colpitts oscillator help

[falcone]

Hye,
I've build this oscillator 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

 

[rogerynt]

What is your tuning frequency ?

Did you try to change 65pF VC to a fix SMD cap. of 62 pf or 68pF. Then tune another VC only. It's difficult to trim both vc at the same time. Besides, the parasite inductance due to vc leads has significant effect on upper frequency limit.

 

[falcone]

hye,
the tuning range is 990 Khz to 2150 Khz (local oscillator for AM broadcoast). I've tried a fix 65pf but I need to have both VC1 & VC2 grounded (This is a gang cap's type)
When I do that, the oscillator dont oscillate on 990 to 2150 ??

 

[Milad-D]

Just make the 2 capacitors constant and add a variable capacitor between drain node and ground to tune the frequency.