EEVblog #61 - Crystal Oscillator Drift

Dave explains the what how and why of measuring crystal clock oscillator drift over time.

[RavenLuni]

Applying logic to solve an unexplored problem. I love this kind of stuff :)

[you]

Nowadays they have chip-scale atomic clocks; the Microsemi SA.45s has 0.05 ppb accuracy and 0.3 ppb/month aging at 115 mW power consumption. The application list includes "Underwater sensors for seismic research or gas and oil exploration", seemed quite obscure 'til now :P

[egbertgroot]

Funny to see the dramatic change of the looks of the equipment in the background compared to the current video's in 2020 !!!

[hobbified]

Your method is basically an higher-tech version of the old trick of taking the two clocks, plugging them into the two axes of an X-Y scope, and timing how long it takes the eye to "wink" (every 180-degree change in phase is half a cycle slipped), which gives you the difference in the two frequencies.

A higher-precision way is to use a time interval counter (TIC), which basically does the same trick of using a microcontroller to count the number of ticks of a sample clock between the rising edge of one signal and the rising edge of another, but then it adds on some analog electronics to measure time less than one clock (as a simple example, imagine charging a capacitor with constant current, dumping it back to zero once per clock, and having a way to isolate it after the STOP signal comes in. Then you can discharge it at a lower constant current and "stretch" the original time interval by the ratio of the currents). That *is* a device you can buy from Agilent or whoever. Then you can plug it into a PC and run the data into an app like TimeLab and get some nice plots.

[EEVblog]

I may have one somewhere, I'd have to look.
In this application it's only this resultant "skew corrected" data we are interested in. As it's the maximum drift deviation over the recording period that is the concern. And it ain't as "random" as you might think, that's what makes it all very interesting. The one shown is about as "random" as you'll find, the majority are usually more sinusoidal in shape.

[larsniklassonhede]

I now realize that the crystal is not to be trusted. I use an esp32, using a built-in timer and the crystal, which should wake up twice in 24 hours and wait for a reset button. The crystal was drifting so much it was impossible. Changed the program, so now the esp32 wakes up every hour and checks against the internet if the time is less than an hour left, otherwise it goes back to sleep again. In one hour, the drift is so small that this works. My project is very small relation to yours but well explained and it helped. Thanks.

[JosephLorentzen]

I remember in one radar system, the crystal oscillator OCXO in the tracker unit required 20 minutes to stabilize. As little as the grease from the technicians fingers could change the "oven's temperature" enough to send the chassis back for repair. We needed the trackers out ASAP. So failure rates were high once the trackers got into the field.

[inductivethinking]

I think a reason that wristwatches are relatively accurate is the fact that you wear them all the time.
Therefore the temperature of the watch is kept relatively constant (near the temperature of your wrist).
Also maybe due to the fact that shocks etc are pretty random they "cancel" each other out.

[hanymanm]

That's awesome you measured without buying Synchronization tester, you saved a lot of money.

[electronicsradiovideo]

Interesting to see the comparison between the different types of oscillator.

[EEVblog]

That's mains hum through the battery charger. Normally I film on battery, but sometimes I forget and don't hear it until editing time.

[ValSolival]

As far as I know, radioactive isotopes used for accurate time measure. May be you could use some small samples for that :)

[dhpbear]

Isn't the time-reference of each of these units updated perodically via GPS?

[randallrouth]

Usually there is not that much temperature variation. A wrist watch is usually on your wrist and in constant contact with your skin. As such, the temperature is rather constant.

[xjanosikx]

this is amazing I am dropping of high-school and watching this guy so I can actually learn something

[SolutionByEvolution]

Why didn't you maintain an average of several counts to reduce the effects of jitter instead of using a binning technique? The averaging number crunching could have been done by the PIC and the results sent to the PC to reduce interface burdens.

[alkempster]

the attenuation of the GPS signal that far underwater would be difficult to get around. Most GPS sensors cannot work indoors, even if you did get a signal you would then have to worry about the signal bouncing from the environment which can significantly affect its accuracy, its why in built up areas you can loose GPS position accuracy as the signal bounces of of buildings that create a canyon effect. I know that the post is quite old but it may be noteworthy to someone :D

[DannyStaple]

Could pressure, humidity and other environmental factors influence this stability?

[rarbiart]

If you math out all simple temperature correlated stuff bei DTC (and do it right) then you have an output with artefacts which is not temperature correlated (or at least shows just remaining artefacts beyound your algorith).
Anyway: Wouldn't it have been an option to resync the probes sitting on the bottom of the ocean by some radio pulse (low frequency... deep water.. long antenna rods) just before the actual acousting ping takes place?

[MoTheG]

the C in TCOX stands for compensated
Get a 10MHz OX, hook a 1GHz OX via PLL to it, then compare that to a GPS signal.