We use a microcontroller-enhanced crystal oscillator. The Master Clock is locked to a Rubidium source and temperature-cycled very slowly overnight while a computer logs the corrections that are applied to keep the unit locked against temperature. An accurate correction formula is extracted from the data and programmed into the unit. The Master Clock is temperature cycled again to check that the calibration is correct. This gives a frequency accuracy that is typically within +/-0.1ppm and changes by much less than 0.1ppm over the range 10 – 40 Celsius.