12 Figure 5 – Frequency response of the SAW transversal filter shown in Figure 4, where f0 is the centre frequency and N is the number of finger pairs of the IDT .... 20 Figure 17 – Freq
Frequency response characteristics
SAW filters, or Surface Acoustic Wave filters, utilize Rayleigh waves to operate, concentrating mechanical energy within a surface region approximately one wavelength deep These waves travel along solid surfaces at speeds ranging from \$10^3\$ m/s to \$10^4\$ m/s, enabling effective filtering in the VHF and UHF frequency ranges The design of SAW filters features a planar structure with electrodes arranged on a piezoelectric substrate, facilitating the 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
The filter consists of N taps, each separated by delays \(D_n\) and weighted by coefficients \(A_n\) Filtering is performed by passing the signal through multiple delay paths and summing the delayed signals, with the delays corresponding to the positions of IDT fingers on a substrate The coefficients represent the weighting assigned to these IDT fingers The frequency response of the filter \(H(f)\) is determined through a discrete Fourier transformation, as described in Equation (1) at frequency \(f\).
(1) where T n is the accumulated delay at the nth tap
Both amplitude and phase characteristics of the transversal filter are given by two sets of variables: weighting coefficients A n and delays D n of the sampling taps
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
The surface wave in an IDT is generated through the piezoelectric effect and travels in both directions along the substrate surface At the output IDT, this surface wave is converted back into an electrical signal Maximum conversion efficiency occurs when the IDT's spatial period, 2d, is uniform, allowing the surface wave to propagate one transducer period synchronously with one RF signal period The center frequency \( f_0 \) of the IDT is determined by this synchronization condition, expressed as \( df_0 = \nu_s \).
2 (2) where ν s is the SAW velocity
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:
N is the number of finger pairs
Cut-off frequency Centre frequency Cut-off frequency
(MHz) G roup del ay di st or tion
N om inal gr oup del ay
M in im um ins er tion at tenuat ion N om inal ins er tion at tenuat ion P as s- band rippl e
A ttenua tion ( dB ) R el at iv e at tenuat ion G roup del ay ( à s)
Figure 1 – Frequency response of a SAW filter
Weighting methods
The IDT functions as a transversal filter featuring N taps for weighting, utilizing various applicable methods such as apodization, withdrawal, and series (dog-leg) weighting Among these, apodization weighting is a notable technique.
An apodized transducer, illustrated in Figure 6, is primarily utilized for achieving weighting This device generates or detects acoustic waves exclusively in areas where adjacent electrodes of opposite polarity intersect Additionally, it employs withdrawal weighting techniques.
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
R el at iv e bandw idt h ( % )
Figure 2 – Applicable range of frequency and relative bandwidth of the SAW filter and the other filters
Figure 3 – Schematic diagram showing signal flow through a transversal filter
Figure 4 – Basic configuration of a SAW transversal filter
R el at iv e at tenuat ion ( dB ) f 0
Figure 5 – Frequency response of the SAW transversal filter shown in Figure 4, where f 0 is the centre frequency and N is the number of finger pairs of the IDT
Figure 6 – Apodization weighting obtained by apodizing fingers
Figure 7 – Withdrawal weighting obtained by selective withdrawal of the fingers
Figure 8 – Series weighting obtained by the dog-leg structure
Filter configurations and their general characteristics
General
The split-finger configuration, illustrated in Figure 9, serves as an effective alternative to the solid-finger design depicted in Figure 4, minimizing surface acoustic wave (SAW) reflections at metal electrodes This geometry allows for the cancellation of individual reflections caused by acoustic impedance discontinuities within each finger pair As a result, the split-finger configuration has gained popularity in applications such as SAW TV-IF filters.
Ordinary IDTs show bidirectional property These bidirectional IDTs transmit and receive
Surface acoustic waves (SAWs) are generated by a transmitting interdigital transducer (IDT), which converts an electric signal into SAWs that propagate in both forward and backward directions with equal intensity A receiving IDT can capture these waves with the same efficiency, allowing for the estimation of bidirectional loss values at 3 dB for both transmitting and receiving processes.
The bidirectional loss of 6 dB is a fundamental characteristic of a bidirectional two-transducer SAW filter, representing the minimum insertion attenuation Additionally, these standard SAW filters experience significant pass-band ripple due to the triple transit echo (TTE) when the impedances of the transmitting and receiving IDTs are matched to the external loads.
To minimize bidirectional loss and triple transit echo (TTE) in surface acoustic wave (SAW) transversal filters, the implementation of multi-IDT (IIDT) filters, which include three-IDT SAW filters, as well as unidirectional IDT filters, is essential.
(including tapered IDT filters) are utilized
Reflector filters, a type of transversal filter, incorporate grating technology to alter the propagation direction of surface acoustic waves (SAWs) based on specific reflection frequency responses These filters leverage both the inherent characteristics of transversal filters from the transducers and the diverse reflection frequency responses of the reflector's grating structure This combination actively shapes the filter transfer function while effectively minimizing the chip length through a folding technique.
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 8 λ IEC 770/12
Bidirectional IDT filters
In standard bidirectional two-IDT filters, the transmission through the filter is significantly decreased by mismatching the IDTs to the outer loads, which results in a lower TTE at the cost of increased insertion loss This configuration is exemplified in a frequency symmetrical band-pass filter.
The center frequency and bandwidth of an Interdigital Transducer (IDT) are determined by the finger periods and the number of finger pairs, respectively As frequency increases, the phase lag also increases, resulting in a constant group delay within the pass-band A common use of a frequency symmetric band-pass filter is as an intermediate frequency (IF) filter in radio transmission systems, where linear-phase characteristics and a flat pass-band amplitude are essential Figure 10 illustrates the frequency response of a Surface Acoustic Wave (SAW) filter with a nominal frequency of 70.0 MHz, and high-frequency SAW filters are available that offer greater selectivity Additionally, frequency asymmetrical band-pass filters are also discussed.
SAW transversal filters allow for independent design of amplitude and phase characteristics, enabling the creation of asymmetrical pass-band, stop-band, and group delay characteristics relative to a reference frequency through advanced design techniques Notably, SAW TV-IF filters exhibit frequency asymmetrical characteristics.
Comb filters are now available and have been proposed for use in various applications Recent civil spread spectrum (SS) systems, such as wireless LANs, utilize SAW matched filters Additionally, SAW filters featuring Nyquist characteristics have been developed to enhance modern communication systems.
4.3.2.2 Multi IDT/interdigitated interdigital transducer (IIDT) SAW filters
Multi-IDT filters, also known as interdigitated interdigital transducer filters, have evolved from three-IDT filters to meet the growing demand for low-loss filtering solutions To understand this development, it is essential to first explain the concept of three-IDT filters.
