Application of voltage controlled crystal oscillators to DC voltage reference validation

DC voltage references are typically based on calibrated Zener Diodes which are subject to drift and therefore require periodic recalibration against a Primary Standard. Since it is preferable to maintain constant power to such references, the shipping and handling logistics involved in such a recalibration can present an unacceptable burden. Validation is therefore preferred, and typically accomplished by comparison against other local Zener Diodes. However, these Zener Diodes are all subject to correlated aging mechanisms. Voltage to frequency conversion represents an alternative mechanism for validation and accordingly, is not subject to aging highly correlated with the system being validated. Voltage to frequency conversion using a low space, weight, and power Voltage Controlled Oscillator offers a mechanism for identifying voltage reference drift by comparing the voltage-controlled frequency to primary frequency, typically available through GPS. This technique can validate voltage references when comparison against a primary voltage standard is impractical due to system deployment, away from a robust logistics chain. Voltage to frequency conversion may be accomplished by a Voltage Controlled Ovenized Crystal Oscillator. Using commercial-off-the-shelf hardware we constructed a test to evaluate the stability of such an oscillator for 258 days of continuous run-time, without age acceleration measures. Long-term drift was consistent with a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt><mml:mrow><mml:mi>t</mml:mi></mml:mrow> </mml:msqrt> </mml:math> aging model. Sequestering 2/3 of the data to construct an aging model, then comparing sequestered data, yielded a model-to-data difference of 35 ppm (35 μV/V) which may prove acceptable in supporting instruments in the 6.5-digit voltmeter class.