An electronically tunable bandpass filter is presented. It is a combline filter realized in microstrip form. The filter is made tunable by using the variable capacitance of a GaAs fet (or of a varactor). With the use of this technique it is possible to achieve relative bandwidth from 8% to 30%, a tuning bandwidth 'up to 50% and a center frequency up to 18 GHz. A remarkable characteristic of this device is that it requires no driving current and it presents only a capacitive load (of about 10 pF) to its driver. So, we have an electronically tunable fil­ter with a tuning time of a few nanoseconds. This paper describes method of de­sign, simulation and performance of some monolitic realizations of the filter.