US2621324A - Permeability tuned variable inductance - Google Patents

Description

Dec. 9, 1952 WEN YUAN PAN 2,621,324

PE'ZRMEABILITY TUNED VARIABLE INDUCTANCEZ Filed July 21. 1948 L EA D/IVG SECTION v42 P/raw W17 5W0 can.

3nventor IIIEN YUAN PAN Patented Dec. 9, 1952 PERMEABILITY TUNED VARIABLE IN DUCTAN CE Wen Yuan Pan, Oollingswood, N. J assignor to Radio Corporation of America, a corporation of Delaware Application July 21, 1948, Serial No. 39,949

Claims.

This invention relates to variable permeability tuned circuits and particularly to a radio frequency circuit including a coil tunable by a paramagnetic core and having a substantially linear relation between the movement of the core and the resulting variation in resonant frequency of the circuit.

It is conventional practice to vary the resonant frequency of the signal frequency circuit in a broadcast receiver by moving a paramagnetic core relatively to the tuning coil of the circuit. A paramagnetic material is defined as a material having a magnetic permeability greater than that of a vacuum, which is unity. The magnetic permeability of a paramagnetic material may be independent of the magnetizing force, or it may vary with the magnetizing force, in which case the material is called ferromagnetic.

In a superheterodyne receiver the signal frequency or input circuit and the frequency determining circuit of the local oscillator must be tuned in such a manner that there is always a constant frequency difference between the two circuits which difference equals the intermediate frequency. The problem of obtaining proper tracking throughout the tuning range of a superheterodyne receiver would be made considerably less difficult if there were a perfect linear relation between the movement of the tuning core and the corresponding variation in resonant frequency of the circuit. However, it is well known that if a radio frequency coil is wound with a uniform pitch, the rate of change of inductance of the coil caused by the movement of its associated core will not be uniform. When the core first aproaches or enters the leading edge of the coil the rate of change of the inductance of the coil is comparatively small due to the leading end effect. When the leading edge of the core approaches the center of the coil the rate of change of the inductance reaches a maximum. The rate of change decreases again when the leading edge of the core aproaches the trailing edge of the coil.

It is therefore conventional practice to provide a permeability tuned circuit having a coil of variable pitch. In that case, the curve representing the resonant frequency of the circuit plotted against the core position approaches a straight line. However, in order to obtain a perfect straight line frequency curve the pitch of the coil must change continuously. Such coils are expensive and are not adapted to large scale production. Furthermore, for a given core displacement, a permeability tuned circuit having a variable-pitch coil will cover a smaller tuning range than one having a coil of uniform pitch. Accordingly, the design usually represents a compromise between the optimum straight line frequency characteristic and the required tuning range. It is, therefore, an object of the present invention to provide a coil which is constructed so as to compensate for the leading end effect, that is, to increase the rate of change of the inductance when the core approaches and enters the leading end of the coil.

It is a further object of the present invention to provide a radio frequency circuit including an inductance element tunable by a paramagnetic core and wound in such a manner that the variation in frequency of the circuit is a substantially linear function of the movement of its associated core.

Another object of the invention is to provide a permeability tuned radio frequency circuit including a coil having a substantially linear fre-- quency variation as a function of the position of its tuning core and in which the overall tuning range covers an adequate band of frequencies.

A still further object of the invention is to provide a resonant circuit tunable by the relative movement of a core and coil wherein the comparatively small rate of change of inductance when the core enters the coil is increased, thereby to extend the substantially linear portion of the curv relating the frequency of the circuit to the core position.

A radio frequency circuit in accordance with the present invention comprises a tuning inductance element consisting of at least three sections such as a leading, an intermediary and a trailing section. A paramagnetic core is movable relatively to the sections of the inductance element so as to vary the resonant frequency of the circuit over a predetermined tuning range. Each section has a uniform pitch where the pitch designates the number of turns per inch, but the pitch increases in steps from the leading section to the trailing section. Preferably, the pitch of the intermediary section is approximately 1 times as large as that of the leading section while the pitch of the trailing section is between 2 and 3 times as large as that of the leading section.

