[time-nuts] good ADEV data (was Low-Cost Rubidium Performance)

[WarrenS]

A couple points that some may of missed.
I've seen even slow 24 hr  temperature cycles be the cause of poor ADEV numbers at MUCH less than 24 hr ADEV taus in otherwise low drift Rb's Osc like a LPRO that had showed little ageing drift.
>From what I've seen, more often than not, the "bathtub" turn up at long taus is not due to 1/f which is generally much flatter, but to temperature or ageing, neither of which should really be included in "true ADEV" noise plots.

As Magnus pointed out, The scaling factor for systematic errors is NOT the same as for random noise errors. 
ADEV scales it's answers for random noise,  sort of like the well known fact that RMS sources are not scaled the same as noise or pulse sources when measured with an average responding AC meter.
The values can still be measured and compared IF the waveform stays the same, But the answer is NOT true RMS except for the waveform the average responding AC meter is calibrated for.

John A said : > "I think ADEV is a tool to measure performance in the environment that you want."  
Agree, that is what it is often used for and it can give good "Relative test numbers" for a given repeatable situation" but it is not necessary ADEV.
In addition, if the ADEV numbers can not be repeatable then it is not of much use at all, 
and if the temperature is changing differently between test, when temperature is the major error source, then even the relative ADEV data is near useless.

Jim said > "Isn't the measurement of the "system", which includes the environmental effects".
Yes and NO.
If you want to always strive to make the system better (a nut thing), you need to know the difference between real ADEV things such as 1/f  and the much easier to improve delta temperature effects.
If you can not get the same answers between two like test, Like when the error band is 100%, what's the use of the numbers at all?

I find it is easy enough to remove the effects of temperature when I want useful repeatable ADEV results. 
The most obvious is Don't let the temperature change during the test IF the device under test (or the reference) is temperature dependant.
or the raw data can be post process with the simultaneously taken temperature data much like one does to remove osc aging effects or outliers.
I find using the simple New_Freq = [Raw_Freq plus (K x (temp - nom temp)) works good. 
One  "trick" I use to easily get the correct K temperature scale factor is to take days worth of data so that the temperature effect is repeated several times. 
With a long data run (I find a week works good) and just some basic low pass filtering and plotting,  It becomes real clear which effect is causing what, time, temperature or "noise".
This is something that is easy to do with  LadyHeather and a Tbolt Tic.

My thought is:
ADEV should not be about getting the best possible answers, for me it is just one of many a tools useful for getting the best from a given device when used correctly.
If one wants to get repeatable, comparable and usable ADEV numbers, especially  to compare to some other person, device, or test run, 
I find it best to keep the effects of temperature change out of it or at least keep their effect in a separate plot.

I think Magnus said it best, In short, "keep the random and systematic effects separate" if they are important.
And my golden rule is to ALWAYS also simultaneous record the temperature variation when taking freq data.

ws
  
******************
>
>
> Interesting point you make here. The rising ADEV at 100-1000 second-ish
> tau in a system that should be better is a classic sign (at least around
> here) that temperature effects are showing up.

I regularly see the building AC at 900-1000 s for instance.

> However, how could one remove that effect from the raw data? And isn't
> the measurement of the "system", which includes the environmental effects.

ADEV and friends is there to handle random sources, where as this is a 
systematic source.

> I suppose you could run your widget in a temperature controlled chamber,
> get those numbers. Then run it in a less controlled benchtop
> environment, and get those numbers, and claim that the difference is
> environmental.
>
> But at some point, what you're interested is the performance of the
> system in the environment in which it will be used. If you need good
> ADEV performance at the 1000 second tau, then you need an oven, a vacuum
> bottle, or a better design that's less environment sensitive.

You could also build active systematic effect predictors to lower this 
systematic effect.

By doing proper logging of key environmental effects, build a model for 
how the dominant variations will systematically affect the signal and 
then remove that from the measurements you get a better random jitter 
measurement.

Frequency drift of an oscillator is one such systematic effect. If it 
where linear, processing it with ADEV would cause a sqrt(2) scale error. 
Also, it would not give you a good prediction since usually you follow a 
A*ln(B*t+1) curve which isn't matching the requirement, so you will only 
get first degree compensation of that with HDEV style measures.

Temperature variations is tricky to say the least.

When you have random and systematic effects, separate them and estimate 
them separately and then build a combined prediction from these models.

Random jitter and deterministic jitter are two such aspects. Same 
applies at longer taus as well.

Cheers,
Magnus

********************
Hi Warren --

JimL responded before I did, making pretty much the same point -- I 
think ADEV is a tool to measure performance in the environment that you 
want.  If you want to measure the best-possible performance of a device, 
you want to control for all external factors.  But if you want to see 
the real-world performance, you want to measure in the real-world 
environment.

FWIW, these tests were done in my basement lab.  I don't have my 
temperature monitoring stuff set up yet so don't have data, but the 
furnace isn't cycling too frequently, and there is relatively little air 
flow from the registers into the area (the thermostat is upstairs and we 
just bleed a little air into the basement).  We don't do a huge amount 
of day/night setback at this time of year, so I suspect that the 
temperature is remaining stable within 2 or 3 degrees C.  I suspect the 
seasonal variation is greater than the short term.  But on the project 
list is getting several temperature sensors installed to feed a data 
logger...

On the very long term project list, I'd like to climate control my clock 
room to maintain better than 1 degree C, but that one is down the road.

John A

**************
From: "Jim Lux" 
>  
> Interesting point you make here.  The rising ADEV at 100-1000 second-ish 
> tau in a system that should be better is a classic sign (at least around 
> here) that temperature effects are showing up.
> 
> However, how could one remove that effect from the raw data?  And isn't 
> the measurement of the "system", which includes the environmental effects.
> 
> I suppose you could run your widget in a temperature controlled chamber, 
> get those numbers.  Then run it in a less controlled benchtop 
> environment, and get those numbers, and claim that the difference is 
> environmental.
> 
> But at some point, what you're interested is the performance of the 
> system in the environment in which it will be used.  If you need good 
> ADEV performance at the 1000 second tau, then you need an oven, a vacuum 
> bottle, or a better design that's less environment sensitive.
> 
>  (difference between TRL6 and lower, for those into such things)
> 
> 
******************
From: "WarrenS" <warrensjmail-one at yahoo.com>


> Indeed,
> ADEV is for random freq variation not easily measured by other means.
> Temperature fluctuations do not cause random freq changes and the 
> temperature's effect should be removed if one wants accurate long term ADEV 
> numbers.
> Even daily diurnal cycles due to temperature can have major negative effect 
> on ADEV numbers as low as 2000 to 3000 seconds,
> and if there is an Heater or AC cycling, then any ADEV numbers about a few 
> hundred seconds can be due to TempCoeff, which should not be measured with 
> ADEV or included in ADEV plots.
> This is much the same as a single outlier data point that can screw up the 
> whole ADEV plot and make it pretty much meaningless and unrepeatable.
> Ditto for linear ageing, Should be remove first if one wants true ADEV 
> plots.
> 
> ws
> 
> ***************
> Bob Camp lists at rtty.us
> 
> Hi
> 
> Past 100 seconds I have seen some FE's that look better than your LPRO plot 
> and some FE's that look worse than your FE plots. Running in a +/- 2C room 
> apparently is not the best way to operate them for good long tau 
> performance.
> 
> Bob