IEC 60862-2:2012

Surface acoustic wave (SAW) filters of assessed quality - Part 2: Guidelines for the use

IEC 60862-2:2012 gives practical guidance on the use of SAW filters which are used in telecommunications, measuring equipment, radar systems and consumer products. IEC 60862-1 should be referred to for general information, standard values and test conditions. This part of IEC 60862 includes various kinds of filter configuration, of which the operating frequency range is from approximately 10 MHz to 3 GHz and the relative bandwidth is about 0,02 % to 50 % of the centre frequency. It is not the aim of this standard to explain theory, nor to attempt to cover all the eventualities which may arise in practical circumstances. This standard draws attention to some of the more fundamental questions, which should be considered by the user before he places an order for a SAW filter for a new application. Such a procedure will be the user's insurance against unsatisfactory performance. This edition includes the following significant technical changes with respect to the previous edition:
- Clause 3 "Terms and definitions" has been deleted to be included in the next edition of IEC 60862-1;
- the tapered IDT filter and the RSPUDT filter have been added to the clause of SAW transversal filters. Also DART, DWSF and EWC have been added as variations of SPUDT;
- the balanced connection has been added to the subclause of coupled resonator filters;
- recent substrate materials have been described;
- a subclause about packaging of SAW filters has been added.

Filtres à ondes acoustiques de surface (OAS) sous assurance de la qualité - Partie 2: Lignes directrices d'utilisation

La CEI 60862-2:2012 indique des lignes directrices concernant l'emploi pratique des filtres à OAS pour l'utilisation dans les télécommunications, dans les équipements de mesure, les systèmes radars et les produits de grande consommation. Il convient de se reporter à la CEI 60862-1 pour les informations générales, les valeurs normalisées et les conditions d'essais. Cette partie de la CEI 60862 englobe les divers types de configuration de filtre dont la gamme de fréquences de fonctionnement s'étend approximativement de 10 MHz à 3 GHz et la largeur de bande relative est d'environ 0,02 % à 50 % de la fréquence centrale. Cette norme n'a pas pour but de développer des notions théoriques ni de couvrir tous les cas qui peuvent se poser en pratique. Cette norme attire l'attention sur certaines questions plus fondamentales qu'il convient que l'utilisateur examine avant de commander un filtre à OAS pour une nouvelle application. Une telle procédure servira de garantie à l'utilisateur en cas de fonctionnement non satisfaisant. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
- l'Article 3 "Termes et définitions" a été supprimé en vue d'être inclus dans la prochaine édition de la CEI 60862-1;
- le filtre à TID conique et le filtre à TUDMR ont été ajoutés à l'article des filtres transversaux à OAS. De même, les DART, DWSF et EWC ont été ajoutés en tant que variantes de TUDM;
- la connexion symétrique a été ajoutée au paragraphe des filtres à résonateurs couplés;
- les matériaux récents pour substrats ont été décrits;
- un paragraphe relatif à l'emballage des filtres à OAS a été ajouté.

General Information

• Status: Published
• Publication Date: 06-May-2012
• ICS: 31.140 - Piezoelectric devices
• Technical Committee: TC 49 - Piezoelectric, dielectric and electrostatic devices and associated materials for frequency control, selection and detection
• Drafting Committee: WG 10 - TC 49/WG 10
• Current Stage: PPUB - Publication issued
• Start Date: 30-Apr-2012
• Completion Date: 07-May-2012

Relations

• Revises: IEC 60862-2:2002 - Surface acoustic wave (SAW) filters of assessed quality - Part 2: Guidance on use
• Effective Date: 05-Sep-2023

Overview

IEC 60862-2:2012 is an international standard providing guidelines for the use of Surface Acoustic Wave (SAW) filters of assessed quality. These filters play a crucial role in various industries such as telecommunications, radar systems, measurement devices, and consumer electronics. This standard focuses on practical application advice rather than theoretical explanations, helping users optimize SAW filter performance in real-world scenarios.

