IEC 62575-2:2012

Radio frequency (RF) bulk acoustic wave (BAW) filters of assessed quality - Part 2: Guidelines for the use

IEC 62575-2:2012 gives practical guidance on the use of RF BAW filters which are used in telecommunications, measuring equipment, radar systems and consumer products. General information, standard values and test conditions will be provided in a future IEC standard. This part of IEC 62575 includes various kinds of filter configurations, of which the operating frequency range is from approximately 500 MHz to 10 GHz and the relative bandwidth is about 1 % to 5 % of the centre frequency.

Filtres radiofréquences (RF) à ondes acoustiques de volume (OAV) sous assurance de la qualité - Partie 2: Lignes directrices d'emploi

La CEI 62575-2:2012 donne des conseils pratiques sur l'utilisation des filtres RF à OAV utilisés dans le domaine des télécommunications, des équipements de mesure, des systèmes radar et des produits de grande consommation. Les informations générales, les valeurs normalisées et les conditions d'essai seront fournies dans une future norme CEI. La présente partie de la CEI 62575 inclut différents types de configurations de filtres dont la gamme de fréquences de fonctionnement est comprise entre environ 500 MHz et 10 GHz et la largeur de bande relative est d'environ 1 % à 5 % de la fréquence centrale.

General Information

• Status: Published
• Publication Date: 24-Jul-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: 31-Aug-2012
• Completion Date: 25-Jul-2012

Standards Content (Sample)

IEC 62575-2 ®
Edition 1.0 2012-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Radio frequency (RF) bulk acoustic wave (BAW) filters of assessed quality –
Part 2: Guidelines for the use

Filtres radiofréquences (RF) à ondes acoustiques de volume (OAV) sous
assurance de la qualité –
Partie 2: Lignes directrices d’emploi

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IEC 62575-2 ®
Edition 1.0 2012-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Radio frequency (RF) bulk acoustic wave (BAW) filters of assessed quality –

Part 2: Guidelines for the use

Filtres radiofréquences (RF) à ondes acoustiques de volume (OAV) sous

assurance de la qualité –
Partie 2: Lignes directrices d’emploi

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX T
ICS 31.140 ISBN 978-2-83220-248-7

– 2 – 62575-2 © IEC:2012
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Technical considerations . 6
4 Fundamentals of RF BAW filters . 7
4.1 General . 7
4.2 Fundamentals of RF BAW resonators . 8
4.3 RF resonator structures . 13
4.4 Ladder filters . 15
4.4.1 Basic structure . 15
4.4.2 Principle of operation . 16
4.4.3 Characteristics of ladder filters . 17
5 Application guide . 18
5.1 Application to electronics circuits. 18
5.2 Availability and limitations . 18
5.3 Input levels . 18
6 Practical remarks. 18
6.1 General . 18
6.2 Feed-through signals . 19
6.3 Load and source impedance conditions . 19
7 Miscellaneous . 19
7.1 Soldering conditions . 19
7.2 Static electricity . 19
8 Ordering procedure . 19
Bibliography . 22

Figure 1 – Frequency response of a RF BAW filter . 7
Figure 2 – Applicable range of frequency and relative bandwidth of the RF BAW filter
and the other filters . 8
Figure 3 – Basic BAW resonator structure. 9
Figure 4 – BVD model . 9
Figure 5 – Typical impedance characteristics . 10
Figure 6 – Typical impedance characteristics of RF BAW devices . 12
Figure 7 – Modified BVD model. 13
Figure 8 – FBAR structures . 14
Figure 9 – SMR structure . 15
Figure 10 – Structure of ladder filter . 15
Figure 11 – Equivalent circuit of basic section of ladder filter . 16
Figure 12 – Basic concept of ladder filter . 16
Figure 13 – Typical characteristics of a 1,9 GHz range ladder filter . 17

62575-2 © IEC:2012 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RADIO FREQUENCY (RF) BULK ACOUSTIC
WAVE (BAW) FILTERS OF ASSESSED QUALITY –

Part 2: Guidelines for the use

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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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 62575-2 has been prepared by IEC technical committee 49:
Piezoelectric, dielectric and electrostatic devices and associated materials for frequency
control, selection and detection.
The text of this standard is based on the following documents:
FDIS Report on voting
49/994/FDIS 49/999/RVD
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.

