[time-nuts] Z3805A cooling requirements?
Tom Van Baak
tvb at LeapSecond.com
Mon Dec 24 17:47:35 UTC 2012
> I wouldn't place much emphasis on the Adev data for Tau's of less than 80 seconds.
Actually, just the opposite; the ADEV at short tau is very close to correct.
> I've collected some ADEV data as well but don't entirely trust it yet.
Right, it's the ADEV for longer tau that is completely misleading. Let me explain.
Realize that Allan deviation numbers are statistics; essentially they predict how constant the future frequency might be, based on a sampling of measured frequency in the past. For a statistic like this to be relevant you want to have at least 3, but more likely tens to hundreds of past measurements in order to have confidence in the prediction.
Plotting many Allan deviation statistics, each with a different sampling interval, on a log-log plot gives even more information; the slope of the line reveals noise types.
Now, there is no problem observing transient phenomenon like temperature changes (or phase jumps or frequency jumps or loose cables or pets jumping onto the bench). They show up dramatically in phase or frequency strip plots. You can see how quickly the effect occurs. You can measure the magnitude of the effect. You can measure how long it takes to recover. This is all useful: you get numbers like tempco or thermal Q. But using standard deviation or RMS or Allan deviation, or any other *statistic* on this data is not the right thing to do -- because you have only a sample of one.
On the other hand, if you encounter tens or hundreds of these transients over hours or days or months, then it is perfectly valid to use statistics like standard or Allan deviation to describe the probability of the transient occurring; the magnitude of the effect, etc. Now you have enough events to offer a future prediction based on many samples in the past.
Does this make sense? In your case "removing air flow" is only one event.
I know it's easy to make ADEV/MDEV plots using Plotter or Timelab but that doesn't mean it's appropriate in every case. When your data has an accidental data glitch or an intentional transient, it's best not to use statistics to describe that one event.
----- Original Message -----
From: "Mark Spencer" <mspencer12345 at yahoo.ca>
To: <time-nuts at febo.com>
Sent: Monday, December 24, 2012 6:58 AM
Subject: Re: [time-nuts] Z3805A cooling requirements?
Volker et al..
I placed a small muffin fan near one of my Z3805's. The modest air flow seems to have some affect on the units operation.
I've collected some ADEV data as well but don't entirely trust it yet.
Amongst other issues I had a lengthy power failure at my residence four days ago so both my Z3805's have only been running for a few days and most of my other references and all my counters also lost power. My Z3805's seemed to take several weeks to settle down when they were first powered up.
My FTS 1050 has a separate backup battery system and has been running for a few years so I've used it as the reference for this test.
All that said I've included the Adev plots with this email. Removing the air flow seems to cause more issues than adding it. It took a number of hours for the unit to settle down after the air flow
was removed. I wouldn't place much emphasis on the Adev data for Tau's of less than 80 seconds.
While collecting the data used to generate these plots at times I also compared the reference I was using to two other references to give me some comfort that the reference wasn't drifting.
I'll be interested to see any additional data from Volker.
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