A three-IDT type SAW filter features two identical receiving IDTs symmetrically positioned around a central transmitting IDT When the central transducer is properly tuned to the center frequency, it effectively absorbs the two oppositely directed surface acoustic waves (SAWs), which is the reverse of the process that generates these waves.
SAWs are utilized by a tuned and matched transducer, which enhances insertion attenuation to 3 dB when two receiving transducers are connected and tuned at the center frequency, effectively eliminating TTE Figure 13 illustrates the typical frequency response of a SAW three-IDT filter operating in the 900 MHz range.
This operation principle is extended to the multi-IDT filters b) Multi-IDT/Interdigitated interdigital transducer filters
Multi-IDT filters, or interdigitated interdigital transducer filters, feature input IDTs arranged inter-digitally to connect with output IDTs As shown in Figure 14, this configuration includes (N + 1) input transducers and N output transducers This design significantly reduces the bidirectional 6 dB loss found in two-IDT filters and eliminates the triple transit echo when the input and output ports are properly matched to the outer loads.
When input and output transducers are properly tuned and matched to the circuit, the filter's insertion attenuation, as depicted in Figure 14, decreases to the residual bidirectional loss from the outermost input transducers This loss is inversely proportional to the number of transducers used.
Unidirectional IDT (UDT) filters
Unidirectional filters exhibit low insertion attenuation and superior frequency characteristics due to the directivity of surface wave propagation Ideally, these filters achieve an insertion attenuation of less than 1 dB, allowing for independent control of both amplitude and phase characteristics They can be categorized into two main types, one of which is the multi-phase unidirectional transducer, where electrical fields with varying phase differences are applied.
The other category is the single-phase unidirectional transducer applied with the same phase field a) Multi-phase unidirectional transducers
Three-phase unidirectional and group-type unidirectional transducers exemplify this category, with their unidirectionality stemming from the application of three voltages that are 120° out of phase However, the design involves a third electrode crossing over another via an insulated bridge, which compromises the planarity and reliability of the filter.
The group-type unidirectional transducer, illustrated in Figure 15, effectively addresses previous limitations This transducer consists of several pairs of electrodes, activated with a 90° electrical phase shift, which are considered as a single group Multiple groups can be aligned collinearly, allowing their signals to combine in phase, resulting in a filter that exhibits low insertion attenuation.
Conventional weighting techniques are also applicable in this transducer b) Single-phase unidirectional transducers (SPUDTs)
Single-phase unidirectional transducers (SPUDTs) leverage internal reflections to create unidirectional behavior A schematic representation of a unidirectional transducer employing internal floating electrode reflection, known as a floating electrode unidirectional transducer (FEUDT), is illustrated in Figures 16a-16c.
The transducer depicted in Figure 16a achieves unidirectionality through the offset arrangement of floating open metal strips relative to the positive and negative electrodes Figures 16b and 16c illustrate additional configurations involving floating short metal strips and their combinations Figures 16d and 16e showcase SPUDTs utilizing internal reflection without floating electrodes, with Figure 16d representing a distributed acoustic reflective transducer (DART) and Figure 16e illustrating a different width split finger (DWSF) The electrode width controlled (EWC) SPUDT is similar to the DART, differing only in finger thickness, with DART and EWC widths measuring 3/8λ and 1/4λ, respectively The DWSF, based on the conventional split finger transducer, features varying widths in one split finger pair while maintaining a half-wavelength period, resulting in unidirectionality.
4.3.3.2 Principle a) Multi-phase unidirectional transducers
In a multi-phase unidirectional transducer group, the phase difference between the waves generated by the sending and reflecting electrodes is zero in the forward direction and 180° in the reverse direction The experimental filter configuration demonstrates a minimum insertion attenuation of 1.0 dB and a pass-band ripple of less than 0.2 dB at a center frequency of 99.2 MHz This transducer features four pairs and eleven group electrodes, utilizing a 128° rotated Y-cut X-propagated LiNbO₃ substrate.
50 Ω coaxial cable have been used as a SAW propagation medium and a 90° phase shifter respectively
Figure 17 shows an experimental attenuation-frequency characteristic of a 70 MHz SAW
The article discusses an IF filter designed for a digital-cellular base station, featuring an unapodized multi-phase unidirectional transducer as the input and an apodized bidirectional transducer as the output Both transducers are fabricated on a 128° rotated Y-cut X-propagated LiNbO₃ single crystalline substrate The filter demonstrates an insertion attenuation of 8 dB and a pass-band ripple of 0.2 dB peak-to-peak within a frequency range of 70 MHz ± 1.6 MHz, highlighting its effectiveness in signal processing Additionally, the article mentions the use of single-phase unidirectional transducers.
A single-phase unidirectional transducer exhibits a phase difference of zero (in phase) for forward waves and 180° (opposite phase) for reverse waves, attributed to the bilateral asymmetry of its internal structure This asymmetry is achieved through techniques such as mass-loading effect, reflector array, changes in the electromechanical coupling coefficient, and internal floating electrode reflection These transducers are produced in a single photolithographic process, eliminating the need for external phase shifters Experimental results of a single-phase unidirectional transducer utilizing internal floating, short, and open strips are illustrated in Figure 18.
Figure 16c Tuning is achieved for each transducer with a shunt wire wound inductor of
200 nH The resulting insertion attenuation of 2,3 dB at 97,3 MHz is obtained The bandwidth of the filter is about 3,0 %
R el at iv e at tenuat ion ( dB )
Figure 10 – Typical characteristics of a SAW IF filter for radio transmission equipment
R el at iv e at tenuat ion ( dB )
Figure 11 – Typical characteristics of a frequency asymmetrical SAW filter
(nominal frequency of 58,75 MHz for TV-IF use)
Figure 12 – SAW three-IDT filter
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Figure 13 – Typical frequency response of a 900 MHz range SAW filter for communication (mobile telephone use)
Figure 14 – Schematic of the IIDT (multi-IDT) filter
Figure 15 – Multi-phase unidirectional transducer λ 0 λ 0 12
Figure 16 – Single-phase unidirectional transducers
R el at iv e i ns er tion at tenuat ion ( dB )
Figure 17 – Frequency characteristics of a filter using multi-phase unidirectional transducers
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Figure 18 – Frequency characteristics of a filter using single-phase unidirectional transducers
Tapered IDT filters
One of the classical transversal type SAW filters is a broad-band SAW filter using fan-shaped
The tapered IDT, also known as the slanted finger IDT, features a variable electrode pitch perpendicular to the propagation direction, as illustrated in Figure 19 This design allows for an easy broadening of the filter bandwidth, making it ideal for broadband filter applications The bandwidth is influenced by the range of pitch variation, while the transition width from pass-band to stop-band is typically related to the number of electrodes used.
