US1846701A - Radiofrequency transformer - Google Patents

Description

Feb. 23, 1932. H, WHEELER 1,846,701

RADIOFREQUENCY TRANSFORMER Original Filed June 20, 1929 I INVENTOR flAfioLo A WHEfL ER BY M,M,Zf d

ATTORNEYS Patented Feb. 23, 1932 UNITED STATES PATENT OFFICE HAROLD A. WHEELER, F JACKSON HEIGHTS, NEW YORK, ASSIGINOR TO HAZELTINE GORP ORATION RADIOFREQUENCY TRANSFORMER Original application filed June 20, 1929, Serial No. 372,275. Divided and this application filed Kay 28, 193%). Serial No. 456,474.

This invention relates to high-frequency thermionic amplifiers and more particularly to turnable amplifiers of this type wherein by the utilization of suitable coupling networks containing capacitive and inductive impedance elements associated with the input sections of the successive amplifier stages, the amplification may be caused to vary in a preselected manner as the tuning 1 is varied over a desired frequency range.

This application is a division of m copending application, Serial No. 372,27 5, filed June 20, 1929. I

An object of the invention is to embody in an amplifier having the above characteristics,

neutralizing means for minimizing any fecd back of energy from the output to the input sections of the individual amplifierstages during operation, which energy transfer usually occurs as a result of the interelectrode capacities of the thermionic tubes and tends to set up sustained oscillations with resultant signal distortion, squeals and whistles.

A second object of the invention is to provide an amplifier of the type specified adaptwith the networks associatedwith the suc-' ceeding stages.

A further object of the invention is to furnish specific design data for a high-frequency transformer structure the use of which as a part of the coupling networks will rcsultin a high and substantially uniform degree of amplication over a desired radio-fie quency range.

Other objects will become apparent from the subsequent detailed description.

It is a Well known fact that by interposing between successive amplifer tubes a coupling network having a primary transformer winding across the output terminals of the first tube and a tuned secondary winding'across the terminals of the succeeding tube, an amplifier is obtained wherein the overall a1nplification and also the impedance across the output of the first tube increases as the tuned frequency is increased. The increased am-= plification with frequency is undesirable since it prevents the amplifier from being uniformly sensitive and free from oscillations over a wide range of frequencies The increase with frequency of the primary impedance is the cause of the corresponding increase of amplification andoffeedback currents from plate to grid of the tube.

In order to overcome the disadvantages of the generally known standard type of coupling system, C. E. Trube in his application for United States Letters Patent, Serial No. 120,045, filed July 2, 1926, and Patent No. 1,763,380, issued June 30, 1939, has disclosed improved types of coupling systems, by the pro-per design of which the amplification of a system may be caused to var in a preselected manner as the tuning requency 1s ad uSted over a desired range. These coupling systems are in general characterized by the additional feature that the total primary impedance connected thereby across the output terminals of a tube may be substantially constant or even decreasing with increased frequency, thereby minimizing the tendency for the setting up of sustained 0scillations at the higher frequencies due to the regenerative feedback action of the tubes.

To accomplish the above results, the mentioned Trube coupling systems employ a coupling network one portion of which has a capacitive reactance over a desired high-frequency range and another portion an inductive reactance over the same range. The impedance, and hence the voltage, across the capacitive element, of course, decreases with increase in frequency; while for the inductive element the same factors increase with fre quency. By suitably proportioning the capacitive and inductive elements relatively and suitably coupling the same with the input terminals of the succeeding tube, it is apparent that the resultant effect on the tube input can be caused to vary in a preselected manner with frequency over a desired range. Likewise, if the capacitive and inductive elements be suitably connected across the output terminals of the preceding tube as, for eX- ample, by connecting them in series, then the impedance across the tube output can likewise be caused to vary in a desired manner with frequency, for as the impedance of one element increases, that of the other element decreases and any desired resultant effect may be obtained.

Despite the improvements introduced by the Trube coupling systems, the amplificatio-n obtainable is still limited by the feedin radio receiving systems, it is desirable that back action of the tubes at the higher frequencies, because of the fact that the coupling between grid and plate circuits due to the P interelectrode tube capacity increases with increase in frequency. The present invention, therefore, embodies in an amplifier utilizing the Trube coupling networks, suitable neutralizing means for minimizing transfer of energy from the output to the input of the successive tubes. With such an arrangement all the advantages of the Trube system are available and in addition a much higher de gree of amplification over a desired frequency range is attainable.

