Miguel A. G. Laso

Equivalent-circuit models for split-ring resonators and complementary split-ring resonators coupled to planar transmission lines

In this paper, a new approach for the development of planar metamaterial structures is developed. For this purpose, split-ring resonators (SRRs) and complementary split-ring resonators (CSRRs) coupled to planar transmission lines are investigated. The electromagnetic behavior of these elements, as well as their coupling to the host transmission line, are studied, and analytical equivalent-circuit models are proposed for the isolated and coupled SRRs/CSRRs. From these models, the stopband/passband characteristics of the analyzed SRR/CSRR loaded transmission lines are derived. It is shown that, in the long wavelength limit, these stopbands/passbands can be interpreted as due to the presence of negative/positive values for the effective /spl epsiv/ and /spl mu/ of the line. The proposed analysis is of interest in the design of compact microwave devices based on the metamaterial concept.

New microstrip "Wiggly-Line" filters with spurious passband suppression

In this paper, we present a new parallel-coupled-line microstrip bandpass filter with suppressed spurious passband. Using a continuous perturbation of the width of the coupled lines following a sinusoidal law, the wave impedance is modulated so that the harmonic passband of the filter is rejected while the desired passband response is maintained virtually unaltered. This strip-width perturbation does not require the filter parameters to be recalculated and, this way, the classical design methodology for coupled-line microstrip filters can still be used. At the same time, the fabrication of the resulting filter layout does not involve more difficulties than those for typical coupled-line microstrip filters. To test this novel technique, 3rd-order Butterworth bandpass filters have been designed at 2.5 GHz, with a 10% fractional bandwidth and different values of the perturbation amplitude. It is shown that for a 47.5 % sinusoidal variation of the nominal strip width, a harmonic rejection of more than 40 dB is achieved in measurement while the passband at 2.5 GHz is almost unaltered.

Microwave filters with improved stopband based on sub-wavelength resonators

The main aim of this paper is to demonstrate the potentiality of sub-wavelength resonators, namely, split-ring resonators, complementary split-ring resonators, and related structures to the suppression of undesired spurious bands in microwave filters, a key aspect to improve their rejection bandwidths. The main relevant characteristics of the cited resonators are their dimensions (which can be much smaller than signal wavelength at resonance) and their high-Q factor. This allows us to design stopband structures with significant rejection levels, few stages, and small dimensions, which can be integrated within the filter active region. By this means, no extra area is added to the device, while the passband of interest is virtually unaltered. A wide variety of bandpass filters, implemented in both coplanar-waveguide and microstrip technologies, have been designed and fabricated by the authors. The characterization of these devices points out the efficiency of the proposed approach to improve filter responses with harmonic rejection levels near 40 dB in some cases. It is also important to highlight that the conventional design methodology for the filters holds. For certain configurations, the presence of the resonators slightly lowers the phase velocity at the frequencies of interest with the added advantage of some level of reduction in device dimensions.

Spurious passband suppression in microstrip coupled line band pass filters by means of split ring resonators

In this letter, spurious passband suppression in microstrip coupled line band pass filters by means of split ring resonators (SRRs) is demonstrated for the first time. By etching SRRs in the upper substrate side, in close proximity to conductor strip, strong magnetic coupling between line and rings arises at the resonant frequency of SRRs. This inhibits signal propagation in the vicinity of that frequency, allowing the rejection of undesired passbands by properly tuning SRRs. To test this novel technique, we have designed and fabricated two different SSRs-based filters. In one case, the rings have been designed to suppress only the first spurious band, and SRRs have been etched at both sides of the 50-/spl Omega/ access lines. For the other prototype, SRRs have been etched on the active device region (i.e., surrounding the parallel coupled lines) and have been tuned to eliminate the first and second undesired bands. The measured frequency responses for these devices confirm the efficiency of this technique to suppress frequency parasitics, with rejection levels near 40 dBs, leaving the passband unaltered. Since SRRs are small particles (with sub-wavelength dimensions at the resonant frequency), this approach does not add extra area to the final layouts. Moreover, the conventional design methodology of the filters holds.