Covering a broad operating frequency range from approximately 10 MHz to 3 GHz, and relative bandwidths from 0.02% to 50% of the center frequency, IEC 60862-2:2012 complements Part 1 of the standard by emphasizing filter configurations, substrate materials, packaging, and ordering procedures for effective deployment.

Key Topics

• SAW Filter Configurations
  The standard details multiple types of SAW filters, including:

  • Bidirectional and unidirectional interdigital transducer (IDT) filters
  • Tapered IDT and Reflector filters
  • RSPUDT (Reduced Spurious Pulse Unidirectional Transducer) filters
  • Variants like DART, DWSF, and EWC as variations of SPUDT
    Additionally, balanced connection designs are introduced for coupled resonator filters, enhancing filter performance and adaptability.

• Substrate Materials
  Guidance on the selection of substrate materials covers single-crystal, thin-film, and ceramic substrates. Recognizing their impact on conversion loss, temperature stability, and bandwidth informs optimal filter design.

• Packaging Techniques
  The standard discusses packaging options including metal, ceramic, resin, and chip-scale packages (CSP). Packaging influences mechanical protection, electrical performance, and environmental robustness of SAW filters.

• Technical Considerations and Ordering
  Users are advised on critical technical factors like impedance matching, feed-through signals, input levels, and soldering conditions. The standard also recommends a detailed ordering procedure to minimize the risk of unsatisfactory filter performance.

Practical Applications

IEC 60862-2:2012 provides actionable insights to deploy SAW filters effectively across diverse applications including:

• Telecommunications
  SAW filters are vital in mobile communication systems and radio transmission, offering precise frequency selection and signal filtering in base stations and handsets.

• Radar Systems
  Filters operate within microwave frequency ranges ensuring signal clarity and accuracy for radar measurement and tracking.

• Measuring Equipment
  Precise filtering enhances accuracy and stability in test and measurement apparatus, ensuring reliable data collection.

• Consumer Electronics
  From television IF filters to wireless devices, SAW filters contribute to clear signal processing with compact form factors.

Implementing the guidelines helps engineers achieve optimal signal performance, reduce insertion loss, and ensure long-term reliability.

Related Standards

• IEC 60862-1: Contains general information, standard values, and test conditions for SAW filters, which should be referred to alongside Part 2 for a comprehensive understanding.

• Other IEC and international standards covering electronic component testing, packaging, and environmental requirements may also complement IEC 60862-2 implementations.

---

By adhering to IEC 60862-2:2012, manufacturers and users of SAW filters can leverage standardized best practices that foster compatibility, reliability, and superior device performance across critical electrical and electronic applications.

Standards Content (Sample)

IEC 60862-2 ®
Edition 3.0 2012-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Surface acoustic wave (SAW) filters of assessed quality –
Part 2: Guidelines for the use

Filtres à ondes acoustiques de surface (OAS) sous assurance de la qualité –
Partie 2: Lignes directrices d’utilisation

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IEC 60862-2 ®
Edition 3.0 2012-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Surface acoustic wave (SAW) filters of assessed quality –

Part 2: Guidelines for the use

Filtres à ondes acoustiques de surface (OAS) sous assurance de la qualité –

Partie 2: Lignes directrices d’utilisation

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XB
ICS 31.140 ISBN 978-2-88912-074-1

