N. Ab Wahab

Circuit diagram of a single-mode microwave ring resonator

An electric circuit representation of a side-coupled single-mode microwave ring resonator is presented. The ring resonator is coupled to the feed lines along a quarter-wavelength of its side to exhibit a single-mode bandpass response with two transmission zeros. Using the properties and the circuit model of a coupled-line, the complete diagram of the ring resonator is derived. The diagram will be useful to construct mathematical equations to represent the ring resonator for future development of its synthesis. Example of utilization of the diagram is given to show its feasibility.

Side-coupled single-mode ring resonator with transmission zeros

A single-mode ring resonator which is fed along one of its quarter-wavelength sides is presented. The resonators response presents transmission zeros whose frequencies can be modified by adjusting the line impedance of the ring and the coupling level of the feeding coupled-line. The proposed topology introduced much simpler bandpass filter configuration and reduced the number of control parameters. This idea is tested on FR4 microstrip substrate using the electromagnetic simulation. The measurement results are then presented in this paper to show its feasibility.

Single-mode ring resonator for microwave bandpass filter applications

Topologies of two microwave bandpass filters are presented. The first topology comprises of a single mode ring resonator which is fed along one of its quarter-wavelength sides. The second topology is constructed based on the first topology whereby two series cascaded ring resonators with a quarter wavelength coupled line in the middle of the structure to obtain a 4th order bandpass filter with a good out-of-band rejection level. Observation on the controlling parameters of the 4th order bandpass resonator is conducted to investigate the effect of each parameter in terms of bandwidth, matching level and the transmission zeros. The proposed concept is demonstrated using microstrip technology with operating frequency at 2 GHz. The substrate used in this simulations is Tachonic with characteristics of: εr = 4.5, h = 1.63 mm, tan δ = 0.0035. The filter layout is presented in this paper along with the simulation results to show its feasibility.

Tuning circuit based on varactor for tunable filter

A tuning concept of a quarter wavelength side-coupled ring resonator is proposed by introducing varactor diode as tuning device at the four corners of the ring resonator to allow modification of the ring electrical length. This modification is typically used to allow the variation of the capacitance as a function of bias voltage thereby resulting in center frequency tuning. A complete tunable circuit is simulated using electromagnetic (EM) circuit simulator involving the integration of microstrip technology and lumped elements on Taconic substrate. The proposed tunable ring resonator manages to achieve13% of tuning range.

Dual-band bandpass ring filter

A topology of a dual-band filter is presented based on a series-combination of two identical ring resonators. The circumference of each ring is equal to one wavelength taken at the center frequency between the two chosen operating frequencies. The control of the filter response in terms of bandwidth and separation between the operating frequencies can be done by varying the impedances of the filter elements which are the ring line impedance and the even- and odd-mode impedances of the coupled lines. The proposed concept is demonstrated through a design of a microstrip dual-band filter operating at 0.9 GHz and 1.8 GHz on epoxy substrate with ɛr = 5.4 and substrate thickness of 1.6-mm. Experimental results show good agreement with the theoretical analysis.

A dual-band microstrip filter topology based on quarter-wavelength coupled-lines inter-connections

A dual-band bandpass filter topology based on coupled-lines is presented. In the topology, two identical coupledlines are cascaded in series and duplicated in parallel. There are then ended with another two identical coupled-lines. The dual-path structure exhibits a 2nd order dual-band response with three transmission zeros that separate the passbands from each other, and from the rejection band. The odd- and even-mode impedances of the coupled-lines control the bandwidth, passband and out-of-band responses of the dual-band filter. Finally, to verify the concept, a prototype of a dual-band bandpass filter is designed using EM simulator, fabricated on 1.6-mm-thick FR-4 substrate and measured. The resonant frequencies of the two passbands are centered at 1.82 GHz and 2.38 GHz.

Varactor-based frequency-tuned microwave open-circuited stub resonator

A tuning circuit based on a varactor-diode is proposed to tune the resonant frequency of a microwave open-circuited stub resonator. The circuit is placed at the end point of the stub to give a shunt capacitor effect. As the biased voltage applied to the varactor-diode to control the value of the resultant capacitor, the overall electrical length of the stub can be varied according to the desired resonant frequency. The complete tuning circuit is simulated and presented in this paper along with its tuned frequency-response using the integration of microstrip technology and lumped elements on FR4 substrate. The final tunable stub resonator achieved 66% tuning range.

Series-cascaded single-side-coupled ring resonators

A second- and a third-order filters are presented based on the single-side-coupled ring resonator topology. A simple way to obtain the second-order bandpass response is proposed by adding a quarter-wavelength coupled line immediately at the coupled-line side of the resonator, which forms a half-wavelength resonator, hence an extra pole. Using the similar idea, an extra pole is obtained from a half-wavelength resonator that is formed between two identical resonators cascaded in series. Both filters are designed, fabricated and measured to show the feasibility of the idea, on FR4 substrate using microstrip.

Frequency-tuned microwave ring resonator using varactor diodes

Four identical varactor-based tuning circuits are proposed to tune the centre frequency of a microwave quarter-wavelength side-coupled ring resonator (QWSCR). Each of the tuning circuit is placed at every corner of the ring to give a shunt capacitor effect. The overall electrical length of the resonator can be varied according to the desired resonant frequency through the biased voltage applied to the varactor-diode that controls the value of the resultant shunt capacitor. The complete tunable resonator circuit is simulated using the integration of microstrip technology and lumped elements on FR4 substrate. The proposed concept of the tunable QWSCR filters allows a tuning up to 15% of the nominal frequency of the filter.

Varactor-based frequency-tuned microwave quarter-wavelength side-coupled ring resonator

A center-frequency-tuned microwave quarter-wavelength side-coupled ring resonator based on the varactor diodes is presented. Four identical varactor-based tuning circuits are applied at every entry point of the coupled-lines in the ring resonator to give capacitance effects and cause an increment of electrical length and thus results in tuned center frequency at the desired frequency band. The complete tunable resonator circuit is simulated using EM circuit simulator. The proposed filter is then realized and fabricated on FR4 substrate.