US2477749A - Inductor tuning system - Google Patents

Description

Aug. 2, 1949. F, N, JACOB 2,477,749 INDUcToR TUNING SYSTEM Fi1ed Apri1 4, 19m 4 snags-sheet 1 E ml 11 FREDERICK N. Jco

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Aug. 2, 1949. F, N, JACOB 2,477,749

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INDUCTOR TUNING SYSTEM Frederick N. Jacob, Chicago, Ill., assigner to Aladdin Industries, Incorporated, a corporation of Illinois Application April 4, 1946, Serial No. .659,465

(Cl. Z50-40) 6 Claims.

This invention relates to an inductor tuning system in which the frequency of a tuned circuit is varied by an inductor having a movable magnetic core.v

In the mass production of radio equipment where variable inductan-ce devices are used in the tuned circuits of `a multitude'of like receivers it is customary to :provide substantially identical coil and lcore combinations for any given type of circuit common to these` receivers. Since the distributed capacitances of the coil and core combinations and the inherent capacitances of the corresponding circuits in any one of the multitude of like apparatuses in which these combinations are used are' never the same, it is necessary With said prior arrangements to compensate 4for the slight variations in capacitance by providing a trimmer condenser across the coil core combination and upon connecting the unit in a circuit to adjust the initial frequency value of the circuit by means of the trim-mer condensers when the core has attained a predetermined position relative to the coil. This procedure is costly and has the added element of a trimmer capacitor of questionable stability. The trimmer condenser is made unnecessary by the use of the present invention, in accordance With which both the main condenser and the trimmer condenser are replaced by a single, highly stable fixed condenser, and the initial resonant frequency of the circuit is adjusted by adjusting the initial position of the core from which thecore is moved over the tunning range. This is made possible by the particular construction of the coil or core or the combination of both as with the new construction it is immaterial from which position the core movement begins since with this construction equal amounts of core movement always produce equal amounts of. frequency change.

An object of the present invention is to devise a permeability tuning system in which all of the required capacity in a tuned circuit may be incorporated in a xed capacitor of the type which is free from the effects of vibration and humidity and in addition has a predetermined characteristic under temperature changes. Such capacitors may be of the ceramic type or molded mica construction.

A further object ofthe invention is to devise a novel tuning inductor by which a circuit may be tuned over the same frequency range for the same extent of movement of the core but with different values Yof capacity connected in the circuit.

Another object is to devise an inductor tuning system in whichwequal.displacements of the tuning core throughout different portions of a given tuning range Will produce the same ratio of frequency change.

Still another object of the invention is to devise a novel tuning inductor by which two or more tuning circuits including different capacity values may be tuned over the same range of frequency by the simultaneous movement of the cores of said tuning elements through the same range of movement.

Where a iixed amount of capacity is to be used in a tuned circuit, with no provision for adjustment of this element, adjusting means must be embodied in the tuning inductor for initially establishing the desired resonant frequency of the circuit. Such adjusting means -must be adaptable to use in the manufacture of large quantities of radio ifrequency apparatus incorporating such :circuit elements. I propose to use the relative movement of the core and coil, not only to tune the resonant circuit throughout a substantial range ofi frequencies, but also to establish conformity of this range with a predetermined frequency band.

In order to accomplish the desired result, the tuning inductor must be so designed that the circuit in which it is employed may be tuned over a given frequency range for the same core movement Ibut with different values of capacity connected in the circuit. This means that the core must be capable oi producing the same frequency change in the circuit for a given movement even though the core may start from different initial positions for diierent values of capacity included in the circuit. I have discovered that the desired result may be obtained if the tuning inductor is designed so that Within the desired tuning range, equal movements onf the core produce equal changes in the logarithmic value of the frequency of the circuit. In other words, the tuning inductor must be designed to vary the inductance.

in the circuit in a manner such that there will be a straight line relation between the core displacement from a given position and the logarithmic value of the resulting reasonant irequency of the circuit over a range in excess oi the desired tuning range. Stated in another way, equal movements of the core must produce the same percentage change in frequency throughout the desired tuning range. The inductor must vary the effective inductance of the circuit in a manner to produce a constant ratio between the core displacement and the change in the logarithmic value of the circuit inductance. Where the effective inductance ofthe circuit is change in frequency in each case. It mayalso be noted that the family of curves shown kin Figure 2 provides the same ratio of frequency change for a given core displacement for all curves or for any portion of any curve lying within the useful tuning range.