– 4 – 62575-2 © IEC:2012
A list of all the parts in the IEC 62575 series, published under the general title Radio
frequency (RF) Bulk acoustic wave (BAW) filters of assessed quality, can be found on the IEC
website.
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.
62575-2 © IEC:2012 – 5 –
INTRODUCTION
RF BAW filters are now widely used in mobile communications. While the RF BAW filters have
various specifications, many of them can be classified within a few fundamental categories.
Standard specifications, given in IEC 62575, and national specifications or detail specifi-
cations 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 RF BAW filters with standardized performances. It cannot
be over-emphasized that the user should, wherever possible, select his RF BAW 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.
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 RF BAW filters, so that the filters
may be used to their best advantage. To this end, general and fundamental characteristics
have been explained in this part of IEC 62575.
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 an RF BAW filter for a new application. Such a procedure will be the user's
insurance against unsatisfactory performance.

– 6 – 62575-2 © IEC:2012
RADIO FREQUENCY (RF) BULK ACOUSTIC
WAVE (BAW) FILTERS OF ASSESSED QUALITY –

Part 2: Guidelines for the use

1 Scope
This part of IEC 62575 gives practical guidance on the use of RF BAW filters which are used
in telecommunications, measuring equipment, radar systems and consumer products.
General information, standard values and test conditions will be provided in a future IEC
standard .
This part of IEC 62575 includes various kinds of filter configurations, of which the operating
frequency range is from approximately 500 MHz to 10 GHz and the relative bandwidth is
about 1 % to 5 % of the centre 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
specification. The selection of tuning networks and RF BAW filters to meet that specification
should be a matter of agreement between user and manufacturer.
Filter characteristics are usually expressed in terms of insertion attenuation as a function of
frequency, as shown in Figure 1. A standard method for measuring insertion attenuation is
described in IEC 60862-1:2003, 5.5.2. 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.
___________
This standard (under consideration) is expected to bear the reference number IEC 62575-1.

62575-2 © IEC:2012 – 7 –
Minimum insertion Nominal insertion
attenuation attenuation
Pass-band ripple
relative
attenuation
Attenuation
Specified
Specified stop-band
pass-band
R eference
frequency
Frequency  (GHz)
Cut-off Centre Cut-off
IEC  1444/12
Figure 1 – Frequency response of a RF BAW filter
4 Fundamentals of RF BAW filters
4.1 General
The features of RF BAW filters are their small size, light weight, adjustment-free, high stability
and high reliability. RF BAW filters add new features and applications to the field of surface
acoustic wave (SAW) filters and dielectric resonator filters. Nowadays, RF BAW filters with
low insertion attenuation are widely used in various applications in the gigahertz range.
RF BAW filters are becoming rapidly popular as miniature and low insertion attenuation filters
for mobile communication application. RF BAW resonator filters can realize low insertion
attenuation easily and of a smaller size than that of the RF SAW filters with the same
bandwidth. Their feasible bandwidth is, however, limited by employing piezoelectric materials,
design methods and so on. It is desirable for users to understand these factors for RF BAW
resonator filters. This standard explains the principles and characteristics of RF BAW
resonator filters.
RF BAW filters usually employ a filter configuration called the ladder filter, which is composed
of multiple RF BAW resonators. They are classified into two types: film bulk acoustic
resonators and solidly mounted resonators. In Figure 2, the applicable frequency range and
relative bandwidth of the RF BAW filters are shown in comparison with those of ceramic,
crystal, dielectric, helical, SAW and stripline filters.

