EIA CB6-A-1987 Guide for the Use of Quartz Crystal Units for Frequency Control.pdf

1、EIA COMPONENTS BULLETIN Guide for the Use of Quartz Crystal Units for Frequency CntroI CB6-A (Revision of CB8) OCTOBER 1987 ELECTRONIC INDUSTRIES ASSOCIATION ENGINEERING DEPARTMENT EIA CBb-A 87 3234600 0557562 567 NOTICE IA Engineering Standards and Publications are designed to serve the public inte

2、rest through eliminating misunderstandings between manufacturers and purchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for his particular need. Existence of such Standards and Pub- lic

3、ations shall not in any respect preclude any member or non-member of EIA from manufacturing or selling products not conforming to such Standards and Publications, nor shall the existence of such Standards and Publications preclude their voluntary use by those other than IA members, whether the stand

4、ard is to be used either domestically or internationally. Recommended Standards and Publications are adopted by EIA without regard to whether or not their adoption may involve patents on articles, materials, or processes. By such action, EIA does not assume any liability to any patent owner, nor doe

5、s it assume any obligation whatever to parties adopting the Recom- mended Standard or Publication. This document was developed by the P-11 Committee on Quartz Crystal Devices. Published by ELECTRONIC INDUSTRIES ASSOCIATION Engineering Department 2001 Eye Street,N.W. Washington, D.C. 20006 Printed in

6、 U.S.A. CI36 -A TABLE OF CONTENTS 1 . INTRODUCTION AND SCOPE 1 1.2. Technical Preamble 4 1.1. GENERAL . 1 2 . CRYSTAL HOLDER CLASSIFICATION 6 2.1. HOLDERS . 7 2.1.1. CATEGORIES OF HOLDERS 7 2.1.2. NON-STANDARD HOLDERS . 7 2.2. CRYSTAL CONNECTIONS . 8 2.3. MOUNTING 8 3 . FREauENCY 9 3.1. MODES OF VIB

7、RATION AND FREQUENCY RANGES . 9 3.2. OVERTONE CRYSTAL UNITS . 11 3.2.1. GENERAL . 11 3.2.2. CHARACTERISTICS . 12 4 . FREQUENCY-TEMPERATURE DEPENDENCE . 13 4.1. PARABOLIC CURVES . 14 4.2. AT-CUT CURVE . 14 4.3. SPECIFYING FREQ . TOLERANCE AND OPERATING TEMP 16 5 . FREQ . STABILITY RELATED TO THE COND

8、 . OF USING . 16 5.1. FACTORS AFFECTING THE FREQUENCY STABILITY 16 5.1.1. OSCILLATOR NETWORK 17 5.1.2. CRYSTAL UNIT 17 5.2. TYPE OF OSCILLATORS 17 5.2.1. POSITIVE REACTANCE OSCILLATORS 18 5.2.2. SERIES RESONANCE OSClLLATORS 20 5.2.3. CRYSTAL Q AND TTS IMPLICATIONS 22 5.2.4. SERIES-MODE OSCILLATORS U

9、SING THE MEACHAM . 23 5.2.5. INTEGRATED CIRCUIT CRYSTAL OSCILLATORS 26 5.3. LEVEL CONTROL . 28 5.4. LOAD CAPACITANCE . 29 5.5. LEVEL OF DRIVE . 30 5.6. LEVEL CONTROL 31 6 . SPECIAL REQUIREMENTS . 32 6.1. TOT . FREQ . VARIATION OVER THE OPERATING TEMP . 32 6.2. FREQ . VARIATION OUTSIDE THE OPERATING

10、TEMP . RANGE 32 6.3. FREQUENCY PULLING 33 6.4. SERIES RESONANCE OPER . OF FUNDAMENTAL CRYSTAL UNITS 34 6.5. CLIMATIC AND MECHANICAL EXTREMES . 34 6.6. MARKING . 35 7 . MEASURING METHODS AND TEST CONDITIONS 35 8 . TECHNICAL DATA TO ACCOMPANY ORDER FORM . 35 i EIA CBb-A 7 3234b00 0559564 33T CB6 -A LI

11、ST OF TABLES 3-1: TABLE l IO 5-1: TABLE 329 ii EIA CBb-A 87 3234600 0559565 276 M CB6-A Page 1 1. INTRODUCTION AND SCOPE This document has been compiled in response to a generally expressed desire on the part of both users and manufacturers for a guide to the use of quartz crystal units for frequenc

12、y control so that they may be used to their best advantage. The most conmon error has been the assumption on the part of some users that the crystal unit is an absolute frequency-determining device. This idea has persisted, probably due to the fact that the frequency tolerances in earlier equipment

