US2450192A - Ultra high frequency tuning unit - Google Patents

Description

Sept. 28, 1948. w. c. FREEMAN, JR 2,450,192

- ULTRA HIGH FREQUENCY TUING UNIT Filed June 19. 1943 4 Sheets-Sheet 1 WMsr-tx C. Fxffmw JR y INVENTOR Sep# 28, 1948. w. o. FREEMAN, JR

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Filed June 19. 194s' W. C. FREEMAN, JR

ULTRA"HIGH FREQUENCY TUNING UNIT 4 Sheets-Sheet 4 In vv 4d d0 60 70 J0 f Taf v INVE'NTOR WAL TER C. Fna-m ,w JR

' ATTORNEY Patented Sept. 28, 1948 ULTRA HIGH FREQUENCY TUNING UNIT Walter C. Freeman, Jr., Emporium, Pa., assignor to Sylvania Electric Products Inc., Emporium, Pa., a corporation oi' Massachusetts Application June 19, 1943, Serial No. 491,443

Claims.

This invention relates to radio frequency sig-l naling apparatus and in particular to tuned high frequency circuits having movable metal core coils.

It is an object of the invention to provide means for keeping the circuit Q constant over the full tuning range of a permeability tuning coil.

An object of the invention relates to an in ductance tuned resonance circuit whose band width remains constant over the full tuning range.

It is another important object of the invention to provide means for increasingthe tuning range of' an inductively tunable resonance circuit of given dimensions, fixed band width, and given lower frequency limit over and above the high frequency limit obtainable with conventional permeability tuning coils.

A further object of the invention relates to the design of a single control superheterodyne receiving circuit containing two inductance tuned radio frequency amplifiers and an inductance tuned local oscillator.

Additional features will be presented in the further discussion of the invention.

The simplest form of tuning coil for a permeability tuned resonance circuit consists of a conductor wound helically on a cylindrical surface, and an axially movable core of magnetic material of properly chosen permeability and hysteresis characteristics, whose depth of axialI extension into a cylindrical coil determines the inductance of the coil. When the inside of the coil is lled with the magnetic all iron) core, the inductance is at a maximum, and it decreases continuously as the magnetic core is further and further with-- drawnA Minimum inductance will be reached when the magnetic core is completely withdrawn, leaving an all air core in the coil.

The maximum frequency obtainable with a movable core inductance circuit may be increased by decreasing the number and diameter of the turns of the inductance coil, and by decreasing the capacity supplementing the coil to form the resonant circuit. When electron tubes form part ot' thc tuned circuits, the interelectrodc capacities of these tubes may be introduced as the low capacities required for high frequencies. Comparatively wide frequency tuning rangesl may be obtained with permeability tuned circuits for moderately high frequencies.

There are, however, certain disadvantages connected with iron core inductance tuning, particularly for very high frequenciesv e. g., of the order of 100 mc., where hysteresis losses become so pro- 2 nounced that a reasonable circuit Q cannot be obtained without sacrifice of permeabilityy and thus of tuning range. A closely related disadvantage relates to an excessive variation of the circuit Q from a low value for all iron core to a very high value for all air core. As a result, the band width at low frequencies is large, resulting in decreased selectivity; for high frequencies,

the pass band becomes narrow (eventually too tuning operation the space in the coil from which the iron core is being withdrawn becomes more and more filled with the non-magnetic metal core, and vice versa. In its simplest form my new tuning core consists of a circular rod of convenient diameter and of a length greater than the length of the coil. A length of the core equal at least to that of the coil consists of magnetic material; the rest of the core length consists of a non-magnetic metal of properly chosen high frequency resistivity in the desired frequency range.

The qualitative theory of this composite tuning Core is generally as follows. The increase of the Q of the coil, resulting-from withdrawal of the magnetic core and its replacement by air is considerably reduced due to the positive resistance component coupled into the tuning coil (considered as the primary of a transformer) from the load resistance of the non-magnetic metal core (considered as a secondary load), resulting from the eddy currents therein. The operational inductance oi' the non-magnetic core for the induced currents flowing around its surface appears as a negative inductive impedance coupled into the tuning coil itself. Due to this negative inductive impedance coupled into the tuning coil, its operational inductance becomes less than that of the same coil having an all air core.

Accordingly, it is an object of the invention to increase the tuning range of a permeability tuned coil by attaching a non-magnetic metal slug to one end of the magnetic rod, whereby the Q of the coil is kept from increasing very appreciably over the iron core Q, when the magnetic core is In addition, this Q variation causes moving out of and the non-magnetic metallic core is moving into the coil.

A further object of the invention consists in providing a non-magnetic metal core, fixed to and forming a continuation of the magnetic core,

which tends to decrease the minimum inductance of a tuning coil below that obtainable from an air core inductance coil of otherwise equalI geometrical dimensions.

