ptf 3207A GNS Rubidium vs OCXO Comparison

Application Note – 50

ptf 3207A GNS Rubidium vs OCXO Comparison

The following application note is to compare performance of a ptf 3207A GNS receiver fitted with a Rubidium local oscillator versus a ptf 3207A GNS unit fitted with a high performance OCXO.

Rubidium.

When fitted with a rubidium (Rb) internal oscillator, the unit is designed to be optimized on short, medium, and long-term frequency stability when disciplined by GPS or another satellite constellation.

The internal Rb module is disciplined by the GPS 1PPS, and because of the inherent stability of the Rubidium and the very long (approx. 24hr) loop time constant, the rubidium output is very gradually “steered” towards the long term accuracy of GPS, and after several days can demonstrate accuracies of the order of a few parts in 10-14.

The actual 1PPS phase differences in the Rubidium based instrument can be so small (less than the inherent noise) that is difficult to take an accurate phase measurement between the 1PPS derived from the Rubidium oscillator and the incoming 1PPS from the satellites.

In order to alleviate this the ptf 3207A uses a linear regression technique to calculate phase offset and slope. This coupled with the use of an interpolator (which magnifies the error by 1000 times) allows the instrument to generate a highly accurate control value to be applied to the Rubidium.

As the Rubidium oscillator in the ptf 3207A is driven in a closed loop configuration, this in turn allows the 1PPS output derived from the very accurate 10MHz RF output, to be driven to align very closely with the UTC 1PPS coming from the satellites. Statistically calculated 1 sigma levels of less than 10 nano seconds have been achieved.

OCXO

The alternative configuration utilizing a high performance OCXO as the internal oscillator uses the a three stage, two term, control loop, with a maximum time constant on the slow loop of around 100 seconds.

In order to steer the OCXO (using the OCXO control voltage) the unit takes a measurement once per second between the 1PPS derived from the OCXO 10MHz and the 1PPS direct from the GNS satellite receiver.

In this way the frequency is “steered” to minimize the average error between the two 1PPS signals, by design resulting in a minimal error between the 1PPS derived from the 10MHz and the GNS 1PPS (UTC).

The three stage control approach is used to minimize the time to initial lock, the three stages being fast, medium, and slow.

Summary

For best frequency stability and accuracy performance, a rubidium internal module is preferable, providing a stability around 5 times better than the high performance OCXO, with improved accuracy. Also, due to the excellent inherent stability in the rubidium, the ability for the unit to remain on frequency in the absence of GNS satellite signals (i.e. in holdover mode) is much better, with a frequency drift in the order of a few parts in 10-12 per day.

The OCXO while sacrificing somewhat the frequency stability/accuracy, still provides a highly accurate output adequate for many applications, at a cost that is considerably lower.

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