1. Automatic Gain Control Circuit for Quartz Crystal Microbalance (QCM)By
Abhijat Goyal
&
Saliya Subasinghe

2. What is QCM Sauerbrey Equation Quartz Top Electrode Bottom Electrode
Addition of mass on the surface of the resonator causes changes in its resonance frequency.
Sauerbrey Equation

3. Equivalent Circuit of a QCMLm Rm
Where,
Motional Inductance representing initial mass
Static capacitance
Motional capacitance representing mechanical elasticity of quartz
Motional resistance representing support losses, viscoelastic damping, internal friction, etc.

4. Operation of QCM
In Vacuum
Rm = 100
Rm = 10000
In Water

5. Objective of the Circuit
Maintain the amplitude of oscillation to be constant.
Compensate for the change in motional resistance (Rm) due to change of ambience of the resonating crystal.
We need a circuit with feedback control which can dynamically adjust to changing Rm of the oscillator, in order to sustain oscillations.

6. Other Oscillator Circuits
COLPITTS OSCILLATOR CIRCUIT
PIERCE OSCILLATOR CIRCUIT

7. Problems with conventional oscillator circuits.
Low bandwidth for the range of motional resistances of the resonating crystal.
Crystal Stops resonating when the motional resistance increases or decreases, i.e. it works for only some value of the motional resistance.
Use of inductances and capacitances in the circuit, which alter the natural resonance frequency of the oscillating crystal.

8. Proposed circuit Simple Amplifier Mixer ½ Wave Rectifier
Unity Gain Stage
Integrator

9. Proposed circuit Offset Control Mixer Unity Gain Buffer Output Buffers
Integrator

10. Chip Spec
Design Objective – Maximize the range of Motional Resistance of the quartz crystal that our circuit can handle (1 Ohms – 1 MOhms)
Operating Frequency – Determined by Quartz Resonator (max 100 MHz)
Max Power – 500 mW
Area – 1x1 mm2
Global Pins – Vdd, gnd
Input Pins – to be connected across the QCM
Output Pins – Resonance Frequency, Feedback Amplitude (measure of Q factor)

11. Schedule Layout and Optimization of Unity Gain Op-amp Week 1 (Feb 12)
Layout and Optimization of Mixer Cell
Week 3 (Feb 26)
Layout and Optimization of Offset Control Circuit
Week 4 (Mar 5)
Layout and optimization of Output Buffers
Week 5 (Mar 12)
Layout and optimization of Integrator
Week 6, 7, & 8 (Mar 23)
Integration of the design and debugging

12. Questions?