It is another important object of the invention to provide a rigid core for a permeability tuning coil consisting of three materials joined together along its axial dimension, one magnetic, a second non-magnetic and non-conducting, and a third non-magnetic and conducting. The axial distance between the magnetic and the non-magnetic conducting part is so chosenthat the relation` between the core position in the coil and the frequency becomes linear, or substantially so.

Further objects of the invention will be stated in connection with the description of the drawing. in which:

Fig. 1a shows a longitudinal section of the combination core according to one embodiment oi the invention.

Fig. 1b indicates the shape of the cooperating tuning coil in which the core is moving.

Fig. 1c is an end view of Fig. lb.

Fig. la? shows an assembly of the coil and core within a mounting bracket.

Fig. 1e is a schematic circuit sketch showing one practical arrangement of applying the permeability tuned circuit between output of a rst and input of a second radio frequency amplier.

Fig. 2 gives curves indicating various frequencycore position characteristics.

The curves in Fig. 3 show the variation of the circuit Q over the covered frequency band for various forms of core combinations.

Fig. 4 is an elevation of a radio receiver showing an improved form of permeability tuner.

The cylindrical core L according to the invention is shown in Fig. 1a, and is of circular crosssection of 1A" diameter. The core consists of section i constituted of ultra-high-frequency magnetic iron material usually of compressed powdered iron; a section 3, in the form of a nonmagnetic, conducting metal slug (e. g., brass); and a section 2, in the form of an intervening spacer, made of Bakelite or some other non-conducting and non-magnetic material. The sections l and 3 may be approximately V8" in length and section 2 may be approximately 1A" in length. The dimensions of this composite core are chosen so as to maintain a desired band-width as nearly constant as possible over the whole tuning range..

Attached to one end of the core is a connecting rod 4, preferably of non-magnetic metal, for moving the core axially with respect to the coil, and it may be conveniently operated by means of a spring and cam arrangement shown in Figs. 5 and 6.

Figs. 1b and lc show a preferred form of coil form and mounting, comprising a cylindrical polystyrene coil mounting body 5, into which the coil may be mounted and which provides simple means for sliding and guiding the core axially along the inside of the coil according to one embodiment of the invention. As shown, a bore E is provided, whose diameter is so related to the core diameter as to permit a sliding fit. Bore l l a winding of the coil on the outside or a cylindrical shell form, in that the largest possible inner cross-section of the coil can be completely illled out, either by the iron core or by the brass core. Practically no leakage space is thus lost between core and inner diameter of the coil, with the result that a lower permeability (and thus lower hysteresis loss) iron core can be used for a required maxmum inductance. Coil winding terminals 9 and I0 are carried through a slot in the coil mounting body.

Accordingly, it is an object of the invention to provide a coil mounting body into which the coil may be mounted so as to permit minimum spacing between coil and core Without an intervening ring-shaped cylindrical space which cannot be avoided in conventionally supported coils.

Fig. 1d shows the mounting body 5, with coil l, attached to bracket II by means of screw i2. Bearing Il is provided for rod l, which cooperates with bore 8 to form the two aligned guides needed for a purely axial sliding of the core inside coil l.

Fig. le represents a tuned coupling circuit between two ampliner tubes I4 and l5, consisting of a pi-network including shunt capacities i6 and I1. The variable series inductance is made up of sliding core and coil as described in connection with Figs. la to 1d. Capacities I6 and l1 are formed by the input capacitance of tube I5 and the output capacitance of tube i4, respectively. Also indicated in Fig. 1e, are plate resistor I8 through which B- voltage is supplied to tube I4; blocking condenser i9; and grid-leak resistor 20, for obtaining the proper biasing potential on the grid of tube i 5. The pi-network described here forms part of a heterodyne circuit illustrating a useful application of the invention.

Curves A, B and C of Fig. 2 represent the irequency characteristics of three kinds of inductance cores as a function of depths of core insertion into the coilshown in Figs. 1a to 1d, shunted by a 10 micro-micro-farad condenser. Curve C is the frequency curve resulting from motion of an all-iron slug, i. e., from a conventional permeability tuning coil. The tuning range from position 0, where the iron slug is centered in the coil, to position marked 24, where the center of the iron piece is removed 3A, leaving an all air core, extends from about 40 to 68 mc. Curve B relates to homogeneous brass core, yielding about 95 mc. with the brass core centered in the coil (position 30) and about 67 mc. with brass slug center removed from the center of the coil (position l0).

Comparison of curves B and C shows that the frequency range of the all brass rod tuning inductance is centered about a higher frequency than that obtained with the ultra high-frequency slug permeability tuned circuit. Each type of homogeneous core yields a tuning range of about 30 mc., with the permeability core centered around about 55 mc., with the brass core around mc.

