Need to build a variable frequency colpitts oscillator

[GaryM]

Hey all,

I have to build a colpitts variable frequency oscillator circuit and I am having problems with it. Find attached my schematic. I am using one varactor to acheive variable frequency. But the circuit does not oscillate. Can anyone plese tell me whats wrong???

 

[betwixt]

Without running a simulation, my gut feeling is that C1 is too big by a factor of about 1,000. Try 15pF, maybe 150pF but not 150nF !

R5 and the circuit feeding it also does absolutely nothing, I suspect you should add a capacitor between the junction of R5/C3 and the junction of C1/Q1/R3 or the voltage across the varactor will have no effect.

Brian.

 

[GaryM]

Hi,

Thanks for your suggestion,

you said to add a capacitor between the junction of R5/C3 and the junction of C1/Q1/R3.

could you explain why and how do i calculate the values of these capacitors?

Thanks

 

[vfone]

Just to see the oscillator is working, remove all the varactor components (R5, R6, R7, C2, D3), and place from the junction of C1, Q1, R3 to the ground another 150nF capacitor.

The circuit is oscillating around 45kHz.

Cannot use a varactor at this low frequency to make a VCO because there is no varactor with such high capacitance.

 

[GaryM]

oh so you cant make a VFO using a colpitts oscillator??

 

[vfone]

Yes, you can make a VCO (Voltage Controlled Oscillator) using a Colpitts, but the oscillation frequency should be higher than you get now (45 kHz).

The maximum capacitance of available varactors is in a range of few hundreds of pF.
Using these varactors in your low frequency oscillator you will get very small frequency variation.

 

[GaryM]

Ok...i need a frequency range from 10kHz to 100kHz. Whats the best way to do this using a colpitts?

 

[vfone]

Can build a higher frequency VCO and divide its output to the wanted frequency range.
But this is not valid for your case, where you are looking for a decade frequency range, range which is almost impossible to be covered by an LC oscillator.

Other option is to use an op-amp VCO: