US2163646A - Tuning circuit - Google Patents

Description

June 27, 1939. P. WARE: 2,163,646

TUNING CIRCUIT Filed Aug. 8, 1956 2 Sheets-Sheet l Z-U 5.0 4-0 6 0 6.a 7o da 2o, Za-

.200 Iil ATTORNEY v June 27, 1939. P. WARE `'lUIl-IIK CIRCUIT Filed Aug s, 1936 2 Sheets-Sheet 2 INVENTOR Haw! /fe' BY l,

rij ATTORNEY Patented June 27, 1939 vUNITED STATES PATENT oFFlcE TUNING CIRCUIT vPaul Ware, Indianapolis, Ind., assignor to P. R.

Mallory & Co., Inc., Indianapolis, Ind., a corporation of Delaware Application August 8, 1936, Serial No. 94,928

5 Claims.

This invention relates to improvements in radio tuning circuits having variable inductances.

Certain of its objects are to improve the efficiency and range of such circuits.

Anotherobject is to furnish a means for holding in step, or track, the several circuitsrequired in superheterodyne pre-selection.

' Other objects of theinventionwill be apparent from the following description drawings taken in connection claims.

and accompanying with the appended The invention comprises the features ofconstruction, combination of elements,` arrangement of parts, and methods of manufacture and. op-era- Vtion referred to above or which willk be brought out and exemplified in the disclosure hereinafter set forth including'the illustrations in the drawings.

Figure 1 illustrates a portion of a tuning circuit Vemploying a variable inductance coil;

Figure 2 is a graph having curves representing the Q of certain coil assemblies over a range of frequencies;

Figure 3 illustrates conventional variable condenser tuning;

. Figure i shows a variable arrangement;`

inductance tuning Figure 5 shows another inductance tuning arrangement;

Figure 6 shows a modified arrangement;` I Y inductance tuning Figure '7 shows a radio circuit employing linductance tuning; and

Figure 8V shows -a-modied radio circuit.`

While a preferred embodiment of the invention is described herein, it isfcontemplated that-considerable variation Ymay -be made in the method of procedure andthe construction of parts without departing fromthe spirit of the invention. In the following description andV inthe claims, parts will be identified byl'specic names foi1 convenience, but they are intended to be as generic in their application to simila will permit.

In the drawings, Figure 1 represents rparts as the art a slide wire currently herewith.

A series, arrangement of iixe d end .inductance 4 (Cl. Z50-40) Y and fixed condenser 5 is connected between slip terminals of condenser 5. It has been found i" that a tuning ratio of 10:1 v'is feasible with such an arrangement. quency endr limiting inductance as small as 1% of the total inductance at the low frequency exy As this necessitates' a high fretreme, it is necessary to have the resistance of the end inductance low in order to develop the needed voltage at and near the high frequency extreme. The end inductance might comprise a certain number of unused turns at the high frequency end of the variable coil I, but it is preferable to use a separate component in series with the variable coil unit.V

One advantage of this arrangement' resides vin the fact that the entire variable coil may be utilized for varying the inductance. In addition this arrangement has the advantage of enabling the use of an end inductance of better characteristics than an equivalent part of the unused turns on the variable coil, for the reason that under v practical design limitations the form factor and losses of such unused end turns of the variable unit are worse than those that can be obtained in a practical design of a separate coil. It will thus be apparent` that, by the arrangements described,

the high frequency resistance of end inductance 4 Tis less than the resistance of a portion of the variable inductance l of equal inductive value or, in other Words, the ratio of the reactance to the resistance for coil 4 is greater than for such part of coil l as would be required in the circuit if coil 4 were not used. This ratio is commonly used to designate the characteristics of an inductive coil. The ratio is usually expressed mathematically as f of the coil combination obtained by using aseparate end inductance instead of a number of unused turns at the end of the variable coil. The dotted curves 3l and 32 represent the value of Q over two frequency ranges for a given variable coil withl proper end inductances 4 and capacitances 5 in each range. The solid curves 33 and 34 show the Q of the variable coil over these ranges with the same capacities as used for curves 3l and 32, respectively, but without the use of the separate end inductances 4. It follows that the tuning range for a variable inductance and xed capacity may be increased by the use of the end inductance, because the resista-nce of the circuit in the region of the high frequency extreme has been lowered and hence the limit Where an acceptable value of impedance is obtainable has been moved out to a higher frequency.

