Sang-Won Yun

Varactor-tuned combline bandpass filter using step-impedance microstrip lines

In this paper, a varactor-tuned combline bandpass filter using step-impedance microstrip lines is considered so that the absolute passband bandwidth can be maintained nearly constant within the tuning range. The difference between the odd- and even-mode characteristics of the coupled microstrip line makes it difficult to design a tunable bandpass filter with minimum degradation in passband performance. By using step-impedance microstrip lines, couplings between resonators can be controlled so that the constant bandwidth requirement could be satisfied with reasonable design parameter values. Lumped inductors are used for input and output coupling networks. Design equations are derived, and experimental results are compared with theoretical ones based on these equations.

Miniaturized 3-dB ring hybrid terminated by arbitrary impedances

In the UHF or VHF band, the new design method of a small-sized 3-dB 0/spl deg//180/spl deg/ ring hybrid terminated by arbitrary impedances using both lumped and distributed elements is presented. At the center frequency of 900 MHz, the small sized 3-dB 0/spl deg//180/spl deg/ hybrid terminated by arbitrary impedances is designed, analyzed and tested. Good agreement is obtained between measured and predicted results. Since both lumped and distributed elements are used, the proposed hybrid occupies about 30% of an all distributed-type hybrid. >

A design of wideband 3-dB coupler with N-section microstrip tandem structure

New design equations and a 3-dB microstrip coupler example for N-section tandem connected structure with wide bandwidth are presented. The proposed four-port S-parameters and equations are obtained from a port reduction method. The designed microstrip 3-dB coupler not only does not need high impedance lines, but also uses tight coupling gaps differently from conventional couplers such as Lange couplers, parallel coupled line couplers, etc. The measured data agrees well with the expected data, which show a wide bandwidth of 42%, an amplitude imbalance of /spl plusmn/0.5 dB, a phase unbalance of 1.0/spl deg/, and isolation characteristics of 15 dB at the band of 3.6 to 5.5 GHz.

A Tunable Combline Bandpass Filter Loaded With Series Resonator

This paper proposes a varactor-tuned microstrip combline bandpass filter (BPF) that is loaded with lumped series resonators instead of the short-circuited end of combline. This configuration has the advantage of enhancing the tunability of the suggested BPF by varactor diodes in series resonators, which leads to a wide tuning range even with a small capacitance ratio (Cratio) of the varactor. Therefore, a low controlled bias voltage varactor-tuned BPF can be achieved. In addition, due to the controllable slope parameter of the proposed resonator, the coupling between resonators can be controlled. As a result, the proposed tunable BPF can keep nearly constant bandwidth over the wide tuning range. The insertion loss resulting from the varactors is compensated using an active capacitance circuit with negative resistance. Theoretical analysis and design approach are described. Experimental results of a demonstrative BPF show a 500-MHz tuning range (1.7-2.2 GHz) with Cratio of only 2.6 (1-5-V bias voltage) while maintaining nearly constant absolute bandwidth and low insertion loss. The measured performance of the fabricated filter shows good agreement with the simulated one.

Design of Balanced Dual-Band Bandpass Filters Using Asymmetrical Coupled Lines

In this paper, we present balanced dual-band bandpass filters (BPFs) using asymmetrical coupled lines. The proposed dual-band BPF has an enhanced common-mode suppression level because the proposed asymmetrical coupled line inherently has the bandstop characteristic on common-mode signals without either additional stubs or lumped elements. The proposed asymmetrical coupled line has dual-band performance on differential-mode signals. The common- and differential-mode equivalent circuits of the proposed filter are given and analyzed to demonstrate the common- and differential-mode responses. Two passbands of the differential-mode response are designed based on a lumped equivalent circuit of the proposed filter. Therefore, two bandwidths can be easily controlled. Two second-order balanced dual-band BPFs are designed and tested at center frequencies of 2.4 and 3.57 GHz with fractional bandwidths (FBWs) of 7.5% and 5% and with FBWs of 7.5% and 6.16%, respectively.

Design of an RF low-noise bandpass filter using active capacitance circuit

In this paper, a novel RF active bandpass filter (BPF) is proposed and its noise performance is optimized by noise analysis. In the proposed design, a resonator consists of an active capacitance circuit together with a conventional inductor. The active capacitor is made of a field-effect transistor that exhibits negative resistance as well as capacitance. It can, therefore, compensate the loss of an inductor. Whereas the conventional active filters using negative resistance circuits usually have a high noise figure, the proposed active filter shows good noise property achieved by adopting a novel active capacitance circuit, which is proven by noise analysis and the experimental result. The measured second-order active BPF shows bandwidth of 95 MHz, 0.1-dB insertion loss, 0.3-dB ripple, and a noise figure of 2.4 dB at the 1.9-GHz band, which agrees well with the simulated results

The design of band-pass filters made of both dielectric and coaxial resonators

Two different kinds of resonators can be used to construct a microwave band-pass filter that has a low spurious response. The band-pass filter made with coaxial resonators and disk dielectric resonators shows a good spurious response.

Varactor-tuned hairpin bandpass filter with an attenuation pole

In this paper, a varactor-tuned hairpin bandpass filter using a tapped open stub is studied. In this configuration the filters center frequency as well as the bandwidth can be controlled. The tapped open stub introduces the attenuation pole which can be located near the passband by adjusting the stub length. Varactors loaded at the end of hairpin resonator and the tapped open stub enable us to control the center frequency with constant bandwidth as well as to have a large tuning bandwidth. Based on the detailed analysis, the varactor-tuned hairpin bandpass filter is designed, in which the tuning bandwidth reaches 15 % of the center frequency at 2 GHz.

Hairpin Tunable Bandpass Filter with Improved Selectivity and Tunability

In this paper, a hairpin tunable bandpass filter which has enhanced selectivity and tunability is proposed. The varactors loaded open stubs between quarter-wavelength resonators are introduced to exhibit transmission zeros near the passband and thus better selectivity in the tuning range. In addition, symmetrical filter shape can easily obtained by each of two open stubs which are useful to control notch frequencies. The optimal design of two-stage tunable bandpass filter of varactor loaded hairpin resonator with open stub which works as K- inverter makes possible to meet the wideband coupling relationships in overall frequency tuning range. The measurement results show the proposed hairpin tunable bandpass filter not only has the enhanced tuning range of 23.5%, but also has the symmetrical filter shape in overall tuning range in comparison with conventional hairpin tunable bandpass filter. The hairpin tunable bandpass filter is designed and tested at 1.7 GHz.

Analysis of a novel active capacitance circuit using BJT and its application to RF bandpass filters

This paper presents a novel active capacitance circuit using BJT and RLC series-feedback. It is competitive in the noise performance and the stability by adjusting the feedback elements. We analyze its characteristics in detail and apply it to an RF application by designing a duplexer. The active capacitance circuit can replace the passive capacitors of resonators in Rx and Tx BPF, which provide negative resistance to compensate for parasitic resistance of inductors. With the detailed analysis, we can obtain the frequency range as well as the maximum value of negative resistance. The active duplexer is designed realized at cellular at Rx (824/spl sim/849 MHz) and Tx (869/spl sim/894 MHz) bands.