When utilizing high-frequency amplifiers the antenna system have the same impedance characteristics over the operating frequency range as the output impedance of the successive amplifier stages, since such an arrangement permits the same type of coupling network to be efficiently utilized between the antenna and the first amplifier stage as is used to interconnect the successive stages.

For effective operation, the coupling networks must be designed to work efliciently between the impedances which they inter connect. Thus if, as is generally the case, the

. antenna impedance is quite difl'erent from the output impedance of the successive amplifier stages, optimum design would call for a type of coupling network between. the antenna and the first amplifier tube difierent from that utilized for interstage coupling.

Such a non-uniform arrangement is, of course, undesirable from a manufacturing I standpoint since cheaper set construction could be obtained with the same type of coudesirable since the first coupling network' would tune at a different adjusting position from the others, which would prohibit the use of unitary control means including gang condensers and the like for tuning purposes.

The present invention provides a neutral ized high-frequency amplifier wherein the output impedances of the successive amplifier stages are capacitive over the operating frequency range, due to the use of a relatively large neutralizing condenser connected to each plate. Thus, by suitably proportioning the antenna structure or associating a fixed capacity of proper magnitude with the antenna circuit, the resultant antenna capacity can be made to approximate the neutralizing capacities of the amplifier stages. As a result, the same type of coupling network may be utilized for coupling the antenna and first tube as is utilized interstage and, furthermore, such networks will be identically tunable, thereby embodying in the amplifier the advantages of unit-controlled tuning pointed out above.

Referring now to the drawings:

Fig. 1 shows a circuit diagram of a comlete radio receiving system embodying a unicontrolled, tuned high-frequency amplifier constructed in accordance with the present invention.

Figs. 2 and 3 show modified circuit arrangements which are approximately equivalent to the circuit of 1.

Figs. 4 and 5 disclose the structural details of a special three-winding high-frequency cuit, anda plateP cooperating with the filament or cathode K to form an output section commonly termed the plate circuit of the tube. The batteries 4 supply the necessary negative bias on the grids of the amplifier tubes, while the batteries 5 supply the necessary direct-current voltages for the plate circuits of the successive tubes. Batteries 14 furnish the necessary energy for heating the filaments or cathodes, K.

Associated with the input to each tube is a coupling network comprising capacities C and G together with a three-winding transformer T having a single secondary and two primary windings. The secondary winding L shunted by the variable capacity C constitutes a tunable resonant circuit connected between grid and filament of the succeeding tube. The primary circuit connected between plate and filament of the prccedin g tube, or to the antenna system terminals 11, comprises the winding L, shunted by the fixed capacity (7;, together with the winding L connected in series with L but in the reverse direction. A neutralizing capacity (1 which serves to neutralize feedback of energy from the plate circuit to the grid circuit of the tube is in each case, in Fig. 1, connected from the plate to a point between the primary windings la: and I13.

The capacity C is adapted to tune the secondary circuit over a desired frequency range, say from 550 to 1500 kilocycles, under present broadcasting conditions in the United States. The capacity C is selected to tune the parallel circuit lath to a frequency somewhat lower, usually but slightly lower, than the lowest Frequency of tuning, Under the above assumption li t), would be resonant at about 400 kilocycles. (Ever the operating range of frequencies the circuit Leil thus has a capacitive reactance; which means that the. impedance decreases continu uously as the frequency increases. the

other hand, winding L being substantially uirely inductive, its inipedai ce increases iroportionally with increased frequency.

llue to'the small inductance and small dis- 3Q tributed capacity of coil L its resonant f uency is so much higher than the ripe. limit of the tuning range the coupl system that L may be considered by itse as untuned or eilcctively non-resonant. The coefficient of coupling between L, and L is, however, such that L is tuned Ii -U.

Since the oppositely varying inipcdan serially connected between plate and ment of the tube, the resultant imped be adjusted to vary as desired with quency, preferably decreasing with increase in frequency.

Coming now to the effect produced by nrimary on the secondary circuit 11 C, since 1e coupling between i and is cons Kiel the frequency c voltage 1 ed in L; by L in ith freec 1c due to the increasing a o ge, th

"cases w impedance or portionally grea er voltage drop in i by 1 decreases with q the trequency goes up an increas .entage of the total Cir automatically reducing the coupling between L and L, as the frequency increases.

Thus there are two factors operating on the secondary circuit, one of which increases and the other or" which decreases as the frequency is varied by tuning. By suitably proportioning these factors relatively, the resultant efiect produced in the secondary circuit can be caused to vary in a preselected manner with frequency. In general, it is desirable to have a design such that the overall amplification of the system remains substantially constant over the operating frequency range. Due to the presence of capacity C the current in winding L is in phase opposition to that in L over the operating frequency range. Hence it is necessary to connect windings L and L series opposed in order to obtain additive clfects in windlng L1.

