Hi..

So we have a design problem, we need to create a VCO for FM transmitter (f0=94MHz, 2kHz to 4MHz bandwidth). I'd like to use a Colpitts or a Clapp. According to wikipedia, a Clapp is just a Colpitts with an extra capacitor in series with the inductor. Some sources says (including the link below) that a Clapp is a series tuned Colpitts oscillator. (Can anyone give links for basic analysis of Clapp? Google failed me here.)


So I tried to find answers on the net on topologies that I can use. I found this site:
http://www.qsl.net/va3iul/High_Frequ...Schematics.htm

It really is good, complete with qualitative explanation, except that I'm not really good with analysis.

Look at this picture:
image001.gif

From what I see (after I read the explanation):
L2 is a RFC -- should not be included in small signal model.
C3 is part of the tuning circuit.
C2 -- i don't know if this is just a bypass cap or considered as part of the LC tank
--- Is this a negative feedback circuit?


Here is the small signal model that I did:
LC cut.png

Based on the general form of oscillators, C3 should have been the inductor to get an oscillation, and L1 should have been a capacitor instead, right? So what went wrong with me? I know the statement I just said was wrong, but I don't know how was I wrong. Please help.

Please help me analyze this circuit. Or if you can, please go deeper on the first 3 topologies discussed in the link I gave above.



I just hate it when in the university teaches you over simplified circuits which you CAN'T use in real life cuz they're too ideal. Then give you a hell of problems with projects and exams. Just sayin'.

Thanks in advanced. =)