Design of Compact Dual Band Bandpass Filter with Controllable Bandwidths and Good Selectivity

A compact dual band bandpass filter is presented here, in which an open loop resonator connected with a short-circuited stub loaded in the midpoint is used. The central frequencies of two bands can be regulated by alternating the characteristic impedance and electrical length of the open loop and the short-circuited stub respectively. By introducing new coupling structure, the coupling coefficient of each band can be regulated independently. External coupling for each band is realized through common feed line and can also be regulated independently. Furthermore, source-load capacitive coupling is introduced to produce four transmission zeroes at the adjacent of two passbands. Thus, the bandwidths of this filter are controllable and good selectivity can be realized for narrow band application. A 2.4 GHz/5.7 GHz dual band microstrip filter with equal absolute bandwidths is designed and measured. The measured result agrees well with the simulation. The size of the filter is only 6.26 mm times 5.16 mm, and thus is very compact.