– 2 – 60862-2 © IEC:2012
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Technical considerations . 8
4 Fundamentals of SAW transversal filters . 9
4.1 Frequency response characteristics . 9
4.2 Weighting methods . 10
4.3 Filter configurations and their general characteristics . 13
4.3.1 General . 13
4.3.2 Bidirectional IDT filters . 14
4.3.3 Unidirectional IDT (UDT) filters . 15
4.3.4 Tapered IDT filters . 22
4.3.5 Reflector filters . 23
4.3.6 RSPUDT filters . 27
5 Fundamentals of SAW resonator filters . 30
5.1 Classification of SAW resonator filters . 30
5.2 Ladder and lattice filters . 30
5.2.1 Basic structure . 30
5.2.2 Principle of operation . 31
5.2.3 Characteristics of ladder and lattice filters . 32
5.3 Coupled resonator filters . 35
5.3.1 General . 35
5.3.2 Transversely coupled type . 36
5.3.3 Longitudinally coupled type . 36
5.3.4 Other characteristics of coupled resonator filters . 36
5.3.5 Balanced connection . 41
5.4 Interdigitated interdigital transducer (lIDT) resonator filters . 45
5.4.1 Configuration . 45
5.4.2 Principle . 45
5.4.3 Characteristics . 45
6 Application guidelines . 46
6.1 Substrate materials and their characteristics . 46
6.2 Application to electronics circuits. 50
6.3 Availability and limitations . 52
6.4 Input levels . 53
6.5 Packaging of SAW filters . 54
7 Practical remarks. 56
7.1 General . 56
7.2 Feed-through signals . 56
7.3 Impedance matching condition . 57
7.4 Miscellaneous . 57
7.4.1 Soldering conditions . 57
7.4.2 Static electricity . 57
8 Ordering procedure . 58

60862-2 © IEC:2012 – 3 –
Bibliography . 61

Figure 1 – Frequency response of a SAW filter . 10
Figure 2 – Applicable range of frequency and relative bandwidth of the SAW filter and
the other filters . 11
Figure 3 – Schematic diagram showing signal flow through a transversal filter . 11
Figure 4 – Basic configuration of a SAW transversal filter . 12
Figure 5 – Frequency response of the SAW transversal filter shown in Figure 4, where
f is the centre frequency and N is the number of finger pairs of the IDT . 12
Figure 6 – Apodization weighting obtained by apodizing fingers . 13
Figure 7 – Withdrawal weighting obtained by selective withdrawal of the fingers . 13
Figure 8 – Series weighting obtained by the dog-leg structure . 13
Figure 9 – Split-finger configuration . 14
Figure 10 – Typical characteristics of a SAW IF filter for radio transmission equipment
(nominal frequency of 70,0 MHz) . 17
Figure 11 – Typical characteristics of a frequency asymmetrical SAW filter (nominal
frequency of 58,75 MHz for TV-IF use) . 17
Figure 12 – SAW three-IDT filter . 18
Figure 13 – Typical frequency response of a 900 MHz range SAW filter for
communication (mobile telephone use) . 18
Figure 14 – Schematic of the IIDT (multi-IDT) filter . 19
Figure 15 – Multi-phase unidirectional transducer . 19
Figure 16 – Single-phase unidirectional transducers . 20
Figure 17 – Frequency characteristics of a filter using multi-phase unidirectional
transducers . 21
Figure 18 – Frequency characteristics of a filter using single-phase unidirectional
transducers . 21
Figure 19 – Tapered IDT filter . 22
Figure 20 – Frequency response of a 140 MHz tapered IDT filter . 22
Figure 21 – Various reflector filter configurations . 24
Figure 22 – Z-path filter configuration . 25
Figure 23 – Dual-track reflector filter configuration . 25
Figure 24 – SPUDT-based dual-track filter . 26
Figure 25 – Frequency characteristics of Z-path filter . 26
Figure 26 – Frequency characteristics of dual-track reflector filter. 27
Figure 27 – Frequency characteristics of SPUDT-based reflector filter . 27
Figure 28 – A part of DART electrode in RSPUDT filter . 28
Figure 29 – Distribution of internal reflection and detection inside RSPUDT filter . 28
Figure 30 – Frequency and time responses of a 456 MHz RSPUDT filter . 29
Figure 31 – Structure of ladder and lattice filters . 32
Figure 32 – Equivalent circuit of basic section of ladder and lattice filter . 33
Figure 33 – Pattern layout of ladder filter . 33
Figure 34 – Basic concept of ladder and lattice filter . 34
Figure 35 – Typical characteristics of a 1,5 GHz range ladder filter . 35

