Automatic Gain Control Circuit for Quartz Crystals 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

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

So what do we need?  A mixer  A very good high frequency unity gain buffer  A CMOS based modified half wave rectifier which can not only rectify the incoming signal but can also sense the amplitude of the AC wave.

Rectifier Stage

Results of Rectifier Stage

Integrator

Results of Integrator Simulation (DC)

Integrator combined with Rectifier

Results of Simulation

Variable Gain Amplifier

Results of the Simulation

High frequency unity gain buffer  No DC level shift (could not achieve)  Operation till 100 MHz  Simple Architecture

Results of Simulation

Complete Circuit

It works … (we do not know how???)

Conclusion  An Automatic Gain Control circuit has been realized.  Designing the circuit for operation from 0 to 5 V was a challenge, particularly since the quiescent operating point was not 0V, but 2.5V.  The circuit has a dynamic range of 1.5 to 4.8 volts (in terms of feedback signal).  It can take care of equivalent resistances from 1 ohms to 1M ohms making it a versatile circuit for application with quartz resonators which are used as sensors in aqueous and highly dissipative viscous environments.

Objectives  Design Objective – Maximize the range of Motional Resistance of the quartz crystal that our circuit can handle (1 Ohms – 1 MOhms) (DESIGN OBJECTIVE ACCOMPLISHED)  Operating Frequency – Determined by Quartz Resonator (max 100 MHz) (DESIGN OBJECTIVE ACCOMPLISHED)  Max Power – 500 mW (NOT ACCOMPLISHED – POWER = 3200 W)  Area – 1x1 mm 2 (NOT LAID OUT)  Global Pins – Vdd, gnd (DESIGN OBJECTIVE ACCOMPLISHED)  Input Pins – to be connected across the QCM (DESIGN OBJECTIVE ACCOMPLISHED)  Output Pins – Resonance Frequency, Feedback Amplitude (measure of Q factor) (DESIGN OBJECTIVE ACCOMPLISHED)

Future Work  We have to lay out the circuit.  We have to optimize it further for making the circuit rail to rail, from 0V to 5V.  We have to find ways to reduce power dissipation in the circuit to make it useful for applications in which the sensor is used in the field as a hand held device.  Further work can be done to increase the maximum operating frequency of the circuit.