Quartz Crystal Microbalance Digital Controller

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22 Theory, Operation and Calibration Chapter 2Resistance MeasurementQCM200 Resistance OutputsThe QCM200 system is a stand-alone instrument with a built-in frequency counter andresistance meter. Series resonance frequency and resistance are measured and displayeddirectly on the front panel without a need for an external frequency counter or precisionvoltmeter.Resistance Display (Absolute and Relative Resistance)Range0 to 5000 Resolution 5 digits: 0.001 for R < 100 0.01 for 100 R < 1000 0.1 for 1000 R < 5000 Conductance Output V c * (front panel)Resistance CalculationR = 10,000 x (10 -Vc/5 ) - 75 Resistance range0 to 5000 Impedance1 kVoltage level0 to 10.625 VDC, log scaleConnectorBNC*If desired (due to existing system integration or other issues), the Conductance 7 Voltageoutput (V c ) may be read with a high-precision digital voltmeter 8 with at least 6 digits ofresolution and a computer interface. The calculation of R m is then performed by thecomputer.QCM200 Quartz Crystal Microbalance
Chapter 2 Theory, Operation and Calibration 23Resistance CalculationR sGains:A a x A 1 x A t x A n x A 2 x A f = 1R mR LATTN2:1+ LPFAGC-50dB/VX5 X2.5V cFigure 20. QCM Gain ModelA gain model of the QCM25 Crystal Oscillator is shown in Figure 20. At seriesresonance the reactance of the crystal’s motional inductance and motional capacitancecancel and so the crystal may be represented by just R m , the motional resistance of thecrystal. (It is also assumed that the static capacitance, C o , has been nulled as previouslydescribed.) The circuit will oscillate at the frequency for which the net phase shift aroundthe loop is 360°, provided there is sufficient gain to overcome the circuit losses.Two network elements in the model provide a phase shift. The inverting amplifierprovides 180° of phase shift. The low pass filter is adjusted to provide 180º of phase shiftat 5 MHz. Hence the circuit will oscillate at the frequency for which the crystal isresistive, i.e. at series resonance.The loop gain is the product of the gain (or attenuation) of each of the network elements.If the loop gain is exactly one, then the oscillation amplitude will be maintained at a fixedlevel. An AGC circuit controls the loop gain via a voltage controlled variable attenuator.From left to right, the circuit consists of the following:1. A voltage controlled variable attenuator with attenuation of A a . An automatic gaincontrol circuit generates a voltage to maintain the oscillation amplitude at a fixedlevel of 1 V pp . The attenuator is controlled by this voltage, between 0 and 1 Vdc,providing an attenuation of 50 dB/volt so that A a =10 -Vagc˙50 / 20 . The AGC voltage isamplified by 5x in the QCM25 Crystal Oscillator, and by 2.5 x in the QCM200Controller, before being output via the Conductance BNC on the front panel of theQCM200. Hence, referenced to the voltage V c at the QCM200 Conductance outputBNC, A a = 10 -Vc/5 .2. A fixed gain amplifier with gain A 1 = 45 dB + 20 log (250/200) = 46.94 dB (or 222x.) This inverting amplifier has a bandwidth of 500 MHz, and so introduces verylittle extraneous phase shift.QCM200 Quartz Crystal Microbalance
Page 1 and 2: Operation and Service ManualQCM200Q
Page 3 and 4: Safety and Preparation For UseiSafe
Page 5 and 6: ContentsiiiContentsSafety and Prepa
Page 7 and 8: OverviewvFront Panel OverviewFigure
Page 9 and 10: OverviewviiBack Panel OverviewFigur
Page 11 and 12: OverviewixQCM25 Crystal Oscillator
Page 13 and 14: SpecificationsxiSpecificationsQCM20
Page 15 and 16: Getting Started 1Chapter 1Getting S
Page 17 and 18: Chapter 1 Getting Started 3Quick St
Page 19 and 20: Chapter 1 Getting Started 5Paramete
Page 21 and 22: Chapter 1 Getting Started 7Liquid S
Page 23 and 24: Chapter 1 Getting Started 98. Place
Page 25 and 26: Chapter 1 Getting Started 11Flow Ce
Page 27 and 28: Chapter 1 Getting Started 13Operati
Page 29 and 30: Theory, Operation and Calibration 1
Page 31 and 32: Chapter 2 Theory, Operation and Cal
Page 35: Chapter 2 Theory, Operation and Cal
Page 63 and 64: Sensor Crystals and Holders 49Chapt
Page 65 and 66: Chapter 3 Sensor Crystals and Holde
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Chapter 3 Sensor Crystals and Holde
Page 89 and 90:
QCM Circuit Description 75Chapter 4
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Chapter 4 QCM Circuit Description 7
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Frequency Counter Selection Criteri
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QCM200 Remote Progamming 101Appendi
Page 117 and 118:
Appendix B QCM200 Remote Programmin
Page 119 and 120:
Appendix B QCM200 Remote Programmin
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Quartz Crystal Microbalance Digital Controller

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