US 2081572 A
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May 25, 1937.
sv. J. A. M. BAGNO ATTENUATOR Filed OOJC.. l, 1934 3 Sheets-Sheet l INVENTOR. Samuel JAMBagno Y y .y y
ATTORNEYS May 25, 1937 s. J. A. M. BAGNO V 2,081,572
ATTENUATOR Filed Oct. l, 1934 3 Sheets-Sheet 2 llllllllj' ISO INVENTOR. Samue JAM. Bugnov ATTORNEYS May 25, 1937 l s. J. A. M. BAGNO 2,081,572
TTTTTTTT OR MMIM'AWIVNIVIVIVIVMwww i *C INVENTOR.
Schwe JAMogno Patented May Z5, 1937 UNITED STATES PATENT OFFICE ATTENUATOR Application October 1,
This invention relates to attenuators.
The object of my invention is to generally improve attenuators. A more particular object resides in the provision of an attenuator so arranged that the attenuation obtained may be divorced from the impedance of the attenuator, so that special requirements as to attenuation and irnpedance may be fulfilled largely independently of one another, all while using only a single unit.
One special case of great commercial importance is the provision of logarithmic attenuation. This is valuable for volume control of sound producing apparatus such as radio receivers because it is a well-known physiological law that equal steps of attenuation on a. logarithmic scale sound approximately like equal loudness steps, and logarithmic attenuation therefore acts as an apparently uniform control of volume. In accordance with an important object and feature of my invention, I provide logarithmic attenuation, while utilizing uniform resistance elements of simple form, whereby the attenuator may be manufactured under quantity production conditions at minimum cost.
Another special case of frequent importance is the provision of constant or approximately constant input and output impedance during variation of the attenuator. Itl should be kept in mind that with a conventional unit, for example a potentiometer, the output impedance varies from zero to the actual maximum resistance of the unit, which is an innity percentage variation. In accordance with my invention, however, the impedance may be kept within a` range of say two to one, and this result is obtained while dealing with a unit of extremely simple and inexpensive construction. As will later appear, by a Very slight complication of the apparatus, I may obtain nearly perfect constancy of impedance both for input and output. In accordance with a further object of my invention, I provide both logarithmic attenuation and approximately constant impedance in a single unit of simple and inexpensive construction.
Still further objects of my invention reside in the provision of an attenuator which may be given a high range of attenuation if desired; which may be, and preferably ordinarily is, arranged to pro- Vide continuous variation over the entire range; and which is purely resistive in character, it being non-inductive- Further specialized objects of my invention reside in the provision of an attenuator particularly well adapted for use in oscillators for laboratory purposes and test sets; the further pro- 1934, Serial No. 746,316
(Cl. FX8- 44) Vision of an attenuator particularly suitable for volume control in radio receivers; and the further provision, when desired, of a Volume and tone control combined in a single unit. Still another object of my invention resides in the provision of advantageous structural embodiments of the attenuator, which embodiments are practical, compact and susceptible of manufacture at a cost but little if at all higher than the cost of an equivalent sirnple variable resistance or potentiometer.
To the accomplishment of the foregoing and such other objects as may hereinafter appear, my invention consists in the attenuator elements and their relation one to the other, as hereinafter are more particularly described in the specification and sought to be defined in the claims.
The speciiication is accompanied by drawings, in which:
Fig. 1 is a diagram explanatory of my invention;
Fig. 2 shows the interior of a practical unit with the back plate removed;
Fig. 3 is an enlarged section taken on the plane of the line 3-3 of Fig. 2;
' Fig. 4 shows the interior of a different type of practical unit with the back plate removed;
Fig. 5 is a section taken on the plane of the line 5 5 of Fig. 4;
Fig. 6 is an enlarged section taken on the plane of the line 6 5 of Fig. 4;
Fig. '7 shows one mode of application of my attenuator to an audio frequency, radio frequency oscillator or test set;
Fig. 8 illustrates one way in which the attenuator of my invention may be applied for controlling the volume in an amplifier;
Fig. 9 is a plan view of an attenuator modified to maintain nearly perfect constancy of impedance;
Fig. 10 is a side elevation thereof;
Fig. 11 is a section taken on the plane of the line II-ll of Fig. 10;
Fig. 12 is a schematic diagram of a modied attenuator having non-uniform resistance elements;
Fig. 13 is a schematic diagram of a tapered double unit with tapered resistance elements arranged to maintain nearly constant impedance; and
Fig. 14 shows a combined tone and volume control, and one manner in which the same may be connected in circuit.