The narrow track aperture at the unit frequency point related to the electrode pitch significantly affects the filter response, often leading to a diffraction effect that can result in performance that is unexpectedly inferior to the theoretical expectations.
The implementation of a unidirectional IDT, as illustrated in Figure 16, enables the design of a low loss tapered IDT filter, achieving an insertion loss of less than 10 dB An example of the filter response for this low loss tapered IDT filter is depicted in Figure 20, showcasing an insertion loss of approximately [insert specific value].
7,5 dB with a centre frequency of 140 MHz and a pass-band width of 14 MHz using a 128° rotated Y-cut LiNbO 3 substrate
R el at iv e at tenuat ion ( dB ) 10
Figure 20 – Frequency response of a 140 MHz tapered IDT filter
Reflector filters
Several types of reflection grating filters have been documented, with their fundamental configurations illustrated in Figure 21 These configurations leverage grating reflection functions and serve as both filters and delay lines.
The most popular reflection grating filter can be said to be the reflective array compressor
The RAC filter, illustrated in Figure 21d, utilizes gradually changing array periods along the SAW propagation direction and employs doubly 90° (U-shaped) reflections to facilitate the propagation and reflection of acoustic waves in a U-shape This innovative design is primarily utilized in radar systems.
The Z-path filter, illustrated in Figure 22, is a practical modification of the conventional design depicted in Figure 21e, aimed at reducing chip size In this configuration, an input transducer excites the surface acoustic wave (SAW), while a pair of weakly inclined reflectors, typically at an angle of 4°, facilitates the coupling of the wave from the upper track to the lower track, where it is detected by the output transducer.
The in-line configuration depicted in Figure 21a is ineffective due to the presence of high-level spurious signals from the input transducer to the output transducer In contrast, the dual-track concept illustrated in Figure 23 effectively cancels these direct spurious signals, resulting in a significantly improved filter response.
A variation of the dual-track filter features reflectors positioned centrally between the two tracks, designed to be nearly identical The only distinction between the tracks is the distance of one reflective electrode, resulting in lengths of λ/2.
Transducers are chosen to be single-phase unidirectional transducers (SPUDT, see 4.3.3.1 b)) and hence themselves reflective SPUDT-reflector filters represent an alternative if low attenuation is an additional requirement
RAC filters are primarily utilized as pulse expansion and compression devices featuring dispersive grating arrays, functioning effectively as band-pass filters with reflective responses Z-path filters are particularly advantageous for narrow band filters, offering a relative bandwidth of 0.2% to 1% within frequencies below 100 MHz These filters are constructed from quartz substrate, which helps mitigate the temperature dependence of the reflection angle in the two weakly inclined reflectors, ensuring good temperature stability of the crystal Additionally, they can achieve insertion attenuations ranging from 6 dB to 10 dB.
The frequency response of a Z-path filter at 71 MHz, illustrated in Figure 25, reveals disturbances in the upper stop-band These disturbances are characteristic of Z-path designs and arise from direct acoustic feed-through from the input to the output.
The dual-track filter features four IDTs arranged in a mutually blind configuration, with two input transducers driven 180° out of phase and two output transducers in phase Its transfer function is defined as the product of the transfer functions of the input and output transducers along with the reflector response in the frequency domain This configuration significantly reduces filter length compared to traditional transversal designs, allows for independent design of transducers and the reflector, and achieves excellent stop-band rejection through three cascaded filter mechanisms.
Disadvantages are the additional die width needed for two tracks, a somewhat more complex layout and additional loss from signal reflection
The SPUDT-reflector filter features four distinct propagation paths from input to output, with two paths in each track The first path involves the input signal traveling through the center reflector, reflecting off the output transducer, and returning to the center reflector before detection The second path also includes two reflections before passing through the center reflector This design incorporates four selection mechanisms—input transduction, center grating reflection, transducer reflection, and output transduction—resulting in a stop-band and an impulse response duration approximately twice that of transversal filters SPUDT-reflector filters provide moderate bandwidths ranging from 0.5% to 2% and exhibit insertion attenuation levels between 6 dB and 10 dB.
Figure 27 shows the frequency response of a 110 MHz filter on X-cut 112,2° rotated
Reflector filter designs take advantage of the fact that the reflection function of a grating structure has a time duration that is twice that of the excitation or detection function of a transducer of the same length This occurs because the reflected acoustic signal must travel into the reflector and back out As a result, a total time domain that is double the length of the reflector structure is available for applications such as narrow bandwidths, pass-band shaping, or pulse expansion and compression Consequently, reflector filters are typically shorter than conventional transversal filters.
Figure 21 – Various reflector filter configurations
Figure 23 – Dual-track reflector filter configuration
Figure 24a – SPUDT-based dual-track filter configuration
Figure 24b – Propagation paths on SPUDT-based dual track filter
Figure 24 – SPUDT-based dual-track filter
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Figure 25 – Frequency characteristics of Z-path filter
R el at iv e at tenuat ion ( dB )
Figure 26 – Frequency characteristics of dual-track reflector filter
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Figure 27 – Frequency characteristics of SPUDT-based reflector filter
RSPUDT filters
Classical transversal filters rely on the transfer function from the transmitter side IDT to the receiver side IDT for their filter responses, aiming to minimize internal reflections and multi-reflections, such as triple transit echo (TTE) In contrast, the innovative resonant SPUDT (RSPUDT) filter design has emerged, gaining popularity for its ability to effectively utilize internal reflections and multi-reflections between input and output IDTs.
IDTs in order to achieve required filter response Figure 28 shows a part of DART electrode in
RSPUDT filter In this case, inside the DART electrode, the direction and magnitude of each electrode and its transduction magnitude in each period are changed spatially as shown in
The RSPUDT filter experiences complex multi-reflection, as illustrated in Figure 28, allowing for a total response that is engineered to achieve the desired filter response with a shorter length compared to traditional transversal filters An example of this is depicted in Figure 29.
RSPUDT filter is designed to control the direction and amplitude of the internal reflection and the transduction amplitude
Figure 28 – A part of DART electrode in RSPUDT filter
Figure 29 – Distribution of internal reflection and detection inside RSPUDT filter
The RSPUDT filter, illustrated in Figure 30, demonstrates a filter response characterized by an insertion loss of 9 dB It operates at a center frequency of 456 MHz and features a pass-band width of 7 MHz, utilizing X-cut 112° propagation.