In Figure 2, the portion of curve I between the frequency limits of 1.9 to 1.0 mc. may be considered to bea straight line. This relationship may be expressed by the following equation.

where f is the frequency tuned to P is the corresponding core position on the scale of movement K is the slope of the curve and is a constant fo is the upper limit of frequency for the straight line relationship; in this case 1.9 mc.

Po is the corresponding core position for frequency fo Thisy Equation 1)v may be restatedA If only a portion of the curve is considered,

bounded by end frequencies f1 and f2, for example 1.8 mc. and 1.2 mc., the equations for the limits of this range may be written which shows that for any frequency f1, there is a corresponding core position P1, and for any frequency f2, there is a corresponding position P2. In tuning from frequency f1 to frequency fz, the core displacement will be the difference in the positions P1 and P2, viz:

where L is the value of inductance provided by core position P and C is the circuit capacitance the relationship between Vcore position and circuit inductance may be given as Thus, for any position P1, there is a corresponding inductance L1, and for any position Pz, there is an inductance provided which may be termed L2.

1 1 Pl--Klog 21rfm/lfl-P., 8)

But if a somewhat different value of capacitance, C,- is now assumed, in place of C, whose value falls within the'limits established above, thenv if Equation 6 is'considered,v and the value C is substituted for capacitance C, then to tune to the same frequency a new value of inductance must be found to satisfy Equation 6, and it canl be expressed as That is to say, for any frequency considered, if the circuit capacitance is changed to C', then the core must be moved to a new position which provides a value of inductance which has been modified by the factor the net core displacement is the difference between the new core positions, or,

Equation 17 indicates that under the conditions previously set up, a given frequency range may always be covered, when using different tuning capacities, by equal core movements. Likewise, since f2 need not necessarily be the end frequency on the band, but can be any frequency in the band, the core movement needed to tune from the end frequency f1 to this other frequency f2 falling anywhere in the band will always be the same, and an identical curve shape will result from that secured using the original set of circuit constants, but displaced horizontally from the original. n

By taking Equation 5 a step further, we have A PF 10g 3,-; (18) For any other pair of frequencies. f: and f4, satisfying the relationship the corresponding core displacement always equals P1 minus P2. Stated another way, under this system, for any frequency band having a given ratio of frequency coverage, an identical core displacement is required.

The distinction between the system of this invention and a straight line frequency system will now be shown. Curve 3a of Figure 3 shows the tuning characteristic provided by a core and coil similar to the ones considered above, but designed so as to offer an essentially linear relationship between core movement and frequency of the oscillatory circuit of which the combination forms a part. It will be observed that the abscissa scale of capacity units of micromicrofarads and an ordinate scale in units of megacycle coverage per inch of core travel.

The physical dimensions of the coil and core combination for curve 3e has already been given above. For curve 3f, the same core was used, but the coll had a different winding pattern to secure the straight line frequency characteristic. The coil form had an inside diameter of 0.52 inch and an outside diameter of 0.57 inch. The winding was 11% inches long and consisted of 22.3 turns of No. 30 P. E. wire wound in six sections as follows:

Instead of varying the pitch of the winding turns along the length of the winding, the desired variation in inductance may be obtained by varying the diameter of the turns in different sections of the winding. Such an arrangement is shown in Figure 4 where the pitch of the winding is constant throughout the length thereof but the diameter of the turns varies in different linear sections. The curves shown in Figure 5 illustrate the tuning characteristic of an inductor designed with a variable turn diameter and illustrated in Figure 4. It will be understood that the following example is for the purpose of illustration and not by way of limitation.

The core 1 of Figure 4 is the same as the core for Figure 1. The coil form 5a has a. cylindrical inner bore of 0.505 inch diameter throughout the length, but the outside diameter of different linear sections of the coil form varies in accordance with the following table:

The winding of Figure 4 is formed of No. 30`

P. E. wire and consists of 21.5 turns wound at a pitch of 16 turns per inch, the total winding length being vlift inches. With the core positioned so the end thereof is e12- of an inch removed from section I of the winding, the resonance frequency of the circuit was 2 megacycles with a capacity C of 1910 mmfd. As shown by the curve 5A of Figure 5, the tuning characteristic of the inductor is a straight line from 1.9 mc. to 1.1 mc. For an effective tuning range having upper and lower limits of 1.8 and 1.2 mc., curves 5B and 5C show the tuning characteristics for the limiting conditions where the tuning capacities are 11720 mmfd. and 2080 mmfd. As will be seen from Figure 5, all three curves are straight lines between the limits of 1.2 to 1.8 megacycles and are parallel to each other.