The attenuator of my invention may be described as somewhat analogous to a long line with leakage, for example a telegraph line. Such a line has series resistance along the line and leakage or shunt resistance to ground. These characteristics are accidental and undesirable. In my invention, compact resistances are intentionally provided for intentional and variable control of attenuation. Referring to Fig. l, the attenuator of my invention comprises a Series resistance I2, a highly conductive bus I4 extending along but spaced from said series resistance I2, and a shunt resistance I 6 extending between and cone tinuously connecting the series resistance I2 to bus I4. A terminal A is connected to one end of series resistance I2. A second terminal B is connected to a contact or shoe I8 which is movable along resistance I2 in order to vary the effective length of the attenuator. A third terminal C is connected to bus I4. Terminals A and C may be used as the input or output and terminals B and C as the output or input respectively of the attenuator. One important feature of my invention resides in the fact that the resistances I2 and I6 may be continuous, thus af fording continuous variation of attenuation. The resistance may be a film, or solid, or even Wire-wound, although I prefer the use of a film as being the most convenient. The resistances I2 and I6 may also be uniform and in fact, when these resistances are uniform, the attenuation obtained is logarithmic in character. This simplifies and cheapens manufacture, for it is indeed a simple matter to provide a uniform resistance film. Furthermore, by appropriately spacing bus I4 from series resistance I2, it is possible to make shunt resistance I5 exactly like series resistance I2 in resistivity, or in other Words, it is possible to provide a single film of resistance made of one resistance paint uniformly applied with a uniform thickness for both the series resistance I2 and the shunt resistance I6, thus further simplifying manufacture of the attenuator.
If we assume the simple case of uniform series resistance and uniform shunt resistance, and further assume that the unit is closed by or connected to a load which properly matches the surge resistance of the attenuator, or what is equivalent to the last assumption, if we assume that the total available attenuation is large, (analogous to a very long line) the attenuator may be designed by utilizing the following formulae. The mathematical .derivation of these is given near the end of the specification in order not to disturb the continuity and readability of the disclosure. The designer is ordinarily given the impedance and the total attenuation. The total attenuation is af, where a is the attenuation per unit length and equals e-U-G in which R is the resistance per unit length, and G is the leakage conductance per unit length, which in turn is the reciprocal of the leakage resistance or shunt resistance per unit length, and :c is the length. The surge resistance Ro is equal to the N/Z? G casing 2U lined by appropriate insulation 22. Casing 20 is provided on its forward face with a conventional threaded bushing 24 through which there passes the usual oscillatable control shaft 26. The casing is closed by a back plate which is not shown in the drawings.
A strip of insulation 28 is coated on both faces 3i! and 32 and on one edge 34 with a resistance lm. The coated strip 28 is bent to arcuate form and placed within the casing as is clearly shown in the drawings. Another insulation strip (i5 is wound with fine wire 38 and the windings are interrupted, severed or open-circuited as is shown at 4D in Fig. 3. Wire-wound strip 3B is also bent into arcuate form and placed within strip 28 and contacting directly with the inner resistance iilm 3D. The highly conductive strip of metal or bus I4 is bent to arcuate form and placed within the casing in contact with the lov/cr portion of the outer lm 32. Bus I4, coated strip 28, and wire-wound element 3S are pressed into intimate contact between the wall of the casing and a central disc 42 which is pressed within wire-wound element 3E.
The wire-wound element is slidably engaged by I a contact arm 44 connected by arcuate arms 45 to a disc 43 riveted by rivets 50 to a disc of in'- sulation 52 which in turn is secured to the end of the control shaft 25. A stop arm 54 is riveted in place together with disc 52 and cooperates with an appropriate stop lug on the back plate of the case, thereby limiting the movement of the con tact arm. A brush 55 is bent downwardly from disc 48 and bears against and rides upon metallic ring 58 which in turn is connected with and preferably formed integrally with, the soldering lug or terminal B. Contact 44, arcuate arms 4G, disc 48 and depending brush 5B are also preferably formed from a single piece of metal. Another soldering lug or terminal A is connected with the end portion EI) of wire-wound element 3S and it will be understood that the windings at the end portion 6D are not severed, and accordingly, pro- Vide Contact with the end of the inside resistance film 30. bears against and is connected directly to one end of the highly conductive bus I4.
The operation of this unit will be understood by direct analogy to the schematic diagram shown in Fig. l. end of that part of inside rim 38 which corrcspends in width to the height of wire-wound element 35. The said portion of inside film 3D which. is engaged by wire-wound element 35 corresponds to series resistance I2 in Fig. 1. bus I4 corresponds of course to bus I4 in Fig. 1. The remaining resistance nlm, i. the uppermost part of inside nlm 30, the edge film 34 and the outer film 32 down to bus I4, corresponds to shunt resistance I6 in Fig. l. Contact arm 44 corresponds to sliding contact Iii in Fig. l, and is likewise connected to terminal B.
It will be understood that an important advantage of the present type of unit resides in the fact that a high resistance is obtainable by the use of a resistance nlm, while sliding ccntact is obtained between metallic surfaces, e., contact 44 and wire element S6, thereby elimina*u ing any tendency to wear away or change the resistance value of the resistance nlm.