The increased frequency range made po-ssible by this tuning system introduces another problem in the design of tuning circuits. This is where the variable inductances are employed as pre-selectors in superheterodynes. The pre-selector consists of one or more resonant circuits tunable to the frequency of the incoming signal and another tunable resonant circuit determining the frequency of the local oscillator. It is customary in superheterodynes always to maintain the oscillato-r at a frequency differing from the tunable incoming signal resonant circuits by an approximately constant amount called the intermediate frequency. 'Ihe method or procedure 'of maintaining this frequency difference during tuning is usually spoken of as tracking,

The methods employed to track variable condenser tuning circuits and variable inductance tuning circuits in superheterodynes will be better understood by reference to Figures 3, 4, 5 and 6.

Figure 3 shows the customary method of tracking variable condenser operated pre-selectors in superheterodyne circuits. The variable capacities 8 and 9 are electrically Yand mechanically similar and mounted on a common shaft. The radio frequency circuit comprises variable capacity 6, minimum frequency adjusting capacity 1, and xed inductance 8. In order to track the oscillator over the required restricted range, fixed capacity II is placed in series with variable capacity 8 in the oscillator circuit and fixed inductance I2 in the oscillator circuit is given a smaller value than fixed inductance 8 in the radio frequency circuit. A minimum adjusting capacity IU is connected in shunt with variable capacity 9.

An equivalent pair of circuits for pre-selection employing variableinductances is shown in Figure 4. The radio frequency circuit here comprises variable inductance I4, adjustable minimum frequency inductance I5 (to be referred to as the end inductance) and adjustable fixed capacity I3. The restricted range oscillator circuit consists of variable inductance I'I, adjustable minimum frequency end inductance I8 and adjustable fixed capacity I5. In addition, there is the inductance I9 connected in shunt with the capacity I6. 'I'he variable inductances I'I and I4 are similar electrically and mechanically and are on a common control shaft for tuning purposes.

The variable capacity method of Figure 3 and the variable inductive tuning method of Figure 4 will yield similar tracking characteristics. With either of these methods the frequency difference between the radio frequency and oscillator circuits may be made exactly equal to the inte-rmediate frequency at three different points over la tuning range and by proper design and adjustment these points of exact tracking may be so positioned in the tuning range that the deviation from perfect tracking between these points will be at a practical minimum for this arrangement.`

This condition exists when the maximum deviation between points and at the ends of the tuning range is about equal.

By making end inductances I5 `and I8 as. separate components similar to inductance l in Figure 1, an increased tuning range is made possible but the increased tuning range makes the tracking points more widely spaced thus resulting in increased deviation from perfect tracking between points. An additional tracking correction is therefore desirable in such a system adapted for use for an expanded tuning range.

Figure 5 shows an improvement over the tuning circuit of Figure 4 where in the ganged variable coils 28 and 2l of the type indicated in Figure l and set forth in detail in my above-mentioned co-pending applications have been added, together with a mutual coupling coil 22 coupled to the variable coil 2l.