– 4 – 60862-2 © IEC:2012
Figure 36 – SAW energy distribution and equivalent circuit of transversely coupled
resonator filter . 37
Figure 37 – Typical characteristics of a transversely coupled resonator filter . 38
Figure 38 – Basic configuration and SAW energy distribution of longitudinally coupled
resonator filter . 39
Figure 39 – Typical characteristics of a longitudinally coupled resonator filter . 40
Figure 40 – Configuration of balanced type transversely coupled resonator filter . 41
Figure 41 – Frequency characteristics of balanced type transversely coupled resonator
filter . 42
Figure 42 – Configuration of balanced type longitudinally coupled resonator filter . 43
Figure 43 – Typical characteristics of a balanced type longitudinally coupled resonator
filter . 45
Figure 44 – Schematic of IIDT resonator filter . 46
Figure 45 – Frequency characteristics of a 820 MHz range IIDT resonator filter . 46
Figure 46 – Minimum theoretical conversion losses for various substrates . 48
Figure 47 – Relationship between relative bandwidth and insertion attenuation for
various SAW filters, with the practical SAW filters’ bandwidth for their typical
applications . 52
Figure 48 – Ripples in the characteristics of a SAW filter caused by TTE or feed-
through signal: δf = 1/(2t) for the TTE, and δf = 1/t for the feed-through, where t is the
delay of the SAW main signal . 53
Figure 49 – Example of SAW metal package . 54
Figure 50 – Example of SAW ceramic package . 55
Figure 51 – Example of SAW resin package . 55
Figure 52 – Example of SAW CSP . 56

Table 1 – Properties of typical single-crystal substrate materials . 50
Table 2 – Properties of typical thin-film substrate materials . 50
Table 3 – Properties of typical ceramic substrate materials . 50

60862-2 © IEC:2012 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SURFACE ACOUSTIC WAVE (SAW) FILTERS
OF ASSESSED QUALITY –
Part 2: Guidelines for the use

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60862-2 has been prepared by IEC technical committee 49:
Piezoelectric, dielectric and electrostatic devices and associated materials for frequency
control, selection and detection.
This third edition cancels and replaces the second edition published in 2002 and constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
• Clause 3-"Terms and definitions" has been deleted to be included in the next edition of
IEC 60862-1;
• the tapered IDT filter and the RSPUDT filter have been added to the clause of SAW
transversal filters. Also DART, DWSF and EWC have been added as variations of SPUDT;
• the balanced connection has been added to the subclause of coupled resonator filters;

– 6 – 60862-2 © IEC:2012
• recent substrate materials have been described;
• a subclause about packaging of SAW filters has been added.
The text of this standard is based on the following documents:
CDV Report on voting
49/933/CDV 49/970A/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60862 series, published under the general title Surface acoustic
wave (SAW) filters of assessed quality, can be found on the IEC web site.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
60862-2 © IEC:2012 – 7 –
INTRODUCTION
This standard has been compiled in response to a generally expressed desire on the part of
both users and manufacturers for guidance on the use of SAW filters, so that the filters may
be used to their best advantage. To this end, general and fundamental characteristics have
been explained here.
The features of these SAW filters are their small size, light weight, adjustment-free, high
stability and high reliability. SAW filters add new features and applications to the field of
crystal filters and ceramic filters. At the beginning, SAW filters meant transversal filters which
have two interdigital transducers (IDT). Although SAW transversal filters have a relatively
higher minimum insertion attenuation, they have excellent amplitude and phase
characteristics. Extensive studies have been made to reduce minimum insertion attenuation,
such as resonator filter configurations, unidirectional interdigital transducers (UDT),
interdigitated interdigital transducers (IIDT). Nowadays, various kinds of SAW filters with low
insertion attenuation are widely used in various applications and SAW filters are available in
the gigahertz range.
– 8 – 60862-2 © IEC:2012
SURFACE ACOUSTIC WAVE (SAW) FILTERS
OF ASSESSED QUALITY –
Part 2: Guidelines for the use

