Discussion in 'Electronic Design' started by M. Hamed, Jul 16, 2013.

1. ### M. HamedGuest

I have to admit I am getting confused by the L/C ratio. I read it in another book that for parallel resonance the L/C ratio should be small. A bit counter-intuitive to me.

2. ### M. HamedGuest

Thanks! It's been a lot of fun and a lot of learning

I am seeing a strange effect though. When I am not probing at the FET the frequency is slightly higher (someone at work happened to have a wireless sniffer). So the load on the FET affects the tank frequency. My only guess here would be Miller effect?

3. ### M. HamedGuest

#6
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The fact that every cap in my oscillator circuit seems to be able to affect the frequency bothered me. The fact that I couldn't analyze the circuit bothered me more. I ended up taking a shot at trying to figure out how f is related to all the C's.

After a few agonizing hours with the math and seven pages of symbols, I actually figured it out and came up with a formula.

2pi.f = sqrt ( (c3*c4 + c2*c3 + c2c4) / ( L * (c1*c3*c4 + c2*c3*c4 + c1*c2*c3 + c1*c2*c4)) )

I put down the formula in an Excel spreadsheet that would computer the frequency and it almost always matched simulation especially if the cap ratios are reasonable. C1, C2 are the tank caps and C3,C4 are the tap caps.

Further simplification can be made if C3,C4 >> C1,C2 then the frequency could be simply:

2pi.f = sqrt ( 1 / (L * (C1+C2)) )

This is the first time I analyze such a circuit and probably the most math I have ever done outside of college. It's also the first oscillator that I could mathematically figure out. proud of myself!

4. ### Tauno VoipioGuest

If you do not have it yet, get the ARRL Handbook (a slightly old
printing will do). It will save you countless hours of guesswork.