IEEE TRANSACTIONS ON SONICS AXD ULTRASONICS, VOL. SU-18, NO. 3, JULY 1971 155 Mechanical Filters-A Review of Progress ROBERTA. JOHNSON, MEMBER, IEEE, MANFRED BORNER, AKD MASASIII KONKO, MEMBER, IEEE - Abstract-This paper isareview of electromechanicalbandpass 0 filter, resonator, and transducer development. Filter typesdiscussed include intermediate and low-frequency configurations composed of rod, disk,and flexure-barresonators and magnetostrictive ferrite g and piezoelectricceramic transducers. The resonators and trans- ducers areanalyzed in terms of theirdynamic and material char- 2 2o acteristics. The paper also includes methods of realizing attenuation 2 poles at real and complex frequencies. The last section is a look at future developments. future 30 1 I II l I I 1 IXTRODUCTIOX 100 2W 300 500 1 OW 2000 30W 5ooo FREGUENCY (Hz1 ORE THAN 20 years haw passed since the first Fig. 1. Improvementin phonograph design through the use of practical mechanical filters were introduced by filter design metl~ods[61. Adler [l],Roberts [2], andDoelz [3]. These fibers met an existing need for greater selectivity in the IF stages of AM and SSB receiversdesigned for voice n m / FLEXURE (F" communication. Rlodern receivers and telephone commu- nication equipments carry not only voice but data mes- sages as well, thus requiring lower passband ripple and well-definedand stable passband limits. In addition, greaterselectivity and lon.er loss areoften required. Mcchanic:~l filter development has kept pace with the demands of communicationengineers. For instance, filtershaving passband-~,ipple requirements of 0.5 dB or less or 60/3-dB bandwidthratios of 1.3/1 arenot unusual. Package size has also heen reduced; a 10-pole filter can be designed into a l-c1n3 package. In addition, the lower frequency limits of mechanical filters are now in the audio rangr; the upper limit is that of an AT-cut crystal. Although the mechanical filter was conceived in the US, a great' amount of cleyelopmental work is now being conductet1in Europe and .Japan. Early interestin the use of 111cchanical elementsto providepasshand cllaracteristics resulted from the de- velopnlent, of the electrical bandpass filter by Campbell and Wagner in 1917 [4].A number of patents relating tospring-mass systcms [5] were filed soonafter, but FREOUENCY - the most significant work in terms of practical deyices Fig. 2. Distributedelemrmt mecllanicnl filtcr (Mnson, 1941). wasdone by Maxfield and Harrison on phonograph filter shown in Fig. 2 [TI.Pome of the significant, features M:~nuscripl rrceivetl .J:ln11:ly F, 1971. of this design includedthe use of piezoelectrictrans- R. A. Jnhnson is with the CollinsRadio Company, Newport ducers,distributed-Flement llalf-wavelength resonators, Bcnch, Calif. M. F3iirner iswith AEG-Telefunken Rescarcl1 Institute, Ulm; and an attenuation-pole-producing coupling element. Crrmnny. Just a fev7 years later, the first practical IF mechan- M. Iconno is with theDcpartmcnt, of ElectricalEngineering, Pnm:lgataUniversity, Yonemws, Japan. icalfilter, shownFig. in 3(a)! wasdeveloped by Adler. 156 IEEE TR.INSACTIONS ON SOSICS AND ULTRASONICS, JULY 1971 STATIC CAPACITY COUPLING TRANSDUCER COUPLING REGION COIL Wi RE ELECTRODE PAIR This fi1tc.r Inrltle we of ldf-mrclcngth platw coupled hy short small-tli:unctrr wires. The plate resonators can be representcc1 bp p:~raIlel tuned circuits, the coupling wires by seriw inclurtors, as shon-n in the electrical equivalent circuit of Fig.3 (b). The nickel end platesact’ a.: magnet.o- strictive input and output transducers. The frcqucncp response cllaracteristics are monotonic, that is there arc no finite attenuation poles. Adler’s filter is typic:ll of most modernmcchanical filters in that distrihutetlparameter resonators arc coupledthrough nonrcsorlnnt lincs. ‘I‘here is a striking similarity betmeen theplnte-\\-irc filter ant! the latest t;vi)c of mech:anical filter, thc n~onolithicor quartz mechanic:zl filter shown in Fig. 4(:~’l. uset1 modes are the one-nodal circle (-50-200 The quartz nlcclmlical filter cleveloped by Beaver and kHz) and the two-circle mode (200-600 kIIzl. The Sykcs [S1 and K:‘:lkaza~:l [91 makes use of the cnergy- resonators are coupled h>- small-diameter wire tht :11’(’ t,rapping concept; whcre standing wa~ware set 11p under ?pot n.cltlcrl around the circumftwnce of the disks. As a eacheloctrode pair. 111 the regionshetn.ecn the elec- rwult of the yery complex displacement at the ctlgc of trodes, tllc, acoustic, xivc clcc‘ays oxponcntinlly,the the tlisk, the coupling mode is a con1hinat)ion of extension electro(]ctl rc,gions actlike r(~sonntOrs, andthe non- and flexure and is,therefore, very difficult to analyze. clcmcnts. clcctrodcd regions actlike coupling The elec- The earliest designs made uw of small-diameter odd- t,ric:il cyuivnlcnt circnit of tI1v platc-n-i1,c and the quartz nun1l)er quarter-wawlengtll iron-nickel alloy transducers. mechanirnl filtcxr sllonn in Fig. 4(b) are itlentical except Latclr dcsigns, such 5s tllat shown in Fig. 5(a),use onc- t,llat in the plate-wire case the input and output capac- half and full-wawlcngth ferrite transducers. itors are replaced coils. The monolithic filter also has by Ferrite transducers are also used on the rod-wire filt,er monotonic frequency responsecharacteristics. A con- shown inFig. 