1 Scope
This part of IEC 60862 gives practical guidance on the use of SAW filters which are used in
telecommunications, measuring equipment, radar systems and consumer products.
IEC 60862-1 should be referred to for general information, standard values and test
conditions.
SAW filters are now widely used in a variety of applications such as TV, satellite
communications, optical fibre communications, mobile communications and so on. While
these SAW filters have various specifications, many of them can be classified within a few
fundamental categories.
This part of IEC 60862 includes various kinds of filter configuration, of which the operating
frequency range is from approximately 10 MHz to 3 GHz and the relative bandwidth is about
0,02 % to 50 % of the centre frequency.
It is not the aim of this standard to explain theory, nor to attempt to cover all the eventualities
which may arise in practical circumstances. This standard draws attention to some of the
more fundamental questions, which should be considered by the user before he places an
order for a SAW filter for a new application. Such a procedure will be the user's insurance
against unsatisfactory performance.
Standard specifications, given in IEC 60862 series, and national specifications or detail
specifications issued by manufacturers, define the available combinations of nominal
frequency, pass bandwidth, ripple, shape factor, terminating impedance, etc. These
specifications are compiled to include a wide range of SAW filters with standardized
performances. It cannot be over-emphasized that the user should, wherever possible, select
his SAW filters from these specifications, when available, even if it may lead to making small
modifications to his circuit to enable standard filters to be used. This applies particularly to
the selection of the nominal frequency.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
None.
3 Technical considerations
It is of prime interest to a user that the filter characteristics should satisfy a particular speci-
fication. The selection of tuning networks and SAW filters to meet that specification should be
a matter of agreement between the user and the manufacturer.

60862-2 © IEC:2012 – 9 –
Filter characteristics are usually expressed in terms of insertion attenuation and group delay
as a function of frequency, as shown in Figure 1. A standard method for measuring insertion
attenuation and group delay is described in 4.5.2 of IEC 60862-1:2003. In some applications,
such characteristics as phase distortion are also important.
Insertion attenuation characteristics are further specified by nominal frequency, minimum
insertion attenuation or maximum insertion attenuation, pass-band ripple and shape factor.
The specification is to be satisfied between the lowest and highest temperatures of the
specified operating temperature range and before and after environmental tests.
SAW filters are classified roughly into two types: transversal filters and resonator filters.
Transversal filters are further classified into five types: bidirectional IDT filter, unidirectional
IDT filter, tapered IDT filter, reflector filter and RSPUDT (resonant single-phase unidirectional
transducer) filter. Also resonator filters are further classified into three types i.e. ladder and
lattice filters, coupled resonator filter and IIDT resonator filter. Fundamentals of SAW
transversal filters and SAW resonator filters are described in Clauses 4 and 5 of this standard,
respectively. In Figure 2, the applicable frequency range and relative bandwidth of the SAW
filters are shown in comparison with those of ceramic, crystal, dielectric, helical and stripline
filters.
4 Fundamentals of SAW transversal filters
4.1 Frequency response characteristics
A brief description of SAW filters is given here to help users unfamiliar with these filters to
understand their operating principles and characteristics. The SAW filter uses a surface
acoustic wave, usually the Rayleigh wave. The mechanical energy transported by the wave is
concentrated in a surface region of the order of a wavelength in depth. The wave travels on a
3 4
solid surface at a velocity, 10 m/s to 10 m/s, which offers the possibility of filtering
operations in the VHF and UHF regions in practical SAW filters. The SAW filter has a planar
structure, in which electrodes are formed on one surface of a piezoelectric substrate,
incorporating a suitable configuration of electrodes as a means of conversion between
surface acoustic waves and electrical signals.
Figure 3 is a diagram showing the signal flow through a transversal filter. The filter consists of
N taps separated by delays D . Each tap is weighted by a coefficient A . Filtering is achieved
n n
by passing the signal through a number of delay paths and adding these delayed signals. The
delays correspond to the positions of IDT fingers on a substrate. The coefficients correspond
to weighting coefficients given to the IDT fingers. The frequency response of the filter H(f) is
given by a discrete Fourier transformation, expressed as the following Equation (1) at a
frequency f:
N n
(1)
H( f ) = A exp (− j2πfT ) T = D
n n n i
∑ ∑
i=1
n =1
where T is the accumulated delay at the nth tap.
n
Both amplitude and phase characteristics of the transversal filter are given by two sets of
variables: weighting coefficients A and delays D of the sampling taps.
n n
The SAW transversal filter is essentially constructed with a pair of transducers on a
piezoelectric substrate as shown in Figure 4. When an electrical signal is applied to the input
IDT, the surface wave is generated by means of the piezoelectric effect and propagates in
both directions along the substrate surface. The surface wave is converted again into an
electrical signal at the output IDT. If the IDT spatial period 2d is uniform, maximum conversion
efficiency can be achieved at the frequency for which the surface wave propagates one

