Philips Components Quartz crystals General Introduction INTRODUCTION For practical reasons, technical information on piezoelectric quartz devices is divided into two handbooks which have the following titles: filled with dry nitrogen gas can piezoelectric quartz plate (blank) PA10 - Quartz crystals electrode conducting cement PA11 - Quartz oscillators. mounting structure A quartz crystal consists of a quartz crystal element with electrodes, mounted in a hermetically sealed holder with connecting pins or leads. Quartz crystals are normally used in oscillator and filter circuits. The quartz crystal element is a vibrating resonant plate which relies upon the piezoelectric effect to couple it to electrical circuits. Crystal elements are normally cut in the form of plates. The dimensions of these elements and their orientation with respect to the axes of the crystal give the characteristic of the element. The dimensions are such that the mechanical resonance frequency equals the desired electrical frequency. There are a large number of crystal cuts but the most advantageous orientation is the so-called AT-cut. The frequency range that can be covered is from 1 to 250 MHz. The crystal element may vibrate in the fundamental vibration mode or in the third, fifth or higher overtone. Special cuts for temperature sensors used in digital temperature measurement equipment, are also available. The intrinsic properties of quartz make it a unique device for accurate and stable frequency control and selection. As the properties of quartz (temperature coefficient, ageing, high Q factor) are very stable, the ultimate performance of the element is largely dependent on the environment and the associated electrical circuits. The design of an oscillator requires high technical skill to give the maximum possible efficiency out of the 1996 Oct 02 base hermetical glass-to-metal seal connection leads hermetical seal (resistance welded) CCA360 Fig.1 Section of a quartz crystal unit (metal holder: HC-49/U). transparant glass bulb, vacuum sealed piezoelectric quartz plate (blank) mounting structure electrode all glass base connection lead CCA355 Fig.2 Section of a quartz crystal unit (glass holder: HC-26/U). connection between the crystal and the circuit. A range of oscillator circuits have been designed for all kinds of applications with an optimum pack of specifications. It is advised 1 that any particular application should be discussed with the crystal manufacturer at the earliest possible stage of the design. Philips Components Quartz crystals TERMS AND DEFINITIONS IN ACCORDANCE WITH IEC 122-1 Resonance frequency (fr) The lower of the two frequencies of the quartz crystal alone, under specified conditions, at which the electrical impedance of the quartz crystal is resistive. Anti-resonance frequency (fa) The higher of the two frequencies of the quartz crystal alone, under specified conditions, at which the electrical impedance of the quartz crystal is resistive. Load resonance frequency (fL) One of the two frequencies of a quartz crystal in association with a series or parallel load capacitance, under specified conditions, at which the electrical impedance of the combination is resistive. This frequency is the lower of the two frequencies when the load capacitance is in series and the higher when it is in parallel (see Fig.4). For a given value of load capacitance (CL), these frequencies are identical for all practical purposes and given by: L1 C1 ( C0 + CL) 1 --- = 2 ----------------------------------------C1 + C0 + CL f Nominal frequency (fnom) The frequency assigned by the specification of the quartz crystal. Working frequency (fw) The operational frequency of the quartz crystal together with its associated circuits. Overall tolerance The maximum permissible deviation of the working frequency from 1996 Oct 02 General Introduction nominal frequency due to a specific cause or a combination of causes. Adjustment tolerance (f/f nom) The permissible deviation from the nominal frequency at the reference temperature under specified conditions. Ageing tolerance The permissible deviation (of the working frequency) from its initial value, observed with the passage of time under specified conditions. Tolerance over the temperature range The permissible deviation over the temperature range with respect to the frequency at the specified reference temperature. Resonance resistance (Rr) The resistance of the quartz crystal alone at the resonance frequency (fr). Load resonance resistance (RL) The resistance of the quartz crystal in series with a stated external capacitance at the load resonance