5(h’l. Thetorsional rod-wire filter was structionsimilar to that of themonolithic filters rle- developedindepcndcntly hy Biirnerin Gerrnnny [lZ], scribed abore 11as been used byBorner and Schussler [ 131, and Tanaka in .Japan [l41 ; the Tanaka design uses Langeyin metal alloy/ceramic transducers in place BASICFILTER STRCCTURES of the ferrite.. Thecylindrical rod resonators are de- signed to vihratein R half-marclengthtorsional mode Monotortic Designs at frequencies up to 250 kHz and in a half-wavelength Adler’s tlevclopn~cntof the plate-wire mechanical filter longitudinal mode whenthe filters are designed to was soon followed by Doelz’s disk-wire filter [ 111, shown operate at 4.55 kHz. Use of the torsional mode results in in Fig. 5 (a). The disk-wire filter makes use of flexural longitudinal coupling between resonators, whereas at 455 modes of vibration of the disk resonators. The two most kHz the coupling involves bending or flexure. JOHKSON et d.: REVIEW' OF MECHANICAL FILTERS 157 Use of a wireto connect the transducer to the end 14@ r resonator, as shown in Fig. 5 (h), results in a reduction of 12c t -- - spurious or unwanted responses, particularly if it is nttacl~td at :I 11od:d point of thestrongest, unwanted 1'- -l- ---- - - modes. This technique, which is often used on disk-wire 2 GC filters, rcsults in r1)urious modes being suppressod more U t 4@ than 60 dB, as shown in Fig. 6(a). In contrast to the excellcntspurious response rc>jection of thetorsional rod-wirefilter is the response [Fig. Gcb)] of therod- 2:o--.- 200 400 G@@ 8W l0U0 neckfilter of Fig. 7(a), which is drivenin a similar FREOUENCY :kHz, manner, but, i.4 subject) to bro:ld banclwidthhn(1ing (:L) modes. The rocl-ncrk fi1tc.r was tlcwloped hy Roberts [2] at the same time the disk-wire filter was being developed. The basic clcl-ign concept is that of couplinghalf- wavelengthtol,sionnl or lorrgituclinal resonatorswith c~u:~rter-w:~rcl(~~l~tl~nc~ks:.Tllc, two mjor probltwls of the early designs were the easily excited spurious bend- ing modes [l 51, :~nd thedifficulty in construct,ion and tuning due to hing turned out of a single rod. In adtli- t,ion, at lon.er frcquencicu like 100 kHz, the filtersbecome extremelylong. In oulcrto reduce the length,Tanakn. developed tllc folrlccl line filter show1 in Fig. 7(h). Thc resonatorsvihrntt: in n I~alf-n-:~wlengthlongitudinal mode and are coupled hy relatively large-diamctcr short wires. The usc of 1ongitutlin:ll-mode Langevintrans- ducers plus the rcduced ovcrall length rcsults in :L greater rejection of unwanted moden. Like the rod-wireand foldeddesigns, the disk-wire filter has a relatively low spurious response levc.1. All four filters discussed so far operate at' frequcncies above 50 kHz. With the exception of some work reported hy MaPon and Konno, tllcre was littleactivit,y at fre- quencies hclow 50 kHz hefore 1960.. One of thefirst practical tlesigns was that of Mason and Thurston [ 161, who used antisymmetric mode flexural resonators coupled in t,orsion as sllon-11 inFig. 8(a). Theantisymmetric mode makes this clesign less susceptible to rnicrophonic excitation. A more widrly used design is the symmetrical mode filtershown in Fig. 8h).Work by Iconno [ 171, [IS], Y,zkuwa [19], andAlhsmeier [20], hwe made this a wry practical tleyice in thefrequency range of 300 Hz to 30 kHz. The symmetric-mode filter is driven hy pictzoelectric-ceramic transducers. The coupling wires :m attached to tlx resonators at the nodal points, which resuks in torsional coupling. This type of filter is wry sensitive to chngcs in t,he position of the coupling wires so conderahle care is taken in manufacturing to ensure that bending modes are not propagated. The microphonic problenl has been solved, in part, by supporting the filter with high-damping silicon-ruhher supports. Inaddition to the flexural-har/wire lorn-frequency mechanicalfilters, a considerableamount of work has been done in ,Japan on the development of tuning-fork I mechanical filters [21], such as those shown in Fig. 9(a) (h) and (b). The three-prong filter is interesting in that it Pig. 8. Low-frcqrlrnrymeehnnicnl filters. (a) Antisymmet.ric acts like a coupled two-resonator filter, thus providing a mode [161. (b) Fundamental mode [181. 158 IEEE TRANSACTIONS ox SONICS ASD ULTRASONICS, JULY 1971 (b) Fig. 9. Tuning fork mechanical filters. (a) U-shapedcoupling type. (b) Two-pole three-prong type. ATTENUATION greateramount of selectivit'ythan a simpletwo-prong device. The tuning fork and flexural-bar filters are widely used in Japan for remote and automatic control, selective v callingand paging, telemetry, measuring instruments.