AParallel-Coupled Microstrip Bandpass Filter WithSuppression of Boththe2ndandthe3rdHarmonic Responses

Thispaperpresents aparallel-coupled microstrip bandpass filter withsimultaneous suppression ofboththe2ndand the3rdharmonic passband responses. Half-wave open-ended resonators areusedtoprovide transmission zeros forthe2ndand the3rdharmonics sothatthespurious passbands atthe2ndand the3rdharmonic frequencies arereduced. Inaddition toplacing theresonators, thefilter isbenttwice by90degrees toachieve morecompactsize andhigher suppression ofthe3rdharmonic passband. Thismethodcanbeapplied totheconventional parallel-coupled microstrip bandpassfilter withoutany significant change oftheconventional filter design. IndexTerms Harmonicsuppression, open-ended resonators, parallel-coupled microstrip bandpass filters. I.INTRODUCTION Theparallel-coupled microstrip bandpass filter isoneofthe mostcommonly usedfilters andhasbeenwidely usedinthe last fewdecades, because ofits planar structure, simple design andimplementation, andawidebandwidth range. Butithasa disadvantage ofhaving spurious passbands attheharmonics of thedesigned center frequency (Jo) (1). Thisgreatly limits the useoftheparallel-coupled microstrip bandpass filters in broadband systems operating overa frequency bandwidth including the2ndandthe3rd harmonics ofthedesigned center frequency ofthefilter. Because recent communication systems usewider bandwidth andfilters areessential components in theRFfront endofthesystems (2), this phenomenon becomes amoreserious problem. Manymethods havebeenproposed tosolve this problem. To mention a few,themethodin(3)provides different electrical pathlength fortheevenandtheoddmodeto suppress only the2ndharmonic passband, whicharises from thefact that theelectrical pathlength ofthecoupled lines is different fortheevenandtheoddmode.Themethod in(4) usesa uniplanar compactphotonic-bandgap (UC-PBG) structure toreject boththe2ndandthe3rd harmonic passbands at2foand3fo, respectively. Butthephysical dimensions of theparallel-coupled microstrip bandpass filter should be redesigned. Additional etching andaccurate alignment arealso required because oftheUC-PBGstructure intheground plane. Recently, wiggly-line bandpass filters arereported tobe effective inmultispurious rejection (5), butitalso increases thedesign complexity. Since allofthemethods mentioned abovedemonstrate their performance forthefilters designed around 2GHz,itisnotsurethat they canworkaswellifthe designed center frequency ofthefilter increases suchthat the harmonic frequencies areinthemillimeter-wave region. Furthermore, whentheharmonic frequencies areinthe millimeter-wave region, thefabrication precision required for themethods mentioned above becomes morestringent. A novel andsimple method isproposed inthis paperthat requires neither change intheconventional filter design nor moreprecise fabrication forsuppression ofspurious passbands eveninthemillimeter-wave region. Half-wave open-ended resonators areplaced neartheedgesoftheconventional parallel-coupled bandpass filter tosuppress boththe2ndand the3rdharmonic passbands whichappear at2foand3fo, respectively. Inaddition toplacing theresonators, the conventional filter isbenttwice forbetter suppression ofthe harmonic passbands. Bybending thefilter, amorecompact filter canbeobtained. Thereisnoaddedcomplexity in fabrication compared tothatofconventional filters. A parallel-coupled bandpass filter centered at12GHzusing the proposed topology is designed and measuredfor demonstration.