FR4-Based Microstrip Anti-Aliasing Filters for Ultra-High-Speed Analog-To-Digital Converters

In this work, passive low-pass filters implemented with microstrip transmission lines are investigated for their usability in ultra-high-speed analog-to-digital converters. Compared to other works which utilize expensive radio-frequency (RF) specified substrates based on e.g., ceramics, the most-frequently used material for printed circuit boards (PCBs) FR4 is used in this work. The filters were designed using analytical estimations and electro-magnetic field (EM) simulations. The most promising filter structures are high-order Chebyshev and Legendre type filters. We designed them conventionally and with additional stubs acting as band stop filters to suppress spurious passbands. The filters were physically implemented and measured up to a frequency of 67 GHz and show good performance with a high stopband attenuation for critical frequencies of more than 45 dB and a high skirt steepness. The insertion losses between 1.8 dB and 2.1 dB are slightly higher than those of the reference filters. On the other hand, the effect of spurious passbands is less severe for FR4. These effects are discussed in detail and a comparison to the more expensive RO4350B-filter counterparts is given.