LiTaO 3 Its amplitude response is shown in Figure 30a and its impulse response is shown in
Figure 30b Figure 30b shows very clearly that RSPUDT filter’s time domain response is not symmetric like a normal transversal type and the response continues in very long time
Ins er tion at tenuat ion ( dB )
R el at iv e at tenuat ion ( dB )
Figure 30 – Frequency and time responses of a 456 MHz RSPUDT filter
5 Fundamentals of SAW resonator filters
Classification of SAW resonator filters
SAW resonator filters are becoming rapidly popular as SAW low insertion attenuation filters for mobile communication application in addition to the conventional SAW transversal filters
SAW resonator filters offer low insertion attenuation and a compact size compared to transversal filters with equivalent bandwidth However, their bandwidth is constrained by substrate materials and design methods, and their amplitude and phase characteristics cannot be independently designed Understanding these factors is essential for users of SAW resonator filters This standard outlines the principles and characteristics of these filters.
SAW resonator filters come in various types, all of which can be modeled near the pass-band using resonant circuits composed of lumped elements such as inductance (L), capacitance (C), and resistance (R) The primary distinction among these filters lies in the configuration of the basic resonators This resonant circuit concept is widely applicable to other piezoelectric filters, including crystal filters For a deeper understanding of these principles, consulting IEC 60368-2-1 is recommended.
SAW resonator filters are primarily categorized into two types: ladder and lattice filters, which consist of multiple one-port SAW resonators arranged in ladder and lattice configurations, and coupled resonator filters along with IIDT resonator filters, which leverage multiple modes within a single cavity to simplify the filter structure.
The ladder and lattice filters share the same concept and equivalent circuit as crystal filters, with the key distinction being the replacement of the crystal resonator with a one-port SAW resonator.
The practical constitution and the filter characteristics are given in 5.2
Coupled resonator filters leverage multi-mode resonances within a single surface acoustic wave (SAW) resonator, with a range of resonances defining their pass-band These filters can be categorized into transverse mode and longitudinal mode types based on their resonant modes Transverse mode coupled resonator filters typically employ double modes that occur perpendicular to the SAW propagation direction, resembling the characteristics of monolithic crystal filters In contrast, longitudinal mode coupled resonator filters utilize resonant modes aligned with the SAW propagation direction, resulting in stronger mode couplings and potentially wider bandwidths compared to their transverse counterparts The details of the constitution and filter characteristics for both types are elaborated in section 5.3.
IIDT resonator filters consist of several small-pair IDTs arranged in a line, interspersed with grating reflectors positioned outside the IDTs This design facilitates strong coupling between the input and output IDTs while leveraging multiple resonant modes Further details on this type can be found in section 5.4.
SAW filters utilizing shear horizontal (SH) waves can benefit from using substrate edges as grating reflectors, which aids in the miniaturization of SAW filters, as outlined in section 5.1 of IEC 61019-2:2005.
Ladder and lattice filters
Basic structure
Two types of one-port surface acoustic wave (SAW) resonators with slightly varying resonance frequencies are designed for integration into ladder or lattice circuits The lattice filter is particularly utilized for balanced circuit applications, while the ladder filter serves as an essential component in these configurations.
Figure 31a illustrates a filter structure, while Figure 32a depicts the equivalent circuit of a half-section of a ladder filter, assuming negligible resistance This half-section comprises a series-arm resonator (R1) and a parallel-arm resonator (R2), with the series-arm resonator exhibiting a slightly higher resonance frequency than the parallel-arm resonator The resonator features one interdigital transducer (IDT) positioned between two reflectors, and the electrodes of the SAW resonators are fabricated on a piezoelectric substrate.
Figure 33 The resonators R1′ and R2′ are synthesized resonators R1′ has half-static capacitance of R1, and R2′ has twice static capacitance of R2 b) Lattice filter
This type of filter comprises a pair of series-arm SAW resonators (R1) and a pair of parallel-arm SAW resonators (R2) electrically coupled to form a lattice circuit shown in
The equivalent circuit of a lattice filter, depicted in Figure 32b, assumes negligible resistance The frequency shift is selected to align the resonance frequency of one pair of resonators with the anti-resonance frequencies of the other pair.
Principle of operation
Figure 34a illustrates the relationship between X s and B p as a function of frequency, highlighting that the anti-resonance frequency (f ap) of the parallel-arm resonator closely matches the resonance frequency (f rs) of the series-arm resonator The image transfer constant γ is defined in terms of X s and B p through a specific equation.
X s is the equivalent series reactance of the resonator;
B p is the equivalent parallel susceptance of the resonator
The theory of image-parameter filters indicates that a filter exhibits a pass-band characteristic when the corresponding equation yields an imaginary number, while it demonstrates a stop-band characteristic when the equation results in a real number Specifically, the condition \(0 < B_p X_s < 1\) defines the pass-band, whereas the conditions \(B_p X_s > 1\) or \(B_p X_s < 0\) indicate the stop-band.
Figure 34b illustrates the frequency-dependent variations of \$X_s\$ and \$X_p\$ In this case, the anti-resonance frequency (\$f_{as}\$) of the series-arm pair closely matches the resonance frequency (\$f_{rp}\$) of the parallel-arm pair The image transfer constant \$\gamma\$ is defined in relation to \$X_s\$ and \$X_p\$ through a specific equation.
A filter shows a pass-band characteristic when Equation (5) has an imaginary number
However, it shows a stop-band characteristic when Equation (5) has a real number
Therefore, the condition X s /X p X s or
X p < X s gives the stop-band when the Equation (5) has a real number Condition
X p = X s gives maximum insertion attenuation shown in Figure 34b.
Characteristics of ladder and lattice filters
The performance of ladder and lattice filters is significantly influenced by the substrate material used, with a high electromechanical coupling coefficient being essential for achieving a wide pass-band The insertion attenuation of these filters is primarily determined by the quality factor (Q factor) of the resonators Additionally, the stop-band attenuation is influenced by the capacitance ratio between parallel-arm and series-arm resonators, as well as the number of resonator stages connected In lattice filters, maximum stop-band attenuation occurs when the static capacitance of series-arm resonators (R1) equals that of parallel-arm resonators (R2).
As a ladder filter example, the RF filter was designed and fabricated for portable tele- phone terminals An Al-Cu sputtered film for the electrodes and a 36° rotated Y-cut
X-propagated LiTaO 3 crystal for the piezoelectric substrate was used Figure 35 shows the frequency characteristic of a 1,5 GHz band-pass filter for a digital system The minimum insertion attenuation of less than 3 dB and the voltage standing wave ratio of less than 2 were obtained without an external matching circuit b) Lattice filter
A lattice filter operating in the 1.5 GHz range was designed and fabricated on a quartz substrate It exhibited a measured insertion attenuation of 3 dB, a stop-band attenuation exceeding 35 dB, and a 3 dB bandwidth of approximately 1 MHz.
