Electronically Tunable filter

I want to design an electronically tunable filter ,900MHz---1400MHz,can u give me some examples and aricles?thank u?
is it hard?

Just have a look at this paper. Hard is relative. If you have some skills in making PCBs and some mechanics (for the case) or at least someone at hand who can do this for you, the thing is not too complicated. But don't ask further details about this design, i haven't realized it yet.

https://www.edaboard.com/viewtopic.php?p=389383#389383

You can do a very simple tunable filter using shunt varactors and inductors in parallel. Reverse biasing the varactors will allow you to move the center frequency.

You can also couple a few of these networks together to produce a sharper rolloff. Another option is to add a gain block between a couple of these resonators. One might do this in a receiver front end for example.