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I have been using Google to find a hint to the reactance desired for the coil in the FET oscillators under discussion. Not found yet.
Here is a page with a lot of depressing math.
**broken link removed**
Download the pdf for the full article and other articles included. It's a small file.
EDN PDF
I have been using Google to find a hint to the reactance desired for the coil in the FET oscillators under discussion. Not found yet.
Here is a page with a lot of depressing math.
**broken link removed**
Download the pdf for the full article and other articles included. It's a small file.
EDN PDF
I don't think it's cause he's a brit Space Varmint =)
SV, I know the generalities. The only specific thing I'm asking is a good ac resistance to shoot for for the coil. I could look at a lot of circuits and work backwards to see if it exists.
I'll check the pdf Mike suggests. Perhaps it is in there.
It will be different for fet vs bi-polar and I would think it will be different if the coil in the tuned circuit is in the emitter/source or base/gate or collector/drain sub-circuit. It just seems a good idea to me to know what reactance to design for.
It just seems a good idea to me to know what reactance to design for.
If you think application, than what you are stuck with is a variable capacitor as one half of your frequency determining components. Sure the impedance is a factor but to a lesser degree than the actual X of L and X of C values. And so, with most of the variable capacitors I've seen, they are always a very low value. You want to work with that because if you start adding parallel capacitance to the tank resonant circuit, you will significantly reduce the oscillator bandwidth. Meaning, try not to pad the oscillator. Use only the variable cap if possible. We are talking VFO (variable frequency oscillator).
Therefore your inductor value will be contingent upon the capacitor more than anything else. So your usual formulas will apply.
f = 1/ (2pi * square root of LC)
So start with Xc for a given mid frequency. So your cap is static within it's assigned range. Take a mid-range value of approximately 150pf. Then use the formula Xc= 1/ (2pi * fC)
example:
frequency = 15 MHz
Capacitance = 150pf
then XC = 70.7 ohms
Next you want to match the inductive reactance of the coil because:
f= [2pi*fL = 1 / (2pi*fC)]
When the inductive and capacitive reactance are equal and using the formula directly above, it can be broken down to:
f = 1 / (2pi * square root of LC)
So:
XL = 2pi*fL
solving for L we get:
L = XL / 2pi*f
70.7 ohms / 6.28 * 15 times 10 to the 6th = .75 uH
That should put you in the ball park except for one thing. Your mid rage frequency for your local oscillator will be offset by the frequency of the crystal filter depending on if you use high side or low side injection. The formula will apply directly to a signal generator for testing the hf band.
Hi, Mike
I just tried the Resonant Frequency Calculator and inputing any cap value between 2 and 250 pf and frequency 8 mhz I only get NaN for an inductance. (Firefox v3 java enabled).
L = 1 / (C*[2pif]squared)
what's in the brackets is squared...kapeesh?
It is spelled capisci if your Italian...
If you pay attention you will note.
L= 1/(4pi²CF²)
2 squared is 4 and C x f squared is same thing as you wrote above. Capisci?
If you prefer, use attached formula, same thing.
Hey Mike? How do make that little 2 ? The squared sign.
Hey Mike? How do make that little 2 ? The squared sign.
I am sorry, I don't quite understand your question...