Reference Disciplining

Time-based measurements and periodic signal sources are only as accurate as the frequency reference that is driving the measurement or signal. Oscillators used to provide the reference can vary greatly in type and frequency precision. In addition to the physical type of the oscillator, environmental conditions, such as temperature, can effect both long term and short term frequency stability.

Reference disciplining is a technique of using a highly stable frequency standard to adjust the frequency of the internal oscillator of an instrument to near (typically within one decade) that of the external reference. Using reference disciplining, the long term stability and accuracy of the internal oscillator and thus the accuracy of the measurement or signal source can be greatly improved.

For example, C&H’s MA209 100MHz Pulse Generator has a typical frequency stability specification of 50 parts per million (ppm). However, with external reference disciplining, the frequency stability can be improved to 0.01ppm.

With the MA209, an external 1 Pulse Per Second (PPS), 100PPS, or 10MHz signal can be used to discipline the internal clock. The 1PPS or 100PPS can come from a GPS timing receiver such as C&H’s M213. The 10MHz can come from an ovenized oscillator such as the M207, from a Rubidium frequency standard or from one of many other precision 10MHz frequency sources on the market. The MA209 requires an initial 10 minute module warm-up period, after which the internal clock will discipline, in typically 30 seconds, to within one decade of the external reference, up to the specified stability.

It is important to distinguish reference disciplining from the more complex technique of phase locking which is typically performed with a phase lock loop (PLL). The same results can certainly be achieved with a PLL and a PLL can provided further benefits; however, we typically do not desire the added complexity nor need the added benefits. In contrast, reference disciplining is a straightforward solution that uses the short term stability of a common voltage controlled oscillator and the long term stability of an external reference to create highly stable frequency source.

Reference disciplining is incorporated into a number of C&H’s M-Module instruments including the MA204, MA208, MA209 and the forthcoming M227. In any instrument where the stability of the frequency source is important, reference disciplining is key to achieving the most accurate and precise measurements.