Colpitt question

[matise]

Hi!
Im building an Colpitts Oscillator and its a problem i dont know how to solve.
Using a J310, t37-2 1.7 uH and two 220 pF in serie.
It oscillate perfekt around 8Mhz and i now want to use a variable cap in serie with induktor and it just stop oscillate...
I tried a capasitor with small valdes just to try to get i work but even with a 10pF in serie it just stop oscillate.
Anyone have a suggestion what i should try to do now?

[bd139]

Can you draw what you currently have if possible. It’s easier to communicate these things visually.

[iMo]

If you put 10pF in series with L the total LC tank capacitance will be "10pF in series with 110pF", where 110pF = "220pF in series with 220pF".
Thus you are changing the C in LC from 110pF to 9.16pF.
Not counting parasitic capacitance(s).

What you want to achieve?
What should be the tuning range (in MHz) with your tuning capacitor?
What is the value of your tuning capacitor (from-to) in pF?

[Z80]

You need to put the tuning cap in parallel with the inductor.  See this doc for some examples:
http://www.gqrp.com/ColpittsVFO.pdf

[matise]

Yes z80.
I have tried that but When i connect c3 the oscillation stops. 😳
Its so Weird.
Should i try bigger inductance?
It really works perfekt until i Put a capacitor like c3...
Its something i dont understand here. 🙂

[Z80]

Ok that sounds a bit odd.  Does it stop oscillating if you touch the coil?  Can you measure your tuning cap just to make sure it isn't shorted.  Posting the schematic would help.

[james_s]

That does seem odd, I once lashed together a Colpitts oscillator out of random parts I literally found on the floor and it worked on the first go, it seems to be a very robust circuit.

[rf+tech]

Hi matise,

So what C3 value have you tried?

Are your two 220 pF capacitors C1 and C2? Or are they in parallel with inductor L?

Following the hints in the Colpitts VFO document linked by Z80, C3 is to be five times the capacitance of VC/C4 combined capacitance.

10 pF for C3 is way too small. This adds nearly 2kΩ reactance in series with the LC tank and that is most likely why it will no longer oscillate. Follow the rules of thumb in the document and your oscillator will work. If the 110 pF (two series connected 220 pF) is in parallel with your inductor, then C3 needs to be 470 or 510 pF.

Really, please post your schematic. Having to guess what parts are where by referencing a third-party schematic, makes finding the solution to *your* problem harder for those trying to be of assistance. One picture can convey 1000 words. 

RF+ Tech