Switchable bandpass filter having stepped-impedance resonators loaded with diodes

#1. Field of the Invention

#The present invention relates generally to bandpass filters, and more particularly, to a switchable bandpass filter having stepped-impedance resonators loaded with diodes.

#2. Description of Related Art

#FIG. 1 shows the circuit structure of a stepped-impedance resonator loaded with a load at one end;

#FIG. 2 shows the circuit structure of a stepped-impedance resonator loaded with a diode at one end;

#FIG. 3 shows resonant conditions for the stepped-impedance resonator loaded with a given capacitive load;

#The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparent to those skilled in the art after reading the disclosure of this specification.

#FIG. 1 shows the circuit structure of a stepped-impedance resonator R loaded with a load ZL at one end. The stepped-impedance resonator R is composed of two transmission lines L1 and L2 of different line widths. The load ZL is connected to one end of the transmission line L1. If the input impedance of the stepped-impedance resonator R seen from the open end (i.e. from L2 to L1) is defined as Zin, the parallel resonance of the stepped-impedance resonator R occurs when 1/Zin=Yin=0, and the resonance condition may be written as Z01(Z02−Z01 tan θ1 tan θ2)+j(Z02 tan θ1+Z01 tan θ2)=0   (1), where Z01 and Z02 are the characteristic impedances of the two transmission lines L1 and L2, and θ1 and θ2 are the electrical lengths of the two transmission lines L1 and L2, respectively.

#By defining the stepped length ratio α of the stepped-impedance resonator R as