I have an LC oscillator which runs at 120kHz. I want it to be 125kHz. The coil has 52 turns.
Can you confirm my maths that 50 turns would do it?
If so, can I test this by adding two turns in the opposite direction to the rest of the winding?

 

I agree. The inductance will increace by the ratio of (52/50)^2 But for a fixed value of capacitance the frequency is proportional to 1/(square root of the inductance) So the frequency should be 124.8 Khz. If the coil is on a hollow former then inserting a brass slug into the inducter should increase the frequency. (In the same way a an iron dust core would reduce the frequency.)

Les.

 

Try spreading the turns out a bit (assuming it's an air core coil?)

 

Thanks Les. I will try the two reverse turns.

 

Try spreading the turns out a bit (assuming it's an air core coil?)

It is varnish impregnated, so even removing turns would be difficult.

 

Is pruning an option?

 

1.st option : prune the wire very small part shorter. Each 1 cm of leads have about 10 nH.
2.nd option : shift the capacitor to smaller and put in parallel onother two or three smaller thus may variate the capacitance until frequency is ideal.
3rd option : make nearing the coil core made of one of copper, missing, bronze, aluminium. Core size must be at least 4x smaller than winding inner diameter and it must not come too much near to not loss the Q-factor.
However, if You play so tiny like the 5 kHz difference, the temperature probably is more crucial tnan everything other. Try to blow on the hot air from soldering station and look how large is the result. Probably its not bad idea to have L and C thermostabilized to a certain higher T?