As an instance of quantitative values, I may refer to a unit like that shown in Figs. 2 and 3, except that coated strip 28 is reduced in Width to the width of wire-wound clement 36 so that inside film 38 corresponds directly to series re- A third terminal or soldering lug C i Terminal A connects with one r The Til
Ll i) sistance I2, while outside film 32 corresponds directly to shunt resistance IS. An attenuator designed for 8O db. attenuation employs a coated strip about three inches long and having a series resistance on one side of the strip totalling G() ohms. This resistance is conveniently measured beiore painting the edge 34. The edge and outer side of the strip are then painted like the inner side of the strip. The shunt resistance is given a total value of 20 ohms. This is readily tested by placing a copper plate under the series resistance, applying a copper bus at the lower edge of the shunt resistance, and then moving the copper bus upwardly slightly until the total shunt resistance reaches the desired value. The unit proportioned as aforesaid provides 80 db. attenuation in accordance with a logarithmic characteristic; and provides approximately constant impedance. By approximately constant impedance I mean a variation of the order of say two to one, for example from 50 to 100 ohms. This be considered approximately constant for ordinary volume control or attenuation purposes, because it is to be contrasted with the iniinity ratio of impedance variation obtained with conventional volume control units such as the ordinary potentiometer.
To measure a constructed unit in order to test its resistance to check the same against design values, the shunt resistance is readily determined as previously mentioned by covering the series resistance with a metallic plate and measuring the total shunt resistance for the entire shunt band. The reciprocal of this value is the leakage conductance of the band, and that divided by the length of the band gives the leakage conductance per unit length or G. Similarly, the total series resistance may be divided by the length to obtain the resistance per unit length or R. W hen the shunt resistance is high, it is not essential to disconnect the shunt resistance from the series resistance when measuring the value of the series resistance, and a fairly close reading may be obtained by simply measuring the resistance between terminals A and B. For more accurate work, of course. the shunt resistance is preferably disconnected from the series resistance during the measurement. For measuring the surge resistance of the physical unit, it is merely necessary to apply Ohms law, introducing a test potential and test current on the terminals of the unit. During this measurement, it is necessary for strict accuracy, to close the unit or connect the same toa resistance equal to the surge resistance, or in other words, to match the impedance of the unit. However, i find that for units with a substantial range of attenuation, say more than db., the unit may be considered analogous to a very long or infinite line and no appreciable error results even if the precaution of matching the impedance of the unit is neglected.
Another practical form in which the attenuator may be embodied for volume control and like purposes is illustrated in Figs. 4, 5, and 6. This unit differs from that heretofore described primarily in dispensing with the use of the wirewound element and instead providing sliding contact directly with the surface of the series resistance nlm. Referring to the drawings, the resistance elements are provided upon an insulation ring 62 which is preferably rectangular in cross-section. The outer periphery is coated with a resistance lm i2, while the forward face is coated with a resistance film I6. A highly conductive metallic bus I4 is provided at the inner peripheral edge of shunt resistance I6.
Ring E?. is placed within an insulation case 64 and rests against the front wall thereof. Case '54 is in the form of a fiat cylinder and is closed by a metallic back plate 6G, which has been removed in 4, but is shown in Fig. 5. Ring 52 is held against the front wall of the case by screws iid and the holes receiving said screws are preferably counterbored slightly as shown at lil in Fig. 6 in order to prevent contact between shunt resistance IE. Series resistance I2 is engaged by a wiper or contact shoe 12 carried at one end of an arcuate resilient arm 14 which in turn is riveted at 'i6 to an oscillatable arm 18. This is downwardly -dished to form a hub 80 which is insulatively secured to control shaft 82 between insulation washers 84 and 8S. A stop arm 83 is preferably also riveted to control shaft 82, the said arm cooperating with a stop lug Si! struck inwardly from back plate 65. Metallic hub lill is engaged by an upwardly bowed resilient ring 9D (Fig. 5) which is connected to and preferably formed integrally with soldering lug B (Fig. 4). Another terminal or soldering lug A extends through the wall of the casing and is clamped between ring 52 and the casing. A layer of rnetallic paint is extended from one end of series resistance I2 along the periphery of the ring as is indicated at 92 and thence around the lower face of the ring as indicated at 94, thereby providing direct contact with soldering lug A. The conductive bus I4 may be stamped from thin metal or may be applied to the unit in the form of metallic paint, and in either case, is preferably enlarged at 95 where it contacts directly with the inner end of a third terminal or soldering lug C.
