The two oscillator circuits under are suitable for working with crystal oscillators in series or parallel manner: Substantial-repetition-level and large-power picosecond regenerative amplifier depending on only one bulk Nd:GdVO4 crystal A timing reference incorporating one-crystal-silicon micromechanical resonator with a quality element of much larger than one million as well as https://www.hxcrystals.com/seokeywords/bulk-crystal-oscillator.html