In this study, a varactor-loaded slotted ground structure is investigated and utilized to construct a new tunable common-mode (CM) suppression filter for differential signals. A four-port distributed equivalent circuit model is developed for interpreting the working mechanism of CM signal suppression. It is found that the proposed simple structure is capable of tunable CM suppression with a wide frequency tuning range. The parameter selection and design principle are also given. Finally, the design theory is well vindicated by a common-mode filter using three periodic varactorloaded slots. Simulated and measured results, showing good agreement, exhibit a tuning range from 0.80 to 2.10 GHz, corresponding to the fractional tuning range of 89.7% and more than 30 dB CM rejection level.
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