If you change the load capacitance and drive current on the crystal you may shift it by a few percent.
Del

If you could take apart the crystal you could then grind it on a flat surface with a very fine silicon carbide paste to increase the frequency. This is a pretty difficult task but I have done it with old military surplus crystals that could be disassembled. Probably won't work for what you have.

What Del suggests will work and is called pulling but has limits and introduces temperature dependencies.

Increasing and decreasing crystal frequency by altering its mass is sure to work. Crystals are enclosed in metal or plastic casing such that dust does not alter its frequency. Quart Crystal Mass balance use this property to measure small mass deposit.

Perhaps it may also be possible to increase the Gold contact coating thickness of higher frequency crystal to get right frequency. Grinding idea sure is a very good one.

Crystal need to be encapsulated after all alterations else it will just not work for long.

Hi,

Crystal filters use small parallel capacitors to change the frequency of one to match the others. This is usually only e few hertz. It may not work for the amount you want. Good luck.

S

The frequency of a piezoelectric crystal as already said, is mainly a question of its thickness (not mass). The vibration is either the fundamental frequency or any of its harmonics, so the thickness must be an odd multiple of half wavelength: λ(2n+1)/2.

To calculate the thickness required, the formulas is: t = n.K/f

Where: t is the crystal thickness required, n is the harmonic order (1 for the fundamental), f is the frequency required and K is the "frequency constant" of the piezoelectric material (2,87 MHz/mm for quartz, 1,65 for lead metaniobate, and so on).

You may find the piezoelectric frequency constant of your specific crystal just googling "piezoelectric materials" or "piezoelectric constants".

So, as already said too, the best way is to see what material you have, measure its thickness and grind it carefully (all ceramics are fragile) to the calculated thickness.

I've made some calculations supposing quartz material and the differences in thicknesses from your actual crystals would be in the order of few μm.

Cheers

You don't say what type of Crystal you have, but for main piezoelectric materials here you have the resonant frequencies/mm of thickness

Kwetz,

You replied:

So, as already said too, the best way is to see what material you have, measure its thickness and grind it carefully (all ceramics are fragile) to the calculated thickness.

Grinding the crystal will increase it's frequency. Suman Roy Chowdhury wants to lower the frequency by 50 kHz. I don't believe there is a practical method of increasing the thickness of the crystal.

Of course you're right but if you see the formula in my post is just what it demonstrate, thickness and frequency are inversely proportional!!!

I made the calculation between 5 and 4,95 just to see the order of thickness variation (while I used the calculator I haven't the post in front of me and I remembered that two figures) But our original poster has two crystals: one thicker and other thinner than needed. I supposed any CR4 member could assume that to get a specific thickness by grinding, you must depart from the thickest!!!

This equation is for same dimension crystal where only thickness changes and thickness becomes proportional to mass. When you add mass of Gold or Tin electrodes then it is no longer thickness alone that fits the equation. If mass does not play a role then QCM simply will not work.

Can you justify your claim?

Sorry, You're right. The formula included in my post is obviously applicable just to Crystal thickness. I'm in no way very specialist on Quartz Crystal Microbalance (I think you mean that when talks about QCM).

Another colleague posted about his own experiences increasing thickness (and then mass) by rubbing a graphite pencil lead (graphite is conductive too and cheaper than gold).

If the conductive layers are in the opposite sides of the crystal in the direction it vibrates, increasing the thickness of such layers the total mass increases too, but through increasing the thickness. I have made no experience of that but I think that wave produced gets into the conductive layer and then interferes depending on the relative acoustic impedances with the original wave.

If the conductive layer would be larger in surface dimensions than crystal surface, would it change the frequency? Does it means that you can vary the resonant frequency of a 1" circular crystal by increasing the surface of the conductive layer say to 10" diameter?

QCM does allow to measure small deposit quantities of different stuff over the crystal, but because it changes the overall acoustic impedance and it is possible only if you take into account the increasing overall thickness (which is proportional to mass as section remains constant). Obviously, the calculation must then take into account the deposited material thickness and acoustical impedance.

It seems you are a very specialist on this field, could you please tell us if the above assumptions are right or wrong?

Thank you in advance

Think in terms of strings of Guitar having same length, but different mass and tension, they vibrate with different frequencies. Quartz crystal is like a tunning fork. Quartz crystal mass balance senses small change in mass by measuring change in frequency.

Just replying anonymously, but the method used by amateur radio operators in time past with the FT-243 crystal (you could unscrew the package and the actual wafer was betwen two spring contacts) to increase a lower frequency crystal to higher frequency was to CAREFULLY polish it on a paper towel with powdered sink cleanser, wash thoroughly with soap, rinse with lots of water, reassemble, test, do over-and-over until correct. To DECREASE frequency they used a very fine smudge of graphite from a hard pencil lead. To increase durability, I would guess (!) this technique would work with a fine smudge from rubbing a piece of solder on the crystal surface. This does not increase the thickness much, but does increase the mass. Please note, this works with FUNDAMENTAL mode crystals. I bet overtone crystals (3rd, 5th) are another animal altogether. Good luck!!

Keith in 29Palms California

Try phase lock loop.

Regards all !

I remember that a Trimmer-capacitor was used for "TRIMMING" or you can say "FINE-Tunung" in circuits in parallel to X-tal.

But the effect was very minor.

This I have also seen in Clocks circuits on sale in market.

Regards!

"

I need a presize 4.95 MHz crystal for building a OOK receiver.

but I have with me a 5MHz and 4.915Mhz crystal.

Is there any way to change the crystal frequency slightly so

that i can achive the desired working frequency. "

1.Cuttng & grndng of -Tal is not that simple job, but a professional jobreuiringa lot of knowledge, practical experience & a costly setup [Mechancal & Electronic].

I have seen one such setup. Needs "Dust-Free" Environment ...

I remember one Term -Cut, Y-cut & how many cuts for refinements, stabillity, Deviation/ year ...

1. The solution for original problem seem to be "Trimming" of freuency in Electronic-Circuit.

2. Lower freq-Tal can be ground to a higher freq so 4.915Mhz X-Tal can be worked out.

3. As I said in #15 its freq can be trimmedup to some limits.