INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 16)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
GENERAL DESCRIPTION 1-2
1.1.4 MULTIPLE CRYSTAL MEASUREMENT CHANNELS
The RQCM can be configured to measure up to three crystal measurement channels
simultaneously.
1.1.5 ELECTRODE CAPACITANCE CANCELLATION
The total quartz crystal impedance includes a shunt capacitance (due to the capacitance of
the crystal electrodes, cable and holder) in parallel with the series resonant arm. The total
current through the crystal is the sum of the current through the shunt capacitance plus
the current through the series resonant arm. The physical motion of the crystal is reflected
in the values of the L, R and C in the series arm of the crystal only, and therefore we want
to subtract out or otherwise cancel the current through the shunt electrode capacitance.
The Crystal Measurement Card includes a method of canceling the electrode capacitance
insuring that the measured crystal current does not include the current through the
electrode capacitance and therefore is essentially the current through the series resonant
arm of the crystal only.
1.1.6 “AUTOLOCK”
When the PLO loses lock, the VCO (Voltage Controlled Oscillator) is ramped up to the
maximum frequency at which time it is automatically reset to the minimum frequency
and a new scan is initiated.
To insure that the VCO ramps up in frequency, a small amount of quadrature current is
injected into the current to voltage buffer whenever the PLO is unlocked. This current is
equivalent to a shunt capacitance of about 1.5 pfd. As soon as lock is detected, the
quadrature current is turned off.
1.1.7 CRYSTAL FACE ISOLATION
The Crystal face is galvanically (transformer) isolated from earth ground. The Crystal
Face connection allows the crystal face electrode to be easily connected to an external
voltage or current source such as a potentiostat.
1.1.8 FULLY INTERGATED COMPUTER SOFTWARE
Computer software is included with each RQCM, allowing the user to set up, graph and
log frequency and resistance of the crystals from a computer. It also allows the setup,
graphing and logging of temperature and analog data – if the hardware is installed.
1.1.9 INPUTS AND OUTPUTS CAPABILITY
As an option, the RQCM can be outfitted with an I/O Card. This card provides eight
remote discrete inputs and eight relay outputs. These I/O’s can be used to monitor or
control external instruments and peripheral devices.
1.1.10 DATA ACQUISITION
To support the simultaneous logging and display of additional analog information, such
as voltage, current, or temperature, the RQCM can be outfitted with an optional Data
Acquisition Card. This card supports three types of temperature sensors (RTD,
Thermocouple and Thermistor) as well as five scalable analog inputs.