Figure 31 – Structure of ladder and lattice filters
Figure 32a – Ladder filter of half section
Figure 32b – Lattice filter of full section
Figure 32 – Equivalent circuit of basic section of ladder and lattice filter
Figure 33 – Pattern layout of ladder filter
A ttenua tion ( dB ) f ap f rs
Figure 34 – Basic concept of ladder and lattice filter
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Figure 35b – Pass-band response of amplitude and group delay time
Figure 35 – Typical characteristics of a 1,5 GHz range ladder filter
Coupled resonator filters
General
The operation of coupled resonator filters is similar to that of monolithic crystal filters (MCF)
Acoustic coupling between identical resonators generates various resonance modes with distinct frequencies, including symmetric, anti-symmetric, and higher order modes These modes, characterized by different frequencies and opposite phases, can create a band-pass filter when properly terminated.
Transversely coupled type
A transversely coupled filter featuring two one-port resonators positioned closely in the transverse direction generates both a zero order transverse mode (symmetric mode) with a symmetric SAW amplitude distribution and a first order transverse mode (anti-symmetric mode) with an anti-symmetric distribution The frequency difference between these modes is influenced by the distance between the resonators, the aperture of the IDTs, and the degree of energy trapping This filter, depicted in its equivalent circuit, exhibits narrow bandwidth characteristics and typically employs stable substrate materials like quartz to maintain the pass-band at a specified frequency Despite its narrow bandwidth, this filter is significantly smaller than a transversal filter, whose size is inversely proportional to its bandwidth.
Longitudinally coupled type
In the case of a longitudinally coupled resonator filter with two IDTs arranged in series between grating reflectors as shown in Figure 38a, the zero order resonance mode
The symmetric and first order resonance modes, also known as anti-symmetric modes, are generated similarly Typically, the resonance frequency of higher order longitudinal modes is lower than that of lower order modes The frequency difference between these two modes is primarily influenced by the number of IDT fingers and the extent of energy trapping.
The double-mode filter configuration, illustrated in Figure 38b, utilizes zero order and second order longitudinal modes, resulting in a wider pass-band compared to first order modes due to the lower frequency of the second order mode Figure 39 depicts the transmission characteristics of a longitudinally coupled resonator filter, which exhibits stronger acoustic coupling between interdigital transducers (IDTs) and consequently a broader pass-band than transversely coupled filters To achieve a wider pass-band, it is essential to reduce the capacitance ratio of the resonators, which can be effectively accomplished by using substrate materials with a high electromechanical coupling coefficient, such as LiTaO₃.
Other characteristics of coupled resonator filters
The insertion attenuation of both types of filters is determined by the Q of the resonators
A higher quality factor (Q) results in reduced insertion loss in filters These filters can exhibit various spurious responses, primarily due to the arrangement of interdigital transducers (IDTs) and reflectors, which may include higher-order inharmonic resonance modes and the inherent responses of the IDTs and reflectors.
The next ones are caused by different kinds of waves generated in IDTs or converted from
SAW in IDTs and reflectors or at the edge of the substrate
Figure 36a – Basic configuration and SAW energy distribution of transversely coupled resonator filter
Figure 36b – Equivalent circuit of transversely coupled resonator filter
Figure 36 – SAW energy distribution and equivalent circuit of transversely coupled resonator filter
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Figure 37b – Pass-band response of amplitude and group delay time
Figure 37 – Typical characteristics of a transversely coupled resonator filter
S A W ener gy di st ribu tion
Figure 38a – SAW energy distribution of longitudinally coupled resonator filter using zero order and 1st order modes
Figure 38b – Resonator filter using zero order and 2nd order modes
Figure 38 – Basic configuration and SAW energy distribution of longitudinally coupled resonator filter
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Figure 39b – Pass-band response of amplitude and group delay time
Figure 39 – Typical characteristics of a longitudinally coupled resonator filter
Balanced connection
In cell phones etc., amplifiers or mixers used are often balanced type in order to reduce noise
To connect without an unbalanced-balanced conversion circuit like a balun, a balanced terminal is essential for a SAW filter Although achieving a balanced terminal on a ladder type filter is challenging, a coupled resonator filter allows for an easy implementation of a balanced type without the need to ground either terminal on the IDT.
A transversely coupled type, with the structure shown in Figure 36a, divides its common grounding electrodes to input and output separately, and is configured as shown in Figure 40
Figure 41 illustrates the frequency characteristics of a transversely coupled type, which shares similarities with an unbalanced type However, its advantage lies in its ability to easily achieve attenuation Additionally, a longitudinally coupled type features a distinct electrode structure.
Figure 38b demonstrates the ability to achieve balanced terminals without grounding either terminal on the IDT A longitudinally coupled type operating in zero order and second order longitudinal mode utilizes a configuration, as depicted in Figure 42a, that enables balanced output of the IDT on both sides, as illustrated in Figure 42b.
To achieve a balanced output, it is essential that the amplitudes are equal and the phases are 180° apart This requires the IDT and the shape of the bus bar to be arranged in a geometrically symmetrical manner.
43a shows an example of the frequency characteristics of the longitudinally coupled type, while Figure 43b shows an example of amplitude and phase balance characteristics And
Figure 43c shows an example of stopband attenuation characteristics across a wide band range that is a benefit of the balanced filters
Figure 40 – Configuration of balanced type transversely coupled resonator filter
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Figure 41 – Frequency characteristics of balanced type transversely coupled resonator filter
Figure 42 – Configuration of balanced type longitudinally coupled resonator filter
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Ins er tion at tenuat ion ( dB )
Am pl itude bal anc e dB
A m pl itude bal an ce (dB ) P has e ba lanc e ( °)
Figure 43b － Amplitude and phase balance characteristics
Ins er tion at tenuat ion dB
Ins er tion at tenuat ion ( dB )
Figure 43 – Typical characteristics of a balanced type longitudinally coupled resonator filter
Interdigitated interdigital transducer (lIDT) resonator filters
Configuration
The IIDT filter outlined in section 4.3 exhibits residual bidirectional loss due to the outermost electrodes To mitigate these losses, various configurations are suggested For instance, Figure 44 illustrates an IIDT filter featuring grating reflectors on both sides of the IIDT setup It is essential to achieve an increase in out-band rejection while simultaneously reducing losses.
Principle
Grating reflectors, as illustrated in Figure 44, effectively reflect surface acoustic waves (SAWs) emitted from the outermost transducers, minimizing the residual bidirectional loss at these transducers Adjusting the placement and the number of finger pairs in the transducers can lead to decreased SAW power flow densities at the outermost transducers, further contributing to loss reduction.