The winding L: in addition to its function as forming part of the primary circuit'of the coupling network in the manner explained above, also cooperates with the capacity C to neutralize the coupling etl'ect occurring between the plate circuit and grid circuit of a tube as a result of the interelectrode capacity G Regenerative feedback occurring between the plate and circuits of a thermionic tube, as is well known, is prcvented when he circuit arrangements are such that a vol c active in the p e circuit of the tube can produce no (llQCb the grid circuit thereof. This con M ion is commonly referred to as neutralizat on. Inferring to Fig. l, high-potentia winding :5 be of opposite alte rent polarity to that of wind' 0 Li gener tive feedback will occur when lowing relation obtains:

I c T wnerc M tween windn a n natiofor exact neutralization will vary considerably with frequency.

In order that the amplifier have a uniform gain over the operating frequency range, it is required .that the inductance L be relatively large, and L be relatively small as compared to L In practice, L amounts to only a few turns of wire. This means, of

' course, that the mutual inductance between windings L and L will be quite small as compared to the self-inductance of winding L and if reference be had for a moment to Equation (1) above, it will be seen that under such conditions the neutralizing capacity required will be very large as compared to the plate-to-grid interelectrode capacity C In an actual design neutralizing capacities of "also as compared to the impedances introduced between plate and filament thereof due to the interlectrode tube capacities occurring between the plate and grid, and'plate and filament electrodes, all of which impedances are effectively in parallel with the impedance of theneutralizing capacity C In" general, the impedance of C over theoperating frequency range is about 1000 ohms, as

compared to a filament-plate resistance of about 10,000 ohms, a filament-plate interelectrode capacity negligibly small, and a grid-to-plate interelectrode capacity C of 3 to mmf.

The apparent output impedance of the tube is, therefore, approximately that of the capacity 0 throughout the operating frequency range. More accurately, this apparent 01ftput impedance of the tube is represented by the total capacity, C +C plus that introduced by'leads and wiring.

The usual capacity of an antenna used with ordlnary commercial receiving sets is of the order of 200 mmf. Referring to Fig. 1,. by connecting in a suitable capaclty 8 of say 250 mmf. in series with such an antenna 7 having a capacity of about 200 mmf., the resultant capacity of antenna and wiring is of the order of 130' mmf., so that the antenna circuit will have a capacitive impedance over the operating frequency range which simulates the apparent output impedances of the successive tubes as approximately determined by the capacity'C The advantage of such a design, of course, hes in the fact that the same type ofcouphng network may be utilized between the antenna circuit and the first tube 1 as is util zed for interconnecting the successive tubes, with the resultant advantages in construction and operation pointed out above.

The fixed capacity 8. is optional, and may v be omitted if the antenna used in a specific case has a low capacity value, say of the order of 130 mmf. In practice, it has been found sufficient to provide a single capacity 8 of approximately 250 mmf. If the antenna impedance happens to be high, the capacity 8 is connected in series therewith, and if'it'happens to be low, the capacity 8 is short-circuited.

It willbecome apparent from the discus sion given above that the circuit arrangement disclosed in Fig. 1 is very efficient and economical in that the winding L serves a double purpose, i. e., as a portion of the primary circuit and also as a portion of the neutralizing circuit. Likewise, the capacity C serves a double purpose, i. e., as a portion of the neu-' tralizing circuit and as a means for furnish= ing a capacitive output reactance, thereby permitting identically constructed and identically tunable coupling networks to be utilized throughout the receiver.

It has been found that the best results are obtained when the coupling between windings L1 and L is not too loose. The coupling under such conditions might be termed moderate and will be so referred to in the appended claims. A moderate coupling, in this case, is one wherein the coupling coeflicient has a value lying between the limits of about 10% and.50%. On the other hand,-the coupling between windings L and L should'be loose, i. e., having a value less than about percent, while still fulfilling other requirements. In actual coil construction, this figure can usually be met due to the fact that the coupling between windings L and L, can be as loose as is consistent with the necessary degree of coupling between windings L and L and between windings L and L The ideal condition is attained when the coupling coefficient between windings L and L equals the product of the coupling coefiicients between windings L and L and between wind-- ings L and L i. e., when where K represents the coupling coeflicient, and the subscriptsdenote the windings between which the coefiicient is given. This equation is approximately satisfied by the design shown in Figs. 4 and 5, as will be seen from the description below.