frequency (fL). Note: The value of RL is related to the value of Rr by the following expression: C0 2 R L = R r 1 + ------- CL Drive level dependent resonance resistance (Rdld) Maximum resonance resistance value in a specified range of drive level, over 10-16 W to 10-4 W. Operating temperature range (Toper) Level of drive The range of temperatures as measured on the holder over which the quartz crystal must function within the specified tolerances. A value of the amplitude of motion imposed upon the quartz crystal expressed in terms of dissipated power. Operable temperature range (Top) Note: In special cases, the level of drive may be specified in terms of crystal current or voltage. The range of temperatures as measured on the holder over which the quartz crystal must function within though not necessarily within the specified tolerances. Reference temperature (Tref) The temperature at which certain crystal measurements are made. For controlled temperature crystals, the reference temperature is the mid-point of the controlled temperature range, for example +70 C. For non-temperature controlled crystals, the reference temperature is normally 25 2 C. 2 Unwanted response (Rn) A state of resonance of a crystal vibrator other than that associated with the working frequency, expressed in the ratio Rn/Rr or in dB (being 20log Rn/Rr). Philips Components Quartz crystals General Introduction Load capacitance (CL) The effective external capacitance associated with the quartz crystal which determines the load resonance frequency (fL). The parameters of the series branch are termed the `motional parameters' of the quartz crystal. The parameter C0 is termed the `parallel capacitance'. The capacitance in parallel with the motional arm of the equivalent circuit. The equivalent circuit has two resonance frequencies at which the electrical impedance is resistive: the `resonance frequency' (fr) and the `anti-resonance frequency' (fa). The resistance of the equivalent circuit at the resonance frequency (fr) is termed the `resonance resistance' (Rr). Motional inductance (L1) 1 For R 1 < ----------C 0 Motional capacitance (C1) The capacitance of the motional (series) arm of the equivalent circuit. Shunt capacitance (C0) The induction of the motional (series) arm of the equivalent circuit. ELECTRICAL PROPERTIES AND BEHAVIOUR Quartz crystal equivalent circuit The equivalent circuit, which has the same impedance as the quartz crystal in the immediate neighbourhood of resonance, is usually represented by an inductance, capacitance and resistance in series, this series branch being shunted by the capacitance between the terminals of the unit. The parameters of the series branch are usually given by L1, C1 and R1. The parallel capacitance is given by C0 (see Fig.3). handbook, 2 columns C1 C0 L1 R1 symbol CCA352 Fig.3 1996 Oct 02 Quartz crystal equivalent circuit. the following relationships hold: 1 f r = ------------------------2 L 1 C 1 1 f a = --------------------------------------C1 C0 2 L 1 -------------------C1 + C0 (1) (2) (3) Rr = R1 Load capacitance and frequency pulling During manufacture, definable limits are set to the accuracy of frequency. In an oscillator, a load capacitance (CL) is required to trim the working frequency (fw) to the nominal frequency (fnom). Figure 4 shows the quartz crystal equivalent circuit with a load capacitance in series and parallel. Each combination has two resonance frequencies at which the electrical impedance of the circuit is resistive. The lower of the two frequencies when the load resistance is connected in series and the higher with the load connected in parallel, are termed `load resonance frequencies' (fL). At these frequencies the resistance of the combination with the load capacitance in series is termed `load resonance resistance' (RL). 3 For 1 R 1 < ----------C 0 1 f L = ----------------------------------------------------C1 ( C0 + CL) 2 L 1 ---------------------------------C1 + C0 + CL C0 2 R L = R r 1 + ------- CL (4) (5) For a given value of CL the load resonance frequencies of the series and parallel combination are identical. In practice, however, the parallel combination shown in Fig.4 (c) rarely occurs in an oscillator. From equation (4) another two parameters of vital concern can be derived: the difference (fL) between load resonance frequency (fL) and resonance frequency (fr), termed `load resonance frequency offset': f L = f L - f r (6) respectively the `fractional load resonance offset': C1 fL - fr D L = ------------= -----------------------------2 ( C0 + CL) fr (7) and the sensitivity of load resonance frequency with