In a third modication the desired inductance variation is secured by a special core construc- 10 diilerent linear sections thereof. One speciiic example of core design will be given by way of illustration. This arrangement is illustrated in Figure 6 and the tuning characteristics are shown in Figure 7.

The coil for the inductor of Figure 6 is wound of No. 30 P. E. wire at a pitch of 19 turns per inch and includes a total of 25 turns. The coil form has an outside diameter of 0.535 inch and an inside diameter of 0.505 inch. Capacitor C for curve 7A had a value of 1940 mmfd.

The core 'la of Figure 6 is built up of six sections #1 toit, each 1A; inch in length but varying in outside diameter. The core sections are assembled on an insulating rod 'lb of 0.187 inch diameter passingthrough a center hole common to all of the sections. Beginning with the section adjacent the tuning coil as No. 1, the outside diameters of the core sections are 0.42 inch, 0.44 inch, 0.47 inch, 0.5 inch, 0.5 inch, 0.49 inch. All of the core sections are formed of compressed ferromagnetic material (Aladdin No. 521) having a density of approximately 5 gr./cc. It will be understood that the core may be formed as a single compressed body instead of being assembled in sections.

Curve 1A in Figure 7 illustrates the tuning characteristic of the inductor of Figures 6 where the capacity C has a value 1940 mmfd. This curve is substantially straight from 1.85 mc. to 1.15 mc. If the effective tuning range is to extend from 1.8 mc. to 1.2 rnc., then the values of capacity C will be 1785 mmfd. for curve 1B and 1990 mmfd. for curve TC. All three curves are essentially parallel and straight from 1.8 mc. to 1.2 mc.

The various core sections of Figure 6 may all have the same external diameter but diiferent sizes of central openings so as to secure the necessary variation in cross-sectional area of the core along the length thereof.

Figure 8 illustrates another arrangement for securing the desired'variation in inductance in which the winding .pitch and turn diameter of the coil are constant throughout its length, and the inside and outside diameters of the core are constant, but the density of dilerent linear sections of the core varies. One speciiic example of construction will be described by way of illustration for producing a tuning characteristic according to Figure 9. In the construction diagrammatically illustrated in Figure 8, the coil consists of 22.5 turns of No. 30 P. E. wire uniformly wound over a length of 11% inches on a cylindrical coil form having an outside diameter of 0.607 inch and an inside diameter of 0.505. The core 1a is formed of four sections mounted on an insulating rod 1b which passes through a central hole common to all of the sections. The different sections of the core are formed of compressed ferromagnetic material (Aladdin No. 521) and of varying lengths and density according to the following table:

It will be understood that the core may be formed as a single -composed body instead of being astion which the diameter of the core varies in sembled in sections. t

Curve 9A of Figure .9 shows the tuning characteristic of the inductorof Figure \8 where thecircuit has a frequency of2 mc. when the end `of section I of the core is positioned 'als -oi an .inch from the end of the coil 6 and with a value of condenser C of 1900 mmfd. This-curveis essentiallystraight over the range 'extending .from 1.9 mc, to 1.2 mc. For a tuning range varying .from 1.8 .-mc. to 1.2 mc., one limiting value of condenser C will be 2115 mmfd. which results in a characteristic according to curve .9C,'and the other :limit-ing value of condenser C will be 1900 mmfd. which produces a tuningcharacteristic according to curve 9A. Curve 9B illustrates the characteristic obtained :by using an intermediate value of (le-.2000

mmfd. .Again'the three curves `'are substantially parallel to one another.

While the :four `specific examples given above involve a coil and .core combination tor use over the range of 1.8 to 1.2 megacycles, which represents a frequency ratio of 1,5 to 1., it will be understood that the same principles can .be applied for covering a tuning .range .having .a .frequency ratio as greatas 3 to l.

In the arrangements represented in Figures .1, 4, 6 and 8 the condenser C is connected directly across the inductor coil, and the effective inductance of the tuned circuit is providedentirely by the variable inductor. It will be understood, however, that the ,principles ofmy .invention may be applied to a circuit in which a part oi the circuit inductance may be embodied in a separate inductor connected 'in Vseries with the condenser and the tuning inductor.

Where all of the circuit lnductance is provided by the variable inductor, 'the design for 'the inductor must satisfy Equations 7., 8 and 9 above. The .relation between the distance D of travel of the core between ,positions P'I andIPZ and the circuit constants may lbe obtained by subtracting Equation v9 from Equation i8 which results in change Vin core position .and the change in the 5 logarithmic value vof the .ind-uctance -of the inductor. In other words, the diiierence between the logarithms `of. the inductance values Ll and L2 at two different positions Pl and PL2V ofthe core is proportional to the distance D between 'the two positions. Stated more simply, the curve fof logarithmic value oi inductance against core position `is a straight line having a constant slope. Also, the curve showing the logarithm ofthe reciprocal of the inductance against core position is a straight line. having a `constant slope but in the opposite direction.

