This article proposes a dielectric waveguide bandpass filter (BPF) with good stopband suppression based on different kinds of dielectric waveguide resonators (DWRs). Three distinct types of DWRs are modified from the traditional rectangular DWR, i.e., one with a metallized blind hole, one with a metallized U-shaped slot and another one with a pair of parallel ridges. These resonators are designed that their fundamental mode frequencies are basically the same and their higher-order modes are staggered. As a result, the higher-order modes can be suppressed to a certain extent when conducting BPF designs. For verification, a sixth-order BPF with an operating frequency band ranging from 3.4 to 3.5 GHz is designed, fabricated and measured. It is composed of the above three distinct types of DWRs, with a deliberate arrangement that prevents the DWRs of the same type from being adjacent to each other, guaranteeing that the harmonics are well suppressed. In measurement, the in-band return loss is better than 12 dB, the minimum in-band insertion loss is about 1.0 dB. Besides, the 20 dB out-of-band suppression reaches 6.9 GHz, which is almost twice the center frequency.

A novel high-selectivity ultra-wideband (UWB) balanced bandpass filter (BPF) with harmonic suppression characteristics is proposed. The differential-mode UWB passband is constructed using quarter-wavelength asymmetrical parallel-coupled transmission lines, resulting in sharp filtering selectivity, while common-mode noises are suppressed by loading the T-type stubs. A simple design method of integrating bandstop filter branches into the BPF for third harmonic suppression without increasing the circuit size is described and confirmed. A planar high-selectivity balanced BPF prototype is manufactured and measured at 2.6 GHz with a measured 3-dB fractional bandwidth of 108% to validate the framework principle. More importantly, with third harmonic suppression characteristics, the measured out-of-band rejection is more than 20 dB, spanning 4.35 GHz until 12.4 GHz.

The current paper describes the design of a fourth-order compact narrow-band hairpin-line bandpass filter centered at 2.5 GHz and enhanced stopband suppression ~38 dB required for wireless local area network (IEEE802.11b). The size reduction of 24% has been achieved for a conventional hairpin-line filter with a second-order inward folding of the open-end arms with an improvement in the skirt characteristics. However, the folding mechanism has been overshadowed by the presence of spurious harmonics with an attenuation level of <10 dB. Subsequently, L-shaped spurlines with optimum dimensions have been incorporated in the coupled arms of the folded hairpin-line cells. Three fourth-order filters have been designed with the incorporation of spurlines in different coupling regions, fabricated, and tested experimentally. Accordingly, an extended stopband with a rejection level of 38 dB up to 4.48f0 and a size reduction of 18% over the conventional hairpin-line filter has been achieved.

In this paper, a novel microwave oscillator incorporating miniaturized nested split-ring resonators is proposed. The high-quality (Q) factor and wide spurious-free band of the NSRR contribute to low-phase noise and high-harmonic suppression of the proposed oscillator circuits. In addition, the NSRR is featured by compact size of 0.12λg × 0.12λg, where λg is the guided wavelength of resonance frequency. The fabricated 2.4 GHz oscillator has an output power of 11.7 dBm with 5 V DC supply and 10 mA current consumption. The second harmonic suppression is −45.49 dBc, the phase noise is −110 dBc/Hz @100 kHz, and the DC–RF conversion efficiency is measured as 30%.