respect to load capacitance variations, termed `pulling sensitivity': 1 f L 1 f L S = ---- x ----------- = --- x ----------(8) f r C L f L C L D L C1 = -------------- = ---------------------------------2 C L 2 ( C0 + CL) Philips Components Quartz crystals General Introduction Standard values of load capacitance The standard values of load capacitance for quartz crystals operating at the fundamental frequency of the mode are: handbook, 4 columns C1 (a) L1 C0 R1 0 oo fr fa R1 15 pF, 20 pF, 30 pF and 50 pF. Load capacitances of the values 8 pF, 10 pF, 12 pF and 18 pF may also be used for fundamental mode quartz crystals. f Note: In some countries 32 pF is still in use, but this value should not be considered a standard value and its use is not recommended. C1 (b) L1 C0 RL oo 0 fL fa R1 f Overtone quartz crystals are often operated at series resonance. Where a load capacitance is used, it should be chosen from the above mentioned values. CL (c) C L C0 L1 Pulling Sensitivity (S) Rp C1 oo 0 fr fL R1 f MSA589 reactance resistance Fig.4 Resonance, anti-resonance and load resonance frequency. 1996 Oct 02 In special cases, load capacitance values of 5 pF upwards are available in a narrow distribution. 4 The pulling sensitivity expressed in ppm/pF is a good measure for the frequency sensitivity as a function of load capacitance variations at the working frequency. Figure 5 illustrates the load capacitance, for two quartz crystals having different C1 and C0 values it should be noted that a tolerance of 0.5 pF on a 20 pF load capacitance may lead to an error of 11 ppm. For low values of CL the pulling sensitivity is increased, which means that the frequency is more strongly dependent on the external parameters of the oscillating circuit. Philips Components Quartz crystals Table 1 General Introduction Quartz crystal parameters (see Fig.5) QUARTZ CRYSTAL a QUARTZ CRYSTAL b fr = 9 994.400 kHz(1) fr = 9 998.727 kHz(1) pF(1) C0 = 2.0 pF(1) C1 = 28 pF(1) C1 = 5.6 fF(1) CL = 20 pF CL = 20 pF C0 = 5.0 fL = 10 000.000 kHz fL = 10000.000 kHz S = -22.4 ppm/pF S = -5.79 ppm/pF Note 1. Tolerances on the parameters fr, C0 and C1 are required for calculating the `f' and the `pullability at fnom'. handbook, full pagewidth MSA591 - 1 fL (kHz) fL (kHz) 10 2 5 1 0 0 0 10 20 30 40 50 C L (pF) S (10 6 /pF) S (10 6 /pF) 50 20 25 10 0 0 10 20 30 40 50 C L (pF) 0 10 20 30 40 50 C L (pF) 0 0 10 20 30 40 50 C L (pF) crystal a crystal b Fig.5 Change in frequency (fL) and pulling sensitivity (S) as a function of the load capacitance. 1996 Oct 02 5 Philips Components Quartz crystals General Introduction C1 (fF) 100 C0 S (ppm) CL (pF) 50 0.02 1000 500 0.05 20 0.01 10 200 5 100 0.2 0.5 50 2 1 1 20 0.5 10 2 5 5 0.2 0.1 10 2 20 CCA385 50 Fig.6 Nomogram enabling the determination of pulling sensitivity (S). 1996 Oct 02 6 Philips Components Quartz crystals handbook, full pagewidth f General Introduction CCA386 80 f (ppm) 70 60 50 16' 4' 40 2' 14' 30 0 20 12' 2' 4' 10 10' 6' 8' 0 6' 8' 10 4' 10' 20 2' 12' 30 0 40 14' 2' 50 16' 4' 60 70 80 60 40 20 0 20 40 60 80 100 120 T C) Fig.7 Examples of frequency-temperature characteristics of AT-cuts as a function of the cutting angle. 1996 Oct 02 7 Philips Components Quartz crystals Level of drive The power dissipated in a quartz crystal is termed `level of drive' and is usually expressed in mW. In the level of drive range 10-12 to 10-4 W the drive level dependency of the crystal characteristics is almost negligible. For drive levels greater than approximately 0.1 mW, the crystal characteristics tend to change. For this reason the crystal characteristics are specified at drive levels of 0.05 mW to 0.5 mW depending on the crystal type. Low drive levels When a quartz crystal oscillator is switched on, there will initially be some noise in the circuit. The noise power, which depends on the circuit design and on the components used, will be in the region of 10-16 W. From this level, the oscillatory power builds up in the quartz crystal, passing through a power range of approximately 12 decades to its maximum value. At the extremely low power levels that occur during build up of oscillation, the resonance resistance (Rr) may increase slightly. The quartz crystal oscillator should, therefore, have sufficient loop gain to avoid start-up problems. Typically, a negative resistance of three times the specified Rr(max) value is sufficient. High drive levels For applications requiring high stability, a drive level between 5 W and 100 W should be used. Drive levels exceeding 0.5 W should be avoided, and excessively high drive levels (exceeding 5 mW) may seriously affect the quartz