response to the oscillation of the scanning probe into and out of the wave guide. It is again assumed that the probe is adjusted for balance at the mean position, and a single cycle is shown. Penetration of the probe into the guide being in quadrature with its excursion along the guide, the solid curve b, b b is thus shown, in the same time interval as that represented by the curve a, a in the curves above. The lobes b, b b, indicate the modulating component as seen in the output of the envelop detector. With regard to the response of the phase-sensitive detector when supplied by the envelop wave it will be seen that it will supply no net control signal to the servo which it governs.

The lowest curve of the group, curve c, is the sum of the two components of the rectified wave from the envelop detector. This is the wave form actually delivered to the maplier 47. Being an A. C. amplier, this eliminates the D. C. component, so that the wave fed in parallel to the two phase-sensitive detectors constitutes the alternating component only. lt will be seen that this wave delivers no net control signal when intermodulated with either of the reference waves, as it has equal lobes above and below its axis in any half-cycle of the reference Wave, whatever the phase of the latter may be.

lf, when thc system is put into operation, the carriage is in such a position as to create a maximum possible mismatch; i. e., so that when the scanning probe is in its mean position the two reected waves are exactly in phase and the resultant wave has its maximum amplitude, a longitudinal excursion of the probe within the guide will cause equal decreases, instead of increases, in the amplitude of the resultant wave. Again there will be no control lsignal developed, but the equilibrium is unstable and any deviation will cause a control signal to be developed which will cause the carriage to seek its balance. If the phase angle of the wave reflected by the mismatched load varies only through a limited and known range, it maybe possible to position the transformer at a point such that a complete mismatch can occur only at the extreme ends of the travel of the carriage, in which case the unstable balance just described could not occur. In the general case however, it must be assumed that a maximum mismatch in phase may occur. When it does, and any disturbance occurs which will upset the unstable equilibrium, it is entirely a matter of chance in which direction the carriage will move to seek its balance. If the balance point is near one end of the travel range and the carriage starts to seek a balance in the opposite direction, the probe will reach the end of its slot before the balance is attained.

in order to prevent such a situation resulting in a complete stall, limit switches are provided in the positions shown at 76, 76 in Fig. l. These switches are so connected that when one is actuated the servo-signal is overridden and the carriage is driven to the opposite end of its travel and into contact with the other limit switch. Operation of the second limit switch interrupts the overriding signal and restores the servo-control, and since the carria'g'e'has been moved at least a half wavelength along the line, the reestablished servo-control is in the right direction and moves the carriage away from the second limit switch and into a position of a balance.

Fig. 7 shows schematically one arrangement of the limit switches and the accompanying circuitry, through which they are connected to the servo-motor 25. The switches 76, '76 are double-pole switches which are spring biased so'as to keep one contact normally closed but to open this contact and Iclose the other when actuated by the pressure of the carriage-9 'against them. v Inthe diagram they are shown in their normal position, in which the normally closed contact is connected to a source of direct current (not shown) the other terminal of which is grounded. Associated with each of the limit switches are relays, 77, 77. The relays are identical, each having three sets of double-throw contact arms, and normally closed and normally open contacts are provided for each of the contact arms designated, on the relay 77, as 77-1, 77-2, and 77-3. The switch 77 has contacts designated in the same mannerexcept for the accent. The actuating coil 78 ofrelay 77 connects to the normally open contact of switch 76 through a resistor 79, and the coil is bridged by a condenser 80. The resistor yand relay comprise 4a delay circuit; when the normally open Contact of switch '76 is closed the relay does not respond until the conl denseir80 Vhas had time to charge to some predetermined value through fthe resistor 79. A delay of about 1 second in the lresponse time of the relay is thus provided.

The split-phase winding 69 of the servo-motor 25 is connected to the windings of the magnetic amplifier through the lnormally closed contacts 77-2, 77'-2, with its return circuit through the normally closed contacts 77"-3, 77-3. During the period when the condenser 80 is charging this connection persists, maintaining the pressu-re against the limit switch 76 and holding the contact closed until the relay has had time to act. Actuation of the relay closes the upper, holding contact 77-1 through the normally closed contact of limit switch 76', and therefore holds relay 77 closed until the opposite limit switch is actuated. At the same time the split-phase winding of the servo-'motor is disconnected `from the servo-amplier by the actuation of contacts 77-2 and 77-3, which connect the servo winding directly across the leads of the main Ysupply line in series with a phase-splitting condenser 81. The connection is in such direction that the carriage immediately moves to the opposite end of its trav'ehwhere it operates switch 76 and thus interrupts the current through the holding contact.

