Equipment was developed to measure the phase stability of quartz crystal units in the frequency range from 1 to 110 mc while being subjected to 10 g vibration from 20 cps to 2000 cps. Details of development problems, explanation of circuit operation, and circuit analysis are presented for the tunable phase detector, reference oscillator, crystal-vibration-test oscillator, and the complete phase-locked loop system. Pertinent information also is given on other parts of the system, such as the audio amplifier, oscilloscope, power supplies, and cabinet. System operation is detailed, and performance data are presented. Phase-deviation resolution of 0.75 millidegree at 1 mc and 40 millidegrees at 110 mc was measured in the 4-kc system bandwidth. Application of this equipment will facilitate the development of phase-stable crystal units. It will be a useful measurement tool for the development of circuits and components for crystal oscillators, frequency synthesizers, and other radio-frequency signal sources demanding increased phase stability.