I have found that if the shunt inductance I9 shown in Figure 4 is moved so as to have an inductive relationship with variable coil II the tracking may be materially improved. As shown in Figure this is accomplished by dividing the shunt inductance into two parts, 22 and 23, and disposing the part 22 concentrically inside or outside of variable coil 2I so that it will inductively couple therewith. It will be noted from Figure 5 that the mutual inductance between coils 22 and 2i will vary with the tuning because as the moving contacter is moved upward less and less turns of coil 2l will be in the circuit, the part of the variable inductance 2l below the contactor being short circuited as shown. The coils 22 and 23 constitute a shunt around the parallel resonant circuit. polarity causes the inductance of the whole circuit t'o be reduced instead of increased, and suitable physical conditions are provided, such as the size and location of coil 22 at its optimum position with respect to variable coil 2| and the optimum selection of the sizes of coil 23, end coil I8 and fixed capacity I6, then the tracking is found to be greatly improved in a wide range circuit over that which is possible with the three point arrangement of Figure 4. that the adjustment of components of the oscillator circuit of Figure 5 are made with a given set of component adjustments of its cooperating radio frequency circuit and with a predetermined intermediate frequency.

Figure 6 shows a modification of the circuit of Figure 5 wherein individual adjusting capacities 25 and 21 are shunted around Xed end coils 24 and 26, respectively, in place of the directly adjustable and similarly disposed end coils I5 and ,f

I8 of Figures 4 and 5. This modification permits the use of standard commercial adjustable trimmer condensers and other standard parts.

Figure '7 shows a complete pre-selection system for a superheterodyne radio receiving set employing three variable inductance tuners of the type indicated in Figure 1 and described in my copending cases, mechanically operating in unison on a single shaft (i. e., ganged), thus affording single control for tuning. The tube 88 is a radio frequency amplifier, the tube 82 is a modulator and tube 87 is an oscillator tube. One of the inductive tuning units 6I is employed to tune the circuit of the antenna 50. A second variable inductance unit 'I6 is employed for tuning the output of tube 66 (which is also the input of tube 82) at radio frequency. 'I'he third inductive tuning unit 88 is for the purpose of maintaining the frequency of the circuit of oscillator tube 81 at a substantially xed frequency spacing from the If coil 22 is connected so that its It is to be understood ,1

v:frequency"ofthe two other radio frequency cirvto the'xediintermediate frequency amplifier by circuit 91. Circuit 91 is the primary circuit of the rst intermediate frequency transformer of a Vstandardsuperheterodyne circuit. There may be Vone or more such 'transformers tuned to the intermediate frequency; The intermediate frequency circuit andi the audio frequency output circuit may be of standard design.

Anode power for the` modulator tube 82 is supplied from' the battery |99 connected to the lower part of circuit^91 as shown.

There are a number of single pole two-position switches :disposed throughout the pre-selector and it is understoodthat all of these switches are mechanically attached to a common actuating means so that theyI areeither all simultaneously snapped tov position L or to position H. Position L connects'all of the A'components so as to operate over aflow frequency :wave range which may cover .54 tov2.50 "rnc/s."(megacycl`es per second) and when 'invthe position H the Ycircuit components are ofisuchsize andconnection as to en able tuning'thasystemover a high frequency 'range which maybe adjusted to cover 2.30 to 18.00 .mc/S. Y u