13、were suff- iciently wide that frequency variation imposed by the associated circuit conditions were small enough to be ignored. It is not the function of this brief document to explain theory nor to attempt to cover all the eventualities which may arise in practical circumstances. It does draw atten

14、tion to some of the more fundamental questions which should be considered by the user of quartz crystal units for new applications. Such a procedure will reduce considerably the users risk of unsatisfactory performance. 1.1 GENERAL Although the performance of a quartz crystal unit depends mostly on

15、its design and fabrication its performance is influenced by other circuit components associated with it. For example, small variations in the input impedance of the oscillator circuit may result in modification of the frequency of oscillation beyond the permissible limits The quartz crystal unit is

16、a mechanical vibrating system which is EIA CBb-A 7 3234600 0559566 LO2 CB6-A Page 2 driven by the electrical current supplied to it. The amplitude of the vibration is proportional to the current. If the amplitude is great enough the quartz will fracture resulting in catastrophic failure. Heat is gen

17、erated in the quartz as it vibrates. The resulting rise in temperature is proportional to the square of the current. Driving the unit at levels higher than the specified value may result in excessive frequency changes. An even more serious problem is the effect of thermal gradients in the quartz. Th

18、ese may result from excessive drive levels or from ex- ternal heat sources such as the heaters in ovens. TtL* thermal gradients produce stresses in the quartz which may result in very large frequency perturbations, High drive levels also tend to create or aggravate coupled modes resulting in changes

19、 in the parameters of the crystal unit and in un- desired responses. Since temperature and thermal gradients influence the frequency of a crystal unit some time is required for the frequency to become stable after turn-on. The time required depends upon the design and the drive level. Ordinarily the

20、 frequency becomes substantially stable after a few minutes of operations. Sometimes permanent changes in the frequency occur, especially after the unit has been operated at a high drive level. Effects of this nature may result from various causes. For example: internal fractures in the quartz cause

21、d by excessive strain, loss of electrode metal due to the acceleration forces which may well exceed a million times that of gravity, permanent displacement in the crystal lattice of the quartz, inelastic properties of the mounting systems, displacement of dust particles on the surface of the quartz,

22、 etc. Sometimes the equivalent resistance of a crystal unit is found to EIA CBb-A 87 3234600 0559567 O47 cB6-A Page 3 change with the drive level, This phenomenon is particularly comn in units designed to be operated at very low drive levels. It is thought to be due, in most cases, to surface effect

23、s such as loose Flating or particles of som foreign material on the surface of the quartz. Although a crystal unit should be operated at the drive level for which it was designed, any change of resistance with drive level is usually an indication of inadequate processing technique. Whenever possible

24、 the oscillator circuit should be designed to utilize a standard crystal unit and care should be taken to insure that the crystal unit is operated at its rated drive level. In special cases where no standard crystal unit is available, the circuit designer should work closely with the crystal manufac

25、turer to insure that the crystal unit and the circuit are compatible and that the resulting circuit meets customers requirements. Standard specific crystal data sheets, similar to those given in military documents such as MIL-C-3098, and suppliers technical information bulletins will define the avai

26、lable combinations of frequency tolerances, temperature range and load capacitance, the overtone order in the case of overtone crystals, and whether for series resonant or positive reactance operation. Each sheet will further specify the maximum level of drive and maximum equivalent series resistanc

27、e (ESR) in relation to the frequency of the crystal unit. These data sheets have been carefully complied to include a wide range of crystal units with standardized performances and dimensions. It cannot be stressed too greatly that the user should, whenever possible, select his crystal units from th

28、ese data sheets, even at the expense of circuit modifications, to,permit the use of standard crystal units. EIA CBb-A ? 3234b00 05575b T5 (206-A Page 4 Standardization is a continuing process and, as new requirements arise, new data sheets will be produced to meet these requirements. 1.2 Technical P

29、reamble At frequencies near that of a mechanical resonance, a crystal unit may be represented by the equivalent electrical circuit of Figure 1 which consists of capacitance Ci, inductance L and resistance R in series, shunted by a second capacitance Ce due to the electrodes on the crystal plate. Alw

30、ays present are the distributed capacitances shown between terminais and metal holder or ground if a non-metallic hol der i s employed), The first three parameters Ci, Li, and Ri are termed the “motional parameters“ of the crystal unit. The resulting shunt capacitance Co must be determined by consid

31、ering Ce together with the distributed capacitances. For example, if the metal holder is grounded, some of am“. i i the distributed capacitance will not appear across the “motional The circuit of Figure 1 accurately represents the crystal unit if the four parameters Ci, Li, Ri and Co are constant. C

