how to make crystal work at the third overtone?

[ilikebbs]

3rd overtone crystal

I have a crystal label with 48mhz. But when i use oscillograph to measure its frequency, the frequency is 16mhz. So how to make the crystal work at the third overtone?

now i use two capacitor for the crystal.
thanks in advance.

 

[Phytech]

crystal 3rd overtone

Crystals can work at basic frequency, 3rd overtone, 5th overtone, etc. depending on frequencies, unloaded Q, and motional inductance (Lm). So it's very typical.

 

[ilikebbs]

third overtone crystal

could you more detail explanation about it?
thanks.
it will be better that you can upload some document about it.

 

[rwiggins]

3rd overtone

This may help:

**broken link removed**

Added after 37 seconds:

**broken link removed**

 

[ilikebbs]

crystal 3rd overtone

i do not want to make the crystal by myself.
now i already have one crystal. generally we just need two capacitor to make it works. but now i want to let the crystal works at the third overtone of the frequency.

i appreciate that you can help me.

 

[rwiggins]

cristal+how+inductance

This should make it clear, if I am reading your question correctly:

https://hem.passagen.se/communication/txo.html

 

[ilikebbs]

3rd overtone crystal oscillator

rwiggins,
I have seed the document already. I think that you also get the document by "google".
sure. it is the ansfer for my question. but it is too bad. the circuit is too complex. I hope that it can be simple.

BTW, i connect the crystal to IC. Is it possible that IC has restriction that crystal can not run at the third overtone? thanks in advance.

 

[penrico]

how frequency crystal works

You can make the crystal work in any overtone, using a circuit like this.

You must select L and C to tune in the desired frecuency:

F=1/(2*PI*SQRT(L*C))

There is not need to be exact frequency to it work, crystal will do the job. Only select the nearest coil and capacitor to get your desired frequency. If that frequency is a multiple of crystal main frequency it will oscillate.

For your example, choose:

L = 1uHY -> You can buy a inductor with this value
C=1/((48mhz*2*PI)^2 *1uHy ) = 10.99 pf -> Select a fixed 10 pF capacitor
Q1 could be a BC548 (it works up to 300mhz)

 

[ilikebbs]

third overtone crystal?

thanks, but according to your circuit, how can i connect the crystal to IC?

 

[penrico]

300mhz overtone oscillator circuit

Just put a capacitor between collector of Q1 and Xin input in your IC. Only thing you must care, is that the output voltage of that circuit is allways VCCx2 in the collector of Q1. Maybe a resistor divider will fix the voltage.

 

[FvM]

3rd overtone capacitor

To use an overtone crystal with an usual IC (inverter) oscillator, you need at least an inductor in parallel to the crystal.

 

[ilikebbs]

how a frequency crystal works

penrico,
how to determine the value of inductor and capacitor?
according to your equation, the circuit does not work at all.

 

[penrico]

3rd overtone oscillator

ilikebs,

I didn't send a circuit like you draw. The equation I send is for the circuit that I show you. This equation is the resonance frequency:

F=1/(2xPIxLxC)

In the circuit that I send, the transistor do the job to make the cristal to oscillate. If the resonance is putted near the third overtone, the transistor will oscillate in this frequency.

 

[ilikebbs]

how to make crystal to oscillate

penrico,
anyway, thanks. I appreciate your help.

 

[manve_13]

Hello,

Its not so good bumping an old thread but this is exactly what I need.

I need to oscillate my crystal around 216Mhz, as i calculated i need to use 10 pf capacitor and .17uH inductor. How can i understand my crystal is oscillating. What should i see on oscilloscope when I check from collector of transistor? Im using 3V so should i see 6V at collector?

Thank you.