Replicability of a Millimeter-Wave Microstrip Bandpass Filter using Parallel Coupled Lines

Replicability of filters is critical especially at millimeter-wave (mm-wave) frequencies, as a manufacturing error of a few tens of microns can significantly shift the frequency response in this range and lumped elements are not available at these frequencies. In this paper, seven replicas of a mm-wave coupled-line 3^rd order bandpass filter (BPF) are fabricated and measured under the same test conditions. The filters are designed on a single lot 10 mil-thick Rogers RO4350B substrate. The smallest spacing is 109 μm and the smallest line width is 330 μm. Over seven replicas, the passband is from 34.8±0.2 to 38.1±0.1 GHz; the insertion loss is 3.44±0.32 dB; the typical return loss is 10.18±2.43 dB. The measurement results are in accordance with the EM simulation results. They show that the reflection parameters are relatively sensitive to manufacturing tolerances and connector realization, while the transmission parameters are robust to fabrication variations. This demonstrates a satisfactory manufacturing replicability of microstrip BPF in the Ka-band (26.5-40 GHz), in the scope of radar system design.