Attenuation  (dB)
– 8 – 62575-2 © IEC:2012
SAW filters
Helical
RF BAW filters
filters
Ceramic
Stripline filters
filters
–1
Dielectric filters
Crystal filters
–2
–3
1 M      10 M     100 M     1 G      10 G     100 G
IEC  1445/12
Frequency  (Hz)
Figure 2 – Applicable range of frequency and relative
bandwidth of the RF BAW filter and the other filters
4.2 Fundamentals of RF BAW resonators
a) Acoustic resonance
When a mechanical impact is applied to a solid surface, acoustic waves are generated, and a
portion of their energy is transmitted by propagation of the acoustic waves in the bulk. This
type of wave is called the bulk acoustic wave (BAW). Remaining energy may be transferred
by acoustic waves propagating along the surface. This type of wave is called the surface
acoustic wave (SAW).
There are two types of BAWs: the longitudinal or dilatational BAW, with the displacement
toward the propagation direction, and the transverse or shear BAW, with the displacement
normal to the propagation direction. Acoustic wave velocities in solids are a few hundreds of
meters per second to twenty thousands of meters per second. Usually the longitudinal BAW is
few times faster than the shear BAW for a given material and orientation.
In the case of acoustic wave propagation in a parallel plate, it is known that the plate causes
a mechanical resonance (thickness resonance) when the plate thickness h is half-integer
times the wavelength λ of acoustic waves propagating in the plate normal to the plate surface,
i.e. h= nλ 2, where n is an integer and called the order of modes. We obtain mechanical
resonance frequencies f as
r
f = V λ= nV (2h) (1)
r
where V is the acoustic wave velocity. Equation (1) indicates that in addition to a lowest-order
resonance (n=1) called the fundamental resonance, a series of higher-order (n≠1) ones might
be excited. Since f for n≠1 will be integer times f for n=1 in this case, higher-order
r r
resonances are often called harmonics or harmonic resonances. When the longitudinal BAW
is responsible for the thickness resonance, it is called the thickness extensional (TE)
resonance but when the shear wave is responsible, it is called the thickness shear (TS)
resonance.
Relative bandwidth  (%)
62575-2 © IEC:2012 – 9 –
There are also acoustic waves propagating along the plate top surface. When wave energy is
well confined near the top surface and influence of the back surface is negligible, the waves
are called the surface acoustic waves (SAWs). On the other hand, when wave energy
penetrates into the plate and influence of the back surface is not negligible, the waves are
called plate waves or Lamb waves.
b) Piezoelectric excitation and detection
In the case where a piezoelectric plate is sandwiched between two parallel electrodes (see
Figure 3), when an electrical voltage E is applied between two electrodes, mechanical force is
generated through the piezoelectricity, and acoustic motion will be induced. On the other hand,
electrical charges will be induced to the electrodes by electric fields associated with
propagating acoustic waves.
Upper electrode
Piezoelectric material
h
Lower electrode
IEC  1446/12
Figure 3 – Basic BAW resonator structure
An electromechanical equivalent circuit shown in Figure 4 may be deduced from these
relations. In Figure 4, C is the clumped capacitance originating from the electrostatic
coupling between two electrodes, and C L and R are the motional capacitance, inductance
1, 1 1
and resistance, respectively, originating from mechanical reaction, i.e. elasticity, inertia and
damping, respectively. This circuit is called the Butterworth-Van Dyke (BVD) model.
R L C
1 1 1
IEC  1447/12
C
Figure 4 – BVD model
Figure 4 implies that mechanical resonances described above can be excited and detected
electrically through the electrodes. Namely, this device serves as an electrical resonator. This
type of resonator is called the BAW resonator. Proper choice of the piezoelectric material
offers small acoustic attenuation, which results in long duration of the mechanical vibration.
This mechanical property influences the electrical one as large quality (Q) factor of the
electrical resonance circuit.
Figure 5 shows typical resonance characteristics calculated by the BVD model. It is seen that
a series resonance occurs at a frequency f where the electrical impedance Z between two
r
electrodes becomes pure resistive and very small. From the BVD model, f is given by
r
f ≈ 1 2π L C (2)
r 1 1
On the other hand, at a frequency f slightly above f , a parallel resonance occurs where Z
a r
becomes pure resistive and very large. From the BVD model, f is given by
a
– 10 – 62575-2 © IEC:2012
−1 −1 −1
f ≈ 1 2π L (C + C ) (3)
a 1 1 0
These frequencies are called the resonance and the anti-resonance frequencies,
respectively
f
a
f
r
BVD
10 mBVD
–1
0,98 0,99 1,00 1,01 1,02 1,03 1,04
Frequency  (GHz)
IEC  1448/12
Figure 5 – Typical impedance characteristics
The capacitance ratio r is often used as a measure of the resonator performance, and is
defined by
−1
r=[( f f ) −1] (4)
a r
From the BVD model, and r is given by
r= C C
(5)
0 1
In the filter design discussed later, r limits achievable fractional frequency bandwidth for filter
applications.
At frequencies much lower than f , the resonator is equivalent to a capacitor with the
r
−1
capacitance of C + C = C (1+ r ), which is given by εS / h, where ε is the dielectric
0 1 0
constant and S is the electrode area. Thus C is adjustable only by S because h is mostly
determined by the frequency setting.
It is clear from Equation 5 that r indicates weakness of the piezoelectricity. In fact, full wave
analysis gives a relation between f /f and the electromechanical coupling factor k for the
r a t
thickness-longitudinal vibration of the piezoelectric material as
k = (nπf / 2 f ) / tan(nπf / 2 f ) (6)
t r a r a
When f ≅ f , Equations (3) and (5) become
r a
___________
Frequencies f and f giving minimum |Z| and maximum |Z| are the frequencies of maximum and minimum
m n
admittance or those of minimum and maximum impedance. When Q is large, f and f are almost equal to f and
m n r
f , respectively.
a
Impedance  | Z| (Ω)
62575-2 © IEC:2012 – 11 –
2 2 2

