US2431339A - Method and apparatus for determining the resonant frequencies of networks - Google Patents

Description

I Nov. 25,- 1947. w. A. McCOOL METHOD AND APPARATUS FOR DETERMINING THE RESONANT FREQUENCY OF NETWORKS Fiied July 6, 1944 gwue/wtom I WILLlAM A. MQCOOL Patented Nov. 25, 1947 mn i non mussel-armres .ING JIHE R'ESONANT AEREQUENCIES or 1 NETWORKS ".William AflVlcCool, Washington, D. m plicationnuiyie, 1944;Sei-3QFND53435148 1'5 "Claims.

' (-'Gra'rited underthe act of March 3, 1883, as

amehiiGH'ADfil-BO, 1928; 370 O. G. 757) This invention relates to the deternlinationbf resonant frequencies 'of"reactive-networks "and is particularly directed to the problem of ascertainin'g the f resonant frequency "of transmission lines'ections.

It is accordingly the object of the invention to-provide a "novel method and means ofascertaming the-resonant frequencies dfrea'ctivemet- Works.

.A further' object "of the zinventionis to deterniinetlie velocity of 'propagationfof a wave "along a line section. This is accomp'iiShedby ascertainin'g the' resonatingfrequency for an unterminated line section of known 'length. At such frequency the line section comprises an cold It quarter-wave multiple of the wave length propagated therein. Inasmuch as tl-i'e' order of' magnitude of the velocity-is;ordinarily known from the construction of the cable, the particular multiplefii ay be obtained. Consequently, from -the value of the --excitin frequency, "the velocity of propagatio'nmaylee-calculated. I

'i i n exemplary form of apparatus suitable for elfectu'ating the method of the presentk'invention is ishown in the-drawing, in connecti on with which the invention will be further described.

The system of the present invention includes a shielding cabinet 1 subdivided at 2 into two compartments 3 and 4. Compartment 3 contains a variable frequency oscillator including a triode 5 coupled conventionally with a tuned circuit including inductance 6 and tuning capacitor 1. Balanced capacitor 8 is provided for trimming adjustment of the oscillator frequency.

The oscillator inductance is Very loosely coupled through shield 2 with inductance I0, which forms a tuned circuit with tuning capacitor II and inductance l2. Trimming capacitor I3 is also provided.

The desired coupling of inductances 6 and I0 may be established by employing a sheet metal shielding member 2 suitably apertured. Under such loose coupling the coil l0 may be considered as driven by a constant series voltage.

The coupled circuit includes in series a jack M for receiving connection of the network Whose resonant frequency is to be determined. In the drawing an unterminated concentric line section is shown, which is resonant at frequencies at which it constitutes a quarter wave length or an "odd'r'iiiiiltifile lthere'o'f. LAt ,resonant ..-'frequencies itheiifiipedan'ce appearing acrossjack l.4.wil1iconstitiifte a more I "ista'nc'e which will not afiect the .r souautsrrequenc Lthe :tu'neu icir'c'uit.

As "showhfih'fithe Idra" mg, the Itarik -condense'r l and ithe condenser "I I .6f "the tuned circuit are igangedfltoa lcommonfcoiitrol 1J5. Y'TEor lining .up the. ci'flillits, Taj S6113. ibohdubtoi :maybe ihse'rteli tofshfiftjjaik f4 anuftrimme'rsfaana 13 adjusted YSO that theilfitll'fal frfidlfallit freque cies 6f "the oscillatortank circuit and the *coupled circuit substantially coincide throughout the tuning range. This may be "determined "by a vacuum itubeivoltinter lb "of. any "suitable. type connected, "forinstance; t0 "11 T2. Oh J'cdllfittra'kin'g 'the foscillatori'arid coupled circuit are "tuned -tdsiiher throiighflie rang "and voltmeter *1 6 will indicate "afsubstantially uniform voitagefthroughout. (on connection "'of'jthemetwork "to." jack f4, the ireactanoa'thereof willdetune theicoupled "circuit so -fthat no voltnieter i'ndication"isob tairfed-*-except there'sonaiiEfrequency of thenetwork. Conseq may fthe latter mfaybeaite'rmined immedi- "any adjusting metered l-i-for' ama'inmum deflection of voltmter -Fli.

