US2882501A - Micropotentiometer - Google Patents

Description

April 14, 1959 M. c. sELBY MIcRoPoTENTIoMETER original Filed June 29, 1951 INVENTOR @fr0/7 {f6/b] Y m Rf 0S CI United States Patent() 2,882,501 mcRoPorEN-TIOMETER Myron C. Selby, Boulder, Colo., assignor to the United States of America as represented by the Secretary of Commerce Original applicationy June 29, 1951, Serial No. 234,166, now Patent No. 2,782,377, dated February 19, 1957. Divided and this application February 8, 1957, Serial Nia-639,152

z claims. (ci. 3ssl-21) eral serious disadvantages. One is that the output im" pedance of the generator varies greatly over Wide frequency ranges.` A second disadvantage is the necessity of using attenuators to drop the generated voltage from the order of magnitude of one volt to al1 desired lower values-in this case, a ratio of 1 million to one. The

construction of these attenuators, maintenance of their stability with regard to time, use, temperature,` and hu- Illidity,` and their' calibration entail expensive and difficult procedures. A.4 third disadvantage is the Iuncertainty of theinitial calibration and especially of longtime calibration stability. another reasonfor :inaccuracy is the effect of load'on the frequency of the generator. n

As a` result of these limitations, considerable ditiiculty has been encountered in establishing low-voltage, higlb frequency standards.

.1t is therefore the primary object of this i-nventionfto" provide a voltage standard for low voltages rat highv frequencies that will be ,free from the aforementioned diticulties.

Another object isto provide a high-frequency, lowvoltage standard that willoperate `over a range of fre' quencies from about 3i) to several thousand megacyeles andl over a range .of voltages frorn one to one million microvolts. t

A further object is to` provide a standard with a non inductive f output impedance.-

A .still` further object is to provide a standardwith-xa'n outpn-t'impedance ofthe orderA of milliohms;

Still another object ofthe invention is to provide an inexpensive and uncomplicated high-frequency, lowvoltage standard..

Another object is to-provideaf balancedvoltage standardat highfrequencies .and low voltages.

A still further object is` torprovide a standardthat not seusitivevt'o frequency.

Still another object is tolprovideastandardvthat'is'vv quickly andeasily calibrated.

Stillk anotherobject is to provide a' high-frequency; low-voltage standard that isy calibrated` by the use of af direct current.

yAnother object is to provide astandard withJ a good long-time stability,

In accordance with the present invention there is-pro' vided a unit which will hereinafter be referred to asV ar coaxial micropotentiometer, whichr unit will when used l 2,882,501 Patented Apr. 14, 1959 in conjunction with a radiofrequency oscillator and voltmeter provide thestandardoutput microvolts and millivolts required. The micropo-tentiometer is .composed of three principal parts, a noninductive low-value resistor, a uniformA coaxial waveguide section, and an RF volt- Ineter. The waveguide terminated by the noninductive resistor acts as a load o nthe R-E oscillator ,and lthe standard voltage is taken off across the resistor. `The resistor isv made of a very thin annular ring of conductive material and is connected between the center and outer conductors -of the coaxial waveguide. Since such type of resistor is essentially noninductive, its resistance will ref mainconstant over a very wide frequency range, and thereforey the direct-current resistancev can `be taken as resistance at high frequency without introducing an ap-Y preeiable error.I It should also benotedl thatsince thisv resistorispmade of highly conductive material itsy resistance will be very low. As a result, when the micro' potentiometer is being used to calibrate an instrument whose input impedance is relatively high, the instrument will represent practically no increase in loadvand therefore. the frequency of the RJ? oscillator feeding: energy to'the micropotentiolneter will not vary significantly during the applicationrof the instrumenn At any given fre= quency the vhighfrequency current ilowing throughY this resistor is accurately determined from the measured'highvoltage input into thev coaxiall waveguide and the length 0f thisguide The micropotentiometer therefore includesv a; resistor that does not vary with frequency anda'means for measuri-ng the loadcurrent ywhich is also unaffectedby fre# quency. Then by meansof Oltms law- (EIR) thev value ofvoltage is easily calculated,- ory obviously; low voltage output of the instrument may be calibratedy in* terms of its-z high voltage input. Itfisnot necessary .tov

compu-te the current enteringthe resistor' because the lowvolta-ge output is theoretically precisely-'- equal-*to Vcscai t1) R, is the resistance of the disk", [3io is'fthe" known phasev l tlcally constant, frequency stability" readily obtainable.