4k / nπ n : odd
t
( f − f ) / f ≅ 1/ 2γ≅
(7)

a r a

0 n : even

This indicates three important facts:
1) achievable r is limited by k of employed piezoelectric material;
t
2) even-order overtones cannot be excited electrically; and
3) γ increases rapidly with an increase in n.
It should be noted that Equations (6) and (7) are only valid when a uniform piezoelectric layer
is sandwiched between two infinitesimally thin electrodes with infinite conductance. Since
influence of electrodes is not negligible as will be discussed later, piezoelectric strength of the
resonator structure is often characterized by the effective electromechanical coupling factor
defined by
k = (πf / 2 f )/ tan(πf / 2 f ) (8)
r a r a
t eff
From the BVD model, the Q factor at f is given by
r
Q = 2πf L R (9)
r r 1 1
and is often referred to as the resonance Q or Q . We can also evaluate the Q factor at the
r
anti-resonance frequency, and the value is called the anti-resonance Q or Q . In the filter
a
design, Q and Q determine steepness of the pass-band edges for filter applications.
r a
For resonator characterization, the figure of merit, M is defined as
M= Q r (10)
r
In the filter design, M determines achievable minimum insertion attenuation.
It is interesting to note that the BVD model indicates that
M≅ 2πf C Z ≅ 1 2πf C Z (11)
a 0 max r 0 min
where Z and Z are electrical impedances of the resonator at f (≈f ) and f (≈f ),
max min n a m r
respectively. Thus Z /Z called the impedance ratio is also used for the resonator
max min
characterization.
NOTE 1 This approximated form is valid only when Q and r are large.
r
c) Secondary effects
Basic operation of BAW resonators is simulated fairly well by the use of the BVD model
described above. In real devices, however, various secondary effects occur, and their
influences shall be well-controlled for device design and production. Significant secondary
effects are:
1) Lateral wave propagation
At frequencies close to the resonance, Lamb waves are excited and propagate along the
surface. If their wave energy is dissipated, it will cause Q reduction of the main resonance. If
the resonator structure is designed to confine the wave energy, the resonance Q might be

