L-C oscillator topology

Whew! 20nH of inductance resonates with a huge 3166pF of capacitance at 20MHz, and the resonant impedance is very low, Q times 2.5 ohms. I'd advise using a higher frequency and dividing down if necessary.

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 Thanks,
    - Win

Is a venereal disease ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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(at 20MHz)

No matter haw you attack this, the low Z of the 20nH at 20MHz will be trouble. I suggest you go up to at least 100MHz. The semi-isolated Colpits oscillator would be a good way to go. You could use a, lets say,

50 Ohm, collector resistor to match to a coax directly.

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kensmith@rahul.net   forging knowledge

I read in sci.electronics.design that Tom wrote (in ) about 'L-C oscillator topology', on Mon, 3 Oct 2005:

Clapp.

-- Regards, John Woodgate, OOO - Own Opinions Only. If everything has been designed, a god designed evolution by natural selection.

Also see

Go buy an LCR meter and reverse-engineer it.

Good Luck! Rich

OK, then, how about Pierce? ;-P

Cheers! Rich

(for those who are just tuning in, the joke is, "Isn't that how you get the Clapp in the first place?")

"Tom"

** To keep the frequency lowish and the Q high with practical capacitors, why not used a small fixed inductance in series with your tiny variable one. Seeing as you intend to use a frequency meter, the range of frequency change need not be large.

If you use say, 500 nH in series, then the total L value to be resonated varies from 520nH to 620 nH - which is far more manageable than 20 nH to

100 nH.

With a parallel C of say 1000pF, the frequency will then vary from 7.0 MHz down to 6.4 MHz.

Should be easy enough to get Mr Colpitts to co-operate with you then.

.......... Phil

Hello Tom,

As Win said 20MHz won't really cut it here. Go to a few hundred MHz and place the oscillator on board. The BFS17A comes to mind which is a SOT23 pocket rocket for UHF. If you don't go too high in frequency you may be able to divide it down at reasonable cost with newer logic families.

In case it absolutely has to be low frequency you could consider a crystal oscillator that is "pulled" by that variable inductance. You'd have to think about the circuitry, I have only done that to measure minute capacitance variations but not inductors (yet). The nice thing about this narrow bandwidth scheme is that it can be done in an ISM band to avoid potential EMC scuffles with the FCC.

Regards, Joerg

You have the *what*????

..already hav too many things pierced!