1、BRITISH STANDARD BS 7737-2: 1995 IEC 377-2: 1977 Recommended methods for the determination of the dielectric properties of insulating materials at frequencies above300MHz Part 2: Resonance methodsBS7737-2:1995 This British Standard, having been prepared under the direction of the Electrotechnical Se
2、ctor Board, was published under the authority of the Standards Board and comes into effect on 15 November 1995 BSI 11-1999 The following BSI references relate to the work on this standard: Committee reference GEL/15 Draft announced in BSI News April 1995 ISBN 0 580 24936 0 Committees responsible for
3、 this British Standard The preparation of this British Standard was entrusted to Technical Committee GEL/15, Insulating material, upon which the following bodies were represented: British Ceramic Research Ltd. British Industrial Ceramic Manufacturers Association Electrical and Electronic Insulation
4、Association (BEAMA Ltd.) Electricity Association Federation of the Electronics Industry Rotating Electrical Machines Association (BEAMA Ltd.) Transmission and Distribution Association (BEAMA Ltd.) Amendments issued since publication Amd. No. Date CommentsBS7737-2:1995 BSI 11-1999 i Contents Page Com
5、mittees responsible Inside front cover National foreword ii 1 Object and scope 1 2 Introduction 1 3 Test apparatus 1 4 Test specimen 2 5 Testing procedure 2 6 Evaluation of measured data 3 7 Test report 3 Appendix A Resonators 4 Figure 1 Resonance test assembly Principal circuit diagram 13 Figure 2
6、Re-entrant cavity resonator (schematically) 14 Figure 3 Coaxial cavity resonator 15 Figure 4 Mode-chart (f o d o ) 2= f(K) of the first four resonances of the lowest 10 modes for a cylindrical cavity 16 Figure 5a -cavity resonator 17 Figure 5b TM 010cavity resonator 17 Figure 6 Instantaneous field c
7、onfiguration and cut-off wavelength of some low-order modes of a cylindrical cavity 18 Figure 7 Mode-shape factors of various E lmn - and H lmn -modes in cylindricalcavity resonators versus , penetration depth due to skin effect 19 Figure 8 Open cavity resonator 20 Figure 9 Semiconfocal resonator 20
8、 TE 01 0 Q - d o l $ -BS7737-2:1995 ii BSI 11-1999 National foreword This Part of BS 7737 has been prepared by Technical Committee GEL/15. It is identical with IEC 377-2 Methods for the determination of the dielectric properties of insulating materials at frequencies above 300 MHz. Part 2: Resonance
9、 methods, published by the International Electrotechnical Commission (IEC). A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confe
10、r immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 20 and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table
11、 on the inside front cover.BS7737-2:1995 BSI 11-1999 1 EXPLANATORY NOTE. The requirements of this standard are valid only in connection with the requirements given in IECPublication 377-1, Recommended Methods for the Determination of the Dielectric Properties of Insulating Materials at Frequencies A
12、bove 300 MHz, Part 1: General. 1 Object and scope This standard applies to the procedures for the determination of relative permittivity and dielectric dissipation factor and of quantities related to them, such as loss index, of solid and liquid or fusible dielectric materials in the microwave frequ
13、ency region by resonance methods. The methods described herein apply primarily to low-loss specimens. 2 Introduction The measuring methods to be described in this standard involve the use of resonance apparatus. Such apparatus consists essentially of a transmission line section of a given cross-sect
14、ion short-circuited at both ends at an interval of an arbitrary multiple of one-half the working wavelength. When a test specimen is inserted into the resonator, the working wavelength changes. The frequency shift or the length variation respectively required to re-establish resonance and the associ
15、ated change in the Q-factor are measures of the dielectric properties of the test specimen. The particular advantage of resonance methods compared with other test methods centres on the extremely high unloaded Q-factor, which can be achieved by the use of suitable wave modes and by proper design; by
16、 means of this technique, very low dissipation factors of the test specimen can be measured. Thus, in general, to benefit from the advantages of this method, a resonator is constructed for a particular measuring problem (frequency, shape and dielectric properties of the specimen). To avoid ambiguity
17、 of results, a careful examination of the resulting field configuration is necessary. Hence, the resonance apparatus is necessarily a narrowband device, with the resulting test frequency depending on the quantity and shape, the dielectric properties and the location of the test specimen within the r