crystal's behaviour (see Fig. 9). General Introduction graph showing the temperature coefficient (TC) curve or drift characteristic. In the case of AT cuts, the relation of drift and temperature is approximated by a cubic curve; the drift characteristic of most other cuts is parabolic. Figure 7 shows a number of frequency-temperature curves obtained from AT-cut crystals with various angles of cut ( from -4' to +16' increasing angle of cut). The curves are symmetrical with respect to approximately +27 C. A temperature range which is fairly symmetrical with respect to 27 C (e.g. 0 to 60 C) will, therefore, result in the smallest frequency drift in that range. A small frequency drift over a wide temperature range, e.g. -40 to +80 C, will result in a fairly steep temperature coefficient at room temperature. Advantages of all-glass holders Quartz crystals with all-glass holders show the following advantages over those with metal holders: 1. A lower ageing rate. 2. A lower series resistance, which also means a higher Q-factor, due to the fact that glass holders are evacuated giving less mechanical damping. 3. Better performance under adverse climatic conditions. 4. Smaller adjusting tolerances. Ageing A gradual change in resonance frequency with time is called (an effect of) ageing. Only where very good long-term stability is required should ageing be of consequence. Frequency/temperature characteristics The frequency drift as a function of temperature can be represented by a 1996 Oct 02 8 It should be borne in mind that (with a view to ageing only): 1. Quartz crystals with an all-glass holder have a lower ageing rate than metal sealed crystals. 2. Low frequency crystals are preferred to high frequency crystals. 3. Overtone crystals are preferred to fundamental crystals for the same frequency. Crystal behaviour in an oscillator In the vicinity of resonance, the impedance of a quartz crystal can be represented by a circle (see Fig.8). The circle is shifted downwards with respect to the resistance axis over: 1 (9) X 0 = -----------------2f r C 0 When a load capacitance is connected in series with the quartz crystal the shift is X0 + XL, where 1 (10) X L = ------------------2f L C L The difference between anti-resonance frequency and resonance frequency C1 CL f a - f r ---------- x f r x -------------------2C 0 C0 + CL (11) is assumed to be 100%. It can be seen that the difference between the two frequencies, determined by the phase angle , disappears at fw = 50%. The phase angle in the oscillator should be kept sufficiently small to avoid quartz crystal operation in the uncertain area above 50% (frequency switching). Philips Components Quartz crystals General Introduction Quartz crystals for frequencies higher than 100 to 125 MHz (depending on type) have an impedance circle with a greater downwards shift, even to below the real axis. When the figure of merit given by handbook, 4 columns reactance 85 % fw 90 % 80 % 95 % 70 % 40 % X0 X0 1 M = ------- = -------------------------------R1 ( 2f r ) R 1 C 0 Za 60 % (12) 100 % fa XL resistance is less than approximately 5, the resonance frequency (fr) is arbitrary. fp Indications for use 2 X0 MSA590 R1 0 10 % Zr fr fa fs fp 10 % Fig.8 handbook, 4 columns Enlarged area around the zero point. fa = anti-resonance frequency fr = resonance frequency fs = series resonance frequency fw = working frequency Zr = impedance at working frequency. Working frequency and impedance of a quartz crystal in the impedance diagram. CCA387 20 f f (ppm) 15 10 5 0 10 3 10 2 10 1 1 10 10 2 drive level (mW) Fig.9 Quartz crystal drive level. 1996 Oct 02 9 Keep phase deviations in the circuit sufficiently low to avoid quartz crystal operation in the 50% working frequency area, in particular when phase variation is used for frequency pulling (PLL system). Ensure that the amplification is sufficiently high, particularly when applying phase variation. Keep quartz crystal drive level low (generally 0.5 mW; preferably 0.1 mW), (see Fig.9). Philips Components Quartz crystals General Introduction MEASURING PROCEDURES Several methods of measuring quartz crystals are in use. Because different methods may give various results, refer to the test block diagram of Fig.10. This is the passive method with -network in accordance with IEC publication 444. The accuracy of reproduction of the -network method ranges between 10-6 and 10-8 for frequency measurements, depending on the type of quartz crystal to be measured. Passive method with -network (IEC 444) The principle of this method is very simple. With the equipment shown in Fig.10, a stable signal source (frequency