A coupled complementary split‐ring resonator (CSRR) has been proposed. The proposed resonator consists of two coupled 50‐Ω transmission lines and complementary split rings in the ground plane (bottom layer) as defected ground structure (DGS). The proposed bandpass filter (BPF) has a measured stopband attenuation loss of less than −20 dB in the frequency band between 2.3 and 2.9 GHz and a low insertion loss of 1.15 dB. The measurement is carried by a VNA (vector network analyzer) meter of model ZVA67. Then, the proposed resonator (BPF) is utilized with an active device (SiGe BFP 640) as an output matching network of a class‐F oscillator. Fundamentally, this research aims to design and model an optimum achievement of BPF for optimum oscillator performance. The design simulation of the CSRR oscillator has been obtained by ADS and CST programs.