Also since' thevoltage is measured atl`the niicro'potentioln? et'eritheoscillator'is only required to produce a controlable" amplitude. and" attenuators'is eliminated entirely. Microvolts and Inillivolts of the same 4order ofL accuracy as lin available highlvolt'age voltrneters are readily obtained, and errors approaching 100% andA higher are safely eliminated'.- Var# iations in ca ilibratio'n.` overlongl periods is no longer at serious'prblem since, even if the resistance of the annhlar ring doesl vary, whichv isf' highly unlilgelypthe re` sistance ofthe annular ring. can b`e measured bythe very rapid and uncomplicated D.C.k method'.Vv A

Other uses `and advantages ofv th'e'inventio'n will become apparent upon reference to the specificationand drawings* in which:

Fig l is' anl elevational` view ofv a preferred embodiment of the invention;

Figi. 2 is a cross section of a clamped annular riiztg'ltype of assembly; i i

Fig. 3vis a cross-sectional'rview of a" modification of:

Expensive calibration .ofi the oscillator-V f 3 Pig. 2A in which the annular ring is deposited by plating, tiring,` evaporation, or otherv means;

Fig. 4 shows a modification of the annular ring assembly in which the annular ring or resistor lies in the.

output plane of the waveguide; and

Fig, 5 is a block diagram showing the micropotentiometer according to this invention being used to calibrate a vacuum-tube voltmeter.

Referring specifically to Fig. 5 which is a block diagram showing a micropotentiometer embodying the features of this invention being used to calibrate a vacuumtube voltmeter,l the general operation of the instant invention is as follows: The signal generator 1 provides a current which is adjustable lboth as to frequency and amplitude for the micropotentiometer 2. The adjustable current is obtained by varying the current; output of -the oscillator of the signal generator. Most standard signal generators have an adjustment of this type and' therefore there is no need for the use of attenuators. This immediately eliminates the many problems incident to the useof attenuators, some of which were pointed out in the introduction.

The adjustable current is fed into the micropotentiom stant for any single resistive unit, may be of any desiredI value from 1000 microhms to 1 milliohm or higher if necessary, depending upon the value of voltage desired. This D.C. resistance is accurately measured beforethe start of the'standardizin'g procedure by the D.C. voltmeter-ammeter method generally used for measuring very low resistances. This method requires the use of accurate D.C.l ammeters and voltmeters which, however, are relativelyl inexpensive since the currents and voltages used are relatively large. The voltage output can then be easily calculated by Equation l. For example, if it is desired to obtain voltages of one to 100 microvolts at a given frequency one may use a resistance element of .one milliohm, a waveguide section 1A of a wavelength long having a characteristic impedance of 100 ohms and input voltages of 0.1 to l0 volts` Other combinations' could'of course' also fbe used. This example demonstrates the vextreme ease with which it is `possible to obtain standard voltages by means of the present invention.

Fig. 2 is a cross-sectional view detailing the construction'of the waveguide andannular resistor assembly of the micropotentiomelteraccording to this invention. As indicated, the micropotentiometer resistor assembly' in# cludes a generally coaxial waveguide comprising an outer conductor member 31 and an inner conductor 27. The resistor `assembly 19 is connected to one end of the waveguide as shown in Fig. 1 to form a coaxial micropotentiometer.

The resistive element 11 which is formed of an annular ring of highly conductive material is connected between the outer conductor 31 and the inner conductor 27. The calibrated 'voltage is developed across the annular ring 11 and appears as the output which is taken between the conductors'l and 27.

The micropotentiometer resistance element shown in Fig. 2* is about 3 inches long and 2 inches in diameter.

Other sizes and shapes have been used, some of` which are shown in Figs. 3 and 4 and will be referred to later in the specification. The micropotentiometer resistance element also includes a center post 18 having a ange or shoulder 26. The center post 18 provides a coaxial connecting means for the inner conductor 27 of the co# axial waveguide assembly. In the embodiment disclosed in the present application no current-responsive device outer portions27 and 31 of the unit (except for the such as the thermocouple 9 disclosed and claimed in applicants above-identified patent is employed.

The conductive annular ring 11 is slidably mounted over the center post 18 and rests against the flange 26. The annular ring is held fast against the tiange by means of a hollow cylinder 27 which tits snugly around the post 18 and is held tightly against the ring 11 by means of a nut 21. A circular disk 28, which is composed of a suitable insulating material, lits around the cylinder 27 and seats against-a shoulder 30 forming part of the cylinder. A nut 29, which screws onto the cylinder 27, holds the insulating disk rrnlyin place. The subassembly containingthe parts 18,211, 27, 21, 28, and 29 ts into an outer hollow cylinder 31. The outer portion of the insulating disk 28 seats against a tlange 32 forming part of the cylinder 31 and Vis held tightly against this flange by a nut 33. It will be noted that the disk 28 provides the .only mechanical connection between the inner and annularY ring 11),. The disk 28 therefore provides for a'strong mechanical connection but one that is electrically nonc'onductive. Except for this disk and the annular ring' the inner and outer portions of the assembly are sep! arated by the air space 34.