synthesizer) is adjusted to the frequency at which the signal has zero phase change when passing through the crystal, as measured by the phase meter; this frequency (measured with the frequency counter) is then the resonance frequency of the crystal. - NETWORK VA VB PHASE & VOLTAGE METER frequency modulation input Measuring methods can also be applied by using the following equipment if it is available: SAUNDERS Test Set, type 150 (A, B, C). Crystal Test Set, type TS193A (British Military Standard). Crystal Impedance Meter TS330/TSM (U.S. Army Standard). FREQUENCY COUNTER SIGNAL SOURCE (SYNTHESIZER) For ease of operation, it is possible to phase-lock the system by feeding back the analogue output of the phase error (from zero) to control the precise frequency of the signal source (AFC loop shown by dashed line). analogue output Crystal Impedance Meter TS683/TSM (U.S. Army Standard). A -network test jig is used to connect the quartz crystal to the measuring equipment (see Fig.11). This test jig consists of two -connected resistive pads, carefully manufactured to represent a pure, constant resistance, which is frequency insensitive at the terminals of the quartz crystal. The function of the input and output `pads' is twofold: A.F.C. AMPLIFIER MBA927 1. To match the crystal impedance to the associated equipment. 2. To attenuate reflections from the associated equipment. Fig.10 Test equipment block diagram. For further information consult IEC recommendations, Publication 444. Crystal shielding R3 R3 2 1 VA R1 R2 R2 R1 1' VB 2' Depending on the application, crystal shielding may give rise to frequency deviations, in particular for fundamental mode quartz crystals with a considerable pulling sensitivity. MSA586 In our procedure the metal enclosure of the quartz crystal is normally not earthed. If, in special cases, earthing is required this should be mentioned in the specification for ordering. Fig.11 Test jig. 1996 Oct 02 10 Philips Components Quartz crystals MOUNTING Quartz crystals provided with pins (such as HC-6/U, HC-27/U, HC-29/U and HC-50/U) are for mounting in sockets. Quartz crystals with leads are for mounting on printed-circuit boards. There are basically two methods: horizontal and vertical mounting. Horizontal (flat) mounting gives better mechanical stability whilst vertical mounting uses less printed-circuit board space. To prevent permanent damage of quartz crystals during mounting operations, some precautions have to be taken: * Glass feed-throughs are rather vulnerable so avoid excessive forces on the leads which can cause breakage. If cutting of the leads is necessary, use suitable tools to prevent shock waves in the leads. * If bending of the leads is necessary e.g. in the event of flat mounting, make the bend at least 2 mm away from the body with a bending radius >0.5 mm. * Note that, especially when the component is vertically mounted, the first mm of tinned leads away from the body are not guaranteed for use. When mounting on thin printed-circuit boards (e.g. 0.7 mm), the use of spacers is recommended. 1996 Oct 02 General Introduction Specially designed for surface mounting, there are two constructions in HC-45/U-SMD and HC-49/U-SMD. All crystal types are designed such that they withstand all commonly used soldering techniques, see Chapter "Tests and requirements" in the individual data sheets. Exposing the crystal units to high temperatures for a prolonged time, however, should be avoided. For utmost mechanical stability and electrical reproducibility, metal types can be supplied with a third (top) lead which serves both as a ground wire and a three-point attachment to the printed circuit board. QUARTZ CRYSTAL UNITS AS DIGITAL TEMPERATURE SENSORS The most well known applications of quartz crystal units are those where the crystal is used in oscillator and filter circuits, as a frequency-selective element with an extremely high Q-factor. By correct choice of the cutting angle of the vibrating plate, it is possible to obtain a very low TC over a limited temperature range. Examples of such crystal cuts are: AT, BT, CT and GT cuts. In addition, it is also possible to cut crystal plates so that the resonance frequency is an almost linear function 11 of the temperature. It should be noted, that the first quartz crystal cut to be discovered was in fact a `Y- cut'. However, there are some disadvantages which make this cut unsuitable for temperature sensing, therefore special cuts have been introduced, depending on the application. How to use a quartz crystal unit as a temperature sensor In order to measure temperatures with a quartz crystal