In order to compensate for the leading end effect a further winding may be provided on the leading edge of the inductance element and ahead of the leading section. This winding has a pitch that is larger than that of the leading section and preferably is at least as large as that teristic of this invention are set forth with partic ularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objectsand advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, in which:

Fig. 1 is a sectional view of a radio frequency coil and its associated core in accordancewith the present invention;

Fig. 2: isJa graph illustrating curves showing. the

' relation between the position of a paramagnetic core and the corresponding frequency ofa resonant-circuit; and

Fig. 3 is a circuit diagram of a portion of a superheterodyne receiver utilizing the radio frequency coilof'the invention.

Referring now to Fig. 1 there is illustrated a radio frequency inductance element or coil 1 which is wound, on a cylindrical coil form 2 consisting of a suitable insulating material such as a phenol formaldehyde resin. Paramagnetic core 3isaxially slidable-within coil form. 2 for Varying the inductance of coil I. When paramagnetic core 3- is: moved from le t to right'as shown by arrow 4-, the resonant frequency of acircuit consisting of coil i and a. shunting capacitor (not shown) is decreased. Accordinglytheleft hand edge of coil l which core 3 first enters, may be designated its leading edge, while the right hand edge of the coil may be termed the-trailing edge.

'In accordance with. the present invention, coil I is designed so that the variation in frequency of a resonant circuit including the coil. is a substantially linear function of'the position of core 3'.v If the curve relating the. frequency variation toithecore position should be. perfectly straight, coil I- must be wound with a constantly changing pitch. However, such a coil cannot conveniently r be manufacturedon amass production basis. In accordance with the present invention, coil l consists of a plurality-of sections each having auniformpitch but the pitches of the various sections differing from each other. Such a coil will have a substantially linear frequency scale.

The mathematical equation for apermeability tuned coil having a follows-z where f is the frequency in cycles per second, K1 and f2 are constants and so is the displacement of core 3 insidecoil I in inches. Equation 1 representsa straight line. Accordingly,

which is the constant slope of the curvegiven by Formula 1. is is the upper frequency limit of the tuning range. Ifwe assume a broadcast receiver covering the frequency range from 540 kilocycles (kc) to 1620 kc., fz:1620 kc. K1 then equals the frequency tuning band divided by are linear frequency scale is as which is the maximum displacement or core 3 in inches. Accordingly,

l 108;)0 kc.

Since where L0 is the inductance of coil I in henries with core Sremoved, Lc is the inductance in henries addeditc coil If by core 3 and C is the capacitance of the capacitor shunted across coil l, we find by substitution 1 +f2 2 V u. Accordingly:

L, 1 1+LTW This is the basic relation between the added inductance. L0 and the coredisplacement m for a. perfectly linear-tuning scale.

An empirical formula has been derived for theinductance added to a coil by amagnetic core forany tuning position of the core. This formula may be written asfollows:

.2: 2 I a (I e -4+ W In the above formula K is a constant. depending upon the absolute permeability of core 3, (la is. the effective diameter of coil I in inches, dc is the diameter of core 3. ininches and n isthe winding pitch .in turns per inch.

liormulaifv can be simplifiedv as. follow provided a: is between 0. and .lmn:

. m e. (a) By means of Formulas 2 and 3. or 3A the pitch of coil I: can be calculated. This may bedone by utilizing the formulas to .calculateindividually the pitch of sections or portions of coil I provided the length of each section is. assumed. This may be effected by differentiating Equation 2 and 3 and equalizing them. Of course, Equation 3A may be substituted, for Equation3 as. long. as, Equation 3A is valid. In other words, the rate of change of the inductance with respect to the, core; position at the end of each coil section is, made. equal to the corresponding values derived fromEquation 2,

In accordance with the present invention coil I may consist of leading section 5., intermediary section 6 and trailing section 7 although more than one intermediary section may be provided, Preferably, leading section 5 has an axial length of 40 per cent of the total lengthjof coil I while intermediary section 6 and trailing section I each have an axial length of 30 per cent of that of coil I. Ahead of leading section I there may optionally be provided an extra winding indicated at B in order to compensate for the leading end effect whereby the frequency scale can be made more linear at the high frequency end of the tuning range. Winding 8 may have an axial length of per cent of that of coil in which case the length of leading section 5 is reduced to 30 per cent.