– 12 – 62575-2 © IEC:2012
preserved, while it may cause unwanted resonances often called spurious resonances. Since
lateral structural size is significantly larger than the BAW wavelength in general, frequency
separation between the resonances is narrow. From this property, these spurious resonances
are called inharmonics. The top surface of the resonator is sometimes shaped in an irregular
polygon to smear out spurious resonance peaks.
2) Parasitic impedances
Ohmic resistances, parasitic capacitances, and inductances of the electrodes and pads are
not negligible in radio frequencies (RF).
Figure 6 shows polar plot (Smith chart) of the return coefficient Γ of two RF BAW resonators.
Γ is given by (Z-R )/(Z+R ), where Z is the device impedance and R is the characteristic
0 0 0
impedance of the measurement system. The trace rotates clockwise with the frequency, and
leftmost and rightmost points of the trace correspond to the resonance and anti-resonance
frequencies, respectively. Series of inharmonics are seen in Figure 6 a). It should be noted
that the overtones are only seen above the resonance frequency, and this property called the
cut-off indicates they are due to lateral wave propagation.
Application of an appropriate technology enables to suppress inharmonics almost completely
as shown in Figure 6 b). In addition, it is seen that the trace approaches to the outermost
circle, namely |Γ| close to unity. This indicates the lateral wave propagation can be one of the
most significant loss mechanisms.
resonator
resonator
APLAC 7.80 User: Infineon Technologies AG Jun 08

APLAC 7.80 User: Infineon Technologies AG Jun 08

0,5 2,0 0,5 2,0
0.5 2.0
0.5 2.0
Spurious modes
-0.5 -2.0
–0,5 –2,0 –-0.0,55 -2.–2,0 0
IEC  1449/12 IEC  1450/12
0.0 0.2 1.0 5.0 0.0 0.2 1.0 5.0
S11 S11
a) before suppressing spurious resonances b) after suppressing spurious resonances
Figure 6 – Typical impedance characteristics of RF BAW devices
The BVD model is often modified to take these effects into account. Figure 7 shows an
example, where series resistance R and shunt resistance R are added to express variation
s 0
of energy dissipation with frequency, and L expresses inductance of the interconnecting
s
electrodes and/or bonding wires.
NOTE 2 A modification of the equivalent circuit is not unique. For example, R in Figure 7 is sometimes placed in
parallel with C instead of series.
Even if L and/or R are small, their impact is significant near the resonance frequency where
s s
|Z| becomes extremely small. This modified BVD model gives the resonance frequency f and
r
the resonance Q, Q , as
r
62575-2 © IEC:2012 – 13 –
f ≈ 1 2π (L + L )C (12)
r s 1 1
and
Q = 2πf (L + L ) (R + R ) (13)
r r 1 s 1 s
respectively. On the other hand, impact of R is significant near the anti-resonance frequency
where |Z| becomes extremely large. This modified BVD model gives the anti-resonance Q, Q ,
a
as
Q = 2πf L (R + R ) (14)
a a 1 1 0
C
R L
L R
S S
IEC  1451/12
R
C
Figure 7 – Modified BVD model
Figure 5 compares the electrical impedance |Z| given by this modified BVD model with that
is set to zero. It is seen that f and Q are
given by the original one. In this calculation, R
0 r r
slightly decreased while f and Q are unchanged. It should be noted that the original BVD
a a
model indicates Q≅Q , but this is not true in general.
r a
Since L effectively increases k , it is often used positively to enhance the filter performance.
s t eff
On the other hand, the parasitic capacitance between terminals effectively increases C and
results in decreased k . Thus the device package shall be co-designed with the BAW
t eff
device chip itself so as to optimize the total device performance.
4.3 RF resonator structures
For applications lower than a few tens of MHz, BAW resonators can be mass-produced by
thinning and polishing piezoelectric materials. For higher frequencies, on the other hand,
since required thickness h is reduced to micro metre order, thin film technologies are
applicable instead of mechanical processing. Although use of overtones with n≠1 is another
choice, it results in significant increase in r.
Aluminium nitride (AlN) is widely used as a piezoelectric layer for the RF BAW resonator
because of its several distinct features: low propagation loss, high electrical resistivity, and
possible growth of high quality films on underneath metal electrodes. Although various
materials such as zinc oxide (ZnO), lead zirconate titanate (PZT), etc. have been investigated
extensively, realized performances are much lower than those attained by AlN and far from
practical use.
Lack of material choice limits applicability of RF BAW filters. That is, r limiting the filter
bandwidth is mostly determined by the piezoelectric material as indicated in 4.2. Molybdenum,
Ruthenium, Tungsten, etc., are used for the electrodes because their large acoustic
impedance offers slight decrease in r or increase in k and they act as a good seed layer
t eff
for the AlN growth.
RF BAW resonators are categorized into two types.