For obtaining the velocity of propagation within a cable section, the apparatus may be employed to yield direct results by suitable calibration of control l5. Assuming normal construction wherein the average dielectric constant within cables of the type considered is 2.25, and that measurements are to be taken at about mc./s., the wave length within the cable will be substantially 200 cm., and a three quarter wave section is cm. long. If, therefore, a 150 cm.

section cable be inserted in the jack l4, its resonant frequency adjacent 100 mc./s. as determined by control l5 corresponds to three quarter wave lengths. Since the period required for propagation to the end of the line is %.1/f seconds, the velocity in centimeters per second is 200 f. The velocity expressed in per cent of the velocity in free space is /3 f where f is expressed in megacycles. Control l5 may be directly calibrated in the velocity of propagation, if desired.

Where a number of cable sections are bein tested, the reading of vacuum tube voltmeter I6 varies inversely with their attenuation and will afford a rough indication of the relative properties of the lines in this respect.

Where the constructional characteristics of a line are unknown and cannot conveniently be determined, the quarter wave length multiple may i be found by ascertaining two adjacent resonant frequencies of the section, Since the wave length is inversely proportional to the frequencies, and only the ratio of the former is required, the desired information may be derived from the formula where A is the shorter of the adjacent resonant wave lengths.

, line section.

It will be understood that the other line lengths may be employed and that theillustrations discussed are exemplary only. xItis'further apparent that a short-circuited half wave or multiple half wave section may be employed.

The invention described herein may b'e'm anu 1 factured and used by or for the Government of the United States of America for governmental purposes without the payment of anyroyalties thereon or therefor.

Iclaim:

1. Means for ascertaining the resonant frequency of an electrical network including an oscillator tunable over a range of frequencies, a circuit tunable over saidrange of frequencies coupled to said oscillator, common control means for simultaneously tuning the oscillator and the coupled circuit togetherthrough said range in tracking coincidence, means for indicating the response developed in the coupled circuit, and means for electrically coupling a reactive electrical network with said coupled circuit to detune the latter except at'the resonant frequency of the network.

2. Means for ascertaining the resonant frequency of an electrical network including an oscillator tunable over a range of frequencies, a

circuit tunable over said range? of frequencies coupled to said oscillator, commoncontrol means for simultaneously tuning the oscillator and the coupled circuit together through said range in tracking coincidence, means for indicating the:

response developed inthe coupled circuit, and means for connecting an electrical network in series in said coupled circuit whereby the latter is detuned and unresponsive except at the resonant frequency of the network.

4. Means for ascertaining the resonant frequency of an electrical network including an oscillator circuit tunable over a range of frequencies, a circuit tunable over said range of frequencies coupled to said oscillator, means for simultaneously tuning the natural frequencies of the oscillator circuit and the coupled circuit over said. range in tracking coincidence, means for electrically coupling a reactive electrical network with one of said circuits to detune said circuit ,except at the resonant frequency of the network,

I reactive network.

2 Number and means for indicating the signal level in the I coupled circuit.

5. The method of ascertaining the resonant frequency of a reactive network electrically coupled to a resonant circuit 'excitedby a tunable oscillator comprising the steps of tuningv the oscillator over a range of frequencies, simultaneously tuning the resonant circuit over the same range of frequencies in tracking coincidence with. the

oscillator, and indicating the signal level in the resonant circuit to determine the frequency at which the resonant circuit is nct detuned by the WILLIAM M cooL.

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

UNITED STATES PATENTS Name Date Y Schiepe Aug. 24, 1937 Schrader June 20, 1939 Broaten Oct. 3,1939 Dennis et a1. May 25,1943 Mittelmann 'July 20, 1943

Images