Quartz Crystal Reliability and Application Copyright 1998 Hong

Quartz Crystal Reliability and Application Copyright, 1998 © Hong Kong X’tals Limited. Arthur Lee Technical Director 1

What is a Quartz Crystal ? ? ? • Production Engineers : – It is the first component you replace when you have a failure board ! • Purchasing Officers : – It is a component that you could never get a reliable vendor ! • R&D Engineers : – It is a mysterious little black box ! Hong Kong X’tals Limited 2

What is a Quartz Crystal ? A crystal is a Piezo-Electric Device, i. e. : Energy is converted between electrical and mechanical, millions of times per second. Hence, its characteristic is very different form other components, as oppose to : • Resistor -- Pure Electrical • Screw -- Pure mechanical 3 Hong Kong X’tals Limited

The Structure of a Quartz Crystal Cover Silver Electrode on both sides Quartz Wafer ( Blank ) Silver Epoxy Sealed with Clean, Dry, inert gas Base Hong Kong X’tals Limited Mechanical Energy Stress and Motion Electrical Energy Voltage & Current 4

Equivalent Model of a Quartz Crystal L 1, C 1 : Mechanical Energy -Stress and Motion Electrical Energy -Voltage and Current RS : Energy Loss Hong Kong X’tals Limited Co : Electrode Capacitance Rs : (ESR) Equivalent Series Resistance Co : (Shunt Capacitance) Electrode Capacitance C 1 : (Cm) Motional Capacitance L 1 : (Lm) Motional Inductance 5

Raw Parameters Rs, Co, C 1 and L 1 give you all the information concerning impedance change of a crystal near by its operating frequency. Hong Kong X’tals Limited 6

How to Specify a Quartz Crystal The first thing to do is to understand your circuit : • Application Aspects, then, your products Environmental Requirement : • Reliability Issues. Hong Kong X’tals Limited 7

Your Application Copyright, 1998 © Hong Kong X’tals Limited. 8

Basic Crystal Oscillator Model 3 Main blocks of an oscillator : Amplifier, Phase Compensation Network, and Crystal is only one of the three parts in the whole oscillator feedback loop Hong Kong X’tals Limited 9

Equivalent Model of an Oscillator Hong Kong X’tals Limited 10

Basic Characteristic of your circuit The Amplifier and phase compensation network determines : • Gain & Phase Shift -> Negative Resistance • Phase Shift -> FR or FL ( hence CL ) • Voltage Amplitude -> Drive Level Hong Kong X’tals Limited 11

How to measure circuit characteristic ? Very Difficult !! The crystal is operating at high impedance and small capacitance. Hence, inserting any measurement probes will offset the operating conditions : • Oscilloscope probes ( 2 p. F ~ 20 p. F ) : too much • Vector voltmeter probes ( 50 Ohm ) : too much Send your PCBA to your vendors for measurement. Hong Kong X’tals Limited 12

Negative Resistance and Maximum ESR Negative Resistance of a oscillator circuit is the maximum ESR of a crystal at which the circuit will oscillates. However, this negative resistance may change due to : • Power supply variations • Temperature change • Component variations • Start up • The crystal’s ESR may change due to drive level, temperature So, usually, Max. ESR of crystal = 1/3 ( to 1/10 ) of Negative Resistance Hong Kong X’tals Limited 13

Drive Level Definition • By definition, very simple, Drive Level ( operating ) = Power dissipated in the crystal = vector product of voltage Vx and current Ix • This depends on your circuit, Not the crystal. • Crystal maker will calibrate / test the crystal at this drive level • Specifying Maximum drive level is quite meaningless. • Current probes will affect phase angle ( CL ). Send your PCBA to your crystal vendor for measurement Hong Kong X’tals Limited 14

What is Load Capacitance, CL ? Hong Kong X’tals Limited • Load Capacitance describes the phase shift of your circuit. • A single crystal can operate at any phase shift from 0 up to +90 degree, hence each crystal can be “Series” and “Loaded” resonant. • Crystal makers will calibrate the crystal’s frequency at that phase shift. 15

FS, FR, FL and FA Fs Fa = Frequency at which J part of impedance approaches infinity Fr ( CL = infinity ) FL Larger CL Smaller CL Not necessary to specify FA Hong Kong X’tals Limited 16

FS, RS and FR, RR When at zero phase : Frequency = FR ( Resonant Frequency ) Impedance = RR, No imaginary part RS Co • Below ~60 MHz : FS = FR, RR = RS • Above ~120 MHz : FR and RR do not exist, because No zero phase shift due to impedance of Co 17 Hong Kong X’tals Limited

