Lowpass filter with slow-wave rail coplanar stripline (R-CPS)

Abstract

The coplanar stripline (CPS) suffers from excessively high characteristic impedance which makes the design of filters difficult. Proposed is a slow-wave rail CPS (R-CPS) transmission line for the design of a stepped impedance lowpass filter. The R-CPS exhibits an extended low-impedance limit not achievable by conventional CPS, while retaining the advantages of the CPS. It also improves the performance of the filter in terms of suppression of the spurious harmonic passbands and reduces the filter size.Introduction: Uniplanar transmission lines, such as the coplanar waveguide (CPW) or coplanar stripline (CPS), exhibit several advantages over microstrip lines or striplines. CPW often incorporates a metal plate at the backside of the substrate, thereby forming a conductorbacked CPW (CB-CPW) [1 -3]. This back conducting plane reduces the characteristic impedance. CPS has all the benefits of CPW, and has a balanced configuration which is beneficial in circuits involving differential components, such as dipole antennas, balanced mixers and amplifier pairs. Considering a conventional CPS printed on a substrate of height h and dielectric constant 1 r , its characteristic impedance Z 0 [1] decreases when the gap distance between the strips g decreases, or when h increases and 1 r increases. Very high characteristic impedance can be easily achieved by simply increasing the gap between the two strips. However, it is generally difficult to provide low characteristic impedance for a given substrate because of limitations in the fabrication tolerance of the gap, especially on an electrically thin substrate, typically encountered in standard MMICs. This restriction severely limits the application of CPS in filters, where a large hi-lo impedance ratio is required. A tentative solution would be to introduce a conductor backed CPS (CB-CPS), analogous to the CB-CPW. However, additional common mode may be excited in a CB-CPS, except the expected differential transmission mode.In this Letter we propose a rail CPS (R-CPS) transmission line structure and apply it to a stepped impedance lowpass filter. The R-CPS is composed of a conventional CPS loaded with transverse strips at the back of the substrate, which resembles train rail tracks. It provides a dramatically extended lower impedance limit without reduction of the higher impedance limit, while preserving the CPS benefits of uniplanarity and differential configuration. Benefiting from the proposed structure, a seventh-order Chebyshev stepped-impedance lowpass filter with a cutoff frequency of 5 GHz and a passband ripple of 0.5 dB has been designed and measured. In conventional CPS technology, such a design is impossible on a relatively thin and low-1 r substrate [4,5] owing to the required very low impedance. Moreover, thanks to its slow-wave effect, the R-CPS also reduces the size of the filter. Finally, a broad stop-band, immune to some of the parasitic harmonic passbands, is achieved owing to the stop-bands of the periodic loading strips.