The micropotentiometer resistance assembly shown in Fig. 2 also includes an output connector 20 comprising a sleeve 35 and threaded flange 35a which is screwed down into thecylinder 31. To prevent this part from coming into contact with the annular ring 11 and possibly tearing it because of the twisting motion of 32while it is being screwed into place, a' Washer' 36 is placed between the disk and the part 32. A lead washer (notshown) may be placed between 11 and 36 to insure a better electrical contact if desired. This unit is now com' pleteand is adapted for mounting in the transmission line 51 (Fig. l) by means of the screw threads 31a on the outer surface of the cylinder 31. For this purpose, the

ltransmission line 51 (Fig. 1) may be provided with av 'rotatable collar 52 to facilitate connecting the micropotentiometer resistance assembly (or element).

As shown in Figs. 2 and 3 the resistor element 11 comprises a thin, planar annulus. ring may -be made in several ways and from a number of diiferent materials depending upon the desired resistance. L These annular rings can be made of foils of platinum, tin," silver, gold or any other conductive material, having thick-fy nesses of the order of 0.0003 to 0.001 inch, and havingl resistances ranging from microhms to several mil-I Also carbon elements have been made withv resistances of about `1 ohm, and if required even higher resistances can be used. Actually the resistance of the ring depends upon several facts; such as, the resistivity of the material, the thickness of the ring, and the ratio of the.

liohms.

inner and outer diameters of the ring. The main feature ofthese annular rings is that they are made of conductive elements and therefore must be kept exceedingly thin'to" This feature is very important get the desired resistances. because the annular rings must be kept as thin aspossible so that the current will penetrate the entire thickness jof these rings at all desired frequencies." 'Ihe higher the frequency the thinner must be the element. thickness of the rings'is inversely proportional to the square root of the frequency.A It was anticipated and found to be truey during the development of this invention,

out introducing an appreciable error.

Fig. 3 shows a modification of the assembly19 in which` the annular ring 11a is made by plating rather vthan by forming circular disks out of the appropriate conductive foil. The cylindrical insulatorZSa provides theme'chani-A In practice this annular` The allowable' essffor joining the insulatorsandmetalmay be' used.'l Silver paint is tired onto the inner: andouter cylindrical. surfaces of the insulator 28a.- The silverfiscopperashed- SQ aSiO, protect the silver.. The copper:k surfaces are then finned. Also the surfaces of the postlSa and-conductor 31a that will contact the insulator are tinned. Then-the post 18a and conductor 31a are soldered to the insulator. S far only the mechanical connections between 18a, 28a, and 31a have been made and nothing has, been done toward the plating of the annular ring.

The plating process for the annular-ring 11a may'be'as follows Thev surface of 'conductorl 31a-lying in the plane .A-A of the annular ring and a small portion of the center post 18a lying to the left of the insulator are silver plated. Then a very thin film of silver is evaporated over the entire surface formed by the conductor 31a and insulator 28a in a plane perpendicular to the longitudinal axis of the micropotentiometer resistance assembly. Then another and final silver nish is plated over this entire surface. This silver nish provides the annular ling 11a.

There are other methods of forming the assembly 19 and the anular ring 11. If the post 18 and conductor 31 are made of Kovar metal they can be sealed in glass and therefore the insulator 28 would be glass. Then to form ring 11, fine silver can be deposited right on top of the glass. Also it is well known that metals can be soldered to ceramics and so the unit 19 might be made that way. Insulating cements might also be used.

Many other similar techniques are known or are being developed, but since this forms no part of the present invention they will not be discussed.

As previously stated, one of the major objectives of this invention is to provide a source of standard voltage that has a negligible internal impedance. This becomes increasingly diicult with the unit shown in Fig. 2 as the frequency increases because the inductance of the output connector begins to have noticeable effect at these higher frequencies. In Fig. 3 it will be noted that the annular ring 11 is not in the output plane P-P of the micropotentiometer. In practice this ring lies about one centimeter to the right of the output plane. Therefore at higher frequencies the impedance of this section of the connector ohms at 1000 megacycles) causes a noticeable difference between the standard voltage at plane A-A and the voltage at the out-put plane P. This difference, of course, will depend to a great extent on the input impedance of the unit being calibrated.

