More Obscure Filters

After you get through Low pass filters, High pass filters, Band pass filter, and Band Reject filters you get into some obscure highly specialized hardware. So, there's also SAW Filters, BAW filters, FBAR etc. But point being, we are well into the back pages of the filter catalog. These are highly specialized filters that while not exactly rare are certainly uncommon.

Garnet Filters - More specifically a Yttrium iron garnet (YIG) filter.  This method filters at microwave frequencies from 800 MHz to about 5 GHz. It uses a synthetic single crystal yttrium iron garnet sphere which is comprised of yttrium and iron. The garnet sits on a strip of metal driven by a transistor, and a small loop antenna touches the top of the sphere. An electromagnet changes the frequency that the garnet will pass. So these filter is runable over a fairly wide freqency band.

Atomic Line Filter - Also known as an ALF filter. This is used almost exclusively in filtering laser light. While light is in the electromagnetic spectrum it's not really RF.  But for such high frequencies and precision, the vibrations of atoms must be used.  Atomic clocks use cesium masers as ultra-high Q filters to stabilize their primary oscillators. The alternate method, genuinely used with RF is a ruby maser tapped delay line. This treats the maser like a  very low-noise pre-amplifier. They are cryogenically cooled  to temperatures below 5 kelvin to reduce noise. Some of these can produce 50db of gain on a signal originating 140 million miles away. We've been using these since the early 1960s.