Toward a bandpass filter for 60GHz wireless communications

Metamaterials are artificially constructed sub-wavelength structures that offer the promise of strong and even negative electric and magnetic responses.^{1, 2} By making use of their fascinating properties, researchers have already turned science fiction dreams such as invisibility cloaks,³ super/hyper lenses,⁴ and slow light waveguides⁵ into reality. As a result, metamaterials are increasingly attracting research attention. However, practical issues are preventing metamaterials from being commercialized further. Examples are their narrow band responses,⁶ polarization sensitivity,⁷ and anisotropic properties.⁸

Figure 1. Fabricated two-handed metamaterial (THM) (right panel) and perspective view of a unit cell (left panel). The THM is composed of 17μm-thick copper plates and a Rogers TMM4 board with the relative dielectric constant of 4.5. The geometric parameters of the THM are a_x=a_y=9.8mm, r=4.75mm, d=0.7874mm, h=0.15mm, and w=0.28mm. (b) The retrieved dispersive curves of the effective permittivity, ε_eff(ω)(blue lines), and effective permeability, μ_eff(ω)(red lines), from numerical simulations. ω_M0: Resonant frequencies of the magnetic dipole. ω_MP: Effective magnetic plasma frequencies. ω′_p: New effective plasma frequencies of the metallic structures. ω_E0: Resonant frequencies of the electric dipole.

Figure 2. (a) Magnitudes of the scattering (S-) parameters, corresponding to the reflection and transmission of the THM from numerical simulations (black solid line and black dot-dash line) and real measurements (solid lines in other colors) at different polarizations under normal incidence. (b) On removing one of the metal plates from the THM as a control sample, the left-handed peak disappears but the right-handed one survives. E: Electric field vector. S21: The forward transmission coefficient. S11: The input reflection coefficient. θ: Angle between the x-axis and the electric field.

For every wireless communication application, a bandpass filter is a crucial and mass-used component to allow signals and suppress noises in specific frequencies. In particular, wireless communication in the 60GHz band, an unlicensed spectrum centered at 60GHz with a bandwidth exceeding 7GHz⁹ (regulated in the US by the Federal Communications Commission, FCC), provides a higher transmission rate than other wireless techniques such as Wi-Fi (2Mbps) and Bluetooth (11Mbps) and can even transmit uncompressed high-definition cinema data. However, it is difficult to achieve a high quality factor and bandwidth simultaneously for conventional bandpass filters. We have developed a so-called two-handed metamaterial (THM)¹⁰ that has the potential to act as a bandpass filter for wireless communication.

Figure 3. (a) The unit cell of the designed high-ratio bandwidth squarewave-like (HRBSWL) bandpass filter and its perspective view. The HRBSWL filter is composed of 8mm-thick copper plates and a Rogers board 5880 with the relative dielectric constant of 2.2. The geometric parameters of the HRBSWL filter are a_x=a_y=3mm, l=2.5mm, d=0.07mm, w=1mm, and m=0.1mm. (b) Diagram of complex transmittance for 1–75GHz. A multi-allowed band bandpass filter is presented. The first allowed band is located at 1–9GHz and second allowed band range from 50 to 70GHz.

A THM comprises a left-handed (LH) and a right-handed (RH) allowed band that together radically broaden the bandwidth. An LH band gets its name from the relationship between the wave vector, , electric field, , and magnetic field, : the propagating waves travel in the opposite direction from the energy in that they go toward, rather than away from, the source. This requires the refractive index, n, to be negative.

Our THM is composed of a dielectric substrate with a dual layer of patterned copper plates: see Figure 1(a). It satisfies the three requirements of a well-behaved bandpass filter for wireless communication. First, the effective permittivity, ε_eff(ω), and effective permeability, μ_eff(ω), must be either both negative or both positive: see the shadowed regions in Figure 1(b). Second, the THM impedance should match that of free space, as at the two intersections of ε_eff and μ_eff: see Figure 1(b). Third, the responses of the THM must be independent of incident wave polarization: see Figure 2(a and b). The copper plates result in an inductance/capacitance resonance and the LH allowed band. In contrast, the dielectric substrate results in the cavity mode resonance of the natural material and also the RH allowed band: see Figure 2(b).

Figure 4. (a, b) The simulated S-parameters for the transmission-line metamaterial bandpass filter. (c) The dispersion relations calculated by the simulated phase of S21 within the passband. (d) Calculated group delay from the simulated phase of transmission. θ: Phase change between input and output signals. β (propagation constant) and p (propagation distance) of the composite right-handed/left-handed transmission line filter.

More recently, we further evolved the THM to demonstrate a square-wave-like (SWL) bandpass filter at 60GHz—see Figure 3(a)¹¹—by shrinking the unit cell to achieve a higher operating frequency and by defining different patterns of metal plates to enhance the transition from the LH to the RH allowed band. We obtained an ultrawide allowed bandwidth of 20GHz between 50 and 70GHz, with a sharp band-edge transition of 15dB/GHz: see Figure 3(b). Besides this, another allowed band is located at 1–9GHz, which is the spectrum occupied by currently used wireless communication systems such as Wi-Fi and Bluetooth. As a result, these two specific allowed bands suggest that such an SWL filter could be readily used in 60GHz wireless communication and also be downwards compatible with Wi-Fi and Bluetooth.¹²

Our SWL bandpass filter is hard to integrate into a silicon chip because the electromagnetic wave propagates normal to the substrate. For wireless communication, we can further extend the THM concept to a transmission-line-type bandpass filter, for which the electromagnetic waves propagate along the substrate, so it is easier to combine with other components for an integrated on-chip solution. We combine the RH and LH responses and choose 60GHz as the central frequency to achieve a composite RH/LH transmission line (CRLH-TL) that complies with FCC regulations.¹³ Simulated results show that these coupled resonators provide a passband composed of an LH and an RH band, and provides a 6.2GHz-wide bandwidth between 57.4 and 63.6GHz: see Figure 4(a–c). Moreover, this CRLH-TL filter has an insertion loss as low as –1.08dB, a sharp band-edge transition, a wide stopband—see Figure 4(b)—and a small group delay: see Figure 4(d).

In summary, we have developed a bandpass filter that can be used as an integrated solution for 60GHz communication applications. We are now working to develop a new prototype of pliable filters that can be attached to the surface of wireless communication devices (such as cellphones), and also to develop a practical 60GHz CRLH transmission-line type bandpass filter in integrated chips.