The operation of this unit will be readily understood by analogy to the schematic diagram of Fig. l. Terminal A is connected to one end of series resistance i2 located on the periphery of ring 62. This resistance is varied over its entire width by wiper shoe 12 which may rock on arm 14, and which arm is yieldable, thus providing selfadjusting seatingl of the contact on the nlm. Contact 12 is of course connected through arm 1li, arm 13, hub: 80 and bowed ring SE' to the terminal or lug B. Shunt resistance i6 extends between and is continuously connected to series resistance l2 and bus I4, and the latter is in turn connected directly to terminal C. With this unit, as with that previously described. logarithmic attenuatio-n is obtained with simple uniform resistance films. Films I2 and Iii may be made different in thickness for a speciiic resistance, or
when desired, may conveniently be made the same in thickness, in which case the desired value of leakage conductance is obtained by appropriately adjusting the radial dimension of the nlm.
In Fig. 1 l. show a test set oscillator suitable for use by radio repair men and for laboratory purposes. This oscillator comprises a tube |00 having a tuned radio frequency oscillatory circuit H32 connected between its cathode and control electrode. The radio frequency is controlled by an appropriate calibrated variable condenser H34 while a change in range of frequency is obtained by an inductance switch Iii. The feed-back coil is connected in the plate circuit of tube I0() and is regeneratively coupled to the inductancc Iil of tuned circuit H32.
Audio frequency energy is generated with the aid of a tube I l2 having a tuned circuit l I4 connected between the cathode ISB and control electrode I|8. The audio frequency is adjusted by means of variable condenser |20. Coil |22 is connected in the circuit of anode |24 and is regeneratively coupled to the coil |28 of tuned circuit II4. The radio and audio frequency energies are combined or modulated by tube I I2 which acts also as a mixer tube, the radio frequency energy being applied to another control electrode |28 of the tube. In practice, I prefer to use a multiple grid tube and the extra grids, in this case three, are all connected to anode |24. I find this desirable when using a low potential, say 221/2 volts on the anode, which in turn is convenient so that a relatively small B battery |30 may be used to energize the oscillator. This, of course, makes for portability. The cathodes are heated by a 4% volt battery |32, the circuit of which is controlled by a switch |34.
The modulated energy is supplied to the load or receiver being tested, through terminals I 36. My attenuator |38 is preferably connected in circuit between the oscillator' and terminals |30 in the manner shown, the terminals AC acting as input terminals and the terminals BC acting as output terminals. If `ii, is desired to obtain audio frequency energy rather than modulated radio frequency energy, connection may be made to terminals |40.
The use of my attenuator for the present purpose is characterized by a number of advantages, including the facts that it does not detune the antenna. circuit; that a large range o1'` attenua-- tion is wanted and is obtainable; the impedance is kept below the capacity leakage of the load or set being tested and therefore the calibration of the test set is not upset; the percentage ol accuracy of the calibrated attenuator is constant regardless of the attenuation; and the attenuator is non-inductive.
In Fig. 8, I illustrate the application of the attenuator to a cascade amplifier including vacuum tubes |42 and |44 coupled in cascade by a trans-- former |46 having its primary connected to the output circuit of tube |42 and its secondary connected to the input circuit of tube |44 in conventional fashion. In the present case, the attenuator is connected between tube |42 and the primary of transformer |45. Terminals AC act as the input and the terminals BC act as the output.
It should be appreciated that the attenuator may, if desired, be connected between the secondary of transformer |46 and tube |44. Furthermore, assuming the amplifier to form a part of a radio receiver, it is possible to insert the attenuator between the antenna and the receiver as well as between tubes of the receiver. It may be used in the radio frequency portion or in the audio frequency portion of the receiver. It may also be used between the receiver and the speaker coil, as is later illustrated in connection with Fig. 14. In Fig. 8, the resistance elements are assumed to be constant thereby producing logarit-hmic attenuation. It may be remarked that in such case, there is no tone control effect because the impedance is substantially constant.
It is possible both theoretically and practically to provide constant input and output irnpedance for the attenuator, and a device for this purpose is illustrated in Figs. 9 through Il. Such departure from constant impedance as takes place across terminals AC in the units heretofore described is due to the variable shunting eiect of the load, or from another viewpoint, is due to the effect of the unused part of the attenuator lol-- lowing the movable tap- In other words, if the attenuator were variable in length, the unused part following the tap being disconnected at all times, there would be perfect maintenance of constant impedance both for input and output, Such a result may be obtained for all practical purposes by open-circuiting or making ineilective the highly conductive bus for the unused part of the attenuator.
Referring to the drawings, Figs. 9 through ll, the attenuator comprises an insulation base |50 provided with a series resistance film I2, a highly conductive bus I4 and a shunt resistance nlm I5. Terminal A is connected to one end of series resistance I2. The series resistance is slidably engaged by a Contact shoe |52 mounted on a metallic support |54 slidable on a conductive rod |55 to which terminal B is connected. Bus I4 is in this made el a strip of resilient metal so shaped that it tends at all times to lift from the resistance @im is indicated by the raised ljiortion |58 in Fig il). Terminal C is connected to the fixed end I" which is secured to base |5t. lhe resilient strip is forced downwardly by a roller |62 carried on an insulation arm |54 moua ted on support |54. It will be manifest that the slider is moved along the unit the bus is made effective only for the useful part of the attenuator, only .for the distance between terminal A and the conta-ct 52. The remaining or free end portion el the bus is elevated, thus eliminating the shunting of energy across the unused part of the attcnuater to the bus.
