[time-nuts] Behavior of a disciplined rubidium oscillator?

[AC0XU (Jim)]

Time Nuts-

I have been observing the relative time stability of a Jackson Labs Ln Rb GPS disciplined oscillator and a 5071A cesium reference (undisciplined) , as measured via the 1 PPS outputs and a TIC. See the attached plot for about 8 days of history. Horizontal axis is seconds and vertical axis is nanoseconds.

The Ln Rb has very good specs, qualfied by Jackson Labs with "no vibration", "no air current", etc. I have been surprised to find that the unit responds rather violently to external temperature changes.

(A) represents fluctuations of 100 ns or so when the room ceiling fan is turned on or off. Upper case temperature was about 40C.

(B) represents fluctuations of about 200 ms when I put an insulating blanket around the unit. Case temperature went up to about 65C and remained constant until point (F). After applying the blanket, it took several hours for the timing to stabilize.

(C) represents a steady period with the blanket on and a case temperature of 65C.

(D) represents a fluctuation of  almost 1 us, with no cause that I am aware of. There was no significant change in lab temperature, no mechanical shock or vibration, this is a mystery.

(E) represents a period of fluctuation up and down of size almost 100 ns with a period of around 2 hours. Again, I have no idea why the oscillator started behaving this way.

(F) represents my removing the insulating blanket and placing the unit on a very large heat sink. Upper case temperature dropped to 35C.

(G) represents steady operation on the large heat sink with an upper case temp of 35C.
		
I am assuming that this behavior (aside form the steady downward slope, which I expect is the 5071A needing some steering) is due to the Ln Rb oscillator, and not the 5071A, which was not subject to any fluctuation I am aware of during the test. Does anyone have an idea what might have caused the behaviors at (D) and (E)?

The behaviors at (A) (B) and (F) are evidently due to external temperature changes. It seems less than ideal to me that the device would be engineered so as to cause a large jump in the timing for a couple of hours after modest external temperature changes. A slowly turning ceiling fan 10 feet away does not seem like a big environmental factor. Shouldn't the rubidium oscillator and/or GPS be filtering that out? It appears that I would have to add external temperature control (i.e. environmental chamber) to this device to maintain reasonable stability.

Any thoughts?  Thanks!

Jim

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