Microstrip "wiggly-line" bandpass filters with multispurious rejection

A method to achieve the rejection of multiple spurious passbands in parallel-coupled-line microstrip bandpass filters is proposed. As it was previously demonstrated by the authors, using a continuous perturbation of the width of the coupled-lines following a sinusoidal law, the wave impedance can be modulated so that the first undesired harmonic passband of the filter is rejected, while the desired passband is maintained virtually unaltered. In this letter, the scope of the method is extended to reject multiple spurious passbands by employing different periods in each coupled-line section tuned to the different bands to be rejected. Simulated and measured data show that for an order-seven bandpass filter prototype, a rejection level exceeding 30 dB is obtained in the first four spurious passbands, while the desired pass-band is kept almost unaltered.

Multiple-frequency-tuned photonic bandgap microstrip structures

Photonic bandgap (PBG) structures in microstrip technology have been recently proposed as efficient Bragg reflectors. The periodic patterns employed until now were formed by a distribution of nonconnected holes (cermet topology) etched in the ground plane or drilled in the dielectric substrate, giving rise to single-frequency-tuned band reflectors. In this letter, a novel pattern that follows a continuous profile (network topology) is proposed to simultaneously reject multiple frequency bands. It is formed by the addition of various sinusoidal functions tuned at the design frequencies. Measurements performed for two-and three-frequency-tuned PBG microstrip prototypes show that multiple deep and wide stopbands can be obtained using these novel devices.

Real-time spectrum analysis in microstrip technology

We report on a time-domain analog in microwave lines to the spatial Fraunhofer (far-field) diffraction in paraxial conditions. Microstrip lines are used to design filtering configurations acting as spectrum analyzers. They are based on linearly chirped distributed Bragg coupling between the fundamental microstrip mode and the same but counterpropagating mode. Linearly chirped continuous impedance modulation in a microstrip line with varying upper plane strip-width is shown to yield a mode-coupling location and group delay linearly distributed in frequency. Under the condition of a temporal equivalent to the spatial Fraunhofer inequality, the energy spectral density of the input signal is directly recoverable from the average output (reflected) power. It is only necessary to take into account a linear axis-change, given by the dispersion coefficient (group-delay slope) of the structure, from time to Fourier frequency. Both pulsed and nonpulsed RF signals are studied. Sequential time-gated segments of the input have to be processed in the nonpulsed case. The maximum frequency resolution achievable in this situation is discussed. The devices developed here could have important potential applications in the field of temporal signal processing, such as filtering using time-division techniques.

Chirped delay lines in microstrip technology

In this paper, we report on a design method for chirped delay lines (CDLs) in microstrip technology. They consist in a continuously varying strip width, so that the coupling location between the quasi-TEM microstrip mode and the same but counter-propagating mode is linearly distributed in frequency. High delay/spl times/bandwidth products, over frequency ranges of several gigahertzs, can be obtained following this procedure. Experimental data confirm the design method. Real-time Fourier analysis of wideband pulses can be performed using these CDLs.

Dual electromagnetic bandgap CPW structures for filter applications

A novel coplanar waveguide (CPW) low pass filter based on electromagnetic bandgaps (EBG) with double periodicity has been designed and fabricated. The device consists on a CPW with T-shaped loading capacitances at periodic positions and slot width modulation. By properly choosing the ratio between the two periods of the structure, a huge band gap (more than five times the bandwidth) is obtained through the suppression of spurious frequencies. The fabricated prototypes also exhibit very sharp cutoff, very low insertion loss in the passband and slow wave effect.

Analysis and design of periodic structures for microstrip lines by using the coupled mode theory

In this work, the coupled mode theory is formulated to analyze distributed periodic structures for microstrip lines. After it, several reasonable approximations are introduced giving rise to analytical solutions for the problem. The obtained results, both numerical and analytical, are checked against measurements showing very good agreement. The proposed method is very attractive for the study of these devices since it avoids the time-consuming full-wave electromagnetic simulations customarily employed and provides analytical solutions that are very useful for analysis and synthesis purposes.