Each section 5, 6 and I has a uniform pitch but the pitches of the three sections differ from each other. Preferably, the pitches of leading section 5, intermediary section 6 and trailing section I have a ratio of 4 to 6 to 9 or of 4 to 6 to 11. Accordingly, the pitch of intermediary section 6 is approximately one and one-half times as large as that of leading section 5. Furthermore, the pitch of trailing section I is at least twice as large as that of leading section 5 and preferably between two and three times as large as that of the leading section. The axial length of extra winding 8 may amount to 10 per cent of that of coil I in which case the axial length of leading section 5 would be reduced to 30 per cent. The pitch of extra winding 8 should be between one and one-half and two times that of leading section 5. In other words, the pitch of winding 8 should be between that of intermediary section 6 and that of trailing section I. The pitch ratios of the coil sections are of more significance than the actual pitches. Coil sections 5, 6, I and 8 preferably consist of a progressive universal winding, although winding 8 may be wound as a universal winding. Winding 8 may also be provided on top of leading section 5. Winding 3 and coil sections 5, 6 and I are connected in series to form a single coil Referring now to Fig. 2 there is shown a curve representing the frequency f plotted against the relative core position Curve I0 is representative of the frequency curve for a coil wound in accordance with conventional practice with uniform pitch. It will be seen that the frequency varies non-uniformly with the core 7 position. Curve II illustrates the frequency variation obtained with coil without the extra winding 8. This curve approaches more nearly a straight line. However, when winding 8 is added to coil I, curve I2 which is substantially linear, is obtained. Winding 8 appreciably reduces the leading end effect as clearly shown by curve I2. It will be understood, of course, that coil I consists of 3 or 4 sections which form together a series wound coil.

A multisection coil such as illustrated in Fig. 1 may, by way of example, have the following dimensions. The pitches of sections 5, 6 and 7 may have a ratio of 4 to 6 to 9. Leading section 5 has 49 turns and a winding pitch of 133 turns per inch. Winding 8 consists of 30 turns and a winding pitch of 250 turns per inch. Intermediary section 6 has 74 turns wound with a pitch of 200 turns per inch and trailing section I consists of 111 turns with a pitch of 300 turns per inch. The total length of the coil is 13% inches and core 3 has a length of 1% inches which also equals the maximum core displacement $u.da=.25 inch and dc=.2 inch.

A multisection winding such as illustrated in Fig. 1 will increase the equivalent ratio of coil diameter to coil length. It is well known that this will reduce the effective permeability of coil I and core 3, thereby reducing the tuning range. Thus, in designing a coil a compromise must be found between the desired linearity of the frequency scale and the required tuning range. The tuning range, of course, can be increased by increasing the length of the coil. This increase, however, is usually limited by the space available for the coil and core in a broadcast receiver. Another way of increasing the tuning range is to wind the coil with a wire of smaller diameter. However, the diameter of the wire determines the Q of the circuit and in order to have a high Q resonant circuit, the wire diameter cannot be decreased too much.

The most eifective way of increasing the tuning range is to utilize a paramagnetic core 3 of a material having a high magnetic permeability. Such materials including ferrites are now readily available.