FL, RL and FP, RP At Zero phase (including the external CL) Frequency = FL also known as FP Ideal ( no stray ) External CL ( Load Capacitance ) RL = RP • Always specify CL with FL • Due to big measurement errors in RL, DO NOT specify RL • However, RL is the actual impedance seen by your circuit Hong Kong X’tals Limited 18

How to specify Frequency and ESR ? • Specify the exact Nominal Frequency that the oscillator is going to operate, e. g. if you want a 3 rd o/t 36 MHz, then specify 36 MHz, not 12 MHz. • If the circuit is zero phase specify : – FR, and RR or • If the circuit is phase lag specify : – FL, CL and RR Hong Kong X’tals Limited 19

How to specify Co ? If your application does not require pulling, and the frequency is below 100 MHz, the value of Co is not important. In such case, putting Co < 7 p. F is harmless, and also quite meaningless. Hong Kong X’tals Limited 20

Co : in pulling applications Applications that require to change the oscillator frequency are pulling applications ( e. g. FM, VCXO, PLL FSK etc). • Any ONE of these parameters specifies pulling uniquely : Ts, FL 1 -FL 2, DF • These parameters also specify pulling, but indirectly : C 1, L 1, Co, Q Hong Kong X’tals Limited 21

Do NOT Multi-specify Pulling parameters Pulling ranges • Specify one and only one parameter for pulling. Effective pulling range Co C 1 Ts • Each crystal has only one pulling characteristic, more than one definition would only create confusion or contradiction. Hong Kong X’tals Limited 22

Do Not Specify Co for pulling L 1, Co, Q are indirect parameters affecting pulling. Depending on design, process, and raw material, crystals with the same Co may exhibit different pulling characteristics. So, do Not Specify Co for pulling. Some crystal makers may use C 1 for pulling control, this is acceptable. Hong Kong X’tals Limited 23

Co at Higher frequency At higher frequencies ( about > 100 MHz ), the impedance of Co becomes very small, so Fr does not exist ( no zero phase shift ), and the crystal behaves like a capacitor. Usually an external inductor is required to null the Co, so that zero phase exist again. In this case, specify : Co = Vendor suggested value +/- 15% Hong Kong X’tals Limited 24

Pullability • If your application is PLL, VCO, FM etc. must specify either Ts ( ppm/p. F ), FL 1 -FL 2, DF or C 1. Consider using Fs in these applications. • If your application is a simple oscillator, specify maximum Ts will give you higher effective Q. Hong Kong X’tals Limited 25

Temperature Characteristic AT Cut • For AT Cut, usually +/- 30 to 50 ppm over -10 to +60 • Always refer to 25 degree C as 0 ppm Hong Kong X’tals Limited 26

Temperature Characteristic BT Cut • For BT cut usually over 200 ppm • Common habit is to refer to 25 degree C as 0 ppm • Consider seriously to allow +80 ppm typical for 25 C Hong Kong X’tals Limited 27

Overtone or Fundamental ? Hong Kong X’tals Limited 28

Error Budget for Frequency Tolerances l l Factor 1 : The first most important information is : What is your Total allowable Tolerance from the nominal frequency ? Factor 2 : Is the Crystal working in Fs or FL ( what is the CL ? ) ? This affects the Measurement error. Factor 3 : Does the circuit require frequency pulling ? ( FM, PLL, VCXO etc. require frequency pulling ) This affect the Ts ( Trimming sensitivity ) of the crystal, hence, measurement error and frequency change due to components variations. Factor 4 : Is there any frequency tuning device ( such as VC or VL ) in the circuit ? Measurement error and components variations could be tuned out during production. Hong Kong X’tals Limited 29

Error Budget ( without VC ) for Frequency Tolerance Case A : The circuit does NOT have any tuning device, such as VC or VL Note : Measurement error could be large ! Hong Kong X’tals Limited 30

Error Budget ( with VC ) for Frequency Tolerance Case B : If your circuit has some tuning device, such as VC or VL Note : If the specification of CL is wrong, you may NOT be able to tune the frequency to your requirement ! Hong Kong X’tals Limited 31

Summary : Basic Crystal Specifications · The Basic parameters of a Quartz Crystal : · Nominal Frequency, ESR, CL, Co, L 1 and C 1 · Pullability · Room Temperature and over temperature frequency tolerance · TC : Over Temperature Characteristic · AT or BT cut · Fundamental or Overtones Hong Kong X’tals Limited 32

Reliability Issues Copyright, 1998 © Hong Kong X’tals Limited. 33

Reliability Issues • Sleeping Crystals – DLD : Drive Level Dependency – Starting ESR • Drop Test • Spurious modes • Temperature Cycle Hong Kong X’tals Limited 34