– 10 – 60862-2 © IEC:2012
transducer period synchronously in one RF signal period. The centre frequency f of the IDT
is given by this synchronization condition:
(2)
2df = ν
0 s
where ν is the SAW velocity.
s
When the SAW transversal filter has two uniform identical transducers, its frequency response
is as shown in Figure 5. The transfer function T(f) is approximately expressed as:
 sin x 
T(f) = (3)
 
x
 
where
Nπ( f − f )
and
x =
f
N is the number of finger pairs.

Attenuation
Specified
pass band
Group delay
Specified
stop band
Cut-off Centre Reference Cut-off Frequency
frequency frequency frequency frequency
(MHz)
IEC  1202/11
Figure 1 – Frequency response of a SAW filter
4.2 Weighting methods
The IDT operates as a kind of transversal filter with N taps for the weighting. A number of
weighting methods are applicable, for example apodization, withdrawal and series (dog-leg)
weighting.
a) Apodization weighting
An apodized transducer, as shown in Figure 6, is most commonly used to achieve
weighting. An acoustic wave is generated or detected only in regions where adjacent
electrodes of opposite polarity overlap.

Attenuation  (dB)
Minimum
Relative
insertion
attenuation
attenuation
TTE ripple
Group delay
distortion
Nominal
insertion
attenuation
Pass-band
ripple
Nominal
group delay
Group delay  (µs)
60862-2 © IEC:2012 – 11 –
b) Withdrawal weighting
Weighting is achieved by selectively withdrawing electrodes, as illustrated in Figure 7, to
equate with the desired weighting function.
c) Series (dog-leg) weighting
Weighting is achieved by dividing the voltage by segmenting each electrode pair, as
shown in Figure 8.
SAW filters
Helical
filters
Stripline filters
Ceramic
filters
–1
Dielectric filters
Crystal filters
–2
–3
1 M 10 M   100 M    1 G    10 G    100 G
Frequency (Hz)
IEC  763/12
Figure 2 – Applicable range of frequency and relative bandwidth of
the SAW filter and the other filters
T T T T
1 2 3 n
D D D D
1 2 3 n
Input signal
A A A A
1 2 3 n
Output signal
IEC  764/12
Figure 3 – Schematic diagram showing signal flow through a transversal filter

Relative bandwidth  (%)
– 12 – 60862-2 © IEC:2012
2d
IDT
Input R Output R
s L
Substrate
IEC  765/12
Figure 4 – Basic configuration of a SAW transversal filter

f
Frequency
f /N
IEC  766/12
Figure 5 – Frequency response of the SAW transversal filter shown in Figure 4,
where f is the centre frequency and N is the number of finger pairs of the IDT
Relative attenuation  (dB)
60862-2 © IEC:2012 – 13 –
IEC  767/12
Figure 6 – Apodization weighting obtained by apodizing fingers

