Re: frequency doubler
i would suggest diodes but you have to choose w/c one would you prefer.
the ordinary fast switching diode would do, also a fast recovery junction diode & ultra fast, likewise a schotty barrier or a varactor.
but for me the best choice would be the schotty barrier due to its low flicker noise in addition to its low barrier potential.
for ultra fast switching speed i can recommend the FD700. it switches at 0.7ns and a junction capacity of 1pf, it can handle up to 250ma with a reverse voltage of 30v.
for fullwave rectification, a varactor will do it for you becoz it has a high conversion efficiency and has an odd harmonics rejection. and finally filtering is easy.
Here is the pdf for frequency doubler