EEVblog #646 - Gravity Detection Using A Frequency Counter!

[Dr. Frank]

Yep, here I found one of several papers explaining the gravitational effect:

http://www.ieee-uffc.org/frequency-control/learning/filler_paper.html

It confirms, that this effect is caused by interaction between the crystal plate and the mounting.

Frank

[tszaboo]

I think that 1 bpm change is not really significant to talk about, while we have thermal EMF, aging, temperature coefficient, mechanical stress, michrophonic effects etc... on ther components which are several order of magnitudes bigger than this.

Also, I never really understood why we can get crystals in hermetic packages for 0,1 USD, while a resistor or a reference voltage is 10+.

[rs20]

I think that 1 bpm change is not really significant to talk about, while we have thermal EMF, aging, temperature coefficient, mechanical stress, michrophonic effects etc... on ther components which are several order of magnitudes bigger than this.

I think you mean ppb (parts per billion), not bpm (beats per minute). :-)

Also, I never really understood why we can get crystals in hermetic packages for 0,1 USD, while a resistor or a reference voltage is 10+.

Supply v demand / economies of scale? Most consumer electronics get by just fine with the plethora of 0.001 USD 1% resistors and 0.1 USD 5% regulators out there, so regulators (I'm considering references and regulators to be close relatives here) and resistors with higher specs simply have very little demand. On the other hand, quartz-grade oscillators are essential for just about any kind of Wi-Fi, bluetooth, USB (sort of), anything that keeps track of the time of day, etc etc.

So even though a high-end resistor might only cost 0.05 USD in parts and materials, the demand is so low that they must charge a huge amount more to cover factory time, intial engineering outlay and capital costs, and so on.

[tszaboo]

I think that 1 bpm change is not really significant to talk about, while we have thermal EMF, aging, temperature coefficient, mechanical stress, michrophonic effects etc... on ther components which are several order of magnitudes bigger than this.

I think you mean ppb (parts per billion), not bpm (beats per minute). :-)

Also, I never really understood why we can get crystals in hermetic packages for 0,1 USD, while a resistor or a reference voltage is 10+.

Supply v demand / economies of scale? Most consumer electronics get by just fine with the plethora of 0.001 USD 1% resistors and 0.1 USD 5% regulators out there, so regulators (I'm considering references and regulators to be close relatives here) and resistors with higher specs simply have very little demand. On the other hand, quartz-grade oscillators are essential for just about any kind of Wi-Fi, bluetooth, USB (sort of), anything that keeps track of the time of day, etc etc.

So even though a high-end resistor might only cost 0.05 USD in parts and materials, the demand is so low that they must charge a huge amount more to cover factory time, intial engineering outlay and capital costs, and so on.

It was more of a ranting than a real quesiton.

[rs20]

Is there a way to 'counteract' the effect of gravity/acceleration? Like using a second crystal oscillator placed 'upside down' respect to the first one, so that the two variations cancel out?

Isn't turning the crystal upside down (180 degree change) the same as switching the leads?  So, if you had an oscillator that would periodically switch the crystal leads and average the results, wouldn't that solve the 'problem'?  It wouldn't fix the 90 degree change, though.

No? The natural frequency of the crystal is a property of the crystal itself, independent of whether the leads are wired to anything (capacitative loading aside), let alone which way around they're wired. It's a reversible component like a resistor or capacitor. I believe using two crystals in opposite physical orientations probably would indeed cancel out errors if the entire arrangement is flipped entirely upside down, but I agree it's unlikely it would handle all orientations perfectly -- that would require assuming that the frequency shift was precisely a linear function of the gravity along a single axis and that axis alone.

(Stating the obvious; I suspect that in the real world, it's better to buy a crystal that inherently has required immunity to gravity.)

[PE1RKI]

if you would go a level higher in the building and it then changes a digit, isnt that gravity detection ?
this is more a tilt detection.

[rs20]

if you would go a level higher in the building and it then changes a digit, isnt that gravity detection ?
this is more a tilt detection.

Weak answer: False dichotomy. It's the direction and magnitude of the force of gravity on the crystal that causes the effect, so it's perfectly fine to call it a gravity effect.
Strong answer: If you take a pendulum/mercury switch (a classic 'tilt detector') into space, it'll sit in some undefined position and be useless. However, the frequency generator will see the absence of the 1 g and deterministically output a change accordingly. Thus, just like a single-axis accelerometer*, it's detecting the force of gravity, not just the direction, so it's much more a gravity** detector than a simple tilt detector. 

Dave, you should build a centrifuge and stick the frequency counter inside!!

* It is a single-axis accelerometer.
** According to some kind of relativity, gravity and acceleration are equivalent/indistinguishable.

[KedasProbe]

if you would go a level higher in the building and it then changes a digit, isnt that gravity detection ?
this is more a tilt detection.

I don't know how high your building is but assume you have 100m
Earth radius 6371km
6371000² vs 6371100²
or about 31ppm of 1g

2g can change it a few counts, so it's safe to say that 0.000031g won't make a noticeable difference.

P.S. gravity is a vector not just a number.

[PE1RKI]

yes now it makes more sense to me.
learning everyday