– 14 – 62575-2 © IEC:2012
The first type is called film bulk acoustic wave resonator (FBAR), which employs a free
standing membrane supported at side edges. Three kinds of FBAR structures were proposed:
Figure 8 a) shows the one employing an air cavity created by back-side etching of the
supporting substrate. Figures 8 b) and c) show the ones employing an air cavity created by
etching of a layer underneath the resonator structure after completing its fabrication.
Upper electrode
Piezoelectric material h
Lower electrode
Supporting
substrate
IEC  1452/12
Figure 8 a) – Back-side etched

Upper electrode
Piezoelectric material h
Air gap
Lower electrode
Supporting
substrate
IEC  1453/12
Figure 8 b) – Front-side etched

Upper electrode
Piezoelectric material
h
Air gap
Lower electrode
Supporting
substrate
IEC  1454/12
Figure 8 c) – Sacrificial-layer etched
Figure 8 – FBAR structures
The second type is called the solidly mounted resonator (SMR), which employs an acoustic
mirror giving acoustic isolation from the substrate and tight physical contact with it. Figure 9
shows the SMR structure. This mirror is composed of multiple layers with different acoustic
impedances. For example, the combination of W and SiO is suitable for the use, and a few
layers are enough for sufficient reflection. Each of the layers is designed with about a quarter
wave thickness for an optimal reflection at the intended operation frequency.

62575-2 © IEC:2012 – 15 –
Upper electrode
Piezoelectric material h
Lower electrode
Lower Z layer
a
Higher Z layer
a
Supporting
substrate
IEC  1455/12
Figure 9 – SMR structure
4.4 Ladder filters
4.4.1 Basic structure
Ladder type filters are comprised of series and parallel BAW resonators in a ladder type
arrangement. Basically, it can be represented in and near the pass-band with resonant
circuits using the BVD model. Although various kinds of RF BAW resonator filters have been
proposed, only the ladder type is put into practice.
Figure 10 shows an example of a filter structure and Figure 11 shows an example of an
equivalent circuit of a half-section of a ladder filter assuming that the resistance is negligible.
The half-section of the filter consists of a series-arm resonator (R ) and a parallel-arm
resonator (R ). A series-arm resonator has slightly higher resonance frequency than that of a
parallel-arm resonator. The resonators R1′ and R2′ are synthesized resonators. R ′ has half-
static capacitance of R , and R ′ has twice static capacitance of R .
1 2 2
R′
R R R
1 1 1
R′
R R R
2 2 2
IEC  1456/12
Figure 10 – Structure of ladder filter

– 16 – 62575-2 © IEC:2012
Z =jX
s s
R
R
Y =jB
p p
IEC  1457/12
Figure 11 – Equivalent circuit of basic section of ladder filter
4.4.2 Principle of operation
Figure 12 shows the variations of X and B as a function of frequency, where X is the
s p s
reactance of R , and B is the susceptance of R . Here, the anti-resonance frequency (f ) of
1 p 2 ap
the parallel-arm resonator is nearly equal to the resonance frequency (f ) of the series-arm
rs
resonator. The image transfer constant γ is expressed with X and B in the following equation:

s p
tanhγ= B X (B X −1) (15)
p s p s
According to the theory of image-parameter filters, a filter shows a pass-band characteristic
when γ is an imaginary number. On the other hand, it shows a stop-band characteristic when γ
is a real number. Therefore, the condition 0 < B X < 1 gives the pass-band, and the condition

p s
B X > 1 or B X < 0 gives the stop-band shown in Figure 12.

