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

Hi Usually Rb’s are spec’d in terms of something like baseplate temperature. Your measurement of the case temperature may or may not correspond to the way the device is specified for temperature performance. Put another way - you may be out of spec on the high end of temperature ….. Bob > On Feb 3, 2019, at 6:06 PM, AC0XU (Jim) <James.Schatzman@ac0xu.com> wrote: > > <LnRb5071A_1pps.pdf>

> 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. Coincidentally, I just measured one of the LN Rb units for 24 hours in undisciplined mode against a couple of 5065As. It performed well relative to its +/- 600 ns holdover spec, staying within about +/- 60 ns over 24 hours: http://www.ke5fx.com/lnrb_undisc.png I didn't attempt to maintain a constant temperature, just normal HVAC activity. About 80 ns of drift occurred between 75K-80K seconds after opening a window a few meters away; opening the same window again near 85K seconds started another downturn. It's about -2 degrees C outside right now, so opening the window dropped the room temperature substantially. The event near 31K seconds didn't correspond to anything in particular, but it only amounted to about 15 ns. A couple of points to consider: - How long was the unit running before you began the measurement? Traditional rubidium standards sometimes benefit from several days' worth of aging before being put to the test. I don't know if this is still the case with a CPT-based device like this one. - Small rubidiums are designed for low power consumption, so it's not too surprising that tempco suffers to some extent. If the relevant holdover specification doesn't specify a temperature spread -- and AFAIK it doesn't -- then it may not be treated as a key target for optimization. - It might be interesting to measure your LN Rb in undisciplined mode in order to rule out any peculiarities in GPS signal quality. Ideally, if you want to make a disciplined measurement, you could use a splitter to drive a separate GPSDO whose performance is being measured at the same time. That's next on my list of things to try with this one. -- john, KE5FX Miles Design LLC