I'm used to an air core inductor having a positive tempco of inductance around +120 PPM/k or so, just from the physical expansion of the copper increasing the diameter.
But this Coilcraft midi-spring part
Well, it makes it easier to temperature compensate my oscillator. Out on my bench, it's measured 49.9944 MHz, exactly the same, for 3 hours.
Just a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature?
You could heat it above the curie point, if the inductance decreased suddenly...
You can make the longitudinal expansion of the plastic compensate for the circumferential expansion of the copper.
To within a couple of percent, L(uH) = (a**2 N**2)/(9a + 10 b), where a is the mean radius of the coil, b is the length of the winding (both in inches), and N is the number of turns.
CTE(a) is controlled by the copper, CTE(b) by the plastic.
so we get
dL/dT = 2 N**2 a da/dT/ (denom) - N**2 a**2 *( 9 da/dT + 10 db/dT) / denom**2
da/dT = a*CTE(Cu) db/dT = b*CTE(plastic)
Dividing by L and collecting terms, we get
TCL = (dL/dT)/L = (2 CTE(Cu) - (9a CTE(Cu) + 10b CTE(plastic))/(9a + 10b).
With CTE(Cu) = 17 ppm/K and CTE(plastic) probably 100 ppm/K, you get zero TC somewhere about a/b = 4.5, i.e. diameter ~ 9 x length. (That is, unless I've made a blunder, which is a definite possibility at 1 AM.)
Cheers
Phil Hobbs
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