Consideringv the antenna tuning circuit which is connected to the input terminals of radio fre-l quency amplifier tube-Bgwhen the switches are *in the "lowf frequency position L signal current entering" the system by way' of antenna. 5U is divided just tothe'fleft of :switch |60 and part of the currentpasses-throughcapacitance 5| and `chol'ec'oil53 to-g'ro'und'. The choke coil 53 is designed--toresonate at a frequencyrbelow the ilowerlimitofthe low'frequency range. Current alsofpasseswin thev other Vbranch "of the divided antenna circuit'througlr'capacitanjce 52 to a junction betweenfcapacitance*55and grounded capacitance 55 whi'ch'ftogetherlform the resonant circuiti iixed capacitancel for f the low frequency range.f lThe induct'anc'eofthis range comprises `adjosta-ble end inductance 54 and the variable ini VV'The' steppedLup'voltage developed V'across the tuned impedance of this resonant circuit is then Lled by'wayAof switch |00 andlcapacitance 62 to 'the control vgrid of tuba-66 and byway of ground Vand capacity 65 to "the cathode of tube 59. "Throughout most of this range-coupling from the vantenna is supplied -through the branch containingucapacity 52' and nearthej low frequency end of this rangey resonantfchoke 53;,coupled to end ance'll are connected acrosscapacities 58 and 59.- The voltage-across'the tunable impedance thus formed is led through' switch 109 and capacity 62 tothe control gridof tube 65 and by way .of ground/and Vcapacity 6'5`to the cathodey of tube `96 asy previously described. Resistance 63,- high compared with the resonantcircuit` impedance, ris for the purposeof 'supplying automatic volumecontrol voltageinthe usual manner across Athe input terminals of tube '66.- Capacity 52 is for passing ra'diofrequency and preventing the short. circuiting of the automaticvolume lcontrol 16, and for the high-frequency range capacitance 14, end linductance 15 and likewise variable inductance 16. `The choke' coil 1|-is effectively connected across the output terminals of tube 66 and is employed to supplythat tube with anode power from the battery |99; The characteristics of the choke 1 are such' as to afford the highest impedance over the entire tuning range of the system Without causing resonance at any frequency within that range. The' capacitance 1li s for the purpose 'of passing 'radio'frequency current and blocking the D..C. current' from battery |99 from being short-circuited by thetuning components. Capacitance and kresista-nce '18' are employed in' the same manner for tube 82 respectively as capacitance 62 and'resistance 63 are in the case of tube 66. The path ofthe radio frequency current betweeny tubes 66 andf'82 is from the 'anode of tube` 66 by way of capacitances 10 and 11 to the control grid of tube 82 as shown, Vwith the tunable impedanceinterposed between this line and ground as shown. AThis variable impedance which is tuned in unison with the antenna input "circuit already described accomplishes two purductance 88, end inductanc'e* 92 and xed'capacitance 9|. In shunt witlrl'iixed` capacity 9| is the shunt inductance fdivided into two parts 89 vand |5|. The part of the 'sl'ufnt inductance |6| isthe equivalent offc'oilf2'2'in Figures 6 and 5. When the switches-arein thehigh frequency position H the tunable resonant; circuit of the oscilf lator comprises the' same' variable inductance 88,

the end inductance93 land 'fthe 'fixed capacity 94, in Vshunt with which -isthe shunt in-ductance 95. The feed-back or tickler circuit for the low frequency range rimsfromithe anode of tube 81 through the switchl and the tickler coil 90 to the positive pole of battery |99, and for the high frequencyk range fromfthe "anode through' tickler inductance 96 tothe-positive pole of the same Ibattery. vOnthelow frequencyrange the polarity ofY coupling between coils 99 andv|39 is such as vto produce -oscillationsand'likewise on the high frequency. rangev the' coupling-between coils- 96 and 19'5 is of` suchpolarity-as to1 produce oscillations; 'The resonantjtuning circuits kjust de- 'scribed are connectedV by. way of fswitchrv |05 throughfcapacit'an'ce 85 to V-the'c'ont'i'pl gri-d of tubee1 anditheresistance 89 is-for the purpose of producing' `a suitable "bias. They oscillations thus. developed by the4 circuits in cooperation' with tube 81 vare-supplied by wayof capacitance 84 to the oscillator grid'of" modulator 'tube 82. The resistance 89 is for the purpose of additionally adjusting the bias oftube 82 as the strength of the oscillatoryoltag'e may vary. f

Tube 92 maybe one of the well known pentavgrid types suchas 6A'1 or 6A8'wherein theusecond vgrid out from thefcathode is known as the anode grid. The connection shownv in Figure 7 where fthis grid is connected to2-the' cathodev is not unusual, and enables retention of the desirable conversion gain properties of the pentagrid with the use of a separate oscillator tube as 81.

