Wireless coupled LC sensor & ATmega32

Hi all,

Currently I am working on system which should consists of reading device based on atmega32 and LC humidity sensor. They are coupled by wireless (see attached picture). I do not have experience with RF design and for that reason I am asking if anyone has something to share with me- literature, schematic, etc.
block scheme.pdf
Thanks in advance.

In the proposed "LC humidity sensor", which circuit parameter is expected to vary with humidity?

Capacitance.

Regards.

My researching is now on grid dip meter realization in digital form. I think that it can be solution. Any schematic for digital grid dip meter?

Thanks in advance.

I know what a grid dip meter is, but what's exactly a "digital" grid dipper?

In general terms, I would describe the instrument as a impedance meter.

It means that realization should be microcontroller based. Thanks.

Yes, why not. You didn't mention a frequency range for your dipper circuit, so "microcontroller based" can mean a lot. You'll need a continuously tunable generator, most likely the processor can't generate the excitation in the intended frequency range, so there must be some external circuitry, e.g. a synthesizer or DDS generator. You also need a level and/or phase detector.

Many thanks for your reply. Resonant frequency range for specific sensor can be wide (10-300 MHz) but resonant frequency variations due the humidity are about 2 MHz for relative humidity change 1-16 %. Experimental results are: resonant frequency changed from 27.11 to 25.07 MHz for RH change from 1 to 16.4 %.

What do you think about usage of AD5933?

Would be a good solution for a different (frequency range) problem.

If you have any influence on resonance frequency determination, you'll hardly use this wide range, because it reduces the chances to design a dedicated reader with limited effort. Everything (frequency synthesizer, coupler, receiver) must be pretty wideband. Of course, it's no problem to use general purpose instruments like vectorial or scalar network analyzers.

I did not write clearly- we have developed different sensors with frequency range from 10 to 300 MHz but of course I will use specific sensor with known resonant frequency (desired value). I posted this to be sure what is proper solution and then I will choose sensor according to device capabilities.

Do you have any experience with AD5933? Datasheet is very good and I do not have doubts about configuration and communication but I can not found what is maximum distance for sensor detection? What about power of output signal?

Thanks.

No, I didn't use it yet. But the specification is clear enough to calculate everything from known coupler parameters. In a wider sense, you can take AD5933 as an example of a single chip fully digital vectorial impedance analyzer. Of course you can implement the same function with separate synthesizer and receiver building blocks. For higher frequencies, the receiver part must no necessarily use a homodyne topology. You can refer to the design of industry standard impedance and network anaylyzers, e.g from Agilent. There's a lot of support literature available, including instrument and service manuals with detailed block diagrams.

As a first step, you'll would determine the characteristic of coupler geometries, determine SNR and respective measurement accuracy. I would use a standard network analyzer for this measurements, as already suggested.