**Jie Cui, Haojie Chang and Renli Zhang \***

School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; cuijie@njust.edu.cn (J.C.); njusteo\_changhj@njust.edu.cn (H.C.)

**\*** Correspondence: zhangrenli@njust.edu.cn

Received: 15 August 2020; Accepted: 17 September 2020; Published: 28 September 2020

**Abstract:** A multilayer bandpass filter with high selectivity is proposed in this letter. Discriminating coupling formed by slot-coupled quarter-wavelength and half-wavelength resonators introduces a zero at 3f0 (f0 is the center frequency) and the second harmonic is also suppressed due to the quarter-wavelength resonators. Owing to multilayer structure, source-load coupling is introduced to improve selectivity. Then an extra coupled line path is added with the same amplitude as the discriminating coupling path while they are out of phase. Thus signal cancellation produces three extra transmission zeros, with the selectivity and suppression performance further improved. To validate the design, a prototype bandpass filter centered at 2.49 GHz with 3 dB fractional bandwidth of 8.1% is fabricated. Both simulated and measured results are in good agreement and show good performance of the proposed bandpass filter.

**Keywords:** bandpass filter; discriminating coupling; high selectivity; source-load coupling
