Quartz Crystal Microbalance Digital Controller

20 Theory, Operation and Calibration Chapter 2Frequency MeasurementQCM200 Frequency 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 the need for an external frequency counter or precisionvoltmeter. There is also an analog output proportional to the Relative Frequency whichcan be used to interface with potentiostats (i.e. for EQCM applications). The FrequencyOutput port (BNC) on the front panel is for direct connection to an external frequencycounter (if desired).Frequency Measurement (Internal Reference)Accuracy±1.5 ppmStability0.01 Hz for 1 sec0.05 Hz for 10 sec or longerFrequency Analog Out (back panel, DC)Range (full scale)±10 VResolution20-bitScales (V out = Relative Frequency/Scale) 20 kHz/V, 10 kHz/V, 5 kHz/V, 2 kHz/V,1kHz/V, 500 Hz/V, 200 Hz/VFrequency Output* (front panel, AC)Frequency5 MHz (nominal)LevelTTL (square wave)Source impedance50 ConnectorBNC*If desired (due to existing system integration or other issues), an external frequencycounter may be used to measure the frequency of the square wave provided at the frontpanel Frequency Output. Consult Appendix A for recommendations on the selection offrequency counters for QCM measurements.Error AnalysisThe QCM25 Crystal Oscillator will oscillate at the frequency such that the phase shiftaround the entire loop is 360º. Referring to Figure 20, important sources of phase shiftwithin the network include:1. 180º from the inverting amplifier A 12. 180º + [37 µº/Hz deviation from 5 MHz] from the low pass filter3. 0º + [0.20º/pF (R m = 40 ) or 0.81º/pF (R m = 375 )] from uncompensated C o4. 0º + [0.20º/Hz (R m = 40 ) or 0.04º/Hz (R m =375 ) deviation from seriesresonance] from the crystal in the R s / R m / R L network.If an extraneous phase shift (2) or (3) is present, the oscillator will operate away fromseries resonance allowing the crystal network (4) to cancel the extraneous phase shift.QCM200 Quartz Crystal Microbalance