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LC Oscillator Question

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jasonca2

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I understand how to calculate the oscillation frequency. The calculations are straightforward enough, but I don't seem to be able to find any information on how to select the component values.

I assume that certain values of inductance and capacitance are more suitable for certain frequency ranges.

The resonance equations would indicate that any value of inductance would do if you had the right capacitance, but I doubt that could be the case. I imagine you wouldn't for example attempt to produce a 50MHz signal with a one henry inductor.

My assumption is that if you don't use an appropriate amount of inductance and capacitance for the frequency you are looking to achieve that your output waveform would not be symmetrical. Is that the case or will the oscillator just not work if you use too much capacitance vs inductance or too much inductance relative to capacitance?

Anyone have any ideas where I might look for some details on this subject?
 
Generally, it is the impedance at resonance to fit the oscillator that dictates the L and/or C. For example, if using a common emitter transistor as the gain block to create the oscillator, you would not want the base-emitter input impedance of the transistor to degrade the Q of the tuned circuit too much.
 
It's also related to practical values of inductance and capacitance. For example, you typically want to use air core inductors to give good Q (and a stable oscillator frequency) and they have a relatively low upper limit on their value without getting excessively large. Also capacitors below about ten pF or above a few tenths of a µF are not generally used for practical stable oscillators.
 
Air core inductors, above a certain size, also have a poorer Q than cored inductors because the winding resistance is higher for a given inductance. Magnetic losses only become apparent above a frequency determined by the core material.
 
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