With this arrangement very nearly or prac-- tically constant impedance .is obtained, by which l2 mean a constancy as near perfect as is ordinarily obtainable or expect-ed with practical work-- ing There is a slight departure from absolute perfection because of the slight leakage in the resistance i'llni itself even though not grounded by a` conductive bus.
It possible to use tapered rather than uni- `form resistance elements and such arrangement is schematically illustrated in Fig. 12, in which the series resistance element I2 tapers from a high resistance at terminal A to a low resistance at its opposite end. The shunt resistance I6 likewise tapers from a high resistance to a low resistance in the same direction as series resistance I 2. rl`his arrangement has the advantage of minimizing the effect of the unused portion of the attenuator for the reason that this effect is signincant only at the low attenuation or high volume end of the unit when the unused part of the unit comprises nearly all of the unit. With a tapered unit such as is shown in l2, when the slider is moved near to terminal A for lov.7 attenuation, the resistance values are high and but little loss takes place.
However, with this arrangement the impedance across terminals BC is variable although the impedance across terminals AC may remain approximately constant.
I may provide constant output as well constant input impedance by utilizing in effect, two tapered units, and such an arrangement is shown in Fig. 13. In this arrangement, the series resistance tapers from the center towards its ends and the same applies to the shunt resistance. The series resistance is provided with a pair of sliders I'Iil and I'I2, which Iare preferably simultaneously and oppositely moved by appropriate mechanical link mechanism schematically shown by the dotted lines IM. Termina-1 A is connected to one of the sliders and terminal B to the other.
In practice, the units fi U (ill
teristic resulting may of course be arranged side by side instead of end to end, this facilitating simultaneous movement of slidersl10 and i12. Such a unit is characterized by equal and constant input and output impedances, the constancy obtained being nearly as good as that obtained in the unit of Fig. 9. The connections of the bus i4 are, of course, permanent. The unit of Fig. 18 may therefore be thought of as a wholly different approach to the problem of obtaining constant input and output impedance, for in the unit of Fig. 9 a mechanical change is introduced for varying the bus connection, whereas in Fig. the change is an electrical one by tapering the resistance Values.
Reverting now to Fig. 12, the variation of irnpedance is sometimes permissible and even desirable. One example is the case of a conventional L-type resistance unit which, however, requires two variable resistances, whereas the unit of Fig. l2 requires only one variable resistance. Incidentally, I may point out that it is possible to obtain logarithmic attenuation even when using tapered instead of uniform resistance values, provided that the resistance values are properly adjusted. It is, of course, also possible to obtain different desired rates of attenuation by appropriate tapering of the resistance values. As an extreme case, I may mention the possibility of obtaining uniform attenuation by utilizing logarithmically tapering resistances.
Another application or use for a tapered unit such as shown in Fig. l2, is for a combined tone control and volume control for a radio receiver. The logarithmic attenuation of a uniform resistance unit such as is shown in Fig. l, is highly desirable and is sui'licient when dealing with a single frequency because, as has been already mentioned, the relation between sensation level and intensity of a sound is approximately logarithmic. However, in a radio receiver, we deal with a wide band of frequencies, and the requirements become somewhat different when we wish to obtain an attenuation vs. frequency characin an apparently uniform pitch-intensity level relationship. The response of the ear is such that it is desirable to increase the relative loudness of low frequencies at a low volume level and vice versa at a high volume level. Complicated multiple units have heretofore been designed in an effort to obtain this result. With my invention, the requirements may be satisfied by a simple tapered unit and such an arrangement is illustrated in Fig. 14. The attenuator is there shown connected between output transformer it of the audio frequency amplifier and the movable or voice coil |52 of an electrodynamic speaker 184. The conventional field coil is omitted from the drawings. By utilizing a tap-ered unit such as is shown in Fig. 14, at maximum volume, the shunt resistance is at maximum and there is accordingly best response for high frequencies. At minimum volume the shunt resistance is at minimum and there is best response for low frequencies, the high frequency energy being dissipated to a larger extent in the shunt resistance; also, a larger proportionate drop occurring across the leakage reactance of the transformer for the high-er frequencies. The unit may be proportioned to obtain logarithmic attenuation together with a beneficial tone control effect. It will be understood that the attenuator is not itself a tone control but acts as such only in cooperation with the associated external circuit.
Considered broadly, the attenuator is a combined attenuator control and variable impedance passive transducer, so arranged that the latter property of impedance change is used to vary the frequency-gain characteristic.