Where some of the circuit inductanc'e is einbodied in a separate inductor having a value 'of La, the design of the variable inductor must still satisfy Equationsl and 9 above, but the expressions \/LC, \/L1C and x/LzC in these equations would assume the form of \/(La|-L)C', \/(La-l-L1)C and x/(La-l--LzlC respectively. In this case, the equation for the variable inductor, corresponding to Equation 33 above, would be Log1o(L1-|-La) -logmiL-z-iI-La) aD For all of the arrangements described above it wil-l be understood that more than one winding layer may be used if desired. Also, the `desired turn density along the length of the coil .maybe secured by rst winding a layer of turns of constan-t pitch and then winding additional turns :on top of this layer and distributed along the length of the 'winding space to secure the desired turn distribution.

It is obvious that my invention is not limited to the use of cores formed of compressed, nelydivided ferromagnetic :material but cores of the laminated type may :be employed.

A permeability tuning combinationrof coil and movable core may be so designed as to exhibit substantially the same temperature coeicient for any position of the core with respect to the coil. 'By temperature coeiiicient is mean-t the rate at which the ind-uctance of the combination device changes with unit change in temperature. YI'his combination likewise'may be treated to make it relatively immune 'from the effect -o-f high degrees of humidity to which it maybe subject. It is inherently resistant to vibration. Through the use of a iixed capacit-or, matching these Yproperties (having a compensating temperature coeiiicient), an exceedingly stable tuned circuit may ,be evolved, and through proper designa vacuum tube oscillator .incorporating Such a tank circuit may be made which approaches the `stability of crystal controlled oscillator systems.

While mention has been made of a relatively restricted rband of frequencies, i. `e., 1.8 to 1.2 mc., it is to be understood that the same principles may be employed in radio .apparatus having more than one oscillatory circuit, as 'for instance, in the radio .frequency stages oi a tuned radio frequency receiver or a superheterodyne receiver, operating over more extended ranges. In the latter case 'the local oscillator ,circuit will follow a semi-logarithmic straight line relationship of irequency vs. movement as disclosed above. The other preselector stages will also have such a characteristic, 'but lsince they operate over a frequency band which is usually lower than that of the local oscillator frequency spectrum by an amount equal to the intermediate frequency, some amount of misalignment will result between the circuits, although the resultant attenuation will not be severe.

'These principles may also be advantageously employed iin the master `osc-i-llator and succeeding amplifier .stages of radio frequency transmitters. Here, the "cumbersome and costly a-i-r dielectric capacitors could be dispensed with, thus saving -space and improving reliability. In t-he master oscillator, the use of :my system improves Vfrequency stability.

In various applications of my invention, it is necessary to operate two or more -inductors from a common actuating member, and the operating means foreach .inductor should include provision for varying the initial setting "of the core with respect to the coil. `One suitable arrangement for simultaneously operating the cores of two inductors which .may be included in separate tuning circuits is illustrated in Figures 1l) to 13, inclusive, but it will be understood that other arrangements are possible.

Refer-ring to Figure 10, two substantially identical inductor units havin-g coils Aa and Ab and movable cores Ba and Bb are connected in separate tunable circuits including condensers Ca and Cb which may have different values. The coils of the two inductors are carried by brackets Da and Db which in turnV are mounted for 13 vertical adjustment on a supporting panel E. Each of these brackets is provided with rack teeth on one edge thereof by which the bracket may be adjusted in a vertical direction by means of a suitable pinion tool, as will be more clearly understood from the patent to Schaper 2,272,433. The cores Ba and Bb are supported from a horizontal cross-arm F by means of a pair of threaded rods or stems Fa and Fb having threaded engagement with erom-arm F which in turn is mounted upon vertical slide Fc. Any suitable means may be employed for operating slide Fc to move the cores in and out of the coils, such as a rack and pinion arrangement, or an arrangement may be employed like that disclosed in the patent to Kirk et al. 2,286,283. 'I'his arrangement involves a tuning spindle G which drives a pulley H through a string K, and the pulley H operates a suitable mechanism to produce rectilinear motion of slide Fc. A horizontal frame or panel L is mounted on panel E and carries a frequency scale La. A movable frequency pointer or indicator Lb is supported from the top edge of the panel L and is adjustably clamped to an operating string Le mounted directly behind the panel L and extending parallel to the upper edge of the pane] L. The string Le passes around suitable guiding pulleys and is driven from a driving pulley Ld mounted on the shaft of a pinion Fd having engagement with rack teeth formed on one edge of slide Fc. Thus, vertical movement of slide Fc is translated into horizontal movement of pointer Lb and there is a, iixed linear relationship between the movement of the pointer' and the movement of the inductor cores.

