i am trying to build a 3rd overtone as the diagram below.

According to the text on http://hem.passagen.se/communication/txo.html

I am supposed to tune inductor L1 to the exact frequency required. Does anybody know the equation to obtain this frequency using L1 and capacitors in the circuit?

Appreciate all the help given.

Attached Images

3rd_overtone.gif (6.9 KB, 218 views)

 

Here is my take on this: (without looking at the link page)

C2 should be about 47pF (not critical) at the upper end, and around 68pF -100pF at the lower end.

R3 x C3 = 0.5/F (Make R3 variable during tests, take precaution for R3=0! by inserting safe fixed value in serie)

Where R(ohms), C(uF), F(MHz)

C1 and L1 will be series resonant at the required frequency using the basic formula Fr = 1/2*Pi*SQRT(L*C)

Making C1 smaller will raise the frequency. Making L1 bigger will introduce some more phase-lag an lower the freguency.

You have to choose your values close enough cause you will only be able to "pull" the crystal frequency by a couple of hundred Hertz

R4, in most cases can be left out

 

Thanks. It works!!

I set it my inductor to 0.33uH but it oscillated at 13.5mhz, and around 43mhz if without the crystal in the circuit.

Changed the R3 resistor to 390 ohms using your formula and it oscillated at 27mhz 8) But the output is only around 200mv...

 

It may have to do with the values you selected for the series L and C

Large L and Small C will give a higher Q factor but also more insertion loss.

Try a combination still using the basic formula, but make C1 higher. You will now have a slightly smaller value for L1 according to the formula and higher output.

With what are you measuring the output level?

Also post all your final values, to be helpful to other members.