Because the inductanceof coil 78 tends to maintain the ow of current, condenser 80 discharges very rapidly and the relay releases almost immediately, before the delay circuit 79', 80 will permit relay 77 to close. The control current of the yservo system immediately takes charge, moving the carriage away from limit switch 76', therefore reopening the limit switch before relay 77' can operate.

Relay 77' is connected in a manner *which is identical with that of relay 77, except that closure of the normally open contacts 77-2 and 77 '-3 reverses the connection of the split-phase servo winding with respect to the supply line, and therefore operates the carriage in the opposite direction when relay 77' closes. y

Ordinarily limit switches are not required in connection with the movementof the probe 21. If, in some application, it should be necessary, they can be actuated in a similar manner by a similar circuit.

An embodiment of the invention which is capable of matching impedances within a more limited range, but which, on the other hand, is capable of handling much more power is illustrated in Fig. 5. In this embodiment of the invention the line 81, like the line 3 of Fig. l, is illustrated as awave guide, but it will become apparent that the same general arrangement can be utilized with coaxial lines, and, although with somewhat less convenience, with parallel-Wire lines. f

The line 81 is supplied by a generator 82 and feeds a load '83. Branching otf from line 81 are two `stub lines, 85 and 85', spaced an odd number of eighths of a guide wavelength apart at the mid-frequency of the band which the systemis intended to utilize. Within each of the stubs isa shorting plug, 87, 87'. Each of these plugs should make good effective contact lwith the stub, so that the resistance of the contact is as low as possible. Various types of plugs for this purpose are shown in Microwave Transmission Circuits, cited supra, and hence a diagrammatic showing is Iall that is here necessary. The shorting plugs are carried on plungers 88, 88', by which their position within the stubs can be adjusted. The ends of the plungers extend outwardly through the ends of the stub lines to carriages 89, 89', and the carriages are mounted for movement with respect to the stubs by means of servo-motors 90, 90', each driving a lead screw 91, 91', threading a nut on the carriage.

Mounted on each carriage is a vibrator, 92, 92', and the ends of the plungers 87, 87 are secured to the armatures 93, 93 of the vibrators. The coils of the vibrators are excited in quadrature from a two-phase source (not shown), so that lthe plungers oscillate, in quadrature, through a very small amplitude within their respective stubs. This same source is used to supply the reference frequency waves for the two phase-sensitive detectors. The remainder of the circuitry is not shown since the detection and `servo-circuits may be identical with those used in connection with the first described embodiment of the invention. Limit switches as described, can, of course, be used with this embodiment in the manner already described.

As has been stated, the form of the invention here shown is capable of tuning only through a limited range, since it cannot increase the effective conductance of the line to a value greater than twice that of the line. It can tune out completely impedance mismatches which produce VSWRs which do not exceed 2, and will effect reasonably acceptable matches with VSWRs up to about 2.5.

From the diverse forms of equipment in which the invention can be embodied, Iand the comments which have been made with respect thereto, it should be evident that the particular forms described are merely illustrative of a few of the many which the invention may take. There are many known equivalents for each of the elements that have been described, and those which have been described in detail constitute only a few of such possible equivalents. The invention, therefore, is considered to lie in the combination and not in the individual elements themselves, and` to be limited inV scope only by such limitations as are specifically expressed in the claims i which follow.

What is claimed is as follows:

a control voltage supplied thereto, probe means mounted on said carriage and movably mounted to vary the distance into said wave guide to which it extends, a secondA servo-system adapted to vary said distance in either direction in response to the polarity of a control signal applied thereto, additional means on said carriage for simultane.

ously and cyclically varying in quadrature the positions of said probe means withY respect to said carriage bothlongitudinally of said 'wave guide `and as to its extension` therein, means for generating a pair of reference Awaves in phase respectively with the cyclic variations in position, a directional coupler to said line adapted to respond to waves reflected from said probe means, means for detect-A ing the modulation envelop of waves exciting said directional coupler, and means for intermodulating said reference waves and said envelop to provide control signals to the respective servo-systems.

