The two oscillator circuits under are suitable for employing crystal oscillators in series or parallel method: Superior-repetition-level and high-electrical power picosecond regenerative amplifier based upon just 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