Fundamental to all frequency and period measurements is the requirement for a very
accurate frequency source. In most bench top instruments, the frequency reference is obtained from a quartz crystal oscillator operated under conditions of strict temperature
control in an oven generally kept to 0.01C or better. For example, the Hewlett-Packard
HP105B quartz frequency standard has buffered sinusoidal outputs at 5, 1 and 0.1 MHz,
and a short term stability measured over at 5 MHz of 5 parts in 1012 (measured over a
1s averaging time). The ‘ageing rate’ is 55 1010 per 24 hours for this oscillator.