Referring now to Fig. 3 there is shown a circuit diagram of a portion of a superheterodyne receiver utilizing inductance coil I. The receiver comprises antenna which impresses a radio frequency wave (R. F. wave) on R. F. amplifier 2| to which is coupled frequency converter 22. Antenna 20 is connected to ground through capacitor 23 and is coupled to resonant circuit by a capacitor 24 for matching the impedances of antenna 20 to that of resonant circuit 25. Resonant circuit 25 is coupled between control grid 25 of amplifier 2| and ground through blocking capacitor 27. Resonant circuit 25 consists of coil and shunting capacitor which may be adjustable as shown. Circuit 25 is tuned to a desired resonant frequency by paramagentic core 3. Core I is identical with that shown in Fig. 1. An automatic volume control voltage (AVC) is fed to control grid 25 through grid leak resistor 3 I.

The output load of R. F. amplifier 2| is represented by resonant circuit 33 consisting of coil identical with coil I and shunted to ground by capacitor 34 which may be adjustable as shown. Resonant circuit 33 is tuned to the desired R. F. wave by paramagnetic core 3'. The lower terminal of coil l' is connected to a suitable positive voltage supply indicated at +3 The amplified R. F. wave is impressed upon signal control grid 35 of pentagrid converter tube 22 through coupling capacitor 36. An AVC voltage may also be impressed on control grid 35 through resistor 31.

Cathode 38 and control grid 40 of converter tube 22 form the oscillator section of the converter. Cathode 38 is connected to ground through choke coil 42. Resonant circuit 43 consists of two capacitors 44 and 45 connected in series and across coil 46 which may be tuned by paramagnetic core 47. Trimmer condenser 48 may be connected across coil 45. The junction point of capacitors M, 45 is connected to cathode 38 and the high alternating-potential terminal of tuned circuit 32 is coupled to control grid 40 by coupling condenser 53. Control grid 45 is connected to ground by grid leak resistor 49. The oscillator described operates as a Colpitts oscillator.

load consists of resonant circuit 522 also connected to +13 and tuned to the intermediate frequency which may be derived from circuit 52 in any conventional manner;

Paramagneti-c cores 3, 3' and '4'! are preferably moved in unison as indicated at 53 to tune the receiver to a predetermined frequency. Since coils l and l have an almost linear relationship between the frequency and the core movement, the design of oscillator coil it is considerably facilitated. Thus, oscillator coil 56 may be designed similarly to coil 1.

There has thus been described a radio frequency coil tunable over a predetermined frequency range by movement of a paramagnetic core relatively to the coil. The coil consists of several sections each having. a uniform pitch but the pitches of the various sections differ from each other. Preferably, the coil is provided with an extra winding on its leading edge to compensatefor the leading end effect. A coil of this type will display a variation in frequency which is a substantially linear function of the core position.

What is claimed is:

l. A tunable circuit comprising a single tuning inductance element consisting of a leading section, an intermediary section and a trailing section, said sections being arranged adjacent to each, other without spacing, and a paramagnetic core movable in succession relatively to said leading section, said intermediary section and said trailing section to vary the resonant frequency of said, circuit over a predetermined tun-ing range, each of said sections having a uniform number of turns per inch, the number of layers of windings in said intermediary section being largerthan that of said leading section and the number of layers of windings in said trailing section being larger than that of said intermediary section, whereby the variation in frequency of said circuit is a substantially linear function of the movement of said core.

2. A radio frequency circuit comprising a tuning inductance element consisting of a leading section, at least one intermediary section and a trailing section, and a paramagnetic core slidable in succession into said leading section, said intermediary section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform number of turns per inch, the number of layers. of windings in said intermediary section being larger than thatof said leading section and the number of turns per inch in each layer of said intermediate section being approximately one and one-half times as large as that of said leading section, the number of layers of windings in said trailing section being larger than that of saidv intermediate section and the number of turns per inch in each layer of said trailing section being at least twice as large as that of said leading section, whereby the variation in frequency of said circuit is a ranged axially' within said element and slidable y in succession into said leading section, said intermediary section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform number of turns per inch,

the number of layers of windings in said intermediary section being larger than that of said leading section and the number of turns per inch in each layer of said intermediate section being approximately one and one-half times as large as that of said leading section, the number of layers of windings in said trailing section being larger than that of said intermediate section and the number of turns per inch in each layer of said trailing section being between twice and three times as large as that of said leading section, whereby the variation in frequency of said circuit is a substantially linear function of the movement of said core.