The two radio frequency tuning circuits employed in Figure 7 are for the purpose of affording a much better superheterodyne image ratio than could be obtained with a single radio frequency pre-selection circuit. Two important advantages obtained with a variable inductance method over the customary condenser tuned method are considerably wider tuning range per switch position, and improved radio frequency amplification on the shorter waves. The Wider tuning range ma-de possible with this system results in greater economy due to the reduction in the number of circuits required, and also enables the use of considerably longer short wave dial scales with greater accuracy than is possible with present condenser tuned systems.

An important advantage of inductive tuning especially where wide frequency tuning ranges may be covered, .as with this system, is that a substantial amount of xed capacitance remains in the circuit at all times. At the high frequency en-d of a range Where condenser tuning is employed, a large part of the total resonant circuit capacitance is due to the wiring and input or output electrodes of the yassociated tubes. On the other hand with inductive tuning a much less proportion of the xed capacitance is made up of the circuit and tube capacitances.

In certain applications it may be desirable to employ a shunt coil across the xed capacity of the radio frequency circuit, as across the capacitance I3 of Figure 4 as Well as the xed inductance I9 across the xed capacity |6 of the oscillator circuit.

It has been found that the presence of the choke coil 1| in the output of tube 66 of Figure '7 has aided the tracking of this stage with the antenna input tuning stage tunable by variable inductance 6|. This is because the slight frequency range restricting effect of choke 1| about equals the restricting effect of the antenna circuit by coupling to the tunable resonant circuit.

Figure 8 shows a superheterodyne pre-selector employing two variable inductance units |29 and |50. These variable inductances are 'similar electrically and mechanically and are mounted on a common shaft for single control of tuning.

These variable inductance units are individually shielded as in the case of the three variable inductance units shown in Figure 7.

The radio frequency signalsenter the pre-selector through antenna V||0 and pass- ,through wave trap This trap is resonant to-the intermediate frequency and is customarily employedin single pre-selection circuits to prevent reception of signals in the region of this frequency. On the low frequency range all of the switches are in the position L and the antenna `signals for this range pass through'switch ||2 and are divided through capacitances ||1 and H8. Those passing through capacitance ||1 go to ground through coil |26 which is designed to resonate below the low frequency limit of this range. Those passing through capacitance I8 go through inductance ||9 to the joint between series capacitances |24 and |25, capacitance |24 being connected to ground by switch |30 on this range. It will be observed that the capacitances |24 and |25 in series constitute the resonant circuit flxed capacitance for the low frequency range. The remainder of the low frequency resonant circuit is through switch ||6 and end inductances` |28, |21 in series and the variable inductance unit |29.

The size of inductance ||9 is chosen so that the antenna circuit effective capacitance in conjunction with capacitance ||8 and capacitance |24 will resonate outside the high frequency limit of the low frequency range. The coupling of the antenna circuit to the input resonant circuit over the low frequency range is by mutual inductance between coils |26 and |28, and by the common capacitance |24.

On the high frequency range all of the switches are in the position H and the operation of the antenna input circuit is as follows: From antenna ||0 and through trap and switch H2, signals are fed through capacitance ||3 to the joint between capacitances ||4 and. H5, which in series constitute the fixed capacitance of the resonant circuit for this range. In shunt with capacitances ||4 and ||5 is the end inductance |21 (end inductance |28 being shorted out on this range by switch H6) and the variable inductance |29. The coupling between the antenna circuit and the input resonant circuit on the high frequency range is by common capacitance ||5.

The resistance |32 is for conveying automatic Volume control voltage to the control grid of tube |33, and the capacitance |3| is for passing radio frequency current and preventing the short circuiting of the automatic volume control voltage by the tuning components.

'Ihe oscillator tuning circuit operates over the low frequency range by Variable inductance |50, end inductance |42 and the xed capacitance |44. 'Ihe oscillatoi1 circuits are arranged to run at a higher frequency than the radio frequency circuits. To restrict its range a shunt inductance is connected across and is made a part of the low frequency resonant circuit. This shunt inductance is divided into two parts, coil |5| and the mutual coil |60. The mutual coupling between coil |60 and the variable inductance |50 varies with the tuning in the manner explained in the description relating to the coil- 22 in Figs. 5 and 6.