Sleeping Crystals • Sleeping crystal is a well known phenomenon in Crystal Industry : – The Crystal will NOT start, but after the crystal is started by some mechanical or electrical excitation, it works fine for some time, then, after some unknown time, it sleeps again – not functioning. – The time that it begins to sleep again is unpredictable, could be minutes or months. – Not all crystal makers know how to avoid producing sleeping crystals, that’s why not many crystal makers like to talk about this. • Root Cause : Improper Cleanliness • Cure : None, a sleeping crystal will sleep again. • Test Method : DLD, or high drive ( note high drive is a test, not a cure ). Hong Kong X’tals Limited 35

DLD or Starting ESR • Same root cause as Sleeping Crystal, less serious. • If your circuit has starting problem, you should specify extra requirement of ESR at a lower drive level. • This specification depends on your circuit’s drive level, and starting gain. • Do NOT over specify, it costs. Hong Kong X’tals Limited 36

At 10 n. W Rs=37 At 100 u. W Rs=31, non-ideal process, depending on applications, could be good or no good Crystal DLD Typical Example Changes due to Design limit, not because of poor process Kolinker KH 1200 Test data Hong Kong X’tals Limited 37

Typical DLD Specification • Ask your crystal vendor the operating Drive level (DL) of your circuit, and also whether your circuit has a good starting gain. • If your circuit has a good starting gain, specify : ESR change from 1/3 DL to DL < 30% +2 Ohm • If your circuit does not has a good starting gain, specify : ESR change from 1/10 DL to DL <30% +2 Ohm • This specification also safeguard against Sleeping Crystal Hong Kong X’tals Limited 38

Drop Test • Very Important Reliability Specification • Do Not over specify, it costs ! • Typical Drop shock performance : Clip Mount : – – Below 12 MHz >= 12 MHz 50 cm 35 cm Spring Mount – – Below 16 MHz >= 16 MHz Hong Kong X’tals Limited 75 cm 50 cm 39

Spurious Modes ( Couple Modes ) • Couple modes always exist ! – Couple modes are caused by energy coupling from the main mode of vibration ( AT cut thickness shear ) to other unwanted vibration mode. Just like when you hit a guitar wire, other then the fundamental frequency, it will always produces other harmonic and inharmonic frequencies. • Must specify Minimum ratio of : ESR(worst unwanted mode) / ESR(operation) > 3 • If the unwanted modes are too active, the oscillator may : – jump from one frequency to another – produce impure spectrum that may jam your circuits. • Do Not over specify, it costs ! Hong Kong X’tals Limited 40

Couple modes at one temperature Main mode : AT Cut, Thickness Shear Couple Mode is also known as : • Spurious mode • inharmonic • unwanted mode All other couple modes are NOT AT Cut, Thickness Shear Hong Kong X’tals Limited 41

Couple modes at room temperature Good Example Main mode : AT Cut, Thickness Shear Kolinker KH 1200 Test data Hong Kong X’tals Limited All other couple modes are NOT AT Cut, Thickness Shear 42

Couple modes over temperature Main mode When Temperature Change : Frequency changes, ESR changes All other couple modes change according to the TC curves of each of their own modes Hong Kong X’tals Limited 43

The Frequency and ESR of the Main mode is affected by how the other couple modes are changing over temperature. If a crystal is not designed properly, the crystal may exhibit frequency jump or sever ESR change over temperature. Hong Kong X’tals Limited 44

Couple modes over temperature Good Example Kolinker KH 1200 TS Test data Hong Kong X’tals Limited 45

Couple modes over temperature Bad Example Kolinker KH 1200 TS Test data Hong Kong X’tals Limited 46

Temperature Cycle Good Example Kolinker KH 1200 TS Test data Hong Kong X’tals Limited 47

Temperature Cycle Bad Example Kolinker KH 1200 TS Test data Hong Kong X’tals Limited 48

Reliability Parameters that Can NOT be Screened • • • Sleeping Crystals : nearly impossible or too costly DLD : too costly Drop test : destructive in nature Spurious modes : too costly Temperature Characteristic : too costly Reliability performance of commercial crystals can NOT be 100% screened. Good Production Process and Design is the only way. Hong Kong X’tals Limited 49

Summary : • Do NOT copy specification sheets. Discuss with your vendor, and send your PCBA to them for measurement. • Crystal can NOT be screened, good crystals must be manufactured with a good process and control. Hong Kong X’tals Limited 50

Technical Support from HKC Send HKC your production failures Send us your latest PCBA for crystal specification verification. E-mail : HKXtals@Hong. Kong. Crystal. com FAX : ( 852 ) 24985908 Copyright, 1998 © Hong Kong X’tals Limited 51