Glossary

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A

Acoustic absorber

An element is made of material with high acoustic loss. This element is placed on a definite part of piezoelectric for acoustic absorption purposes.



Aperture of transducer

Maximum IDT finger overlap length.



Apodization of IDT

Response weighting produced by the change of finger overlap along the length of the IDT to suppress the transverse spurious mode.



Attenuation characteristic

The dependence of the attenuation on frequency.

 
 
 

B

Band-pass filter

Filter having a single pass band between two specified stop bands.



Band-stop filter

Filter having a single stop band between two specified pass bands.



Bandwidth

A specified frequency range over which the amplitude response does not vary more than a defined amount.



Bulk wave signal

Unwanted signal due to bulk-wave excitation.

 
 
 

C

Comb filter

Filter having two or more pass-bands between three or more stop-bands.



Center frequency (f0)

Frequency of the pass band at which the transfer function is at a maximum. The center frequency is determined by the period of IDT fingers and the acoustic velocity.



Cut-off frequency

Frequency of the pass band at which the relative attenuation of a piezoelectric filter reaches a specified value.

 
 
 

E

Electode of IDT

Finger of IDT.

 
 
 

F

Feed through signal

Unwanted signals from the input appearing at the filter output due to stray capacitances and other electromagnetic couplings.



Finger of the IDT

An element of the IDT comb structure.



Finger overlap

An element of the IDT comb structure.



Finger overlap

The length of a finger pair between which electromechanical interaction takes place.

 
 
 

G

Group delay

Time equal to the first derivative of the phase difference between input and output waves, expressed in radians, with respect to the angular frequency of an acoustic device.

 
 
 

I

Insertion attenuation

The logarithmic ratio of the power, current of voltage delivered to the load impedance before insertion of the filter to the power, current of voltage delivered to the load impedance after insertion of the filter.



Insertion Loss

The logarithmic ratio of the input pulse power to the output power of the main pulse expressed in decibels.



Interdigital transducer (IDT)

A comb-like conductive structure that is deposited on a piezoelectric substrate and consists of interleaved metal electrodes whose function is to transform electrical energy into acoustic energy or vice versa by means of the piezoelectric effect.

 
 
 

M

Multistrip coupler (MSC)

An array of metal strips disposed on a piezoelectrically active substrate with lengths transverse to the propagation direction. In this case the propagating wave is displaced by along its propagation path and acoustic power is transferred from one acoustic path to an adjacent parallel path.

 
 
 

P

Passband width (Δf)

Band of frequencies in which the relative attenuation of a piezoelectric filter is equal to, or less than, a specified value.



Passband ripple

Difference between the peak value and the minimum value of attenuation within the pass band of a filter.



Piezoelectric surface acoustic wave filter

filter characterized by a surface acoustic wave, usually generated by an IDT, and propagating along a substrate surface to a receiving transducer.



Phase delay

time of propagation of a sinusoidal oscillation of a certain frequency between two points.

 
 
 

R

Reflection coefficient


dimensionless measure of the degree of mismatch between two impedances Za and Zb given by the expression

Reflection coefficient





where Za and Zb represent respectively the input and source impedance or the output and load impedance.



Relative attenuation

difference between the attenuation at a given frequency and the attenuation at the reference frequency

 
 
 

S

SAW filter

A filter characterized by the use of surface acoustic waves that are usually generated by an interdigital transducer and propagate along a substrate surface to a receiving transducer.



SAW reflector

SAW reflecting component which normally makes use of the periodic discontinuity provided by a metal strip array (shorted metal strip array or open metal strip array) or a grooved array.



SAW resonator

A type of SAW filter with a high quality factor Q(f0/Δf).



Sidelobes

Undesired amplitude responses that lie outside a specified frequency or time interval.



Shape factor

ratio of the two bandwidths of a band-pass filter or a band-stop filter limited by two specified attenuation values.



Stop band

band of frequencies in which the relative attenuation of a piezoelectric filter is equal to, or greater than, specified values.



Surface acoustic wave (SAW)

An acoustic or Rayleigh, propagating along a surface of a substrate whose amplitude decays exponentially with substrate depth.



Surface acoustic wave coupling coefficient


Reflection coefficient





the electromechanical coupling coefficient, where Δv/v is the relative change in acoustic wave velocity produced by short-circuiting the surface potential from the open circuit condition.

 
 
 

T

Transmission factor

the ratio of voltage and current at the output load resistance of the piezoelectric filter to the voltage (current) at the input load resistance.



Triple transit echo

unwanted signals in a SAW device, which have traversed three times the propagation path between input and output IDT’s and are arise from reflection of the wave at the output and input transducers.

 
 
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