Oscillator design guidelines for implantable medical device applications Flavien Heu+er Sales & Applica+ons Engineer 1 EFCC 2015
Agenda o Introduc/on o Key parameters in specifying quartz crystals o Suggested screenings for quartz crystals 2
3 INTRODUCTION
Evolu/on of the quartz crystals for medical implantable device applica/ons CX1 (8.0 x 3.5mm) CX3 (6.6 x 2.4mm) CX4 (5.0 x 1.5mm) 1973 1983-86 1998 4 First Lithium powered And Non invasive Programmable (Telemetry) pacemaker Pacemaker with Temperature and Motional sensors 3 chamber pacing implemented
Evolu/on of the quartz crystals for medical implantable device applica/ons CX9 (4.1x1.5mm) CX11 (3.2x1.5mm) CX16 (2.0 x 1.2 mm) CX18 (1.55 x 0.95 mm) 2005 2009 2010-2013 5 Medical Implantable Communication Service (MICS) devices Retinal Implants BlueTooth Low Energy (BLE) and new generation of MICS. 1 st Leadless pacemaker
Quartz Crystal theory: Modes of Mo/on Flexure Mode Extensional Mode Face Shear Mode 6 Thickness Shear Mode Fundamental Mode Thickness Shear Third Overtone Thickness Shear
7 Tuning Fork Crystal Flexure mode
8 AT-cut crystal Thickness shear mode
9 Quartz Crystal Construc/on
Agenda o Introduc/on o Key parameters in specifying quartz crystals o Suggested screenings of quartz crystals 10
KEY PARAMETERS IN SPECIFYING QUARTZ CRYSTALS 11
Key parameters in specifying quartz crystals o Load capacitance o Gain o Drive level 12
Key parameters in specifying quartz crystals o Load capacitance o Gain o Drive level 13
Crystal Model and C L Symbol for crystal unit C L Total Reactance = X + X CL = 0 C 0 Static Arm Motional Arm C L 14 C 1 L 1 R 1
F L = F s 1+ Load Capacitance F L at C L C1 2( C + C 0 L ) + Area of usual operation in an oscillator TS = 1 F df L dc L = C 1 2(C 0 + C L ) 2 Reactance 0 Resonance, f r Antiresonance, f a Frequency TS is the Trim Sensi/vity - 15 1 2πfC 0
How to measure C L? 1) Measure the crystal parameters C 1, C 0, and F s with a network or impedance analyzer. 2) Install the measured crystal in the oscillator circuit and measure the oscillation frequency f 3) Then calculate the load capacitance C L. For example with F s = 32.7644 khz C 1 = 2.3 ff C 0 = 1.5 pf f = 32.768 khz we find C L = 9.0 pf. C L = C 1 2 F s C 0 f F s 16
Impact of incorrect C L Considering a 32.768 khz CX11 crystal : C 0 = 1.0 pf, C 1 = 3.5 ff, calibrated for C L1 = 9 pf. Customer circuit at C L2 = 11 pf, what is the change in frequency? 17 29 ppm 1 1 2 0 1 0 2 1 1 2 = + + = C C C C C F F F L L L L
Importance of C L Right choice of C L for proper operating frequency Crystal with small load capacitance starts faster Large load capacitors increase the power consumption 18
Key parameters in specifying quartz crystals o Load capacitance o Gain o Drive level 19
Pierce Oscillator R f Fout Amplifier R A C G Crystal resonator C D C d 20
Gain Formula g m 4π 2 f 2 C G ( C D + C d )R e + (C d + R e C D )R A R O + C G C D (1+ R A R O )+ C d 1 R O 4π 2 f 2 C d C D R A 1+ R A + R e R O 4π 2 f 2 C d C D R A R e L 1 C 1 R 1 Oscillator AC equivalent circuit with the crystal I D electrical equivalent circuit R O X D C 0 = C 0 +C s X G 21
Gain Main part of the Formula g m 4π 2 f 2 C G C D R e + C G C D R O 1+ R e R O R f Fout R e L ' 1 C0 1 ' R + CL C + C = C ' L s 2 Xe Re I R O D ' C C + 0 = 0 C s X D C G X G CX R A C D C d 22 Pierce Oscillator AC Equivalent Circuit (R A omitted)
Compare minimum gain to drive sealed and unsealed 32.768 khz CX11 crystal Sealed Unsealed g m = 4π 2 f 2 C D C G R e + C G /(C D R o )(1+R e /R o ) f Hz C D pf C G pf R 1 Ω R e R1(1+C 0' /C L' ) 2 R e Ω R o MΩ C L =C L' +C s C L pf C s pf C 0' =C 0 +C s C 0 pf g m 1 g m 2 g m g m 1 g m 2 g m g m µs (µmho) 0.61 0.54 1.15 5.46 0.82 6.28 32'768 32'768 20 10 20 10 50'000 445'000 72'359 643'992 1 1 7 7 0.3 0.3 1.06 1.06 23 6.97
Key parameters in specifying quartz crystals o Load capacitance o Max allowed crystal resistance o Drive level 24
Drive Level Defini/on Crystal drive level is a measure of how hard the crystal is being driven. It can be defined as the current passing through the resonator, but is most o_en defined as the power dissipated within the resonator. The maximum allowed drive level is given by the crystal manufacturer and is dependent upon the physical size, geometry, mode, and frequency of the resonator. 25
Max Drive Level for CX11 Tuning Fork CX11 32.768 khz : 0.5 μw max AT-cut CX11 24.0 MHz : 200 μw max 26
Drive-Level Dependency Crystal drive-level dependency (DLD) refers to the dependency of the quartz crystal parameters (frequency and series resistance) to the with current through the crystal. High drive could irreversibly damage a quartz crystal 27
Drive-Level Dependency Resistance R 1 Anomalous starting resistance Normal operating range Drive level effects 28 10-3 10-2 10-1 1 10 100 I X (ma)
Drive-Level Dependency Even with the steps we take to mi/gate this, there can s/ll be some parts that exhibit DLD behavior. We recommend customers requiring some levels of tes/ng to ensure reliable start-up of the oscillator. 29
SUGGESTED SCREENINGS FOR QUARTZ CRYSTALS 30
Suggested Screenings for Quartz Crystals o DLD Testing o Thermal Shock o Hermeticity Testing o PIND Testing 31
DLD Tes/ng 35 Purpose : To remove crystals that might have start-up issues in the oscillator. Method : Measure the crystal from a low drive level to the nominal drive level and back, looking for maximum resistance and changes in frequency and resistance.
Thermal Shock 36 Purpose : To determine Resistance of the component to expose at extreme temperature Per MIL-STD-202, Method 107 Method : Stress the mechanical structure (package and quartz bonding) and check effect on crystal parameters. -55 C to +125 C / 5 cycles
Herme/city Tes/ng Purpose: Determine the effec/veness of seal process detec/ng poten/al herme/city issue through ceramic, preform or seal rim. Fine and Gross Leak Tes/ng : Per MIL-STD-202, Method 112 Method : Parts under high pressure before measuring leakage 37
PIND Tes/ng Purpose : Non-destruc/ve test to detect loose par/culates, if any, inside a component cavity. Method : Detect loose par/culates of sufficient mass acous/cally upon impact with the cavity. Par/cle Impact Noise Detec/on per Mil-STD-883, Method 2020 Condi/on A 38 Need special test environment to reduce ambient and external noises.
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