Coil design for inductive coupled circuits

[north2012]

Hi,

I would like to design antenna for inductive coupled circuits.

To be more specific, I want to connect device for impedance measuring with passive LC humidity sensors. On the sensor, capacitance is variable due the presence of moisture.

Resonant frequency of the sensors is in range 20-80 kHz (I have several) and impedance measuring system can generate voltage with peak to peak values 200 mV, 400 mV, 1 V or 2 V in frequency range 1-100 Khz. On the board I have also DFT and ADC.

I am wondering how to design coil to be able to measure changes of impedance of the coil due the change of Fres of LC sensors (because of moisture).

Any suggestions?

Thank in advance.

 

[chuckey]

I think what you are after is a "grid dip oscillator". This was a hand held variable oscillator, whose oscillations "dipped" when it was near a circuit which was tuned to its frequency and hence absorbed energy.
So you have to sweep your generator frequency and look for RF being absorbed from it. I would make the inductance of the L as high as possible and resonate just higher then your top frequency. Put a tap at 10% where you feed your generator in. Measure the RF voltage at the live end with a high impedance and low capacitance buffer in the way. Oh the coil should really be wound with litz wire.
If your transducer produces a modest change in frequency, say 90 - 100KHZ, then if you resonate your LC circuit at 100KHZ, then change to 90 K will produce a large change in output. The trouble is that you metering system will to, but you can calibrate this out.
I am not sure that you can get enough coupling for this to work. What type of transducer are you using?
Frank

 

[FvM]

I have no doubts that an impedance analyzer can detect the sensor resonance frequency similar to a classical grid-dip meter.

You'll want a detector coil with sufficient inductance and high coupling factor to the sensor coil. I have no idea about the actual sensor geometry and application constraints. At first sight, it sounds unlikely that a 20-80 kHz resonator coil has only a few turns and no core as suggested by your block scheme. If so the resonance Q would be unconveniently low.

 

[north2012]

Hi,

Thanks for your replies.

I am more interested for realization based on impedance analyzer because I have developed complex impedance measurement system based on IC from Analog Devices AD5933. Limitation of on-chip DDS is 100 kHz... I added two resistor networks (calibration and feedback) to ensure fine tuning and accuracy of 0.5 % in range 1k-5M Ohms.

 

[FvM]

I added two resistor networks (calibration and feedback) to ensure fine tuning and accuracy of 0.5 % in range 1k-5M Ohms.

I don't realize at first sight what it's good for in the present application. But anyway, the question is primarly about coil design, quite rightly, I think.

Design, calculate, simulate, measure!