To obviate this difficulty micropotentiometer resistors have been developed in which the annular ring lies in the output plane. Such an arrangement is shown in Fig. 4. The output connector 31a is modified so that one surface of the insulating support 68 for the center post 58 is lo cated in the output plane P-P. The annular ring 11a is plated over the insulating support 68 and over the entire surface of the output connector in the output plane P.

This embodiment of the invention can be used over the entire range of from zero to several thousand megacycles.I Without encountering any serious frequency effects as explained above.

The over-all construction of the embodiment of the invention in this case is shown in Fig. l. The micropotentiometer comprises a known length of coaxial trans mission line 51 terminated in an annular ring assembly 19' of one of the types shown in Figs. 2, 3 and 4. The R-F input cable is indicated at 50, While the vacuum tube voltmeter probe for the circuit set-up described in con.

nection with Fig. 5 is shown at 55.

In one `lform, the coaxial transmission line is made equal to an odd number of quarter wavelengths of the frequency of the input. For a line with negligible losses (this is: usually true in practice) the output voltage across the annular ring is equal to the input voltage times the ratio of theannuIarfresistanceftoztlre enarticter'stic-inipeditrice'l of the'line: The equationflforthis relation isl vinvfgvaus Y (2.5'

where:

As indicated by Equationmtisxnecessarwthe input'voltag'e inrthisuform of.` rnicropotentiometeriA How#- ever, in this instance such restriction does-not present a difficult problem. since theinput .voltage-isfofithe ordenar magnitude of one volt. The characteristic impedance of coaxial transmission lines may be ohms. When used with an annular resistance of l milliohm the term'R/Zo has a value of 10-5. Therefore to obtain a voltage as low as 10 microvolts, an input voltage of one volt is used. Vacuum-tube voltmeters that will measure one volt even at very high frequencies are readily available.

It must be remembered, however, that Equation 2 holds true only when the length of the coaxial line is equa'l to an odd number of quarter wavelengths of the impressed sfrequency. However, the output voltages at intermediate frequencies may be computed from the Equation 1 above which can also be rewritten as VFWcscKf vous (a) where:

V1 is the input voltage V2 is the output voltage R is the resistance of the annular ring in ohms Z0 is the characteristic impedance of the line in ohms K is a constant equal to 2.09 10-8L l being equal to the length of the line in meters for an air dielectric line. In case the line has a solid dielectric having a dielectric constant of K relative to air, the value of l has to be multiplied by the square root of K.

j is the applied frequency in cycles per second.

A-gain the ratio of R to Z0 is so small that the input voltage can be relatively large. Obviously Equation 3 can also be written as where K is equal to 2.09 1O-8 for air.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made within the scope of my invention as defined in the appended claims.

What is claimed is: v

1. An adjustable amplitude constant-voltage source for producing accurately controllable voltages over a wide frequency range comprising a coaxial transmission line, a source of radio frequency current having a voltage V1 applied to one end of said transmission line, a micropotentiometer resistance assembly in the form of a waveguide connected to the other end of said transmission line, said micropotentiometer resistance waveguide assembly comprising coaxial center and outer input elements connected respectively to corresponding elements of said coaxial transmission line and having coaxially arranged output terminals corresponding respectively to said input terminals for connection to a load, a resistor electrically joining and coaxial with the center and outer coaxial input elements of said micropotentiometer resistance assembly, said resistor comprising a thin, planar annulus of conductive material having a finite resistance R, the magnitude of which is of an order which is negligible compared to the impedance of the load to which said output terminals are 8 connected, the thickness of said annuius being inversely K lis a constant determined -by the dielectric material in.- proportional to the square root 'of they-highest frequency said coaxial line being 2.09)(10-8 for'air, employed to permit complete current penetrations through l isthe 'length of said transmission dine vin meters, Vand the thickness of said annulus throughout said frequency f :is the frequency of said applied radio frequency current;`

range, the length of said coaxial transmission line being :i

predetermined 1n accordance with the equatlon: end of said coaxial transmission line. l

V220 En 2-. The invention of claim l in which said resistor lies l=fm 1 y 1 inthe plane of the output terminals of said constantvoltage source. where: 10

References Cited in thele of this patent V2 corresponds to the output voltage,

V1* corresponds to the voltage of said applied radio fre# i UNITED STATES PATENTS quency current, 1,850,980 Carter Mar. 22, 1932 Reisv the-resistance of said thin planar annular resistor,` i 15 2,557,122 Leiphart June 19, 1951 andir'nea'ns for d'etermining'the voltage input to said on i

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