June 2005, Issue 179

Precision Frequency Meter
Cypress PSoC High Integration Challenge 2004 Contest Winner

FREQUENCY REFERENCE

You need a precise reference clock to accurately measure a frequency. Crystal oscillators are good, but they’re still only on the order of 50 ppm. High-accuracy crystal oscillators are available, but they’re still only on the order of a few parts per million. Accuracy relates to stability, absolute accuracy, and aging. Stability relates to the oscillator’s stability amid varying environmental conditions such as temperature, supply voltage, and output loading. The Precision Frequency Meter addresses this by controlling the oscillator’s environment. It’s mounted in a crystal oven along with a dedicated voltage regulator and output buffer.

Absolute accuracy relates to the oscillator’s accuracy with respect to a recognized standard. It’s addressed by measuring the oscillator frequency against a known standard such as the hourly time pips broadcast via public radio. The pips are derived from the national standard. The meter counts the oscillator cycles from hour to hour and then calculates the true oscillator frequency.

Aging relates to the oscillator’s stability over time. This is also addressed by way of time pips because the meter measures the oscillator frequency on an hourly basis.