Colpitts variable oscillator

[capone]

Hye,
I've build this oscillator (for AM broadcoasting reception) and it works perfect but I need to switch the positions of the 2 variables capacitors to have the 15p-280p grounded (I have some Hf signals coming from antenna) The problem is when I permute the two CVs, I dont have the same frequency range ?? What can I do without changing the circuit? Thanks

 

[k7elp60]

I assume that VC1 and VC2 are ganged together. So when the shaft is rotated both capacitors rotate. If this is the case then I don't see how the circuit is working properly, as the out terminal is shorted to the junction of VC2 and C2.
I also assume that L2 and its associated circuitry is the antenna tuning as my calculations confirms it tunes the standard broadcast band. Q1 and its associated circuitry are the local oscillator as my calculation confirm it tuning the normal local oscillator frequencies.
You might try interchanging the position of VC1 and VC2 with c1 and c2 .

 

[capone]

I assume that VC1 and VC2 are ganged together. So when the shaft is rotated both capacitors rotate. If this is the case then I don't see how the circuit is working properly, as the out terminal is shorted to the junction of VC2 and C2.
I also assume that L2 and its associated circuitry is the antenna tuning as my calculations confirms it tunes the standard broadcast band. Q1 and its associated circuitry are the local oscillator as my calculation confirm it tuning the normal local oscillator frequencies.
You might try interchanging the position of VC1 and VC2 with c1 and c2 .

thanks for the answer I already try to interchange VC1 with C1 and VC2 with C2 but the problem is the oscillator work in an other frequency range ???

 

[Nigel Goodwin]

To be honest the small piece of circuit you've given doesn't look correct at all?, perhaps you might also clarify EXACTLY what you're trying to do? - is it tune the local oscillator and front end for an AM radio?. Assuming it is?, one side of the capacitors (the common) should be connected to zero volts/ground - and the circuit arranged accordingly.

There are thousands of AM radio circuits about, why not try looking at one to see how it's done?.

 

[capone]

yes it's a local oscillator with the front end, I have some difficult to see the AC equivalent circuit.. .

 

[JimB]

Have you actually built this thing using real components, or just playing on a simulator?

I am with Nigel on this, the circuit is nonsense.
If you want a MW radio circuit, there are thousands out there on the internet and on good old paper!
Have a look at some of them - they work.

JimB

 

[capone]

yes the circuit is build and the oscillator work perfect in the original configuration, I used a gang capacitor and I need to have a common ground pin. I dont want to recalculate another oscillator, if it is possible to conserve this one with modifications, so tell me. Thanks a lot.

I use the simulator only to send images.

 

[Nigel Goodwin]

yes the circuit is build and the oscillator work perfect in the original configuration, I used a gang capacitor and I need to have a common ground pin. I dont want to recalculate another oscillator, if it is possible to conserve this one with modifications, so tell me. Thanks a lot.

Like we both said, the circuit you posted doesn't make much sense? - presumably you drew it correctly?.

Also like we both said, check some of the thousands of circuits available, and you will see where you're going wrong.