Warren, Most all Rubidium standards do not have active current drive. The HP 5065A is the exception. However the mod I installed is not active. The C-field coils are roughly temperature compensated as they are near the cell oven that is regulated. If you check out Poul-Hennings "hacking the 5065a" you will see that the C-field coil does exhibit some temperature effect even with active current regulation, and the stock C-field circuit also introduces some effects. Now these amount to a small coefficient but we are Time Nuts here so we can do better! My circuit gives a precise current into the coil (for a fixed temperature of the coil), and with the active temperature stabilization I am adding to the outside of the sealed optical unit enclosure those effects will be eliminated. Cheers, Corby