Oscillator with an inductance with nonzero resistance

meereck

Hello,
I need to measure ONLY a change in inductance of a loop (long wire) which has got resistance about 30-200 ohms.
I have tried to build the oscillator according to this schematic : http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
but the problem is it works well with an inductance with almost zero resistance only.
If I connect my loop, it behave wrongly. I guess because of the energy disappation?

Therefore, I need to modify that circuit, or use another another one. Unfortunately, I havent found anything about how to make an oscillator with an resistive inductor.

I read about using a negative resistance circuit but I have no clue how to employ that.
I have also heard about using a transformer (perhaps it is called tapped inductors?).

To summarize my approach:
I only need to detect a change in inductance of a wire loop.
I prefer to have an oscillator with TTL output and to measure therefore the frequency (in a microcontroller)

May I ask you for some hints on that?
Thanks in advance,
Best regards Meereck

berkeman

What is the range of inductance that you are trying to measure, in addition to the 30-200 Ohms of real resistance?

I would probably approach it by driving a signal from a known source impedance, and measuring the in-phase and quadrature components of the divided AC voltage waveform. That will give you values for both the real resistance and the reactive inductive impedance.

meereck

thanks for a reply.
The inductance will be about 100-300 microH, its pure resistance will be 30-200Ohm.
>>measuring the in-phase and quadrature components of the divided AC voltage waveform.
Right, but this will probably be quite difficult to measure by a common microcontroller such as a PIC. Would you have any supporting circuit for that?

cheers M.