Free Interdigital Bandpass Filter Tool
Synthesize quarter-wave interdigital bandpass filters. Calculate even and odd mode impedances ($Z_{0e}, Z_{0o}$), resonator bar spacings, and input tapping height.
📶 Filter Specifications & Bandwidth
📊 Resonator Dimensions & Couplings
Recommended Tools & Equipment
Tested hardware and components for high reliability
Principles of Interdigital Bandpass Filters
Interdigital filters consist of parallel arrays of transverse electromagnetic (TEM) quarter-wavelength line resonators. Each resonator bar is grounded at one end and open-circuited at the opposite end, with groundings alternating on opposite sides of the cavity.
Advantages of Interdigital Architecture
- Compact Form Factor: Resonators are quarter-wavelength ((lambda/4)) rather than half-wavelength ((lambda/2)), cutting filter volume in half.
- Wide Second Harmonic Suppression: Because alternate ends are grounded, the first spurious passband is pushed out to the third harmonic ((3f_0)), completely eliminating the second harmonic response inherent in combline filters.
- High Power Handling: Machined from solid aluminum or invar and electroplated with silver, cavity interdigital filters routinely handle hundreds of watts of continuous transmitter power.
Frequently Asked Questions
Why are tuning screws placed above the open ends of the resonators?
Tuning screws increase the capacitive fringing field between the open tip of the resonator bar and the cavity ground wall. Turning the screw inward adds shunt capacitance, slightly shortening the required electrical length and allowing precise center frequency and return loss alignment.
What determines the input tapping height l_tap?
Tapping directly onto the first resonator bar acts as an autotransformer. Near the ground end, impedance is 0Ω; at the open tip, impedance is high. Tapping at approximately 15% to 25% of the bar length matches standard 50Ω coax lines directly without needing a separate discrete matching network.