Filters are essential for sorting signals in both transmit and receive functions in communications, although modern system requirements demand that filters constantly be made smaller and with wider bandwidths. Fortunately, work at China's Xidian University has led to the development of a compact bandpass filter, formed by combining circuits for highpass and lowpass filters. It draws from substrate integrated waveguide (SIW) technology for its highpass characteristics and cascaded defected ground structure (CDGS) technology for its lowpass characteristics. The novel filter, which measures only 34 x 20 mm, has a relative bandwidth of 78.18% with low insertion loss and strong out-of-band rejection.
Microwave filters have been fabricated with low profiles, small size, and low cost by means of SIW technology on standard printed-circuit-board (PCB) materials. An SIW structure consists of a dielectric substrate which is comprised of a pair of metal plates and connected via holes.1 This configuration confines the filter's electromagnetic (EM) field inside the structure.
The use of defected-ground-structure (DGS) elements in microwave circuits can provide numerous benefits, including a size reduction over conventional planar circuits and high suppression of unwanted spurious responses. Conventional DGS elements feature a dumbbell-shaped pattern etched in a circuit's ground plane. The CDGS elements employed in the proposed bandpass filter are based on a conventional U-shaped DGS unit.2 As with traditional DGS elements, this type of CDGS element can significantly suppress the spurious responses in the stopband, at the same time leading to a filter much smaller in size than would be possible with traditional planar circuit techniques.
Bandpass filters have been widely used in various wireless communication systems. For example, in ref. 3, the researchers created an ultrawideband (UWB) filter by combining individually designed highpass and lowpass filters. Since both of the filters were fabricated in a cascaded configuration, this approach did not lend itself to small size.
In this article, the authors present a new compact bandpass filter that is formed by the combination of SIW and CDGS technologies. The field distribution in the SIW circuitry is similar to that in conventional rectangular waveguide, forming a highpass filter.4 And since CDGS supports the construction of a lowpass filter that can be fabricated in the ground plane of an SIW circuit, the proposed bandpass filterwhich combines SIW and CDGS approachescan be fabricated with a relatively compact size.
The proposed bandpass filter combines individually designed highpass and lowpass filters. The highpass filter is based on a SIW approach, while the lowpass filter is based on the use of CDGS circuit elements. The broad passband of this novel bandpass filter extends from 6.7 to 15.3 GHz. A prototype of the bandpass filter was fabricated on a commercial, high-frequency dielectric substrate material with 1-mm thickness, relative dielectric constant (er) of 2.65, and loss tangent of 0.0027.
On this material, a microstrip line width of 2.7 mm corresponds to a 50-O microstrip transmission line on a conventional ground plane.
Figure 1 shows an SIW structure cell and its geometric parameters. The SIW consists of two linear metallic via arrays on a dielectric substrate with a height of h. These metallic via arrays confine the EM fields within the SIW circuitry.4 Based on the method of lines, a relationship between conventional rectangular waveguide and SIW can be used to initially determine the geometric parameters for the SIW.5 This relationship can be described as shown in Eq. 1:
The SIW has a width of a. The metallic via has a diameter of d and the space between the adjacent via is p.
As mentioned previously, the EM fields in SIW behave similarly to the EM fields found in rectangular waveguide. A conventional rectangular waveguide behaves as a highpass filter so, using the similarity between SIW and rectangular waveguide, the cutoff frequency for SIW can be defined as shown in Eq. 5:
where:
c = the velocity of light in a vacuum, and
er = the relative dielectric constant of the SIW structure.
According to Eqs. 1-5, it is possible to design a highpass filter using an SIW approach, with the physical parameters of the highpass filter listed in Table 1.
The proposed double equilateral U-shaped DGS unit is shown in Fig. 2. It has a 50-O microstrip line on the top and two equilateral U-shaped patterns symmetrically etched in the ground plane. Each U-shaped pattern consists of three etched lines with the same widths (g) but different lengths (L1, L2, L3, and L4). By setting these two U-shaped patterns with different lengths, the smaller ones can easily be embedded inside the larger ones using an open-end alignment.
The two U-shaped structures offer dual finite attenuation poles with reduced defected ground area. Similar to two dumbbell-shaped DGS lines connected in a cascade configuration, this proposed CDGS circuitry can be designed as a lowpass filter, and was used to form the CDGS-based lowpass circuitry shown in Fig. 3. By properly choosing of the length of the cascaded two U-shaped structures, near ideal lowpass response can be realized. Table 2 shows the physical parameters for this lowpass filter.
Figure 4 shows the configuration for the bandpass filter based on the SIW and CDGS circuit approaches. In the design process of the proposed filter, the geometric parameters for SIW and the CDGS cells are selected independently, which increases the design flexibility. The dimension of the whole length is determined by the CDGS elements. Compared to filters fabricated with cascaded lowpass and highpass filters, this new approach leads to a bandpass filter with compact size, occupying a circuit area of only 34 x 20 mm.
Based on CAE simulations, the bandpass filter designed by a combination of SIW and CDGS circuit elements should provide a relative bandwidth of 78.18% (from 6.7 to 15.3 GHz). The computer simulations suggest insertion loss of 0.95 dB within the passband at 10 GHz and 1.41 dB at 14 GHz. The predicted return loss is better than 13 dB for the bandpass filter. Even though it uses a combination of SIW and CDGS elements, the bandpass filter can be designed and fabricated with standard PCB material processing techniques.
A prototype bandpass filter built in this way is shown in Fig. 5, while Fig. 6 shows simulated and measured results for the prototype. Over the wide frequency range from 6.7 to 15.3 GHz, the predicted and measured results are in close agreement. According to the measured results, the filter features good return-loss performance with low insertion loss across the whole passband.
In conclusion, the similarity between SIW and rectangular waveguide has helped to define parameters for constructing the highpass portion of a broadband bandpass filter, while CDGS elements have been used to form the lowpass portion of the filter. Combined, the two filter segments form a compact bandpass filter with broad passband.
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