A rubidium frequency standard based on unreal time control approach

Abstract

In conventional rubidium frequency standard and some passive atomic frequency standards the voltage control crystal oscillator VCOCXO exporting the frequency signal of the standards is real time controlled. Because in the locking loop there are frequency transformation circuits, servo amplifier, testing circuits, and physics package, although the output signal of the standards is very accurate and with a good long-term performance, its short-term stability and phase noise performance are poorer than those of VCOCXO. There have been some techniques to filter or control the servo noise, but it is difficult to obtain both good long-term and short-term specifications at the same time. Because a highly stable crystal oscillator not only has a good short-term stability, but also can keep the high accuracy in certain period. Therefore, as an accurate assistant frequency source, the frequency of VCOCXO does not have to be controlled in real time. The unreal time control approach means the servo voltage from the servo amplifier following physics package does not control the frequency of VCOCXO directly. Based on the ability of VCOCXO to keep the accuracy in certain period the control signal can keep unchanged, the good short-term performance of VCOCXO in the standard is taken advantage. In the new rubidium frequency standard, we use microprocessor and I/O units (A/D and D/A converters). This microprocessor can provide the capability of both processing and isolating. It can compare the mean value of output voltage signals of servo amplifier with former voltage signals that control the VCOCXO. The noise of output voltage of servo amplifier can be eliminated through the software processing. The difference between the output voltage signals and the current control voltage signals is related to the regulation value to VCOCXO. The regulation is completed evenly in step according to the least voltage stepping adjusting value in a given adjusting period. This method can guarantee the original stability of VCOCXO and even frequency locking. The rubidium frequency standard can show 1 ~ 2 times 10-12/s stability and better than 5 times10-11/month drift. It is possible to improve its performances further