[time-nuts] Thunderbolt performance vs temperature sensor
holrum at hotmail.com
Sun Mar 8 13:40:52 EDT 2009
The test that I did was meant to reflect how a typical user would be using a Thunderbolt... sitting out in free air on a piece of poly foam, covered by a cardboard box to provide a little isolation from environmental transients. Tbolt internal temperature changes were around 3 deg C per day and 0.3C per hour. My chosen figure-of-merit for the tests was how much the PPS signal changed (as reported by the Thunderbolt message) over an hour.
The unit had run continuously for several months. Before I swapped the temp sensor chip, I made several runs where at 11:00 AM I put it into manual holdover mode and started the log. I recorded its self-measured data for around 23 hours, stopped the log, put it into normal operation mode for an hour.
Next, I shut it down and swapped the temp sensor chip. This took about 10 minutes. I then restarted it, let it stabilize for a day, and did a factory reset. I let it run for four days in GPS locked mode and four days with the 23 hour holdover mode cycles. This (hopefully) gave it some time to learn about the new temp sensor. Next I repeated the "official" 23 hour holdover test runs.
The average of the 1 hour PPS holdover deviations with the newer low res temp sensor was 1.73 uS per hour. The average of the 1 hour PPS deviations with the older high res sensor was 0.82 uS per hour.
Yes, it would be nice to put the unit into an environmental chamber and put it through its paces and discover all sorts of neat things about it. I was more interested in how the change in the DS1620 temperature sensor chip affected the units in a way that I might actually see in the way that I use them. I have already swapped the temp sensor chip three times in this unit (original E2 chip, another E2 chip, and the D1 chip) and don't really want to risk damaging it with two more swaps (back to E2 and then D1).
Tests on a single unit are hardly conclusive, but it appears that you can double your holdover performance by using the older DS1620 chip. Also, by looking at the DAC and TEMP plots the temp sensor chip readings ARE used in generating the DAC voltage in GPS locked mode... so I am assuming that the performance also would be improved in GPS locked mode (but by how much and how to test that have yet to be determined).
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