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8/9/2019 Saw Devices

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SAW DEVICES

Presented by:- Ashish Shavarna

Dept. of Electronics & Communication Engineering, JIIT

University,Noida-324005,INDIA.


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Outlines of Presentation:

Introduction

Problem Modeling

SAW Resonators

SAW Oscillators


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Introduction SGAW sensors utilize mechanical wave as sensing mechanism on the

piezoelectric substrate. Both direct & converse piezoelectric effect is taken

into consideration.

SGAW sensors involve SAW & APM devices

SAW device : wave propagate on the surface of substrate, waves have

longitudinal as well as shear component which can couple with the

medium placed in contact to the surface. Highly sensitive becauseacoustic energy is confined within one wavelength.

APM stand for acoustic plane mode, here energy of wave is confined

b/w upper & lower plate of the substrate. Therefore detection on

either side, preferably isolating the side containing IDT.

SAW sensor SH-APM sensor


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Wave propagation

Rayleigh wave exhibits both longitudinal as well as transverse component,

which can be seen in the figure.


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Problem Modeling

Designing a high Q-SAW resonator

- Resonator Geometries & modes

- Resonators specifications

- Analytic method to design

- SAW resonator using layered structure

Designing a Single-mode Fixed-Frequency SAW oscillator with high

spectral purity & frequency stability.


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SAW Device


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Components of SAW Devices

IDT (Inter Digital Transducer)


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SAW Resonator

Prime requirement of surface wave resonator is an efficient reflector of

waves. Since, there is no practical localized mirror available for Surface

Waves; so E.A. Ash introduced the concept ofreflection grating.

With reflection grating, SAW resonator offers less insertion loss & high

quality factor .Loss mechanism that limits Q value includes SAW to BAW

conversion in gratings, resistive loss in electrodes & viscous damping insubstrate.

Centers of reflection in gratings are at the edges of grating elements while

in excited IDT, they are the mid finger position.


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SAW Resonator

Transducer geometries for one &two port resonator circuit:

In grating reflector, scattering of SAW into bulk waves will result in

increased insertion losses.

Power Handling capability of SAW resonators are limited because

excessive SAW power levels lead to a frequency shift of resonator &/or

destructive failure

Single mode operation can be achieved by reducing the rsonant spacing

between IDT to support only one mode within the width of grating stop

band.


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SAW Resonator

For design of SAW Resonator:

- use of Scattering Matrix {S}

- Cavity Analysis

- Coupling of Modes (COM) Analysis

One COM approach employing an unweighted 2X2 complex grating matrix

{G} , in conjunction with a 3X3 transducer matrix {T} and a transmission

line matrix {D}.

Here, Grating Reflectors can also introduce undesirable transverse mode

responses which can be eliminated by use of tapered gratings.


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SAW Oscillator

They are finding increasing applications because of high spectral purity,

large fundamental frequency range, low power drain etc.

Feedback system for an oscillator

The oscillation frequency of SGAW oscillator delay line ,fn =n. v / L where

L is the acoustic path length, v is the phase velocity. Modification of thephase delay lines can be measures as frequency shifts using the above

equation.

Although very precisely measures the acoustic wave velocity, but do not

provide any information about signal amplitude


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SAW Oscillators

SAW oscillators using high Q Resonators yield the best stability

performance while SAW oscillators employing delay lines have stability

performances at higher frequencies ( > 1 GHz).

The aging rate of SAW Resonator in SAW oscillator is a function of

operating power level. To reduce the effect of migration of the IDT at higher power level, we use

AlCu alloy film rather than pure Al in the IDT itself.

Low phase-Noise performance can be achieved using high Q free running

SAW oscillator.

An alternative of attaining low noise performance is by injection locking anoscillator with poor phase noise to a low noise source.

With the above implementation, the overall noise performance is entirely

that of the injection source


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SAW Oscillator

Source of noise in oscillator includes:

- White phase (f0 )

- Flicker phase (1/f)

- White frequency (1/f2)- Flicker frequency (1/f3)

- Random Walk (1/f4)

White & Flicker frequency are due to perturbations within the bandwidth

of the feedback loop caused by White & Flicker phase noise. Random Walk

noise generates long term aging.

Most predominant noise in SAW devices is flicker phase one. However,

SAW resonators are invariably found to have 1/f2 also.

The level of noise in Quartz resonator depends on transducer

metallization used as well as film stress


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