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Oscillation of a crystal in an RF field

07/23/2022 7:55 AM

Does a galena or quartz crystal,or (other crystal) mechanically oscillate when exposed to an RF field?

I know they will under mechanical stresses,but will they oscillate under the influence of a strictly RF field?

I understand how the cat whisker detector works as a diode,but this is not the issue I ]am concerned with.

All positive,helpful feedback will be sincerely appreciated,so thanks in advance.

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#1

Re: oscillation of a crystal in an rf field

07/23/2022 6:07 PM

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#2
In reply to #1

Re: oscillation of a crystal in an rf field

07/23/2022 6:36 PM

Thanks,SE,but I know how radio waves are produced and encoded/decoded.

My interest is whether a crystal PHYSICALLY oscillates when exposed to a an electromagnetic field. For instance,would a Piezoelectric crystal oscillate and generate a voltage in the presence of an oscillating magnetic field?

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#3
In reply to #2

Re: oscillation of a crystal in an rf field

07/23/2022 7:01 PM
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#5
In reply to #2

Re: oscillation of a crystal in an rf field

07/23/2022 7:39 PM

My interest is whether a crystal PHYSICALLY oscillates when exposed to a an electromagnetic field.

Here's what I'm thinking...

The quartz in a crystal assembly is a small slab of the proper crystal orientation carefully machined to the proper dimension for the desired resonant frequency. Metal electrodes connect it to a circuit and the coupling between the physical vibration of the quartz and the voltage applied from the circuit provide feedback for an oscillator that oscillates at the resonant frequency.

https://www.analogictips.com/quartz-crystals-oscillators-part-1-crystal-basics-faq/

If the plates were not connected to the quartz but an RF field were applied of the exact resonant frequency and the exact orientation to the crystal, theoretically the crystal should vibrate at that frequency.

However, without the electrical feedback from the connecting plates, it would be difficult to maintain the RF source at the resonant frequency, IMHO.

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#6
In reply to #5

Re: oscillation of a crystal in an rf field

07/24/2022 6:04 AM

OK.So if an external RF,at the exact resonant frequency is applied externally,with no physical connection,simply an external magnetic field,the crystal does oscillate.

I am not trying to use the crystal as a detector diode for feedback or to an oscillator for a frequency lock,just wanted to know if the crystal physically changed dimensionally in cadence with the external rf field.

Thanks for answering my question,and thanks to all others that contributed to my knowledge on this matter.

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#4

Re: oscillation of a crystal in an rf field

07/23/2022 7:14 PM

The practical answer is not as you imply. There's a very good reason why a crystal needs wire leads, amplification, and an antenna to select an RF signal. Once one provides all of that circuitry then the crystal does oscillate.

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#7

Re: oscillation of a crystal in an rf field

07/24/2022 6:29 AM

Here is my suggestion for what it is worth. Get an RF xtal, read the basic oscillation frequency from the case and then remove the case which protects the xtal and shields it. Now excite a coil with the base frequency and measure if the xtal generates voltage with a high impedance meter, I have one with a 2uA movement but you may even have access to VTVM which may be as good.

Let us know if you do experiment with the above setup. Maybe even a clock xtal will work but these are generally smaller and harder to sergically remove.

I have my own IFR communication monitor so doing the above would be good to try but right now I am into other mechanical projects and have radio on the back burner.

Have fun if you get the urge to produce some good vibrations.

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#8
In reply to #7

Re: oscillation of a crystal in an rf field

07/24/2022 8:31 AM

The German POW's used a razor blade floating in a water glass,and the filament from a light bulb to receive radio broadcasts to get news from outside.It was connected to a home made headphone set .

Necessity is the mother of invention.

It took a lot of fiddling I am sure, to get anything but they were determined and they had nothing else to do at night.

That indicates to me that just about anything will oscillate at some rf frequency.

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#9

Re: oscillation of a crystal in an rf field

07/24/2022 5:19 PM

In a Royal Institute digitized (archived) video of a demonstration/talk by Sir Lawrence Bragg he states that electromagnetic radiation (RF is only a part of of the full spectrum of frequencies) exists independently of matter. Thus, it propagates through empty space and hence would not be expected to interact with matter.

One can conclude from this that the atoms within a crystal or other solid would not be subjected to any forces that would physically translocation their positions or cause their locations to oscillate. However, the presence of matter (mass) diffracts EMR and Bragg is famous for the fundamental Law underlying the science of crystallography using X-ray diffraction to determine crystal structure and dimensions.

Find this interesting video at Electromagnetic fields By Bragg video.

Thanks for so many stimulating discussions on this forum!

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#10
In reply to #9

Re: oscillation of a crystal in an rf field

07/25/2022 5:08 AM

"would not be expected to interact with matter."

So how do radio receivers work?

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#11

Re: Oscillation of a crystal in an RF field

07/25/2022 5:19 AM

I remember reading a few articles, about 5 yrs. ago about using piezo materials to TX and RX rf under 1 MHz. Piezo antennas. For the first time the length of a low freq antenna was not needed for efficient TX and RX of LF radio waves.

These freq were 10 kHz to 400 kHz. Thru ground and thru rock, low power links. Tunnel and mine rescue. Low power has long range on these freq. If I recall, the antennas were like 6 in long.

Also used for direct antenna modulation and antenna de-mod or detection circuits.

phys.org was the site.

Conventional methods use a physical length of free charge on the antenna.

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#12
In reply to #11

Re: Oscillation of a crystal in an RF field

07/25/2022 6:54 AM

Check out"foxhole radios".

There were many ingenious methods of receiving radio transmissions without external power.A rusty razor blade forms a Schottky diode which was the basis for some of them,although other means were also used.They were also very creative in constructing headsets from whatever was available.

These methods are very interesting,but my main interest is determining the physical effect on a crystal,not for radio reception.I have other ideas about this characteristic.

They did not know about Fractal antennas back then,but it would have been a fantastic advance if they had known.

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#14
In reply to #12

Re: Oscillation of a crystal in an RF field

07/25/2022 7:09 AM

Here is an interesting link to some of the ingenious methods used by soldiers in WWII.

https://en.wikipedia.org/wiki/Foxhole_radio#/media/File:Foxhole_radio_from_WW2.jpg

Interestingly,there have been reports of receiving RF on loose dental fillings.Of course these claims were dismissed off hand as paranoia,but later proved to actually be true.Sensitive microphones were used to pick up the audio from the acciDental diode.Fortunately,it did not Schottky him very much.(pardon the puns).

Excuse me,gentlemen,I must go. I have a secret message coming in on my left molar.

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#13
In reply to #11

Re: Oscillation of a crystal in an RF field

07/25/2022 6:59 AM

VLF is used for underwater communications from land by the militaries world wide.The only deficit is that the information carried is limited also by the frequency,so is mainly used for brief messages,like alerts,etc,.

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#15

Re: Oscillation of a crystal in an RF field

08/04/2022 9:32 AM

Actually crystals are isolators and attached electrodes on a crystal application consists a capacitor that has not linear characteristics, but changes following the dimension changes due to piezoelectric phenomenon and its mechanical resonant frequency is what makes this intersting since you can make a high Q filter out of it. But the crystal between the electrodes IS actually subjected to a RF field, and that's what makes it tick. Now the open air RF field should be pretty strong to "see" the oscillation, plus resonance (where this oscillation maximizes) is very sharp, but oscillate it will. S.M.

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#16
In reply to #15

Re: Oscillation of a crystal in an RF field

08/04/2022 10:45 AM

I knew there was a reason for adding the wires.

GA

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