Mathematical treatment The following mathematical treatment resembles that heretofore developed in connection with telegraph lines, but so far as I am aware, it is wholly new to apply the same for purposeful attenuation in a volume control or like compact structure which does not involve a long line and which is intentionally provided with shunt leakage resistance. In the following treatment it is assumed that inductance and capacitance are ignorable.
t a distance of t millimeters from the beginning of the attenuator, let the steady potential to the low resistance return or conductive bus be E volts, and let the steady current along the series resistance element be I amperes. Let R 'be the series resistance per unit length, in which case the resistance of an element dm will be Rd ohms. Let G be the leakage conductance across the shunt resistance in ohms per millimeter, in which case the leakage conductance in the element da: is Gdr. The drop in potential in the element is IRdx volts, while the drop of current in the element in EGd amperes.
This may be expressed:
(l) i1- IR volts per millimeter dI (2) EG amperes per millimeter Differentiating (l) with respect to we obtain:
12E dI But inserting (2) in (3), we obtain:
E- GRE Diferentiating (2) with respect to zc, we obtain:
LPI dE 5) f2-*Gta Substituting (1) in (5), We obtain:
d2I (6) =GRI Equations (4) and (6) are simple linear differential equations of the second order and may be solved either exponentially or in terms of hyperbolic functions. Equation (4) may be solved exponentially as follows:
dxz mZema Substituting this in (4), We get:
1:12am ERG emR G Therefore,
m2 R G And The solution of Equation (4) is therefore:
(7) I? #muse-4R51 wherein A and B are arbitrary constants of integration.
Students of diierential equations will recognize (7) as a solution of (4) directly. From (7) it follows that when 33:0, the maximum or input potential is (8) E0=A+B Since we desire logarithmic attenuation, it follows that when 2t=1, E1=aEo, Where a is the attenuation per unit length, and when x=n, E=aE0 Therefore, using (7),
oz=e` To test the validity of this assumption divide (9) by (10) arl-1 'I'hls is true only if A is zero. tion (7),
Taking Equa- Assume an infinitely long attenuator, then since the first term increases with it is evident physically that if a: increases without limit, the voltage cannot rise without limit, whence the constant A must be zero.
Which justifies the assumption that:
(12) @mem/E5 'I'his denes the attenuation per unit length, and is one of the equations referred to in the rst part of the specification.
From (8) and (11), it follows that Rewriting (7), we obtain:
(13) E=EOe^JTGfc Differentiating (13) and substituting the same in (1), we obtain:
1 dE Rdx Which may be simplified as:
(14) Imax/ @fw/WI This equation denes the current at any point along the attenuator.
Now, assuming that the attenuator is properly matched in impedance, or in other Words, conlR-ENsee-vx) nectecl to or closed by a resistance equal to the surge resistance, the surge resistance at any point along the attenuator may be expressed by:
E (15) Ro- Substituting (13) and (14) in (15), We obtain:
E 1 Rg--*F E From which We conclude that the surge resistance is (16) Rufwhich is another value used in the first part of the specication.
The solution in terms of hyperbolic functions may be made more general. The solutions of (4) and (6) are respectively as follows:
and We may call the current and voltage at this point I0 and En, respectively. Since cosh (o)=l, and sinh (o) :0, it follows that:
Substituting these values in Equation (18), We get:
This equation clenes the current at any point :t along the attenuator.
Substituting the Values of (20) and (21) in (19), we obtain:
These equations define the potential at any point along the attenuator.
Equations (22) and (23) are general in all respects, except the assumption of uniform resist- `ance and leakage conductance, and the assumption that the capacitance and inductance are ignorable.
General Equations (22) and (23) may be simplied if we assume the attenuator connected to `a properly matched surge resistance so that R0=\/ (Equation 16) and (25) I=I0 cosh ax--O sinh ax amperes These Equations (24) and (25) define the potential and current at any point along the line, with matched impedance.
Equation (24) which is in terms of hyperbolic functions may be converted to exponential form as follows, thus showing the correctness of Equation (13) above.
In Equation (24), IuRo may be replaced by En, giving:
E=Eo (cosh aac-sinh am) But Cosh ax sinh .ax e-ax: e W/RGI Whence we obtain:
(13) E: Eoefi/R-GI In connection with the non-uniform or tapered resistance units, it will be understood that the mathematical treatment is to be modified by recognizing in Equations (l) and (2) that R and G instead of being constant, are variables which are themselves a function of the distance along the device, a2.
