Microwave Engineering Europe - November 2007 - (Page 32)

32 SAW FILTERS Specifying the proper SAW ﬁlter By Stephen F. Magno, SAW Products Marketing Manager & Bob Holloway, Senior Product Support Engineer, IDT MicroNetworks Division oday’s crowded radio frequency spectrum from baseband through 3 GHz requires system designers to meet stringent regulatory requirements without sacriﬁcing performance. Filter requirements demand high selectivity, low insertion loss, ﬂat passbands, and uniform group delay to meet performance criteria. In addition, these ﬁlters must be highly repeatable, small size, low cost, and operate in adverse environmental conditions. Surface Acoustic Wave (SAW) ﬁlters are a perfect match to such requirements. Commercial applications of SAW ﬁlters are legion. They include, but are not limited to, telecommunications (base stations and hand-helds), WiMAX, set top box and cable modems, navigation (GIS/GPS), automotive, and medical. There are also Space and Military applications, including functions from simple time-delay to dispersive devices for complex matched ﬁltering applications. Unlike conventional RF ﬁlters that depend upon electrical parameters such as inductance and capacitance, SAW ﬁlters depend upon the mechanical properties of piezoelectric crystals. Through transducers deposited upon the crystal, SAW devices convert an RF signal into a mechanical displacement, creating a surface wave across the device, and convert back again to an RF signal. The ﬁltering characteristics of SAW devices depend upon the well known properties of the selected crystalline material, the length of total displacement, and the design, placement, and thickness of the transducer. As a result, SAW ﬁlters can be readily fabricated using modern semiconductor manufacturing techniques to an accuracy that is impossible to match using electronic components. Specifying the proper ﬁlter While SAW devices can greatly simplify RF ﬁlter designs, their successful use does require that they be properly speciﬁed. In addition to providing the center frequency and bandwidth for the ﬁlter, designers should consider a number of other factors that may affect system operation. These include the SAW ﬁlter’s insertion loss, signal group delay, out-ofband rejection ratio, and thermal stability T Figure 1: Many of the normal ﬁlter speciﬁcations are easy to address in SAW ﬁlter design because of the wide bandwidth and linear phase response of the crystal material. as well specifying the allowable ripple in the amplitude, phase and group delay responses. These factors affect the ﬁlter’s in-system operation as well as cost, and interactions among some of the factors may require that designers make tradeoffs. A typical ﬁlter is speciﬁed as shown in Figure 1. The wide (20 MHz to 2.6 GHz) operating range of SAW ﬁlters makes specifying the center frequency (CF) and the ﬁlter bandwidth (PW) quite straight forward. Other parameters, however, are more constrained. The stopband level (SL), for example, is limited by the device’s ability to dampen undesired vibrations. The typical value achievable is about -55 dB to -60 dB. If a design requires greater rejection, two or more SAW ﬁlters must be run in cascade, either in the same package or as two separate devices. Two cascaded ﬁlters will achieve -70 dB to -80 dB, but the cost of cascading to increase rejection is accumulating insertion loss. Insertion loss in SAW devices is a key parameter because these devices are passive; there is no internal ampliﬁcation to compensate for the energy lost in the piezoelectric coupling that converts the signal between electrical and mechanical forms of energy. The magnitude of the insertion loss is principally a function of ﬁlter bandwidth along with the crystalline material used (see Table 1). Material choice affects more than insertion loss, however. One signiﬁcant material-dependent factor is the ﬁlter’s group delay. The time required for a signal to pass through the ﬁlter is determined by the wave’s velocity through the material as well as the ﬁlter length. For a given ﬁlter length, then, material choice requires making a tradeoff between insertion loss and group delay. Another primary factor in choosing a substrate material is the fractional bandwidth (FBW) of the device. The relationship between the FBW and the substrate is a function of the piezoelectric coupling factor which is a measure of the coupling between an applied voltage and the resulting mechanical stress Because group delay depends both on material-dependent wave velocity and ﬁlter length, SAW device designers can ameliorate the loss-delay tradeoff somewhat by shortening the ﬁlter to reduce delay. The slope of the ﬁlter’s transition from passband to rejection, however, depends on ﬁlter length. The longer the ﬁlter, the more surface barriers can be placed to attenuate unwanted frequencies and the sharper the cutoff. Thus group delay, insertion loss, and ﬁlter cutoff slope along with package size are all intertwined. The ﬁlter’s temperature stability is also related to material choice, further complicating the tradeoffs. Quartz is Microwave Engineering Europe ● November 2007 ● www.mwee.com 032_033_MWEE.indd 32 26/10/07 12:32:33 http://www.mwee.com

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Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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