The oscillator high frequency range resonant 'circuit is by the same vehicle inductance |50 and by end inductance |43 and fixed capacitance |45. The oscillator on this range also runs at a higher frequency than its cooperating radioV frequency circuit, and for this purpose its range is restricted by the shunt inductance |46.

The tube |33 may be'one of the well known pentagrid types 6A7 or 6A8 wherein the first and Ysecond grid from the cathode are respectively the oscillator grid and oscillator anode.

'I'he feed back orV tickler circuit for the oscillator low frequency range is from this anode grid through coils |41 and |52'and resistance |39 to positive terminal of battery |31, capacitances |53 and |38 being for by-passing purposes. The oscillator high frequency range feed back circuit is the same as that used on the low frequency range excepting that the coil |52 is effectively shorted to ground by way of by-passing capacitance |49 when the switch |48 is in the position I-I. The polarity of mutual coupling between coils |41 and |46 and also between coils I 52 and |5| is such as to produce'oscillations. 'Ihe mutual coil |60 is shorted Vto ground on the high frequency range by switch |30.

The resistance |34 and the capacitance |35 are connected as shownrand arefor the purpose of producing the desired oscillator grid bias in tube |33. The resistance |32 -is for the purpose of conveying automatic volume control voltage to thecontrolgrid of tube- |33 and the capacitance |3| passes'radio frequency current but prevents theshort circuiting to ground of the automatic volume control voltage by way of the tuning components.

of tube |33 are connected to a tap on the battery |31 as shown.

The coil |20 which is coupled to coil |I9 is connected between the cathode biasing resistor |2| and the cathode of tube |33, resistance |2| being by-passed by capacitance |227. The `lcoil |720 is employed only on the low frequency range and its size and degree of mutual coupling with inductance ||9 as well as its positioning along the coil ||9 are such as to improve the superheterodyne image ratio of the circuit. This is desir- '1 able where a single pre-selector circuit is used as in Fig. 8, and it may also be employed in a double pre-selector, as Fig. '1, altho it is not shown in Fig. 7.

The several variable inductance units shown in connection with the circuits of Fig. 1 and Fig. 8 are individually shielded to prevent pick-up of undesired signals and noise as well as to protect them from atmospheric dust and dirt. The circuits just described could be extended to cover more than two frequency ranges if desired.

The coil size for the variable inductances is limited by the highest frequency to which it is desired to tune and must be such as to ensure that the tuning range does not pass over its natural frequency. At the same time it is desirable to make the coil as large as possible in order to have the average impedance high over the tuning range. If any turns are in the circuit while tuning through the natural frequency, there is usually enough mutual inductance between these turns and the shorted unused part of the coil to cause a serious loss in performance at and near this point, and for this reason the coil geometry and number of turns are chosen so asto have this natural frequency fall just outside the high frequency limit of the highest frequency range to be tuned.

There are methods of tuning through this natural frequency point, and these are described in my co-pending application Serial No. 95,332, filed August 11, 1936.

In operating over the two ranges with the same variable inductances by 4switching between two sets of suitable circuit components and arrangements, it has been found desirable to divide the total frequency coverage so that less range is I covered in the low frequency than in the high frequency switch positions. This is'because the considerably larger fixed capacity required to tune the low frequency range renders the average inductance the greater the advantage described in connection with the curves of Fig. 2. y

It will be observed in Fig. 8 that the tracking aid coil |60 (and likewise coil |6|.in Fig. '1) is in circuit only on the low frequency range as it is not foundto be necessary onthe high frequency range. In general tracking errors are less the farther removed the tuning range is from the fixed intermediate frequency, and the narrower the tuning range. quency of coil |60 (and coil IBI) must be such when shorted out on the high frequency range by switch |30 in Fig.` 8 (or switch. |03 in Fig. '1) that it is outside the limits of the high frequency range.

