Electrically Controlled Variable Inductors for Applications in Tunable Filters

Tunable filters have been mostly designed with an electrical control. The controlling components are typically variable capacitors. This is due to the fact that variable inductors are still considered something exotic. However, tunability of the lumped-element filters with the only variable capacitors as controlling components is limited. On the other hand, recent advances in development of the electrically controlled variable inductors make them promising for the design of tunable filters with improved characteristics. Filters controlled by variable inductors in line with variable capacitors are able to provide enhanced tunability without a variation of the bandwidth or the in-band reflection coefficient. The paper discusses the needs for variable inductors in the low-pass and band-pass tunable filters. Examples illustrating advantages of the use of variable capacitors in combination with variable inductors are demonstrated. Design of the grounded as well as floating-point electrically controlled variable inductors based on a negative impedance converter loaded with a varactor diode is considered. A three-pole band-pass filter employing such a variable inductor is shown capable to keep the absolute bandwidth and the reflection coefficient level when tuning the center frequency.