Crystal oscillators are loosely divided because of the frequency they perform at and whether they are fundamental oscillators—work within the frequency marked to the can or at even multiples of that (overtone oscillators). Significant-repetition-charge and high-electric power picosecond regenerative amplifier depending on only one bulk Nd:GdVO4 crystal A timing reference https://www.hxcrystals.com/seokeywords/bulk-crystal-oscillator.html