Unlike condenser tuning where the impedance of the circuit tends to be lower at the lower frequency end of a tuning range, the variable inductance offers an additional advantage in the ease of obtaining uniform oscillations over a given range. 'Ihe increasing impedance as the circuit is tuned toward the lower frequency end of a range with this inductive tuner enables the use of a `high frequency oscillator feed back circuit whose natural frequency is outside the high frequency range, such a degree of feed back being sufficient to produce stable oscillations over a much wider range than would be possible without additional feed back aids in condenser tuned circuits. The natural frequency of the feed back circuit is usually determined not only by the feed back coil itself but also by its mutual coupling to any other circuit plus the additional inductance and capacitance ofthe connecting leads and associated tube. The high frequency feed back coils just described are shown as |41 and 96 respectively in Figs. 8 and '1. Stable oscillations are readily obtainable with the system just described over a frequency range of over 10 to l which requires a reactive ratio of over 100 to 1. The feed back or tickler coil may be partly or wholly arranged inside the variable coil as outlined in my co-pending application Serial No. 31,823, or may be coupled to the shunt inductance |46 by coil |41 as shown in Fig. 8.

This applicationy is a 'continuation in part ofl my co-pending application for Radio apparatus, Serial No. 31,823, filed July 1'1, 1935.

While the present invention, as to its objects and advantages, has been described herein as carried out in specic embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A resonant circuit for tuning within a predetermined frequency range comprising a slide wire variable inductance, a fixed inductance in series therewith, whereby the magnitude of said variable inductance lmay be varied between its maximum value and substantially zero during tuning over said predetermined frequency range, and a capacitance bridged across the series arrangement, the Q of said fixed inductance at a given frequency within said frequency range being higher than the Q of said Variable inductance at said frequency when said variable in- The natural freductance is adjusted to an inductive magnitude K L being the inductance and `R being the high frequency resistance of the inductance element or portion of the inductance element in question andf being the frequency.

2. A resonant circuit for tuning within a predetermined frequency range comprisingra varable inductance and a fixed inductance in series, and a capacitance bridged across the series arrangement, the Q of said fixed inductance at a given frequency within said frequency range being higher than the Q of said variable inductance at said frequency when said variable inductance is adjusted to an inductive magnitude equal to that of said fixed inductance, where L being the inductance and R being the high frequency resistance of the inductance element or portion of the inductance element in question and f being the frequency.

3. A resonant radio circuit for tuning within a predetermined frequency range comprising a variable inductance capable of reduction to sub1- stantially zero inductive value during tuning and a xed inductance in series therewith, and a fixed capacitance bridged across said series combination of nductances, the radio frequency resistance of said fixed inductance at a given frequency within said frequency range being less than the radio frequency resistance of said variable inductance at said frequency when the variable inductance is adjusted to an inductive magnitude equal to that of said xed inductance, whereby to improve the impedance of the said resonant circuit in the high frequency region Within the said predetermined tuning range.

4. A resonant circuit for tuning within a predetermined frequency range comprising a slide wire variable inductance, a fixed inductance in series therewith whereby the magnitude of said variable inductance may be varied between its maximum value and substantially zero during tuning over said predetermined frequency range, a capacitance bridged across the series arrangement, and an adjustable capacitance in shunt with said xed' inductance whereby to enable adjustment of the high frequency limit of said tuning range.

5. A resonant radio circuit for tuning within a predetermined frequency range comprising a variable inductance capable of reduction to substantially zero inductive value during tuning and a fixed inductance in series therewith, and a fixed capacitance bridged across said series combina tion of inductances, the ratio of reactance to radio frequency resistance of said fixed inductance at a given frequency within said frequency range being higher than the ratio of reactance to radio frequency resistance of said variable inductance at said frequency when the variable inductance is` adjusted to an inductive value approximately equal to that of said fixed induct-V ance, whereby to improve the impedance of the said resonant circuit throughout. at least the high frequency half of the said predetermined tuning range.

PAUL WARE.

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