Design of Combline Bandpass Filters Interdigital BPF included

Comline Bandpass Filters Typical Combline Bandpass Filter

Equivalent circuit  

Coupled section equivalent circuit Zoe, Zoo

Slope parameter Use the following resonant condition:

Coupling coefficient(1) J-inverter: Coupling coefficient:

Coupling coefficient(2) Coupling coefficient inversely proportional to the frequency

Combline Bandapss Filter

Equivalent circuit for 1st and last stage Interdigital type feeding

Coupling stage Combline type

Equivalent circuit(1) Equivalent circuit

Design equations

Equivalent circuit(2) Reduced circuit

Equivalent circuits

Interdigital BPF-Introduction Attractive features Very compact Required tolerances in manufacturing are relatively relaxed because of the large spacing between resonator elements The spurious response appears at there times the center frequency of the passband The rates of cutoff and the strength of the stop bands are enhanced by multiple poles attenuation at dc and at even multiples of the passband center

Interdigital Bandpass Filter Input feeding – shorted 30% bandwidth or less

Equivalent circuit  

Design procedures  

Design equations

Designed Filter

Input feeding –Open 30% bandwidth or more

Equivalent circuit

Design equations(1)

Design equations(2)

From Filter Specifications

Design Example Open ended feeding At 2 GHz 10% bandwidth, Order 3

Layout of microstrip interdigital BPF SPEC Order : 3 FBW : 10% Center frequency : 2GHz Resonator width : 모두 같게 설계 Resonator length Case 1. Case 2. Substrate Dielectric : 6.15 H=1.27mm TanD : 0.0013

Layout of microstrip interdigital BPF Step 1 에 따른 resonator length를 구한다. Step 2 -slope parameter를 정해 even mode addmittance값을 구할 수 있다.(resonator width) Step 3 -J inverter 값과 even mode addmittance값을 통해 odd mode addmittance값을 구한다.(resonator들 간의 간격 s)

Simulated Results(1) Center frequency : 1.952GHz 1dB BW :210MHz(11%)

Simulated Results(2) Center frequency : 2.040GHz 1dB BW :210MHz(11%)