Characteristics
Recent IIDT filters exhibit insertion attenuation ranging from 2 dB to 2.5 dB in a 50 Ω circuit without external matching elements, provided that the fractional bandwidth is suitable and a high-coupling piezoelectric single-crystalline substrate, such as the 64° rotated Y-cut, is employed.
X-propagated LiNbO 3 ) The frequency characteristics of this type are shown in Figure 45 A three-transducer-configuration filter with reflector gratings, as a kind of IIDT, also shows small insertion attenuation lower than 2 dB Some configuration variations and optimization methods for IIDT filter designing are discussed in the scientific literature
Figure 44 – Schematic of IIDT resonator filter
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Figure 45 – Frequency characteristics of a 820 MHz range IIDT resonator filter
Substrate materials and their characteristics
Various kinds of piezoelectric substrates are available for SAW filter applications Piezo- electric substrates for SAW filters are selected according to the following:
– temperature coefficient of delay (TCD) or frequency (TCF);
The constants listed from items a) to e) primarily relate to materials, while items f) and g) pertain to conditions influenced by both materials and substrate fabrication techniques Various types of substrates have been developed and successfully implemented in practical applications.
For optimal performance, a high coupling coefficient and a zero temperature coefficient are ideal; however, achieving this is currently unattainable, necessitating design trade-offs Selecting an appropriate substrate based on specific requirements is essential, and the relationships between material constants and filter characteristics are detailed in the subsequent sections.
Displacement of SAW is composed of three components, propagating direction: L
(longitudinal), vertical to the substrate: SV(shear vertical) and perpendicular direction to both:
SH(shear horizontal) Rayleigh wave, which is the earliest known mode, has L and SV components as its dominant components The dominant component of SH wave is shear horizontal
When a slow propagation velocity layer is formed on a substrate, a kind of SH wave called
Love wave may exist Heavy metal electrode fingers of IDTs and reflectors such as copper or gold act similarly with a slow layer to make effective velocity lower
In thick layered structures, boundary modes can exist where energy is concentrated at the edges These boundary modes have the advantage of not requiring hollow spaces for vibration on the substrate's surface, unlike conventional surface acoustic wave (SAW) modes.
Propagation velocity ν s (m/s) is an important factor, which determines centre frequency f 0
(MHz) given approximately by f 0 = ν s /(2 d) where d (àm) is one-half of the IDT periodic length, as shown in Figure 4
For a given center frequency, lower velocities necessitate a shorter finger period and a smaller chip size, while higher velocities are preferred for high-frequency filters to simplify IDT fabrication The propagation velocity in practical substrates typically ranges from 2,000 m/s to 5,000 m/s Additionally, the coupling coefficient plays a crucial role in the performance of these filters.
The SAW coupling coefficient \( k_{s}^2 \) represents the ratio of electric energy to mechanical (SAW) energy In transversal filters, both the minimum insertion attenuation and the maximum relative bandwidth are influenced by this coupling coefficient.
A large coupling coefficient can significantly reduce insertion attenuation and increase bandwidth in resonator filters It is the key factor influencing the capacitance ratio \( r \) When the substrate's coupling coefficient is sufficiently high, designing a Surface Acoustic Wave (SAW) resonator with a low capacitance ratio becomes feasible, thereby allowing for an expanded bandwidth.
C onv er si on l os s (dB )
Figure 46 – Minimum theoretical conversion losses for various substrates c) Temperature coefficient
The frequency response of filters is influenced by ambient temperature, leading to a significant issue: the shift in the center frequency Substrate materials typically show a linear relationship between temperature changes and relative frequency shifts, where the relative frequency shift is approximately equal to the product of the temperature coefficient of frequency (TCF) and the temperature change Notably, the TCF is nearly equal in magnitude but opposite in polarity to the temperature coefficient of delay (TCD).
Rotated Y-cut (around ST-cut) quartz, Li 2 B 4 O 7 and some kinds of ZnO thin films on glasses have zero TCF at a certain temperature d) Relative permittivity
The permittivity of the piezoelectric material is a second-order symmetric tensor
In the case of a normal IDT whose line and space (metallization) ratio is 1:1, the static capacitance of the IDT, C T , is approximately expressed as
C T = w N (1 + ε r )ε 0 where w is the IDT aperture;
N is the number of finger pairs; ε r is the relative permittivity of the substrate; ε 0 is the permittivity of vacuum
The electric field distributions are complex, leading to the use of an effective relative permittivity, defined as \$\varepsilon_r = \frac{\varepsilon_{11} \varepsilon_{33} - \varepsilon_{13}^2}{\varepsilon_{11} + \varepsilon_{33} - 2\varepsilon_{13}}\$ for analysis The permittivities \$\varepsilon_{11}\$, \$\varepsilon_{33}\$, and \$\varepsilon_{13}\$ represent the tensor components of the material A higher permittivity value correlates with increased static capacitance Typical \$\varepsilon_r\$ values for various substrates are detailed in Tables 1, 2, and 3 Additionally, propagation loss is a critical factor to consider.
Insertion attenuation is influenced by three key factors: propagation loss, beam-steering loss, and air-loading loss Propagation loss is determined by the substrate's material and surface finish For well-polished, high-coupling single-crystal substrates, propagation loss typically remains below 1 dB/às at 1 GHz.
Propagation loss increases with the square of the frequency, while beam-steering loss arises when the phase-velocity vector direction does not align with the acoustic power-flow direction To minimize these losses, substrate orientation is typically adjusted to ensure both directions coincide Additionally, air-loading loss, which results from acoustic waves radiating into the air, is also frequency-dependent but is generally minimal compared to other losses Typical single-crystal materials are often used in these applications.
The properties of single-crystal substrates are influenced by the angle of cut and the surface acoustic wave (SAW) propagation direction due to crystal anisotropy Single crystals offer benefits such as reproducibility, reliability, and minimal propagation loss However, achieving a material that simultaneously possesses a large coupling coefficient and a small temperature coefficient remains challenging Table 1 presents typical crystals along with their recommended angles of cut and material constants for SAW filters, particularly for low-loss RF filters and duplexers.
36° rotated Y cut LiTaO 3 is widely used in the present state
Recent advancements involve the use of high coupling coefficient single-crystal substrates, such as LiTaO₃ and LiNbO₃, in combination with dielectric thin films like SiO₂, which exhibit an opposite temperature coefficient This innovative approach aims to create devices that feature both a high coupling coefficient and a low temperature coefficient For instance, the piezoelectric substrate is often cut at an angle close to the 15° rotated Y cut of LiNbO₃.