Advantages It is believed that the mode of practicing as well as the many advantages of my invention, will be apparent from the foregoing detailed description thereof. The attenuator opens up a new iield cf development in that the attenuation is separated or divorced from the impedance so that particular requirements for each may be fulfilled. Attenuators may be conveniently and inexpensively constructed, which provide constant impedance, with the attendant advantage to the circuits in which the attenuator is used. Attenuators are also readily constructed which provide logarithmic attenuation and particularly convenient is the obtention of logarithmic attenuation with simple uniform resistance elements. The variation obtainable is continuous and may be made of high range. The unit is non-inductive. Attenuators of my invention are particularly suitable for use in oscillators and in ampliers and radio receivers, and in the latter case, a combined volume control and tone control of simple form is made available. A constant impedance attenuator is available for use following the output transformer of a push-pull circuit. With my invention, a constant impedance attenuator is made possible by a unit having only a single control. Logarithmic attenuation is obtainable together with constant impedance and both of these are further obtainable by means of only a single con.. trol. In all of these cases, a continuous variation is available in contrast with the stepped adjustment necessary with ladder or other lumped resistance networks.
It will be understood that while I have illustrated attenuators which employ resistance elements of the film type it is also possible to use various other forms of resistance elements. When dealing with resistance elements of the film type I have illustrated the series and shunt resistances as having equal thickness, but it will be understood that whenever it proves more convenient s0 to do, these resistances may differ in thickness. When they differ in thickness, it is desirable to form the same by first coating the entire area for the series and shunt resistances with a high resistance paint; and to thereafter additionally coat one of the resistance areas with another coat oi paint to `obtain the desired value. I may further mention that when dealing with very high frequency work, it is desirable to shield the unit, or more specifically, to take appropriate precautions to prevent a capacitance transfer of energy across the unit itself.
It will also be apparent that while I have shown and described my invention in preferred forms, many changes and modifications may be made in the structures disclosed without departing from the spirit of the invention, defined in the following claims.
l. An attenuator ccinprising a continuous series resistance, a contact movably related thereto, a highly conductive bus, and a continuous shunt resistance extending between and continuously connected to said series resistance and said bus.
2. A substantially constant impedance attenuator comprising a uniform continuous series resistance, a contact movably related thereto, a highly conductive bus extending parallel to and spaced from said series resistance, and a uniform continuous shunt resistance extending between and continuously connected to said series resistance and said bus.
3. A substantially constant impedance attenuator comprising a uniform continuous series resistance film, a contact slidably related thereto, a highly conductive bus extending parallel to and spaced from said series resistance film, and a uniform continuous shunt resistance film extending between and connected to said series resistance nlm and said bus.
4. A substantially constant impedance attenuator comprising a uniform continuous series resistance film, a contact slidably related thereto, a highly conductive bus extending parallel to and spaced from said series resistance film, and a uni form continuous shunt resistance film extending between and connected to said series resistance film and said bus, said series and shunt lms being made of the same material and constituting structurally a single film.
5. An attenuator comprising an elongated con tinuous resistance element having substantial width, a short movable contact slidable longitudinally relative to and making electrical contact solely with one longitudinal edge portion of said element, and a highly conductive fixed bus exu tending along and making xed continuous ccntact solely with the opposite longitudinal edge portion of said element.
6. An attenuator comprising an insulation base coated with an elongated continuous resistance film, a movable contact longitudinally slidable relative to and making electrical contact solely with a longitudinal strip of said resistance film lying near one edge of said film, `a highly conductive fixed bus bearing against and making fixed continuous contact solely with the opposite longitudinal edge portion of said film, a terminal connected to one end of the first-mentioned portion of the lm, a terminal connected to said slidable contact, and a terminal connected to said bus.
'7. A substantially constant impedance attenuator comprising an insulation base coated with a uniform, elongated continuous resistance film, a movable contact slidable longitudinally relative to and making electrical Contact solely with a strip of said resistance film lying near one longitudinal edge of said film, a highly conductive iixed bus bearing against and making iixed continuous contact solely with the opposite longitudinal edge portion of said film, a terminal connected to one end of the first-mentioned portion of the film, a terminal connected to said slidable contact, and a terminal connected to said bus.
8. An attenuator comprising an insulation ring of rectangular cross-section, a continuous resistance iilm coated on the outer periphery and on one face of said ring to form connected peripheral and face films, a control shaft, a wiper arm mounted on said shaft and slidably engaging one of said iilms, and a highly conductive bus extending along and engaging the free edge of the other` film.
9. An attenuator comprising a flat cylin-drical casing, an insulation ring of rectangular crosssection mounted therein, a continuous resistance nlm coated on the outer periphery and on one face of said ring, a conti-ol shaft in said casing, a wiper arm mounted on said shaft and slidably engaging the peripheral resistance lm, a highly conductive bus engaging the face film at the inner periphery thereof, a first terminal connected to one end of the peripheral film, a second terminal connected to the wiper arm, and a third terminal connected to the bus.
10, A compact substantially constant impedance attenuator comprising a flat cylindrical casing, an insulation ring of rectangular crosssection mounted therein, a continuous uniform resistance film coated on the outer periphem and on one face of said ring, a control shaft centrally mounted in said casing, a wiper arm mounted on said shaft and slidably engaging the peripheral resistance nlm, a highly conductive bus engaging the face film at the inner periphery thereof, a first terminal connected to one end of the peripheral film, a second terminal connected to the Wiper arm, and a third terminal connected to the bus.