4. A radio frequency circuit comprising a tuning inductance element consisting of a leading section, an intermediary section and a trailing section, and a paramagnetic core movable in succession with respect to said leading section, said intermediary section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform pitch, the pitch of said intermediary section being larger than that of said leading section, the pitch of said trailing section being larger than that of said intermediary section, said intermediary and said trailing section being of substantially equal axial length, the axial length of said leading section being larger than that of said other sections, whereby the variation in frequency of said circuit is, a substantially linear function of th movement of said core.

5. A radio frequency circuit comprising a tuning inductance element consisting of a leading section, an intermediary section and a trailing section, a paramagnetic core arranged axially within said element and slidable in succession into said leading section, said intermediary section and said trailing section and back again to decrease or increase the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform pitch, the pitch of said sections increasing from said leading section to said trailing section, and a further winding provided ahead of said leading section and on the leading edge of said element, said further winding, having a pitch larger than that of said leading section, whereby the variation in frequency of said, circuit is a substantially linear function, of the movement of said core.

6. A radio frequency circuit comprising a tun ing inductance element consisting, of a leading section, an intermediary section and a trailing section, a paramagnetic core arranged axially within said element and slidable in succession into said leading section, said intermediary section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform pitch, the pitch of said intermediary section being approximately 1 times as large as that of said leading section, the pitch of said trailing section being between twice and three times as large as that of said leading section, and a further winding provided ahead of said leading section and on the leading edge of said element, said further Winding having a pitch at least as large as that of said intermediary section, whereby the variation in frequency of said circuit is a substantially linear function of the movement of said core.

7. A radio frequency circuit comprising a tuning inductance element consisting of a leading section, an intermediary section and a trailing section, a paramagnetic core arranged axially within said element and slidable in succession into said leading section, said intermediary section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform pitch, the ratio of the pitches of said leading section, said intermediary section and said trailing section being approximately equal to 4 to 6 to 9, and a further winding provided ahead of said leading section and on the leading edge of said element, said further winding having a pitch between that of said intermediary section and that of said trailing section whereby the variation in frequency of said circuit is a substantially linear function of the movement of said core.

8. A radio frequency circuit comprising a tuning inductance element consisting of a leading section, an intermediary section and a trailing section, a paramagnetic core arranged axially within said element and slidable in succession into said leading section, said intermediary section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each of said sections having a uniform pitch, the pitch of said intermediary section being approximately one and one-half times as large as that of said leading section, the pitch of said trailing section being at least twice as large as that of said leading section, and a further winding provided ahead of said leading section and on the leading edge of said element, said further winding having a pitch larger than that of said leading section, said leading section, said intermediary section and said trailing section extending each over 30 per cent of the total 10 axial length of said element, said further winding extending over 10 per cent of said length, whereby the variation in frequency of said circuit is a substantially linear function of the movement of said core.

9. A tunable circuit comprising an inductance element consisting of a first leading multilayer inductance section, a second leading single layer inductance section and a trailing multilayer inductance section, said sections being connected in series aiding relation to form a unitary inductance, and a paramagnetic core movable in succession relatively to said first leading section, said second leading section and said trailing section to vary the resonant frequency of said circuit over a predetermined tuning range, each layer of said sections having a uniform number of turns per inch, and the number of turns per inch in each layer for each of said sections having ratios whereby a substantially linear variation in frequency is obtained as a function of movement of said core.

10. A combination as defined in claim 9 wherein said sections are contiguous.

WEN YUAN PAN.

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

UNITED STATES PATENTS Number Name Date 2,190,048 Sinninger Feb. 13, 1940 2,338,134 Sands et al Jan. 4, 1944 2,363,101 Van Der Heem Nov. 21, 1944 2,383,463 Benin Aug. 28, 1945

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