Hi Here's something I Bet you didn't know. Your humble frequency counter here can actually change frequency depending upon its orientation. Don't believe me? Let me show you. I've got my Agilant 53131a frequency counter here and excellent frequency counter.

It's got a build-in uh High Stability Ovenized uh reference oscillator and it's measuring uh, the 10 megaherz reference frequency from my external uh CSO Rubidium Frequency standard here. Watch what happens if I just increase the tilting B put the tilting bail up. Look at that. it's changed.

Magic, It's changed by Uh 3 MZ there. If I put it back, it's changed back to exactly where it was before. Why is it so well? The reason for this is that the internal quartz crystal oscillator in this thing, be it an ovenized one like this, which is kept at a stable temperature, or be it the uh, generic internal one that you're familiar with with using as you know your 10 MHz reference oscillator on your microcontroller for example. They're all quartz crystal oscillators and they're susceptible to many different forms of uh, environmental, uh things.

like for example, everyone knows this, they change with temperature. Of course they'll have a certain temperature spec, they'll drift with temperature, they have an aging characteristic, so every year they'll age by a certain amount and they have voltage dependency. all sorts of things. and not only that, but also uh, physical shock and vibration as well.

Now as it turns out, I've actually done some research on this at a former company I used to work for and you can actually reset the drift characteristics of a typical quartz crystal oscillator by impacting it with a certain shock or vibration. So what's going on here actually has nothing to do with shock or vibration or temperature thermal gradients inside the other or anything like that. What it is is, uh, related to the shock and vibration, in that the physical crystal inside vibrates and that is actually susceptible to gravity. Believe it or not, Yes, gravity, you can't escape it.

You can actually use your frequency counter to detect gravity. And by physically changing it like that, you're actually changing the vibration characteristics of the crystal because you're changing its orientation relative, uh, to the gravitational field. So if I turn it over like that, for example, we will see it change yet again. And look at this, you'll notice that it's changed by roughly uh.

4 MZ there. If I turn it all the way over, it should double. That difference should double. and yeah, it pretty much does.

So it went from 4 MZ above to basically 4 MZ below or thereabouts cuz we don't have the resolution, we'd have to go to a greater uh gate time there to get better resolution. But you can see that we can actually detect gravity because when you turn a crystal upside down, you're changing its physical vibration properties relative to that gravitational field. So when you actually calibrate instruments like this, you've actually got to calibrate them in a specific orientation. and just changing that tilting Bale like that you think nothing of.

but you could do that. And if you're talking about serious measurement, look I mean we're easily we can get this frequency counter to go to another digit resolution after this, but that's the sort of impact you can that gravitational fields can have on quartz crystals. Who knew? And it turns out that your average quartz crystal has a gravitational change of roughly uh, 1 * 10 - 9 per G So that translates to on a 10 MHz uh signal or 10 Meg Crystal Like this at 001 Hertz basically per G. So take your humble quartz crystal here and let's crack this thing open and actually see what's inside this thing.

And after you carefully slice One open Taada, we're in like Flynn And that is what's inside your typical quartz crystal that you used to using. There's a big circular Slither of quartz there with a couple of electrodes on either side and that thing vibrates. And that's how crystal oscillators actually work. But as you can see by the orientation of that, if you Orient it in that direction, gravity is going to have a different effect than if you orientate it in that direction.

As very minor as that is, as very small as that gravitational effect is, it does actually make a difference. Now, you might be a bit puzzled as to what's actually going on here. Why does it make a difference if we suddenly like flip this axis of the crystal over like this I.E We go like that and we turn it over or whichever way you want to, uh, do, it isn't the force of gravity down towards the center of the earth. For example, like 1G it's still 1G pointing down on the crystal.

Why does that make a difference? Well, you got to remember that the crystal was measured I.E You know, calibrated because this is a frequency. it's a reference for example. Uh, then that was actually calibrated with it in this orientation with 1 G being applied to say this top surface here. So you've got positive 1G coming down on this top surface and when you flip it over like this, it was one frequency in that respect before.

But now you've got one positive 1G on this surface. so you've effectively got negative 1G reference from the other side. So that's why you get a total difference figure a total frequency change of 2G when you flip it over and hence why this phenomenon is called 2G Tip over and it's a very common thing. You flip a crystal over like this on a bench and you get a difference of 2G Now as I said, it does change with the cut.

What's called the cut of the crystal as well and a typical SC cut crystal is as I've said, uh, you know roughly uh, 1 * 10- 9 or one part per billion. But you're say your at cut crystals they can be like an order of magnitude worse than that. Now what we've actually seen here is very low values of G We're just basically changing the orientation like that so it changes by 2G when you flip it over which is basically uh, nothing really. The real problem with these things comes about when you start to move them.

I.E you're up in a plane or something like that. or you're something other thing which is moving accelerating all over the place. Well you can have really big problem problems with stability of your oscillator so it's a really huge deal. Now you might be wondering.

does this 2G tip over effect apply to Atomic frequency standards like this: Rubidium Uh Frequency Standard I've got here uh, does it have that same 2G tip over problem because if you know the block diagram of how a Rubian Frequency standard works, it basically has a regular quartz crystal in there which is then frequency locked to the atomic um Phys package inside there. So 2G tip over will apply of course to the quartz crystal inside this thing. but because it's with inside a survey Loop locked to the Uh Atomic Rubidium Frequency Physics package inside there, then it the frequency will change, the output of the Rubidium standard will change, but it uh, its frequency will be dependent upon how fast the servo Loop can act and actually correct for that change. And there are other physical effects like a if you.

that's if you just rotated the crystal inside there. But of course, if you rotated this entire package, then the Rubidium standard itself. uh, the actual rubidium inside the Uh element in there can, uh, diffuse in a different way and you can get physical effects that way. Um, but it's not the same 2G tip over effect that you get in the quartz crystals, so it can affect it.

So certainly if you were, you know if you're in a calibration standard lab and you had and a Ruidan frequency standard like this, you wouldn't be going tipping the thing over, that's for sure. But really, it's not something you really have to worry about and it's absolutely critical. Uh, talking about your Rubidium uh, standards there certainly not in the same league as what we see here with our uh, quartz crystal ovenized oscillator not even close These things. Yeah, you can just measure it on your basic frequency counter and there is plenty of great research out here on this, and it.

It does make fascinating bedtime reading if you're interested in this sort of thing. So I hope you found that interesting and you've gained a new appreciation for what happens when you tilt your tilting bail. Like that little insignificant change to your instrument can have a significant impact upon your measurements, can even change the calibration of your instrument. There you go.

Fascinating, but true. And it can be a really big deal in, uh, some applications. As I said, when things start moving, we just got a basic frequency counter on the bench here and we can measure it easily. We can even measure it with more Precision than this if we really want to.

So there you go. Hope you found that interesting. If you did, please give it a big thumbs up. Catch you next time.