Transversely Compact Single-Ended and Balanced Bandpass Filters with Source–Load-Coupled Spurlines

Abstract

Multi-function wireless systems demand multi-channel transmit/receive (TR) modules, particularly as multiple functions are required to operate simultaneously. In each channel, passive components, including bandpass filters, must be compact, or at least transversely compact; thus, the entire circuitry of the channel will be slender, and consequently multiple channels can be parallel-arranged conveniently. In this work, single-ended and balanced bandpass filters for multi-channel applications are presented. As a unique resonator, the U-shaped stepped impedance resonator (USIR) can achieve size miniaturization compared with its corresponding uniform impedance resonator (UIR) counterpart. Hence, with the utilization of USIRs, the proposed bandpass filters are able to acquire compact transverse sizes. Moreover, by using the source-load coupling scheme, two transmission zeros (TZs) are respectively generated at the lower and upper sides of the passbands, which is useful for improvement of the selectivity performance. In addition, spurlines are introduced at the input and output ports to produce another TZ to further enhance the stopband performance, which cannot be acquired by the UIR or stepped impedance resonator (SIR). To verify the aforementioned idea, one single-ended and one balanced bandpass filter are implemented, with experimental results in good agreement with the corresponding simulations. Meanwhile, as compared with some similar works, the proposed balanced filter achieves compact transverse size, sharp selectivity skirt, and wide stopbands up to the fourth-order harmonic with suppression over 20 dB, which illustrates its suitability for differential signal transmission application in microwave circuits and systems.

Highly‐tunable and wide stopband microstrip bandpass filters

“…Under the dominant mode of the λ /2 SIR, most of the current are distributed along the low impedance section of the resonator, while centre of the high impedance section acts as a short‐circuited region. Hence, λ /2 SIR is equivalent to λ /4 SIR under the dominant mode and Equation () can be modified as follows 19,26 : …”

“…Balanced components, including bandpass filters, have received much attention due to their immunity to noise and cross talk 18‐20 . A highly‐selective and tunable balanced bandpass filter is reported in 21 .…”

Multi‐spurious harmonics suppression in folded hairpin line bandpass filter by meander spur‐line

“…In this context, as an alternative structure, the bandstop nature of the spur‐lines has been incorporated in the hairpin‐line filters proposed in Reference 19 to achieve a wide stopband of rejection level of 20 dB up to 4 f 0 with a compact size of 0.16 λ g × 0.75 λ g . Afterward, a compact‐sized (0.129 λ g 2 ) bandpass filter with sharp selectivity skirt and wide stopbands up to the 4 f 0 with suppression over 20 dB has been proposed in Reference 20 by combining U‐shaped stepped impedance resonator and source‐load coupling embedded with spur‐lines. However, the achievement of a wide stopband beyond 3.5 f 0 with a rejection level over 35 dB for a narrowband (fractional bandwidth of 4%) and compact‐sized bandpass filter is still a challenging issue.…”

Spurline-embedded compact hairpin-line bandpass filters for wide harmonic suppression

“…[25] (size of 0.16 λ g × 0.75 λ g , stopband rejection level of 20 dB up to 4 f 0 ), and Yan et al . [26] (size of 0.44 λ g × 0.29 λ g , stopband rejection level of 20 dB up to 4 f 0 ). However, the rejection levels in [24–26] were not satisfactory regarding modern-day applications.…”

“…[26] (size of 0.44 λ g × 0.29 λ g , stopband rejection level of 20 dB up to 4 f 0 ). However, the rejection levels in [24–26] were not satisfactory regarding modern-day applications. Thus, the design of compact, highly selective, narrowband (<5%) bandpass filters with a wide stopband beyond 4 f 0 and rejection level better than 35 dB still become a challenging issue for researchers.…”