40 KHZ Crystal..can not get it to resonate

[v2030]

I am trying to use a crystal, but I can not get it to resonate properly, What is the best way to determine the cap values which are used in series with the crystal...for 40 KHZ frequency, Thanks in advance.

 

[blueroomelectronics]

Hmm, you need to post your schematic or shake the crystal 40,000 times a second.

 

[Leftyretro]

I am trying to use a crystal, but I can not get it to resonate properly, What is the best way to determine the cap values which are used in series with the crystal...for 40 KHZ frequency, Thanks in advance.

Yes, a diagram would help. Also when there is a need to set a crystal to an exact frequency sometimes a small variable trimmer capacitor is used in place on one of the load capacitors.

Lefty

 

[v2030]

I have configured it as Pierce-Gate Crystal Oscillator...Please see the following link:
**broken link removed**
The values which are indicated on the schematic are:
Rs=1.5k Rf=1M C1=22pf C2=1nf
Shock it 40,000 times...did not work..just kidding

 

[tunedwolf]

Are you certain you are using a fundamental mode, AT cut crystal ?

If the oscillator is running at a much lower freq than expected, it's possible that it's an overtone crystal and is resonating at it's fundamental frequency.

How are you constructing the design ?, in a way that minimises stray capacitance and inductance ?

If soldered, have you completely removed any flux around the components ?

If construction is on breadboard, try soldering the bits directly together, no sockets or anything else, keeping all wires as short as possible, and just sit the final blob of bits on a piece of card or something. Breadboards are notorious for screwed up analog because of the stray capacitances involved.

Makes sure the other gates in the package are either static, or loaded in some way, tie them to Vss if not.

Power supply is really well decoupled ?

Rf seems a little low for 40Khz...Higher the freq, lower the feedback resistor value, so start with a better value for that, then calculate the other components...



rgds

actually, now that I think about it, are you adding anything that would load the output properly ?

 

[v2030]

Hi tunedwolf, Thanks for valuable info. I have constructed the design on a breadboard...I will remove and solder the related parts as you instructed.
Here is a link to the design...If you could spare a moment and check out, will be greatfull...
https://www.electro-tech-online.com/custompdfs/2009/02/stuart1_150.pdf

Thanks for all your help.

 

[Diver300]

Are you certain you are using a fundamental mode, AT cut crystal ?

If the oscillator is running at a much lower freq than expected, it's possible that it's an overtone crystal and is resonating at it's fundamental frequency.

At 40 kHz, it won't be AT cut. AT cut crystals run at 1664/t MHz, where t is the thickness in µm

That makes a 40 kHz crystal about 42 mm thick, and it would have to be much wider than the thickness, so about half a meter in diameter.

40 kHz crystals are tuning-fork crystal, and they are never used in overtone mode.

 

[tunedwolf]

At 40 kHz, it won't be AT cut. AT cut crystals run at 1664/t MHz, where t is the thickness in µm

That makes a 40 kHz crystal about 42 mm thick, and it would have to be much wider than the thickness, so about half a meter in diameter.

40 kHz crystals are tuning-fork crystal, and they are never used in overtone mode.

Thanks for that Diver.

I am aware that a crystal is said to resonate at it's fundamental frequency, when that frequency is half the wavelength thickness of the crystal.

Perhaps you could give a worked example of how you arrived at your figures ?
Could you also state the material type and velocity of sound for it ?


rgds

 

[Diver300]

I can't remember where I the constant of 1664 comes from. I've been using it for 20 years so I am fairly sure that it is about right.

It is the constant for AT cut quartz, in thickness-shear mode. That is by far and away the most common cut for frequencies above 1 MHz.

I think it is in "Introduction to Quartz Crystal Unit Design" by Virgil E Bottom where the value is derived from the density and stiffness of quartz. Quartz is anisotropic so its stiffness depends on the angle of the cut. The electrical properties do not significantly affect the frequency.

 

[tunedwolf]

I think it is in "Introduction to Quartz Crystal Unit Design" by Virgil E Bottom where the value is derived from the density and stiffness of quartz. Quartz is anisotropic so its stiffness depends on the angle of the cut. The electrical properties do not significantly affect the frequency.

Thanks for that, I'll see if I can find a copy.

rgds