IEC  768/12
Figure 7 – Withdrawal weighting obtained by selective withdrawal of the fingers

IEC  769/12
Figure 8 – Series weighting obtained by the dog-leg structure
4.3 Filter configurations and their general characteristics
4.3.1 General
In some cases, the split-finger configuration, as shown in Figure 9, is used as the
replacement of the solid-finger configuration shown in Figure 4 to reduce SAW reflections at
the metal electrodes. With this geometry, the individual reflections, caused by the
discontinuity in acoustic impedances on the surface, are cancelled in each finger pair. This
finger configuration is now popular in SAW TV-IF filters, etc.
Ordinary IDTs show bidirectional property. These bidirectional IDTs transmit and receive
SAWs to and from two directions respectively. For instance, a transmitting IDT converts an
electric signal into SAWs. The SAW propagates both forwards and backwards with the same
intensities. A receiving IDT will receive either of them with the same efficiency. This means
that bidirectional loss values can be estimated at 3 dB each at the transmitting and receiving
IDT. Therefore, the bidirectional loss of 6 dB is inherent and is the minimum insertion
attenuation in a bidirectional two-transducer SAW filter. Moreover, in these ordinary SAW
filters accompanying the bidirectionality, strong pass-band ripple is induced by the triple
transit echo (TTE) when the impedances of transmitting IDT and the receiving IDT are
matched to the outer loads.
In order to reduce the bidirectional loss and the triple transit echo (TTE) in SAW transversal
filters, multi-IDT (IIDT) filters (including three-IDT SAW filters) and unidirectional IDT filters
(including tapered IDT filters) are utilized.
Additionally, reflector filters (see Figures 21 and 22) can be included as one type of the
transversal filters. Grating technology is widely used as a reflector which changes SAW's
propagation direction with some reflection frequency response. The reflector filters utilize not

– 14 – 60862-2 © IEC:2012
only their own transversal filter characteristics which are derived from the transducers but
also the reflection frequency responses of the reflector in various grating constitutions in
order to actively shape the filter transfer function and to reduce their chip length by folding the
SAW propagation. And the studies of these various reflector filters have brought new filter
technologies called resonant single-phase unidirectional transducer (RSPUDT) filters.
A brief summary of the configurations, the principles and/or the characteristics of individual
types of SAW filters is given in the following subclauses.

λ
λ
IEC  770/12
Figure 9 – Split-finger configuration
4.3.2 Bidirectional IDT filters
4.3.2.1 Bidirectional two-IDT filters
In the ordinary bidirectional two-IDT filters, as shown in Figure 4, the TTE is reduced to a
sufficiently low level at the sacrifice of the insertion attenuation, by mis-matching the IDTs to
the outer loads.
a) Frequency symmetrical band-pass filter
The centre frequency and bandwidth for an IDT are given by the periods of the fingers and
the number of finger pairs of the IDT, respectively. In phase characteristics, phase lag
increases proportionally with frequency. Therefore, group delay is invariant in the pass-
band. One typical application of a frequency symmetric band-pass filter is as an IF filter
for radio transmission equipment. Linear-phase characteristics and flat pass-band
amplitude characteristics are preferable for the system requirement. Figure 10 shows a
typical frequency response of a SAW filter whose nominal frequency is 70,0 MHz. High-
frequency SAW filters are also available with higher selectivity.
b) Frequency asymmetrical band-pass filter
In the SAW transversal filters, the amplitude and phase characteristics can be designed
independently. Asymmetrical pass-band, stop-band and/or group delay characteristics in
relation to the reference frequency are obtainable by means of a sophisticated design
technique. SAW TV-IF filters have frequency asymmetrical characteristics, as shown in
Figure 11.
c) Other filter categories
Comb filters have also been proposed and are available. SAW matched filters are applied
to recent civil spread spectrum (SS) systems, for example wireless LAN, etc. SAW filters
with Nyquist characteristics have been developed for recent communication systems.