The tuner just described provides three adjustments, namely, (1) the pointer Lb is adjustable along string Le and may be clamped in any desired position; (2) each core may be i' adjusted with respect to its associated coil or with respect to the other cores by means of the threaded stem support for the core; and (3) each coil may be adjusted with respect to its associated core or with respect to the other coils by vertical adjustment of the mounting bracket.

Where the two circuits including capacitors Ca and Cb are to be tuned over the same frequency range, and the two capacitors have the same value, the two cores will have the same initial setting with respect to the associated coils. The tuner is then operated until the circuits are tuned to a known frequency within the desired range, and the pointer is then adjusted to indicate the correct frequency. It is obvious that each core-coil combination may be adjusted to the same relative setting by adjustment of either the core or the coil.

In case one of the capacitors has a different value of capacity, the two inductors will require different relative settings, and the proper setting of each inductor may be made by adjustment of either the coil or the core. Even though the two inductors have diierent settings, and the two capacitors are of different values, the tuning of the two circuits will track over the same range provided the two capacities are within the maximum and minimum values as explained above. Accordingly, my invention makes it possible to use fixed capacitors in gang-tuned circuits, and the capacitors may have diiierent capacity values without the necessity of providing additional tuning elements for trimmer adjustment.

I claim:

1. A tuning system comprising a pair of tuned circuits having diierent capacitance values, each circuit including a variable inductor comprising a coil and a relatively movable magnetic core for varying the inductance of said coil, said Variable induotors being substantially identical, and the coil of each inductor having inductance determining structure so related to the inductance determining structure of its associated core that equal movements of said core over different portions of the range of tuning movement produces equal changes in the logarithmic value of the eiective inductance of said circuit, -a movable operating member, and means connecting said cores for movement in unison by said operating member, one of said cores being relatively displaced with respect to the other to tune said circuits to the same frequency.

2. In Ia tuning system, the combination of a variable inductor comprising a coil element having a movable magnetic core element for varying the inductance of said coil, and a condenser connected to said coil to form a resonant circuit, the elements of said variable inductor having inductance determining structure distributed along the lengths thereof to translate equal movements of said core into equal changes in the logarithmic value of the resonant frequency of said circuit.

3. In a tuning system formed of capacitance and inductance included in a closed resonant circuit, a device for Varying the tuning of said circuit comprising a coil forming at least part of said inductance, and 4a movable magnetic core for said coil, the inductance controlling structure of said coil and core elements being so distributed along the lengths of said elements as to provide, over a substantial range of movement of said core, a constant ratio between the core displacement and Athe change in the logarithmic value of the effective inductance of said closed resonant circuit resulting from said displacement.

4. A two-part variable inductor comprising a coil and a relatively movable magnetic core therefor, said parts being formed of inductance determining structure distributed along the lengths thereof to produce, over a substantial range of movement of said core relative to said coil, a constant ratio between the relative displacement between said par-ts and the change in logarithmic value of the resulting inductance of said coil.

5. A variable inductor comprising an elongated coil part and an elongated magnetic core part, one of said parts being mounted for movement relative to the other part to vary the inductance of said coil, said parts Ibeing formed of inductance determining structure distributed along the lengths thereof to produce, over a substantial range oi' movement of said movable part, equal changes in the logarithmic value of the inductance of said coil for equal steps of movement of said movable part over dierent sections of said range.

6. In a tuning system formed of capacitance and inductance included in a closed resonant circuit, a device for varying the tuning of said circuit comprising a coil element forming at least a part of said inductance, and a magnetic core element `for said coil, one of said elements being movable with respect to the other element for varying the inductance of said coil, said coil and core elements having inductance determining structure distributed along the lengths thereof to vary the resonant frequency of said closed circuit in accordance with a straight line graph of the logav15 Y rthm of the 'resonant frequer-ic`v of the circuit plotted against the 'extent of relative movement between said elements from a predetermined position. Y

FREDERICK N. JACOB.

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

Number 'White -4--..4 Sept. 10, 1946

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