Calculating Resonant Tank Values for a Clapp Oscillator

[greg997]

Hi can anybody explain to me how to calculate values for the resonant tank of the clapp oscillator. I know that capacitor in series with inductor sets the frequency. But I ve read that formula 1/Ceq = 1/c1+1/c2+ 1/cs, should be used to get Ceq which should be used for frequency setting. So what values of L, C1, C2, and Cs have i got to use to get like 3 MHz. I also know that Cs has to be much smaller then C1 and C2, much smaller means like 100 times?
What values of C1 and C2 to use?
I tried to make it work in multisim and included the diagram. Is it connected properly, the feedback goes from between thes tow capacitors? the Thank you

 

[vk6kro]

The capacitor in series with the coil should not be there as the coil has to carry DC current for the transistor. Capacitors do not conduct DC current.

As a ball-park figure, I would calculate the size of inductor and total capacitance to have a reactance of 200 ohms at the required frequency. This is arbitrary, but gives a starting point.

In this case L = 11 µH and C = 265 pF should resonate around 3 MHz.

C should consist of two capacitors in series where one is twice the size of the other. The larger one would be the top one in your diagram.

The emitter resistor seems a bit high, but you can juggle that when you simulate it. Try for a current of about 10 mA.

 

[greg997]

Hi again:)
In clapp oscillator the capacitor has to be in series with inductor. They set the resonant frequency, but the purpose of other two capacitor is unknown to me. And in my previous diagram i did not put RFchoke. I attched a diagram of clapp oscillator found on the internet.
The probelm i have is to what steps I should take in order to have frequency like 3 MHz using my first diagram.
Do you think you could try creating a model in spice and see if it works? All i got is nearly stedy value equal supply voltage. Thank you

 

[vk6kro]

That 100 nF variable capacitor C_T should probably be 100 pF. Try that.

This is not the normal Clapp oscillator circuit. Normally, the coil and capacitors are on the base side of a transistor (or the gate side of a FET) and they are between the base and emitter (or gate and source of a FET).

It is usually like this, if you ignore the component values and everything to the right of the oscillator:

All the capacitors are in series across the coil so they all have to be considered when working out the resonance frequency.

Do not forget that the transistor DC operating voltages must be OK before this or any other circuit will work. So zero volts or 15 volts on the emitter would be unacceptable.

The inductor marked RFC in your diagram could be modeled with 1 mH.

 

[vk6kro]

This is the classic Clapp oscillator:

[PLAIN]http://dl.dropbox.com/u/4222062/Clapp%20Oscillator%20BJT.PNG

It seems to model OK in LTSpice.

 

[greg997]

Hi, thanks for that spice model and explanations. That really helped. I think I managed to create a correct model in Multisim. Thanks again.