The composite core according to the invention, increases the frequency range to a, total width of about 55 mc., centered around 65 mc. This follows by inspection of curve A which was taken experimentally with the composite core. The displacement of the center of curve A compared with the center of combined curves B and C, was caused by the loosening of one turn of the coil between the readings taken for curve A and those for curves B and C in the test run for which the curves of Fig. 2 were made.

aesaree It can be seen vfrom inspection of the three curves in Figs. 2 and 3, that with the center of the all brass piece in the coil (see curve B of Fig. 3), the frequency is about 95 mc. and the corresponding Q has a value of approximately 88. When this all brass slug is removed completely from the coil, the frequency decreases to 70 mc. while the Q increases to 205. This large increase of Q with the removal of the brass core is in agreement with the theory given above according to which the homogeneous brass core acts as a secondary load on the coil.

Referring now to curve C of Fig. 3, the position of the homogeneous iron core centered within the coil (which corresponds to position 0 of Fig. 2), yields a frequency of 37 mc., and a Q of near 70. The Q value of 220 at about 70 me. in Fig. 3, corresponds to the position of the iron slug completely removed from the inside of the coil (point of Fig. 2). A frequency of about 67 mc. corresponds to an all air core resulting either from complete removal of the homogeneous brass core,

B or from the removal of the all iron slug C,

When the composite core is used for tuning, the

curve is much less abrupt, as can be seen by curve A of Fig. 3, in which the approximate changes of Q are from when the frequency is about 38 rnc.

(iron core) to a maximum of 110 at a frequency of mc. (Bakelite), decreasing to 90 at frequency of mc. (brass section). The large Q variation over the two shorter frequency bands (of about 25 to 30 mc. each), characteristic of the homogeneous cores corresponding to curves B and C, is avoided in the use of the composite core,

and of course its tuning range is almost doubled I (for the coil and condenser used in the experimental runs as described in connection with Figs. 2 and 3) -Plural ganged permeability tuners that can advantageously be utilizedin a superheterodyne receiver will now be described in connection with Fig. 4. In a chassis 36 formed of sheet metal side walls 38, 39 and 4U, and top wall 4I are placed the radio frequency and intermediate frequency tuning circuits containing plural coils, each in the form of a coil winding and mounting, similar to those shown in detail in Fig. 1d. Each of these tuning coils is enclosed in a separate shielded box. Connecting rods 41 of the tuning cores pass through holes which are provided with bushings and are pressed by springs 56 over cam-followers i! against cams 62. The cams may be rotated simultaneously by turning shaft 65 which itself is rotatable around its axis and carried by bearings 66 mounted in the sidewalls 39 of chassis 36.

By proper choice of the characteristics of the tuning inductance capacities, positioning of the tuning cores, and shaping of the cams, a single control gang tuning of the radio frequency pinetwork and the local oscillator circuit by means of tuning shaft 65 can be obtained. In one actual embodiment, the tuning ranges of the three tuning circuits were: to 175 mc. for the two radio frequency circuits and of the local oscillator from 70 to 145 mc., yielding an intermediate frequency (if,

of 30 mc. The pass bands for the signal frequency and for intermediate frequency had a width of 4 mc. f

Though a specific reduction to practice has been described herein in connection with fixed geometrical dimensions, tube types, and capacitance values, etc., it is of course understood that all these specific `data are given by way of illustration as applied to a receiver in the frequency range of the order of 100 mc.-R. F., and should not be considered as limiting the' scope of the invention.

What I claim is: f'

l. A permeability-tuned coil assembly comprising a cylindrical coil, a cylindrical tuning core only slightly less in diameter than the inside coil diameter, and a body of insulating material having one portion encasing said coil and having a coaxial bore in another portion for`slidably accepting and centering such part of said core as is extended beyond said coil.

2. A tuning unit for radio receivers and the like comprising a composite tuning core including a cylindrical non-magnetic but conductive slug, an axially spaced magnetic slug of like diameter, a coil, only slightly larger in internal diameter than said core, and a coil form encasing said coil and having a coaxial bore at the end of said coil of a diameter slightly larger than the diameter of said core'to thereby slidably support the part of the core at one end of the coil. y

3. A tuningl unit according to claim 2 wherein said core includes an insulating spacer between said slugs, the spacer and slugs being rigidly assembled on a control rod of smaller diameter than said slugs. l

4. A tuning unit comprising a body of insulation having a bore with two sections of different internal diameters, a helical tuning coil supported in the larger bore section interiorly thereof, a core adapted to be reciprocated through said bore for tuning purposes, said core having at one end a magnetic section of a diameter siidably fitting the inside diameter of the smaller section bore, said core having at its opposite end a section of non-magnetic conductive material, and an intervening section of solici non-magnetic insulating material, a pair of spaced uprights to one of which said insulating body is fastened, a guiding shaft to which said core is rigidly fastened.-

said guiding shaft being slidably mounted in the other of said uprights.

5. A unit according to claim 4 in which the three sections of said core are threaded on to the end of said guiding shaft and spring means are provided for holding said core in a normal position, said guiding shaft having a cam follower attached to one end thereof, and a rotatable cam engaging said follower to adjust the position of said core within said bore.

WALTER C. FREEMAN. Jn.

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

UNITED STATES PATENTS

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