Automatic Gain Control Circuit for Quartz Crystal Microbalance

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

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

Equivalent Circuit of a QCM Cm Lm Rm C 0 Where, Static capacitance Motional Inductance representing initial mass Motional capacitance representing mechanical elasticity of quartz Motional resistance representing support losses, viscoelastic damping, internal friction, etc.

Operation of QCM In Vacuum Rm = 10000 In Water

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

Other Oscillator Circuits COLPITTS OSCILLATOR CIRCUIT PIERCE OSCILLATOR CIRCUIT

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

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

Proposed circuit Offset Control Mixer Unity Gain Buffer Output Buffers Integrator

Chip Spec p Design Objective – Maximize the range of Motional Resistance of the quartz crystal that our circuit can handle (1 Ohms – 1 MOhms) p Operating Frequency – Determined by Quartz Resonator (max 100 MHz) p Max Power – 500 m. W p Area – 1 x 1 mm 2 p Global Pins – Vdd, gnd p Input Pins – to be connected across the QCM p Output Pins – Resonance Frequency, Feedback Amplitude (measure of Q factor)

Schedule Week 1 (Feb 12) Layout and Optimization of Unity Gain Op-amp Week 2 (Feb 19) Layout and Optimization of Mixer Cell Week 3 (Feb 26) Layout and Optimization of Offset Control Circuit Week 4 (Mar 5) Week 5 (Mar 12) Week 6, 7, & 8 (Mar 23) Layout and optimization of Output Buffers Layout and optimization of Integrator Integration of the design and debugging

Questions?