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A simple oscillator suitable for use as a primary standard of frequency has been developed. Employing a new quartz crystal unit operating in the fifth overtone mode at 5 megacycles, this oscillator has exhibited a frequency stability better than 1 part per billion over periods of several hours; over extended periods the frequency drifts less than 10 parts per billion per month. The oscillator comprises a single stage Class A amplifier with a pi section feedback network; this circuit minimizes the influence of variations in electron tubes and circuit components. Class A operation is accomplished by amplified automatic amplitude control that also holds the crystal dissipation to less than a microwatt. A novel feature is use of a resistor in parallel with the crystal to suppress oscillation at the fundamental and unwanted overtone frequencies, without significantly affecting the frequency stability.