Paul Burridge wrote in message
...
Hi guys,

I have some surplus axial inductors marked as follows:

uH27 -K
GEL

I would have thought this implied 0.27uH or 270nH? GEL is presumably
the maker. However, there's no resonance at 17.6Mhz with 300pF in
parallel as might be expected of an inductor of that value. There *is*
however, a resonance at 50.6Mhz., which implies that the inductors are
actually 33nH. I'm confused. Anyone have any idea as to how to
determine the value absolutely? Are makers' marking to be trusted at
all?
--

The BBC: Licensed at public expense to spread lies.

0.27uH is 5ohms at 5MHz.

Set to 5MHz and wind up your 50ohm signal generator to max output (say a
100mV or so). Measure it's voltage using the scope.
Short the inductor across the sig genny terminals and measure the voltage
across the inductor.
Ignore all irrelavencies such as phase angles, stray capacitances, Q factor,
series resistance,skin effects, self resonances etc. Just treat the inductor
as if it's a straight forward potential divider resistance in series with
the 50 ohms of the genny.

Eg. Sig genny open circuit output at 5megs is 100 mV peak-peak
Inductor connected and voltage measured is now 10mV peak-peak.
Therefore 1/10th of the available voltage is across the inductor and 9/10
lost across the genny internal 50ohms.
So inductor looks like 5 ohms therefore,
5ohms = 2 x Pi x 5megs x L?
=270nH. Simple !
Using this method it's easy to get down to the odd few nH.
Exact same method as used by a nice 'inductance meter' design that turned up
in Radcom about 12 years ago

regards
john