Tunable and miniaturized microwave filters using carbon nanotube-based variable capacitors

In this paper, we introduce a novel concept of tunable and miniaturized filters which embed, as voltage-controlled elements, state-of-the-art variable capacitors, based on vertically aligned carbon nanotubes (VACNTs). Starting from a theoretical estimation of the voltage-dependent capacitance between two adjacent CNTs, we extended this physics principle to a large matrix of CNTs, suitably placed on the molybdenum electrodes of an interdigitated capacitor (IDC), since molybdenum can withstand the high temperature necessary in the plasma process for the growth of the VACNTs. The IDC is the tunable element of a microwave filter, which must fulfill the need for both reconfigurability (being either a low-pass, a high-pass or a band-pass filter, at discretion) and low-voltage frequency tuning of reflection/transmission coefficients. For all these reasons, a very compact layout made of T-type cells (comprising VACNT-based variable capacitors and distributed inductors) was designed, simulated, fabricated, and tested, targeting the C, X, and Ku bands (4-16 GHz) for wireless and radar applications. Taking as a reference the free-space wavelength lambda_0 at 10 GHz, the band-pass filter has overall dimensions of just 3.19 mm x 3.47 mm (i.e., 0.11 lambda_0 x 0.12 lambda_0), with the minimum of the reflection coefficient shifting of 1.16 GHz (within the X band) for an applied dc bias voltage of just 4 V and spanning between -24.81 dB and -36.13 dB. Furthermore, the maximum rejection is 31.65 dB, and the 3-dB fractional bandwidth is 12.44%. The proposed filters are the proof that nanomaterials can be profitably integrated into microwave components for next-generation transceivers.