Compact copper microstrip filters with spiral resonators

Abstract

[2,3]. When conventional straight microstrip is miniaturized by making it narrower, there is a disproportionate decrease in the volume occupied by the electromagnetic field [3] because this volume shrinks in two dimensions. For spirals, there is less reduction of active volume because the combined fields of several parallel turns extends further down into the substrate, so that simulation accuracy for a given cell size is improved, the effect of over-etch is alleviated, and conductor losses are not so badly affected [3]. In this paper, two copper filters are investigated to see if these advantages still apply for normal metal implementations over a range of bandwidths. In contrast to the superconducting filters, which use relatively new technology, this work demonstrates spirals in the other extreme of very basic fabrication. It turns out that, because typical copper spirals contain only about two to four turns, the improvement (compared with a straight narrow microstrip) is only moderate, but can still be useful since there are benefits in three separate areas. Nevertheless, the main advantage is probably the more compact shape. Parallel-coupled filters tend to be long and thin, thus leading to a large border, which is necessary to separate the device from other circuit elements. Also, the electric and magnetic fields decay faster with distance for a spiral.One other nonsuperconducting spiral filter has been reported [4], but it requires via technology and only a lower-order filter was presented. A spiral has also been used in an oscillator circuit [5], while a recent example of a spiral as a lumped element was presented in [6]. 5% BANDWIDTH FILTER DESIGNThe filter layout is shown in Figure 1. The first and last spirals, which are connected to the input and output, are coupling structures equivalent to the input and output coupling lines in a parallelline filter. They are quarter-wavelength resonators, and the centres of each spiral are open circuit and the outer end is connected to a relatively low impedance of 50⍀. The resonant behaviour does not have a significant effect on the passband shape due to the low Q-factor. The four spirals in between form the four half-wavelength resonators of a 4 th -order Chebyshev filter. The substrate is 3.18-mm-thick RT Duroid 5870 ( r ϭ 2.33, tan ␦ ϭ 0.0012); use of a higher dielectric constant would have resulted in an even more compact device. An arbitrary centre frequency of 947 MHz was chosen, while the bandwidth is 5% (at 0.5 dB below the maximum in the lossless approximation).The design procedure is similar to [3], using coupling coefficients k for a 4 th -order 0.5-dB ripple filter taken from [7]. First, an appropriate spiral size was found by trial and error using a commercial full-wave electromagnetic simulator. As a starting point, the unwound length of the resonator should be a half-wavelength, and the effective permittivity is taken to be the permittivity of the substrate, where most of the electric field resides. For ease of fabrication and to retain the possibi...

Quasi-dual-mode microstrip spiral filters using first and second harmonic resonances

“…A possible partial explanation is that simulations with finite cell width (in this case, 0.125 mm) do not fully allow for current and charge crowding at the edges and, therefore, might, in fact, accurately simulate a narrower linewidth. This was also observed in [4].…”

“…Unloaded , found as in [4], is approximately 130 for both filters. Passband loss is accurately simulated here and in [4], but the simulation may not always adequately deal with current crowding at the edges of the tracks.…”

“…The filter was designed using standard data from [19] together with graphs of , examples of which are given above, together with an iteration procedure [15]. The input and output coupling technique is described in [4].…”

“…Compactness has been enhanced by special types of resonators such as step-impedance hairpins [1], meander lines [2], and dumbbell shapes [3]. A particularly promising compact resonator is the spiral [4], which shows a favorable tradeoff between size and quality factor ( ) [4]. In this paper, the range of fractional bandwidths, which can be implemented with spirals, is extended using two layers of spirals in conjunction with quasi-dual-mode half-wave resonators operating at their fundamental and second harmonic frequencies.…”

Compact bandpass filter using novel transformer‐based coupled resonators on integrated passive device glass substrate

“…Filters based on the transmission-line element have encountered the difficulty on size reduction due to the fact that their sizes are in multiples of quarter-wavelength. Some other studies propose novel structures to reduce filter size, including stepped impedance resonator [4], defected ground structure resonator [5], dualmode ring resonator [6], and spiral-like resonator [7]. However, the resulting designs are complex and not flexible to design.…”