LC oscillator component values

Thread Starter

froten_140

Joined Jun 4, 2006
30
hello all,

when designing LC oscillators, is there a rule of thump on how to select the L & C values? How I can select these values for a specific frquency?

for example:

440pF & 0.5UH will reasonate at 10.7MHz
220pF & 1UH will reasonate at 10.7MHz
110pF & 2UH will reasonate at 10.7MHz
55pF & 4UH will reasonate at 10.7MHz

so which values are the best?

any help is very appreciated
 
Last edited:

hgmjr

Joined Jan 28, 2005
9,027
One thing to consider in chosing the LC combination is that the circuitry used to implement the oscillator will have inherent capacitance as well as inductance. It would therefore be prudent to use a combination in which neither the capacitor nor the inductor are small in value. That way the circuit is likely to operate more consistently with changes in the various components over time and temperature since time and temperature are likely to change these inherent inductances and capacitances.

hgmjr
 

SgtWookie

Joined Jul 17, 2007
22,230
If you want to get a decent Q in the inductor, you will have to chose your toroid material carefully.

If you use an Amidon T22-6 toroid with 14 turns of AWG28 magnet wire wound as evenly as possible, you will have an inductor with a Q of about 170 and around 1uH; parasitic capacitance will be pretty low. You can adjust the inductance somewhat higher by squeezing the windings together somewhat, but this reduces the Q of the inductor.

The T22 part relates to the dimensions of the toroid. The -6 is the type of material.
That particular toroid can be ordered from this page:
https://www.amidoncorp.com/items/28
 

KL7AJ

Joined Nov 4, 2008
2,229
hello all,

when designing LC oscillators, is there a rule of thump on how to select the L & C values? How I can select these values for a specific frquency?

for example:

440pF & 0.5UH will reasonate at 10.7MHz
220pF & 1UH will reasonate at 10.7MHz
110pF & 2UH will reasonate at 10.7MHz
55pF & 4UH will reasonate at 10.7MHz

so which values are the best?

any help is very appreciated

As you have discovered, there are an infinite number of L/C combinations that can give you any given resonant frequency. The issue when designing oscillators is the loaded Q, which is greatly dependent on the load presented to the tank circuit. Each oscillator type is different...sometimes radically so.

The best primer on the whole matter is older versions of the ARRL Handbook. (pre-1980 or so).
 
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