Power characteristics of varactor-controlled tunable bandpass filters on lumped elements

Design of frequency-tunable microwave filters for multi-band wireless communication systems has recently attracted considerable attention. Lumped-element tunable filters seem very promising due to their small size compared to the wavelength. Semiconductor varactor diodes have been widely used as controlling components in tunable filters. Characteristics of varactor-controlled tunable filters depend on the input power and can degrade dramatically when the power increases. The paper presents a comparative study of power characteristics of two tunable lumped-element bandpass filters, one based on series LC-tanks and another one using parallel LC-tanks. The three-pole bandpass filters were implemented on Arlon AD 255 printed circuit board using surface-mount components. The hyperabrupt-junction silicon varactors of SMV 123x series by Skyworks were employed as the controlling components. The center frequency tuning range of 430-760 MHz was achieved for the filter on series tanks and of 450-675 MHz for the filter on parallel tanks. Frequency responses of the both tunable filters were measured under different input power up to +18dBm. Simulated and measured dependences of the output power on the input one are presented. The 1-dB compression point was achieved at the input power of +14dBm for the filter on series tanks and at +7.5 dBm for the filter based on parallel tanks. Besides, the varactor-controlled tunable bandpass filter based of series LC-tanks revealed experimentally lower sensitivity of the frequency response to a variation of the input power.