sensor, the device should be connected to an oscillator circuit which usually consists of one or two transistors or an integrated circuit. The oscillator will produce an output signal whose frequency will change by -40 to +80 x 10-6/K, depending on the cutting angle. There are several ways of processing this signal. Due to excellent stability, low ageing and its 'digital' nature, resolutions of 0.001 K are easily achieved without noise problems. This renders the device especially suitable for measurements of very small temperature differences as in distillation columns and flow meters. Philips Components Quartz crystals General Introduction MBD642 2000 f f (ppm) 1000 handbook, full pagewidth 0 1000 2000 3000 4000 60 40 0 20 20 40 60 80 100 140 120 AT-cut Temperature sensor Fig.12 Frequency change as a function of temperature. handbook, full pagewidth temperature sensor 1 OSCILLATOR 1 MIXER temperature sensor 2 COUNTER OSCILLATOR 2 MLB743 Fig.13 Typical temperature sensing circuit with two sensors. 1996 Oct 02 DISPLAY 12 160 T ( oC) Philips Components Quartz crystals General Introduction handbook, full pagewidth temperature sensor OSCILLATOR 1 DIFF. COUNTER AT-cut ref. crystal DISPLAY OSCILLATOR 2 MSA585 Fig.14 Typical temperature sensing circuit with one sensor and one reference crystal. handbook, full pagewidth temperature sensor OSCILLATOR 2 COUNTER MICRO PROCESSOR output MLB744 Fig.15 Typical temperature sensing circuit with one sensor and one microprocessor. 1996 Oct 02 13 Philips Components Quartz crystals General Introduction handbook, full pagewidth temperature sensor MLB745 Fig.16 Miniature wireless temperature sensing circuit. V DD handbook, full pagewidth CLOCK IC 1 s pulse MLB746 Fig.17 Crystal oscillator in a clock integrated circuit. 1996 Oct 02 14 Philips Components Quartz crystals General Introduction HOW TO SPECIFY A QUARTZ CRYSTAL UNIT General product information Nominal frequency fnom ................................................ Enclosure type kHz style 12NC group 9922 ... ..... series Customer ................................................................................................................................... Application ................................................................................................................................... Related IC-type ................................................................................................................................... Date ........................................... Electrical characteristics Resonance frequency Mode of vibration fr/fI ................................................... fundamental; 3rd; 5th and 7th kHz overtones Level of drive P ..................................................... W (100 W) Reference temperature Tref .................................................. C (+25 C) Load capacitance CL ................................................... pF/series resonance Adjustment tolerance (at Tref) F ................................................ ppm Resonance resistance Rr (max) ............................................ Motional capacitance C1 ................................................... fF Motional inductance L1 .................................................... mH .......% Parallel capacitance C0 ................................................... pF .......% Ageing requirement F ................................................ ppm per year Spurious requirement ........................................................ DLD requirement Rr dld ............................................... Operating temperature range T .............................. C Frequency stability ...................................................... ppm Frequency stability in .................................. ................ ppm Storage temperature range T .............................. C to ............... to ............... .......% Mechanical characteristics Connecting leads standard / cut to .............................. Marking [ mm ] first line [ ] second line [ ] third line [ ] optional line Packaging method bulk/tape-reel/ammopack/blister tape ................................................ .................................................................. Special requirements .......................................... ............................................................................................................................ .......................................... ............................................................................................................................ Remarks 1996 Oct 02 15