automatic gain control circuit for quartz crystal microbalance (qcm) by abhijat goyal & saliya...
TRANSCRIPT
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.
mA
ffqq
202Sauerbrey Equation
Equivalent Circuit of a QCM
C0
Cm Lm Rm
662
2268
ct
AAeC
qm
226
3
8Ae
tL qqm
226
2
8Ae
tR qqm
qt
AC 220
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.
-1000
0
1000
2000
3000
4000
5000
6000
7000
19.56 19.565 19.57 19.575 19.58 19.585 19.59 19.595 19.6 19.605 19.61Millions
Frequency (MHz)
Rs
(a
rbit
ary
un
its
)
immediately after water addition
after 29 minutes and 50 seconds
after 34 minutes
in vacuum
Operation of QCMIn Vacuum
In Water
Rm = 100
Rm = 10000
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.
Other Oscillator Circuits
PIERCE OSCILLATOR CIRCUIT
COLPITTS OSCILLATOR CIRCUIT
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.
Proposed circuit
Unity Gain Stage
Mixer Simple Amplifier
Integrator
½ Wave Rectifier
Proposed circuitOffset Control
Integrator
Mixer
Output Buffers
Unity Gain Buffer
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)
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)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
Questions?