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I am trying to simulate a quartz crystal equivalent circuit.  The series LCR part simulates as expected, but when I add a parallel capacitor the simulation fails.  What am I doing wrong?

FHS

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Message 1 of 6
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hello,

 

try that

good luck

 

Michael

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It would help if you listed your value of lumped elements. Better yet show your schematic. Without seeing what you have, I suspect your parallel cap value is much too large.

Keep in mind, the parallel capacitance is the equivalent C of the holder and electrodes, so a value of around 4.5pf would be typical. The series L and C is the resonant frequency so, Fo= 1/2pi(SQR LC) the resistance is mechanical equivalent.

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Hello, thanks for response.

 

Have you checked the second image?

 

the parralel Cap on schematic is too big.

 

but there is a problem -

according to books Cpar  and Cser should be close.

 

but try to simulate it and take a look on frequencies.

 

in real world in my field (which is ultrasonic welding) - these freqs are very close - f,e for 20Khz range it might be 19,800 and 20,000 or something like that.

 

in order to get these freqs  close in simulation I had to increase Cpar.

 

there are formulas -

 

there are 2 resonant freqs -

 

one for serial resonance( L, R, C_ser)

 

for Fo_ser= 1/(6.28(SQR(L* C_ser)))

 

and second one - for parallel resonance ( L, R, C_ser, C_par)-

 

for Fo_par= 1/(6.28 (  SQR  (  L*  (C_ser * C_par)  /  (C_ser + C_par)))

 

it means that C_par and C_ser are connected in serial for that matter.

 

BTW that why , serial  resonance freq is always higher  then parallel one.

 

in order to get these freqs close to each other -  the one of caps should be much bigger then other, and then  C_ser and Cpar in serial would be close to C_ser .

 

run the numbers your self and see. or run simulations , play with parameters and see the results.

 

and another question - what way \ technick do you use to measure these parameters - I mean - C+ser and C_par.???

 

thanks in advance for your opinion

 

Michael

 

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If I understood the question right, the poster was trying to model a quartz crystal, I assume the type used in a crystal oscillator. I am not sure how close a Piezo transducer is to a quartz crystal used in a oscillator, but I am sure the values for the transducer are larger as the resonant frequency is much lower.

 

I found a App note that describes the quartz crystal model fairly well.  https://www.maximintegrated.com/en/app-notes/index.mvp/id/2127

 

Note, this is a snippet from the App note concerning the parallel capacitance.

 

CL1 and CL2 are the loading capacitors. During the test, CL1 = CL2 and the value varies from 5pf to 59pf with the inverter supply voltage Vcc = 3.1V and Vcc = 2.3V. The crystal in the test has a nominal frequency of 27MHz at load capacitance of 14pF. It should be noted that the actual loading capacitance to the crystal equals CL1 || CL2 plus the parasitic capacitance of board and the terminals of the inverters.

 

 

The pdf for the app note. http://pdfserv.maximintegrated.com/en/an/AN2127.pdf

 

Well, perhaps we are thinking Apples and oranges here, so I may just have misunderstood the original question.

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hello, thanks for response.

 

I guess you are right, we talking apple and oranges here  indeed.

 

But common denominator of crystalls and transdusers  is - they both have the same model, but differt set of paramenters of coarse.

 

it seems to me the best solution - to read some good book aboutpiezzoelectric materials.

 

with hope it helps

 

Michael

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