18、esonator. The following types of resonators are commonly used: NOTEThe limiting values for frequency and permittivity are only approximate and may be exceeded if reduced sensitivity to the dissipation factor or permittivity may be tolerated (see also Clause 4 of Publication 377-1). The various types
19、 of resonators and the associated measuring procedures and their evaluation are discussed in more detail in Appendix A. 3 Test apparatus (see Figure 1, page13) The test apparatus consists of: 3.1 A generator supplying the desired frequency at a sufficient power level. The frequency should be tunable
20、 either manually or automatically (a swept-frequency source) over the desired frequency range. NOTESwept-frequency generators used in connection with display devices (see Sub-clause 3.2.2) are convenient for quick testing. Care should be taken that the apparent shape of the resonance curve is not af
21、fected by excessively high sweeping speeds. The output power should be variable. Means for automatic level control (ALC) are desirable. NOTE 1Manually tuned generators for fixed frequency testing procedures shall have sufficient stability of operation. Stability of1ppm or less in the frequency gener
22、ally suffices. NOTE 2To avoid frequency pulling, insertion of an isolator or an attenuation pad between the generator and the circuit is recommended. To avoid spurious resonances, the harmonic content should be less than 1 %. 3.2 A detector of sufficient sensitivity at the test frequency. Different
23、types are used in connection with manually or automatically tuned generators. Type of cavity Frequency range Shape of specimen Remarks No. Re-entrant cavity Coaxial resonator Cavity (closed) “Open cavity” Optical resonator 100 MHz to 1 GHz 1 GHz to 3 GHz 1 GHz to 30 GHz 3 GHz 30 GHz Disk Tube Disk,
24、rod Disk Plate, sheet rk 10 r 5 A.1 A.2 A.3 A.4 A.5BS7737-2:1995 2 BSI 11-1999 3.2.1 Detectors for fixed frequency measurements shall have sufficient stability of operation. Either a) diode-voltmeters with or without amplification, orb) receivers tuned to the microwave frequency or a low-frequency m
25、odulation of the generator output with or without automatic frequency control (AFC) can be used. NOTE 1In general, broadband detectors are convenient as they need not be tuned to the generator and the resonance device allows sufficient discrimination from external microwave interference. It should b
26、e borne in mind, however, that the input level at the detector is rather low and the screening which may be effective at microwave frequencies may not be sufficient at low frequencies; therefore, in an area with ambient interference a tuned receiver may be indispensable. In any case, care should be
27、taken to avoid earth-loops which may be formed by the power connections of the electronic equipment and the screens of the interconnecting waveguides. NOTE 2A receiver showing the quotient of two inputs, i.e. one stemming from the resonator and the other being derived from the generator, is advantag
28、eous as it avoids errors due to output variations of the generator. 3.2.2 Display devices are used with swept-frequency measurements. As only the rectified output of the resonator is shown, any general-purpose oscilloscope of sufficient sensitivity can be employed. NOTEDual-trace facilities (alterna
29、te mode of operation) are advantageous as they eliminate errors due to output variations of the generator. 3.3 A frequency meter of sufficient discrimination at the working frequency range. 3.4 A 3 dB-attenuation standard or a variable standard attenuator. 3.5 A resonator resonating at the desired f
30、requency. NOTEThere may be no resonance device available commercially for obtaining optimum results with an arbitrary test problem. Therefore, it seems appropriate to give some general instructions on the construction of such resonators; details are given in Appendix A on the particular resonator ty
31、pes. a) For ease of machining with the accuracy required, resonators of a circular cross-section are preferred. b) For material testing purposes, modes of axial symmetry are used exclusively. Hence, with TEM- and TE 0mn -modes, resonators of length/diameter ratio close to unity offer optimum perform
32、ance; for TM 0mn -modes, this ratio is in general close to zero. c) The inner surface of the resonator shall be flat to at least one-fourth of the penetration depth of the electromagnetic field at the working frequency. Thus, polishing will normally be necessary. d) As brass is usually used, the per
33、formance will be improved by electro-plating the inner surfaces with silver or gold (for high-temperature application) to a thickness of about four penetration depths of the electromagnetic field. At higher frequencies, bulk silver may be used for the resonator. e) Sliding contacts lower the quality
34、 of the resonator and, especially at the higher frequencies, adversely affect the reproducibility and accuracy of the setting. They should therefore be avoided whenever possible. Removable parts, in particular covers for openings for introducing the test specimen, should be designed so that their co