Fig. 2 shows an alternative circuit arrangement which is a modification of that shown in Fig. 1, and'may therefore be substituted for thearrangement'of Fig. 1 either as ap antenna coupling system or as an inter-tube coupling system. In Fig. 2, the primary circuit is both inductivelyand conduc'tivel-y coupled to the secondary circuit. The capac- 'ity C is" connected from the plate of the preceding tube to an intermediate tapping point on L while the winding L is connected in a reversed sense between plate and filament of tube 1 as compared to its mode of connection in Fig. 1. The tapped portion of winding L in Fig. 2 takes the place of Winding L in the primary circuit of Fig. 1, and in Fig. 2 the winding L serves as a neutralizing winding only. The circuit of Fig. 2 is electrically equivalent to that of Fig. 1. The only difference consists in combining the separate primary and secondary windings L and L, of Fig. 1 into the auto-transformer arrangement of Fig. 2. At the lower frequencies, the capacity G is, in F ig.v 2, ct iEectively in shunt with the winding L because the reactance of the tapped portion of L is negligibly small at such frequencies. The circuit L 0 is resonant at a frequency slightly below the minimum frequency oi the tuning range of the condenser Q, as the case for Fig. 1.

Fig. 3 shows a circuit arrangement simil r to that of Fig. 2 with the exception that the condenser G is connected from the plate of the preceding tube to the upper termi- -al winding L By suitably selecting the value or? C this circuit may be made elecrically equivalent to that of Figs. 1 and 2. n this case the circuit L -J i is tuned to a quency slightly below the tuning range covered by the condenser G. Figs. 2 and 8, the capacity C is decreased and th inductance L is increased when the on coil is moved upward include more turns. lfhe circuit arrangement of l 2 3 are claimed above-mentioned cation, Serial. No. 372,275 or which the sent application is a division.

The structural details and design data of a ansformer which has been successfully used the circuit arrangement of 1 will he en with reference to Figs. 4.- and more particularly to Fig. 5, th s L L L are of cylindri Cal each comprising a single layer or wire uniformly wound about the tubular sections it, i and j, espectivef.y, of 'nsulating mater al, preferably balrelite. 1l e tubular secone are 0;"- successiveiy decreasing diameter showmand are positioned. coaxially one within another in fixed spacial relationship by means or the bolts a and spacing members in. The secondary winding 1 being of termediate diameter is positioned between the primaries with the larger primary i surrounding the same and the small primary L on the inside.

The diameter of section 7' is only slightly less than that of i in order to provide the highest practicable coeflicient of magnetic coupling between the windings L and L The diameter of section it is considerably greater than that of sections in order to provide moderate coupling between the large primary L and the secondary L By positioning the primary windings L and L, on opposite sides of the secondary L a small degree ofmagnetic coupling is obtainable between the two primaries, while at the same time providing the necessary coupling between the primaries and the secondary. The loose coupling between L and L is further enhanced by positioning L toward no receiv ing system through reduction of Lndesired couplings.

The shielding action oi winding does not increase the eiiective resistance oi wind- L over its valuewith on open circuit, for the reason that with h on open circuit, the magnetic flux distribution due to a current in winding is such as to crowd the current in the turns or wire tow the axis of the coil, causing the same to he shrough only a portion of the conducting cross-sect-ional area, thereby increasing the total effective resistance in the path of the current flow. With winding L short-circuited b capacity C however, there occurs a redis uhution off t'luX caused by a current flowing winding l since owing to-the presence in l e shie the flux external to the coil L tends to tie between the coils L and L This in redistribution of L causes the current to be more uniformly distributed over the cross-section al area of the turns of wire, with esu an de crease in total coil resistance. e other v: hand, the magnetic field set up by the cur- ..gy in rent in L causes a dissipation or a L which appears as an added L The net effect, therefore, is elihctive resistance of is shot whether L is on open or short c tirely removed. This same adva tained also when L has the 03 layer coil surrounding L i with the above arrangement, i1 L were of smaller diameter and thus plac d inside of L the total resistance of winding 1" be increased because both of above described efiects would increase the apparent resistance of L i. e., the current in L would be crowded in the portion of the conducting cross-section toward the axis, and in addition the flux set up by the currentin L would cause a dissipation of energy in L In an actual embodiment of the present invention which operated in a'very satisfactory manner, the following dimensions were utilized in the constructionof coils of the type shown in igs. 4 and 5. The dimensions are indicated by the letters in Fig. 5 and were as follows a=l inch 6 2.42 inches 0= .688 inch =1.25 inches e=2.5 inches ,f=1.5 inches 9 1.25 inches cylinders h, z' and j were each of insulating material having a wall thickness of 1/16 inch.