The selection of an optimal combination of dielectric materials, IDT materials, and their respective thicknesses, along with the cut angle of the piezoelectric and the propagation direction, significantly influences various parameters such as coupling coefficient, temperature coefficient, and propagation velocity Typical thin-film materials play a crucial role in this process.
There are a variety of combinations of thin-film materials, bases and structures in thin-film
SAW filters can be optimized through careful design and material selection, enhancing properties such as coupling coefficient and temperature coefficient Utilizing a substrate with a temperature coefficient that opposes that of the thin film can lead to an overall improvement in the total temperature coefficient, with some combinations achieving a zero temperature coefficient at specific temperatures Polycrystalline zinc oxide (ZnO) is commonly employed as a thin-film material due to its excellent electromechanical coupling, while single-crystal films are developed for high-frequency applications Typical ceramic materials used in these configurations are detailed in Table 2.
Ceramic materials have advantages in that various characteristics can be improved by the selection of material compositions They exhibit a relatively large coupling coefficient
Ceramics consist of tiny crystal grains, typically measuring several microns in diameter, which leads to significant propagation loss at high frequencies, particularly above 100 MHz Relevant data for ceramics can be found in Table 3.
Table 1 – Properties of typical single-crystal substrate materials
Angle of cut Propagation direction Velocity ν s
Table 2 – Properties of typical thin-film substrate materials
Thin-film and base materials and structure
Temperature coefficient Relative permittivity ε r m/s % 10 –6 /K p-ZnO/IDT/glass base 2 576 1,4 –11 10,8
Metal/p-ZnO/IDT/glass base 3 200 0,8 –7 10
NOTE p and s represent polycrystalline films and single-crystal films respectively The glass bases are boro- silicate glass
Table 3 – Properties of typical ceramic substrate materials
Pb(Sn 1/2 Sb 1/2 )O 3 -PbTiO 3 -PbZrO 3 2 420 2,4 –38 270
0,1Pb(Mn 1/3 Nb 2/3 )O 3 -0,9Pb(Zr 0,74 Ti 0,26 )O 3 2 430 2,9 –17 460
Application to electronics circuits
SAW filter characteristics are also governed by the tuning networks and external circuits In order to obtain a satisfactory performance, certain precautions are required a) Insertion attenuation
Insertion attenuation in SAW filters primarily arises from several factors, including transducer conversion loss, ohmic loss in metal electrodes of the IDT, acoustic propagation loss, bulk mode conversion loss, leakage from reflector sides, bidirectional propagation loss, and apodization loss In practical scenarios, particularly with bidirectional IDT filters, conversion loss and bidirectional loss are typically the predominant contributors to insertion attenuation.
The IDT conversion loss depends on the impedance matching between the IDT and the external circuits According to the equivalent circuit model, the impedance of the IDT of
SAW transversal filters are capacitive in nature By tuning suitable coils at the center frequency of the SAW filter, conversion loss can be minimized When impedance matching is perfect, the conversion loss can be effectively ignored.
2 /4 f / f k s > π ∆ where s 2 k and Δf / f 0 denote the coupling coefficient and relative bandwidth, respectively
On the other hand, in the case expressed as:
2 /4 f / f k s < π ∆ the attainable minimum conversion loss is limited and the minimum conversion loss is inversely proportional tok s 2 Figure 46 gives the minimum theoretical conversion losses for various substrates
To minimize the bidirectional loss of 6 dB, a three-IDT structure is utilized, where the output transducers on both ends are connected in parallel, resulting in a 3 dB reduction in loss An ideal unidirectional IDT can achieve a bidirectional loss of zero Additionally, noise figures and other issues may arise in applied circuits.
Insertion attenuations for bidirectional IDT filters are typically higher than those of conventional LC filters When substituting conventional LC filters with SAW filters, an additional amplifier may be necessary to offset the increased insertion attenuation There are two types of amplifiers relevant to SAW filters: pre-amplifiers and post-amplifiers, each with its own set of advantages and disadvantages that users and circuit designers should carefully evaluate.
In a pre-amplifier, amplifying the signal at an early stage can lead to interference from non-linearity, resulting in cross-modulation and intermodulation issues To mitigate this, implementing a negative feedback loop is advisable, and maintaining a low gain is preferable Conversely, a post-amplifier can resolve interference problems, although it may worsen the overall noise figure due to the significant insertion attenuation of a SAW filter If the input signal is attenuated by the SAW filter, the post-amplifier's noise can degrade the system's noise figure To improve this, precise impedance matching is essential as it reduces conversion loss at the SAW filter It is crucial to design front-stage amplifiers with adequate gain relative to the system's noise figure and sufficient linearity to prevent cross-modulation and intermodulation interference.
TTE, an unwanted signal resulting from multiple acoustic reflections between input and output transducers, exhibits a delay of 2t behind the main signal, where t represents the delay of the main signal As illustrated in Figure 48, TTE induces ripples with a period of 1/(2t) in the amplitude and group delay characteristics within the pass-band of a SAW filter A TTE that is 40 dB below the main signal leads to approximately ±0.1 dB amplitude ripple and ±0.02t group delay distortion This delayed arrival of TTE at the output causes "ghost" interference, or duplicate images, on the screen of a television set equipped with a SAW filter in the video intermediate frequency stage.
TTE results from the electrical regeneration of the SAW at the IDT To mitigate this regeneration, increasing the terminating impedances and the IDT conversion loss is often effective The enhancement in TTE suppression can be approximated as double the increase in insertion attenuation measured in decibels For effective suppression of TTE due to regeneration, the terminating impedance must significantly exceed the IDT impedance.
In the case where the insertion attenuation is compensated by an amplifier in front of the filter, the output impedance of the amplifier should be as high as possible
SAW filters using ordinary bidirectional transducers face inherent TTE issues However, unidirectional IDT filters and IIDT filters can effectively reduce insertion attenuation while simultaneously suppressing TTE These SAW filters are optimized for specific impedance matching conditions, as any impedance mismatch can lead to increased TTE and higher insertion attenuation.
Availability and limitations
The relationship between relative bandwidth and insertion attenuation for each type of SAW filter with the bandwidth of SAW filters used in a typical telecommunication system is shown in
A SAW filter features a complex mechanical structure that can lead to various unwanted responses, in addition to TTE, which may negatively impact its characteristics It is essential to suppress or minimize these unwanted responses to maintain optimal performance Additionally, long-term stability is a crucial factor to consider in practical applications.
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Figure 47 – Relationship between relative bandwidth and insertion attenuation for various SAW filters, with the practical SAW filters’ bandwidth for their typical applications a) Harmonic response signals
Harmonic response signals in a SAW filter, similar to those in a piezoelectric filter, can disrupt the stop-band characteristics The level of spurious harmonic response signals is influenced by the metallization ratio and the electrode configuration within the SAW filter Additionally, bulk-wave signals play a significant role in this context.