11. An attenuator comprising a series resistance, means to vary the same, a shunt resistance continuously connected on one side to sai-d series resistance, and means mechanically associated with the means to vary the series resistance for continuously short-circuiting the opposite side of the shunt resistance to an extent dependent upon the position of the slider.
12. An attenuator comprising a continuous series resistance, a Contact slidable relative thereto, a shunt resistance having one edge continuously connected to said series resistance, a shortcircuiting bus adapted to be continuously connected to the opposite edge of said shunt resistance, and means mechanically associated with the slider for making a part of said short-circuiting bus effective and the remainder ineffective dependent upon the position of the slider.
13. An attenuator comprising a continuous series resistance, a contact slidable relative thereto, a shunt resistance having one edge continuously connected to said series resistance, and highly conductive means mechanically associated with the slider for bearing against the opposite edge of the shunt resistance an amount dependent upon the position of the slider.
14. An attenuator comprising a continuous series resistance film, a contact slidable thereover, a terminal at one end of said nlm, a highly conductive bus collateral to but spaced from said series resistance film, a continuous shunt resistance film connected to and extending from said series resistance film to and underlying said bus, said bus being resilient and so shaped as to tend normally to move away from said shunt iilm, and means mechanically associated with the slider for bearing against said bus and forcing the same against said shunt film only as far as the slider has been moved from the terminal, whereby said bus is ineffective over the unused portion of the attenuator.
15. An attenuator comprising a uniform continuous series resistance film, a Contact slidable thereover, a terminal at one end of said film, a highly conductive bus extending parallel to and spaced from said series resistance film, a uniform continuous shunt resistance film connected to and extending from said series resistance iilm to and underlying said bus, said series and shunt films being of the same material and constituting structurally a single film, said bus being resilient and so shaped as to tend normally to move away from said shunt film, and means mechanically associated with the slider for bearing against said bus and forcing the same against said shunt film only as far as the slider has been moved from the terminal, whereby said bus is ineffective at the unused portion of the attenuator.
16. An attenuatcr comprising a continuous series resistance which tapers in resistance from one end to the other, a slider movable thereover, a highly conductive bus arranged collatrally to said series resistance, and a continuous shunt resistance connected at all points between the series resistance and the bus, said shunt resistance tapering in resistance in the same direction as the series resistance.
17. A combined volume control and tone control device for a radio receiver, said control comprising a continuous series resistance which tapers in resistance from one end to the other, a slider movable thereover, a highly conductive bus arranged collaterally to said series resistance, a continuous shunt resistance connected at all points between the series resistance and the bus, said shunt resistance tapering in resistance in the same direction as the series resistance, the slider and the bus constituting one pair of terminals of the control and the high resistance end of the series resistance and the bus constituting the other terminals of the control.
18. An attenuator comprising a resistance element made up of a single continuous body of resistance material, a slider longitudinally movable over only a longitudinal edge part of said element, a fixed metallic bus making fixed continuous contact with only the opposite longitudinal edge part of said element, an additional fixed connection to one end only of the first part of said element, said connection, slider, and element being so arranged and proportioned as to provide substantially constant impedance at-, tenuation.
19. An attenuator comprising a resistance element made up of a single continuous body of resistance material, a slider longitudinally movable over only a longitudinal edge part of said element, a xed metallic bus making fixed continuous contact with only the opposite longitudinal edge part of said element, an additional fixed connection to one end only of the first part of said element, said connection, slider, and element being so arranged and proportioned as to provide logarithmic attenuation by continuous variation with substantially constant impedance solely by movement of the slider.
20. An attenuator comprising two resistance elements in edge to edge conductive contact, a longitudinally movable slider movably contacting one only of said elements, a fixed metallic bus making xed continuous contact longitudinally along the other only of said elements, and an additional fixed connection to one end only of the rst of said elements, said resistances, slider, and connection being so arranged and proportioned as to provide substantially constant impedance attenuation.
2l. An attenuator comprising two continuous resistance elements in edge to edge conductive contact, a longitudinally movable slider movably contacting with one only of said elements, a single control to move the same, a fixed metallic bus making xed continuous contact longitudinally along the other only of said elements, and an additional xed connection to one end only of the first of said elements, said resistances, slider, and connection being so arranged and proportioned as to provide continuous logarithmic attenuation With substantially constant impedance by means of said single control.
22. A resistance net Work comprising a compact continuous resistance, a continuous conductor, and a second compact continuous resistance connected between the first resistance and the conductor, said connection being continuously distributed along both the first resistance and the conductor.
23. An attenuator comprising a uniform continuous series resistance, a contact movably related thereto, a highly conductive bus extending collaterally with and spaced from said series resistance, and a continuous shunt resistance extending between and continuously connected to said series resistance and said bus.
SAMUEL J. A. M. BAGNO.