2. An impedance matching device for use in com bination with a transmission line connecting a generator' of electrical waves with a load, comprising a carriage adapted to be mounted for movement relative to said line, means mounted on said carriage for introducing an; impedance discontinuity in said line so that movement of said carriage will vary the phase of refiections caused' in comparison with the range of movement of said carriage to produce a modulation of waves reflected by said discontinuity, means responsive to the modulation of'said reliected waves for producing an error signal representative of the degree of mismatch of said load as supplied through said matching device, a servomechanism responsive to said error signal for moving said carriage to a position producing an optimum impedance match, a pair of limit switches mounted respectively at each end of said range of movement, and means responsive to the actuation of either limit switch for overriding said error signal to move said carriage to the` opposite end of its range of movement and for re-establishing control of said carriage movement by said error signal upon operation of the other of said limit switches.

3. A self-adjusting matching transformer for use in a wave guide transmission line connecting an oscillation generator and a load, comprising a probe projecting into said wave guide, a first servo-motor mechanically connected to vary the distance said probe projects into said wave guide, means for cyclically varying the susceptance introduced into said wave guide by said probe, a second servo-motor mechanically connected to move the position of said probe longitudinally of said wave guide, means for cyclically varying the effective position longitudinally of said wave guide of the susceptance introduced therein by said probe in quadrature with the cyclic variations of susceptance, a directional coupler connected to said wave guide between said generator and said probe for accepting waves reliected from the direction of said probe, means for developing a pair of reference waves in phase respectively with the cyclic variations in susceptance and effective position of said probe, a pair of phase-sensitive detectors connected to said directional coupler and supplied respectively by said reference waves,

and means for applying the output of said detectors to control respectively said servo-motors.

4. A self-adjusting matching transformer for use in a wave guide connecting an oscillation generator and a load, comprising a carriage mounted for longitudinal movement along said wave guide, a irst servo-motor connected to move said carriage along said wave guide in response to an error signal, a main probe mounted on said carriage and projectinginto said wave guide, a second servo motor geared to vary the projection of said main probe into said wave guide in response to a second error signal, an auxiliary probe mounted on said carriage so as to permit movement of the end thereof in an orbital path, said movementhaving quadrature components longitudinal of and transverse to the axis of said wave guide, a motor on said carriage for continuously circulating the tip of said auxiliary probe in said orbital path at a constant rate, a directional coupler connecting out of said wave guide between said carriage and said generator responsive to waves traveling toward said generator, means for developing a pair of reference waves in phase respectively with said components of the orbital motion of said auxiliary probe, and phase-sensitive detector means for comparing the envelops of waves received by said directional coupler with said reference waves and developing therefrom error signals for controlling said rst and second servo-motors respectively. o

5. In a self-balancing matching transformer for use in combination with a wave guide a carriage mounted for longitudinal movement along said wave guide,` a first mounted on said carriage and projecting into said wave guide, a second servo-motor geared to vary the projection of said main proberinto said wave guide in response to a second error signal, an auxiliary probe mounted on said carriage `so as to permit movement of the end thereof in an orbital path, said movement having quadrature components longitudinal of and transverse to the axis of said waveguide, and a motor on said carriage adapted to circulate the tip of said auxiliary probe in said orbital path at a constant rate. y j

6. In a self-balancing matching transformer for use in a transmission line, a carriage mounted for longitudinal movement along said line through a range of at least as great as one half wavelength within said lwave guide of the waves transmitted thereby, means on said carriage for introducing an impedance irregularity in' said line, a servo-motor geared `to adjust said carriage in response to error 4signalsdevelopved in response to Waves reected by said irregularity, limit switches operative at the limits of therrange of motion of said carriage, and means responsive to theA actuation of the limit switch at either limit of said range of motion for over-riding said error signals and reversing said servo-motor until said carriage has traveled to the opposite end of its range of motion and operated the other of said limit switches.

References Cited in the le of this patent UNITED STATES PATENTS 2,404,568 Dow July 23, 1946 2,611,030 Sontheimer Sept. 16, 1952 2,615,947 Macaluso Oct. 28, 1952 Y 2,680,837 Sensiper June 8, 1954

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