Bulk-wave signals are produced at the input IDT and surface acoustic waves (SAW), being detected by the output IDT after reflecting off the substrate's bottom or directly when propagating near the surface These signals, which are faster than SAW, influence the stop-band attenuation in the upper frequency range of the pass-band To mitigate these signals, it is advisable to roughen the substrate's bottom and/or apply a multistrip coupler between the input and output transducers.
Feed-through signals directly connect input and output circuits through electrostatic or electromagnetic coupling, resulting in an immediate appearance at the output terminal upon applying input voltage Similar to TTE, these signals induce ripple in the pass-band, as illustrated in Figure 48, with a frequency period (\(\delta f\)) equal to \(1/t\), which is twice as wide as that of other signals.
TTE involves a delay in the main signals, which can lead to the filling of traps in the stop-band and negatively impact its characteristics To mitigate these effects, a shielding electrode is typically positioned between the input and output transducers Additionally, reflections from substrate edges can also influence performance.
Such reflections cause ripple in the pass-band, but can be easily reduced by inclining the substrate edges and by placing an absorber on the substrate e) Ageing performance
SAW filters demonstrate remarkable long-term stability, similar to bulk acoustic wave filters The ageing rate of a SAW filter is influenced by factors such as the input level, substrate mounting method, and the surrounding atmosphere For filters with narrow pass bandwidth and low insertion attenuation, hermetically sealed packages are typically employed.
R el at iv e at tenuat ion (d B )
Ripples in the characteristics of a Surface Acoustic Wave (SAW) filter are influenced by TTE (Time-Delay Element) and feed-through signals The frequency variations are represented by the equations \$\delta f = \frac{1}{2t}\$ for TTE and \$\delta f = \frac{1}{t}\$ for feed-through, where \$t\$ denotes the delay of the main SAW signal.
Input levels
Drive level performance is limited by
– frequency shift and/or response change;
The damage to the IDT is irrecoverable, particularly due to the narrow spacing of the fingers, which typically ranges from 5 μm to 10 μm for a 100 MHz IDT Excessive drive levels can lead to flashover between the fingers, triggered by a strong electric field Additionally, intense acoustic strains may cause physical erosion of the electrodes, resulting in changes to frequency and response.
SAW acoustic power is limited to the surface of an elastic substrate, which allows SAW devices to demonstrate non-linear characteristics at lower drive levels more readily than traditional bulk-wave devices Additionally, the impact of DC voltage overdrive can further influence their performance.
Applying d.c voltage to a SAW filter, even at low RF signal input levels, can potentially damage the filter or negatively impact its characteristics It is essential to consult with the manufacturer to determine the appropriate d.c voltage level Additionally, the power durability of the filter should be considered.
Excessive mechanical stress can lead to electrode deterioration, resulting in voids and hillocks This deterioration causes shifts in center frequency, pass-band distortion, and degradation of insertion attenuation It is essential to establish the RF signal drive level in consultation with the manufacturer.
Packaging of SAW filters
Ceramic and metal packages are commonly utilized for Surface Acoustic Wave (SAW) filters The piezoelectric substrates, which host the Interdigital Transducers (IDTs), are affixed within the package using adhesive Electrical connections between the substrate and the package are established through wire bonding.
Figure 49 – Example of SAW metal package
Figure 50 – Example of SAW ceramic package
In other cases, a SAW filter chip is mounted on a metallic lead frame, and then molded by resin Figure 51 shows this structure
Figure 51 – Example of SAW resin package
Flip chip technology plays a crucial role in device miniaturization by utilizing gold stud bumps or printed solder paste on the SAW chip, which subsequently forms solder balls through a reflow process.
Chips are attached to substrates like ceramics using the flip chip method and are subsequently encased in resin or metal Additionally, cavity-type ceramic packages and metal lids serve as alternative structures These compact SAW devices, illustrated in Figure 52, are commonly referred to as CSPs (Chip Scale Packages).
Gold bump Gold or solder bump
Figure 52 – Example of SAW CSP
Further miniaturization is achieved recently by WLP (Wafer Level Package) In WLP, a piezoelectric substrate itself, on which SAW filters are formed, plays a role of some part of package
General
An incorrect usage of a SAW filter may at times result in its unsatisfactory performance It is necessary to take care of direct feed-through, impedance matching conditions, etc.
Feed-through signals
Feed-through signals are caused mainly by the electrostatic and electromagnetic couplings between the input and output circuits
To effectively reduce feed-through, utilizing a balanced (differential) circuit is the most efficient method, as it cancels out unwanted coupling signals caused by stray capacitance and current loops Integrated circuits (ICs) can seamlessly incorporate balanced input and output circuits Connecting a balanced output SAW filter with a balanced input IC significantly enhances the reduction of feed-through.
However, it is not effective to use a balun transformer to connect an unbalanced SAW filter with a balanced IC
To minimize electrostatic feed-through, implementing a shield between the input and output circuits on the printed circuit board (PCB) is an effective strategy Additionally, modifying the circuit pattern, particularly the ground configuration, often enhances performance in practical applications.
To minimize electromagnetic feed-through, it is crucial to design the input and output circuit patterns in a way that completely cancels the electromagnetic coupling caused by the current loop in the input circuit at the output Therefore, the circuit design must focus on reducing or eliminating both electrostatic and electromagnetic couplings.
To prevent the adverse effects of ground loops caused by common residual impedance in high-frequency applications with low terminating impedance, it is essential to design the input and output ground patterns on the PCB separately This approach minimizes the impact of feed-through signals and enhances overall circuit performance.
Impedance matching condition
The impedance matching condition affects mainly the pass-band characteristics and is generally more strict for low insertion attenuation SAW filters than for conventional SAW transversal filters
As for the low insertion attenuation SAW filter, such as a resonator filter, the specified terminating (load) impedances have to be used to obtain the specified performance Such a
SAW filter is designed under specific impedance matching conditions and impedance mismatching increases the amplitude ripple and the insertion attenuation of the SAW filter
In the case of a standard bidirectional IDT filter with high insertion attenuation, the impedance-matching condition is relatively lenient, allowing for a 10% variation in matching impedance without significantly affecting the pass-band characteristics of the SAW filter The focus on impedance matching primarily aims at suppressing the triple transit echo (TTE), which is largely influenced by this condition To effectively reduce the TTE signal, one of the simplest methods is to increase the insertion attenuation by intentionally mismatching the load within the limits of the circuit gain If specific echo suppression levels are outlined in the detailed specifications, it is essential to adhere to the designated terminating impedances to achieve the desired TTE suppression.