32、hanges in temperature, drive level or mechanical or thermal stresses may result in small changes in the value of the parameters. If the frequency extraneous mode of vibration happens to be equal to that for which the unit is designed, the two modes influence each other; i.e., are said to be “coupled

33、“. the motional parameters which are no longer independent of frequency and Figure 1 does not accurately represent the crystal unit. In this case the equations and measuring methods normally used do not apply. The validity of the circuit representation can be determined by measuring and plotting the

34、 impedance or admittance of the crystal unit as a function of frequency. of some The presence of a coupled mode causes large variations in EIA CBb-A 87 = 3234600 0559569 711 CB6-A Page 5 In a well designed and fabricated crystal unit the values of the motional parameters of the equivalent circuit ar

35、e independent of the amplitude, An increase in the value of Ri as the drive level is decreased, or failure of the unit to “start“ at low drive levels is an indication that sufficient care has not been exercised in the fabrication of the unit. Unfortunately, measurements of Ri at low drive levels are

36、 not always repeatable because the vibration of the quartz redistributed the offending surface material. The maximum tolerable variation in the value of Ri should be specified. Frequency-temperature characteristics are, to a first approximation, determined by the temperature coefficients of the dens

37、ity, the dimensions, and the elastic modulus of the quartz plate, When the resultant of these three properties becomes zero, the stability of the frequency with respect to temperature will be optimum. A major part of the design consists of achieving this optimum condition over a specified range of t

38、emperature. The frequency generated by a given crystal unit varies slightly with the impedance presented to it by the circuit (usually called “the load capacitance“). This effect may be useful or troublesome depending on the application. The frequency may be “trimmed“ slightly by varying the load ca

39、pacitance but inadvertent changes in the load capacitance may cause undesired changes of the frequency. The frequency stability with respect to differing load capacitance is determined largely by the capacitance ratio A crystal unit with large capacitance ratio is said to be a “stiff“ crystal unit;

40、i.e., its frequency changes little with variations of the input impedance, and vice versa. Co/Ci. Some crystal units exhibit large increases in the value of R. at 1 EIA CB6-A 7 3234600 0559570 633 CSG-A Page 6 certain temperatures; The increase of Ri is usually accompanied by perturbations of the fr

41、equency. The phenomenon is termed “an activity dip“. It is caused by coupling between two modes of vibration which happen to have the same frequency at a certain the unit may even fail to operate. temperature. The problem is especially acute in units in which the ratio of the lateral dimensions of t

42、he blank to its thickness is too low, Since the thickness is determined by the frequency and the lateral dimension by the holder dimensions, it is sometimes difficult, or even impossible, to fabricate low frequency units in small holders. The customer should consult the crystal unit supplier before

43、designing around units in which the diameter/thickness ratio is less than about 20. If low frequency is required one should consider going to the Tuning Fork or Strip AT design to avoid the above problem. It will be readily understood in view of the comments made so far, that it is in the interests

44、of manufacturers and users equally, to decide upon a suitable design of crystal unit in terms of the users requirements, using data sheets as a basis. Questions of availability of crystal units to the chosen specifications may modify, to some extent, the users original requirements. 2. - CRYSTAL HOL

45、DER CLASSIFICATION Some or all of the following information will normally be given Ort the crystal unit: - Frequency - Manufacturers name and c3de - EIA code and/or manufactuiers catalog or type number - Any other information necessary to obtain a coqdete definition of the crystal unit - At the opti

46、on of the manufacturer or by special request by the purchaser, a serial number and date of manufacture may be added - Date code (week and year) - Users part number EIA CBb-A 87 3234600 0557573 57T CB-A Page 7 2.1 HOLDERS Standard outlines of crystal holders are given in EIA-RS-192 and IEC Publicatio

47、ns 122-3. 2.1.1 QEGORIES OF HOLDERS Holders generally fall into one of four classes: a) b) c) Plastic cases d) Surface mount packages Hermetically sealed glass or ceramic units Hermetically sealed metal cases using glass-to-metal seals for the terminations or cold welded seals Glass envelopes are us

48、ually evacuated and metal types with reliable seals may be evacuated. This gives increased Q particularly at low frequencies, and improved long term frequency stability. The most popular type of holder is the hermetically sealed metal case. The trend is towards the use of the smaller types of these

49、holders, although in the smaller holders usually demand lower drive levels than those in the it should be borne in mind that crystal units of a given type larger ones. Phenolic holders are adequate for the use in dry temperate Conditions, but are not suitable for more severe climatic conditions since they are not impervious to moisture, nor can they be effectively sealed. These holders should not be considered where a protected environment is required, 2.1 ,2 NON-STANDARD HOLDERS For certain important, though not widespread, applications; e.g., limited prod