35、ntacting surfaces are not traversed by current. f) Coupling elements should be designed so as to excite only the desired mode of oscillation. Variation of the coupling strength should not affect the measured unloaded quality Q u(see Sub-clause 5.2). At resonance, an insertion loss of the resonator o
36、f about40 dB may be considered as adequate. 4 Test specimen 4.1 The shape of the test specimen shall satisfy the conditions set by the resonator and the mode of oscillation to be used. In general, disks or rods of circular cross-section are used. The particular requirements for the different resonat
37、or types are given in Appendix A. NOTE 1Tight fitting of the specimen to the resonator is necessary at surfaces which are perpendicular to the lines of electric field unless the resulting shearing effect can be taken sufficiently into account by calculation. This is of special importance with coaxia
38、l (TEM) resonators and with TM-cavities. NOTE 2Errors in permittivity due to a residual gap between the end surface of the resonator and the adjacent surface of the specimen become negligible if specimens of halfwave thickness are used. NOTE 3Rod-shaped specimens of low permittivity and small diamet
39、er d s(as compared with the cavity diameter d o ) may be used in cavity resonators. 4.2 The test specimens shall be prepared in accordance with the requirements of the particular test method (see Appendix A) and Clause 5 of Publication 377-1. 5 Testing procedure The testing procedure is as follows:
40、5.1 The specimen is inserted into the resonator and resonance is established; the tuned quantity (frequencyf Lor length l Lrespectively) is then recorded. 5.2 The half-power bandwidth f Lof the loaded resonator is measured by detuning either the resonator or changing the frequency. The Q-factor Q Lo
41、f the loaded resonator is given by: BS7737-2:1995 BSI 11-1999 3 5.3 The specimen is then removed from the resonator and resonance is re-established in accordance with Sub-clause 5.1 (this yields f uor l urespectively). 5.4 The Q-factor of the unloaded resonator is determined in accordance with Sub-c
42、lause 5.2: NOTE 1The coupling to and from the resonator shall not affect the measured half-power bandwidth for any single setting. NOTE 2Accuracy in determining the resonance settings l or f is increased by averaging the half-power points l 1and l 2or f 1and f 2respectively: NOTE 3When the two Q-val
43、ues to be determined differ only by a small amount, which occurs with low-loss specimens, accuracy may be increased and procedure simplified by utilizing the output of a square law detector. If a single Q-value is known, e.g. Q 1 , then: where 1and 2are the deflections of the voltmeter corresponding
44、 to the Q-values Q 1and Q 2respectively. Alternatively, if the resonant voltages U 1and U 2at the detector are kept constant by means of a calibrated variable attenuator, then: where A = 20 (log U 2p log U 1 ) is the required increase of attenuation in decibels. The attenuator should give A to an ac
45、curacy of at least 0.1 dB. 5.5 All measurements should be taken within a temperature interval not exceeding 2 C unless it is possible to correct for the influence of temperature with sufficient accuracy. 6 Evaluation of measured data Measured data are evaluated in accordance with the instructions gi
46、ven for the particular test apparatus as described in Appendix A. 7 Test report The test report is given in accordance with Clause 6 of Publication 377-1. l = l 1 l 2with f = constant f = f 1 f 2with l = constantBS7737-2:1995 4 BSI 11-1999 Appendix A Resonators A.1 Re-entrant cavity A.1.1 Re-entrant
47、 cavity resonators are used in the frequency range 100 MHz to 1 000 MHz. They are suitable for disk-shaped specimens of low permittivities ( rk 10). A.1.2 Principle of operation Re-entrant cavities consist essentially of a fixed length of a coaxial transmission line short-circuited at both ends and
48、loaded by a lumped variable capacitor near the lower end of the centre conductor (see Figure 2, page 14). NOTEThis capacitor is functionally equivalent to the micrometer capacitor of Clause 5 of IEC Publication 250, Recommended Methods for the Determination of the Permittivity and Dielectric Dissipa
49、tion Factor of Electrical Insulating Materials at Power, Audio and Radio Frequencies Including Metre Wavelengths, which forms a resonance circuit in conjunction with the transmission line section in which it is contained. The resonance frequency is determined by the length and the characteristic impedance of the line and by the effective capacitance of the micrometer capacitor. A.1.3 Design Optimum performance is obtained with a ratio of outer to inner diameter of approximately 3.5 (Z 0. 75 7). To avoid excitation of
