Building a Zero-Order BPF with CRLH Transmission Lines (.PDF Download)

Miniaturization of RF/microwave filters helps pave the way to developing smaller wireless devices for internet access. Using metamaterials and circuit structures such as composite-right-left-handed (CRLH) resonators has proven effective in shrinking RF/microwave filter circuits and was demonstrated in the design of a compact bandpass filter (BPF) well-suited for WiMAX wireless applications.

Leveraging third-order coupled CRLH resonators, the filter achieves a center frequency of 5.9 GHz with transmission zero at 6.4 GHz. Measuring just 16 × 24 mm², or about 40% the size of BPFs based on conventional resonators, the filter has a passband insertion loss of just 1.5 dB.

Metamaterials have shown great promise for the fabrication of compact, high-frequency RF/microwave circuits.^1-4 First proposed in 2002, CRLH transmission lines (CRLH-TLs) are forms of high-frequency transmission lines that exhibit backward-wave transmission behavior capable of unusual electromagnetic (EM) wave propagation. Metamaterial approaches to microwave circuit design are typically based on CRLH or negative-refractive-index transmission lines in planar structures by loading a host transmission line with series capacitor and shunt inductive load.

Use of metamaterials and CRLH-TLs enables the design of RF/microwave BPFs with small size, low passband loss, and even low cost. The small sizes supported by metamaterials make possible multiple-band filters that are a fraction of the size of BPFs formed with conventional transmission lines.^5-10 Coupled metamaterial resonators have formed compact BPFs.^8,10 In addition, compact microwave CRLH gap resonators with high quality factors (Qs) show great promise for forming miniature BPFs.^11,12

To demonstrate, by combining high-Q CRLH gap resonators with third-order zeroth-order-resonator (ZOR) coupled resonators, a microwave BPF with extremely compact dimensions was designed and fabricated for WiMAX applications with standard, low-cost circuit materials. The filter was constructed on RT/duroid 6010 circuit material from Rogers Corp. The 1.27-mm-thick circuit material has dielectric constant of 10.8 at 10 GHz in the z-axis (thickness). Commercial electromagnetic (EM) simulation software helped optimize the design, which was characterized using a 50-Ω microstrip feed line and commercial test equipment, notably an RF/microwave vector network analyzer (VNA).

Figure 1 shows an equivalent-circuit model of the CRLH-TL that consists of right-handed inductance (L_R), right-handed capacitance (C_R), left-handed inductance (L_L), and left-handed capacitance (C_L). In a practical circuit design, C_L is fabricated as a four-finger interdigital capacitor, L_L is a viahole in the printed-circuit-board (PCB) material, and capacitance C_R and inductance L_R are parasitic circuit elements essentially invisible in size.