A Compact Low Pass Filter Design with Assigned Transmission Zeros

A combined C-section coupled lines low pass filter by setting transmission zeros to control its stop band bandwidth and frequency response slope near 3 dB cutoff frequency is proposed. The circuit analysis is performed by transmission line model of coupled lines for extracting the equivalent circuits of individual parts of the circuit. The circuit design uses reactances of the equivalent circuits to assign transmission zeros and applies an extra transmission zero to increase frequency slope near 3 dB cutoff frequency. A prototype with 3 dB cutoff frequency at 2.45 GHz and transmission zero at the second harmonic was designed, simulated, fabricated, and measured. The simulated data and the experimental results show a good agreement from 0 to 10 GHz.