p s p s
+
B
p
X
s
f
f ap
rp
f
f f
rs
as
X B <0
X B <0
s p
0 s p
–
X B >1 X B >1
s p s p
f
Frequency
IEC  1458/12
Figure 12 – Basic concept of ladder filter
X , B
s p
Attenuation  (dB)
62575-2 © IEC:2012 – 17 –
4.4.3 Characteristics of ladder filters
The pass-band width of a ladder filter is affected by the employed piezoelectric material.
Ideally it is effective to use an appropriate piezoelectric material having a high
electromechanical coupling coefficient in order to obtain a filter with a wide pass-band.
However, use of AlN is the only choice at least until now. This is because performances
achieved by current AlN films are far beyond those of the other piezoelectric materials. The
steepness of the pass-band edges is determined by the Q factor of the resonators while the
insertion attenuation of a filter is determined by the figure of merit M of the resonators. The
stop-band attenuation is basically determined by the static capacitance ratio of a parallel-arm
resonator to a series-arm resonator and the stage number of the resonators' connection.
Figure 13 shows the frequency characteristics of a 1,9 GHz band-pass filter. The minimum
insertion attenuation of less than 2 dB and the return attenuation of more than 10 dB were
obtained without an external matching circuit.
This filter was designed so as to enhance the rejection around 2,14 GHz at the expense of its
deterioration at frequencies lower than the pass-band. Such characteristic is realized using
tiny parasitic inductances embedded in the filter package.
1,8 2,2
1,9 2,0 2,1
Frequency  (GHz)
IEC  1459/12
Figure 13 a) – Transmission characteristic
1,8 2,2
1,9 2,0 2,1
Frequency  (GHz)
IEC  1460/12
Figure 13 b) – Reflection characteristic
Figure 13 – Typical characteristics of a 1,9 GHz range ladder filter

Return attenuation  (dB)
Insertion attenuation  (dB)
– 18 – 62575-2 © IEC:2012
5 Application guide
5.1 Application to electronics circuits
RF BAW filter characteristics are also influenced by electrical characteristics of peripheral
circuits. In order to obtain a satisfactory performance, certain precautions are required.
Insertion attenuation for RF BAW filters is mainly caused by ohmic loss of metal electrodes,
acoustic propagation loss due to scattering and/or viscosity, leakage loss from reflectors
(SMR case), and lateral leakage loss to surroundings. It should be noted that AlN films are
poly-crystalline, the propagation attenuation is significantly dependent on the film quality.
5.2 Availability and limitations
Because a RF BAW filter has a complex mechanical structure, there are numerous unwanted
responses which may disturb the filter characteristics. Such unwanted responses shall be
suppressed or reduced below a certain level. In practical use, long-term stability should also
be considered.
a) Feed-through signals
Because feed-through signals travel directly between the input and output circuits due to
the electrostatic or electromagnetic coupling, they appear at the output terminal instantly
when the input voltage is applied. They cause ripple in the pass-band, and the frequency
period (δf) is equal to 1/t, where t is the delay of the main signals. Sometimes, they fill the
frequency traps in the stop-band and degrade the stop-band characteristics.
b) Spurious resonances
Because of high Q, excitation of unnecessary acoustic waves will cause spurious
resonances, which generate ripple in the pass-band and/or satellite peaks in the rejection-
band. Inharmonics caused by the lateral wave propagation is a typical example.
c) Ageing performance
RF BAW filters exhibit excellent long-term stability as well as SAW filters. The long-term
ageing rate depends on the input level of a RF BAW filter, the substrate mounting method,
the atmosphere in which the substrate is located, etc.
5.3 Input levels
Drive level performance is limited by:
a) Electrode damage
This damage is irrecoverable. When an excessive drive level is applied, this often causes
a flashover. Sometimes, physical erosion of the electrodes is also caused. This brings
about centre frequency shift, pass-band distortion and insertion attenuation degradation.
The RF signal drive level should be agreed upon with the manufacturer.
b) Frequency and/or response change
RF acoustic power is confined in a small volume. Therefore, RF BAW devices may exhibit
non-linear characteristics at lower drive levels more easily than conventional bulk-wave
devices.
6 Practical remarks
6.1 General
The incorrect usage of a RF BAW filter may at times result in its unsatisfactory performance.
It is necessary to take care of direct feed-through, impedance matching conditions, etc.

62575-2 © IEC:2012 – 19 –
6.2 Feed-through signals
Feed-through signals are caused mainly by the electrostatic and electromagnetic couplings
between the input and output circuits.
There are several ways to reduce the feed-through. The most effective method is to employ a
balanced (differential) circuit to cancel the undesirable coupling signals induced by stray
capacitance (electrostatic) or current loop (electromagnetic). Integrated circuits (ICs) can
easily adopt balanced input and/or balanced output circuits. A balanced out
...