60862-2 © IEC:2012 – 15 –
4.3.2.2 Multi IDT/interdigitated interdigital transducer (IIDT) SAW filters
Multi-IDT or interdigitated interdigital transducer filters have been developed from three-IDT
filters, as demand for low-loss filtering increased. For this reason, a brief explanation for
three-IDT filters is given.
a) Three-IDT filters
A three-IDT type SAW filter provides two identical receiving IDTs, symmetrically placed to
the central transmitting IDT, as shown in Figure 12. When the symmetric central
transducer is tuned and matched at the centre frequency, the two opposite directed SAWs
are completely absorbed, this being the inverse process to the generation of the two
SAWs by a tuned and matched transducer. At the same time, when the two receiving
transducers connected are tuned and matched at the centre frequency, the insertion
attenuation can be improved to 3 dB, and the TTE is eliminated. A typical frequency
response of a 900 MHz range SAW three-IDT filter is shown in Figure 13.
This operation principle is extended to the multi-IDT filters.
b) Multi-IDT/Interdigitated interdigital transducer filters
Multi-IDT or interdigitated interdigital transducer filters provide input IDTs inter-digitally
placed to output IDTs. This filter, as an example, schematically illustrated in Figure 14
comprises (N + 1) input transducers and N output transducers. By this configuration, the
bidirectional 6 dB Ioss in two-IDT filters is reduced to a much smaller value, and the triple
transit echo is eliminated when the input and output port are matched to the outer loads.
When the input transducers and output transducers are tuned and matched to the circuit,
the insertion attenuation of the filter shown in Figure 14 is reduced to the residual
bidirectional loss caused by the outermost input transducers, which is inversely
proportional to the number of transducers, as follows:
dB
10log{(N + 1) N}
4.3.3 Unidirectional IDT (UDT) filters
4.3.3.1 Configuration
Both low insertion attenuation and excellent frequency characteristics in unidirectional filters
are based upon directivity of surface wave propagation. Ideally, the filters have insertion
attenuation of less than 1 dB, and both amplitude and phase characteristics can be controlled
independently. They are divided roughly into two categories. One of them is the multi-phase
unidirectional transducer, to which electrical fields with various phase differences are applied.
The other category is the single-phase unidirectional transducer applied with the same phase
field.
a) Multi-phase unidirectional transducers
The three-phase unidirectional and group-type unidirectional transducers are representa-
tive of the class. The unidirectionality of the three-phase transducers arises from applying
three voltages with phase differences of 120° each. In this case, however, a third
electrode shall cross over one of the other electrodes using an insulated bridge, making
the filter no longer truly planar and less reliable.
The group-type unidirectional transducer shown in Figure 15 is capable of overcoming the
above-mentioned shortcomings. The unidirectional transducer with only a few pairs of
electrodes, excited with an electrical phase shift of 90°, is thought of as one group. Many
groups can then be collinearly arranged, the signal of each group adding in phase with the
signals of all the other groups so as to yield a filter with a low insertion attenuation.
Conventional weighting techniques are also applicable in this transducer.
b) Single-phase unidirectional transducers (SPUDTs)
These single-phase unidirectional transducers (SPUDTs) utilize internal reflections within
the transducer to achieve unidirectional behavior. The basic arrangement of a uni-

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directional transducer using internal floating electrode reflection, which is called floating
electrode unidirectional transducer (FEUDT), is shown schematically in Figures 16a-16c.
The transducer shown in Figure 16a can obtain unidirectionality, caused by the offset
arrangement of floating open metal strips from the centre of positive and negative
electrodes. Similarly, there are other cases of floating short metal strips and combinations
of them, which are shown in Figures 16b and 16c respectively. Other SPUDTs using
internal reflection not by floating electrode are shown in Figures 16d and 16e. Figure 16d
is called a distributed acoustic reflective transducer (DART), and Figure 16e is called a
differ
...