Winding L comprised 200 turns of B & S gauge enameled copper wire, uniformly wound 80 turns per inch; winding L com prised 126 turns of #26 B & S enameled wire, wound 52 turns per inch; while winding L comprised 11 turns of #26 B & S

. enameled wire, wound 16 turns per inch.

Measurements made at 1,000 cycles per second to determine the electrical constants of the various windings gave the following results. The self-inductances of the windings measured in microhenries were:

L 1710 microhenries L 5.4. microhenries L 294 microhenries The mutual inductances wereas follows:

M 375 microhenries M =25.2 microhenries M 18.5 microhenries where M is the mutual inductance, and the subscripts represent the windings between which the mutual inductance is measured as, for example, M refers to the mutual in-- ductance between windings L and L The coeflicients of magnetic coupling between the windings, in percent, were as follows:

K 53 percent K 26 percent K 46 percent The value of K is approximately the upper limit of moderate coupling, as previously defined; and the Value of K is approximately the lower limit for the coupling between L and L as described above. The.

product of K and K is substantially equal to K .In a radio receiving system of the type shown in Fig. 1 utilizing type ,UX-201-A tubes, and coils of the type shown in Fig. 4

having the constants given above, the following values were used for the remaining electrical elements. The capacity G was of 250 mmf. for the neutralizing capacity G a value of 130 mmfwas found satisfactory; for use in the antenna circuit the fixed capacity 8 had a value of 250 mmf.; and the variable condenser C was continuously adjustable between the limits of 30 and 400 mmf.

I claim:

1. A transformer comprising two primary windings and a secondary winding so' positioned relatively that the product of the coupling coeficients between one primary and the secondary and between the other primary and the secondary equals the coupling coeflicient between the two primary windm s.

2. The combination of a transformer comprising a distributed primary winding surrounding an inner secondary winding,

and a relatively low impedance connected between the terminals of said distributed primary winding whereby said primary ,winding acts substantially as 'an electrical shield surrounding said secondary winding.

3. In combination,- a transformer comprising one or more windings positioned within an outer distributed winding in fixed spacial relationship, and a relatively low impedance connected between the terminals of said outer winding for rendering the same effective as an electrical shield substantially surrounding said inner windings.

4. A high-frequency transformer comprising three tubular sections of insulating material of successively decreasing diameters, means positioning said sections coaxially one within another in fixed spacial relationship,

I and windings individual to said tubular sections, each said winding comprising a single layer uniformly wound about the corresponding tubular section and extending over a certain portion thereof, the intermediate and inner windings being situated substantially within the confines of said outer Winding, and

the diameters of said tubular sections being proportioned to provide a desired degree of magnetic coupling between said outer and intermediate windings, and between said in- .termediate and inner windings while at the a relatively large number of turns, the spac-' ing between said intermediate and innermost windings being relatively small as compared with the spacing between said intermediate and outermost windings whereby the coeflicient ofcoupling between said intermediate and innermost windings. is substantially greater than the coeflicient of coupling between said intermediate and outermost windings and between said outermost and innermost windings.

innermost winding and said innermostwind-- ing is positioned adjacent one end of said intermediate wlndlng.

9. A high-frequency transf rmer compris ing a secondary and two primary windings, said secondary winding being positioned within said first primary winding and said second primary winding being positioned within said secondary winding, the spacing between said first primary winding and said secondary winding being substantially greater than the spacing between said secondary windin and said second primary winding, where y close coupling exists between said second primary winding and said secondary winding and moderate coupling exists between said first primary winding and said secondary winding.

10. A high-frequency transformer according to claim 9 in which said windings are helical and the diameter of said second primary .winding is nearly as great as that of said secondary winding and the diameter of said first primary winding is substantially greater-than that of said secondary winding.

In testimony whereof I aiiix m signature.

HAROLD A. W EELER.

CERTIFICATE or CORRECTION.

' Patent No. 1,846,701.

Granted February 23, 1932, to

HAROLD A. WHEELER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, line 3, for the word "turnable" read tunable, and line 73, for "June 30" line 90, for "unit-controlled" read uni-controlled; page 4, line 52, strike out the word "in"; page 5, line 36, for "arrangement" read arrangements; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 5th day of April, A. D. 1932.

(Seal) M. J. Moore,

Acting Commissioner of Patents.

read June'lO; page 2,

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