I hope someone can help with a tricky problem I'm stuck with. I have a custom made 125 khz RFID reader and some commercial tags. The reader itself is very simple, it just excites the tag and then demodulates and extracts the amplitude modulated signal of the tag. I then use a DAQ to sample the analogue tag signal for later analysis on the computer. I'm only interested in the analogue signal and not the digital ID at this stage. The tag just switches on and off giving a stream of equal pulses (50% duty) at a certain repetition frequency.
I'm am trying to measure the effect metals of different conductivity has on the tags response. To do this I place a tag on the surface of a metal and then I read the tags response using the reader coil from a height (R) above the tag. One of the features I used to distinguish and differentiate the different metals is the peak value of the tags waveform (i.e. the average peak value of the pulses). This works well provided I keep the height R constant for all measurements. Even slight variations in the read distance R can cause big changes in the peak value.
I have also looked at different features such as the amplitude in the frequency domain but that too seems to be susceptible to variations in R.
So, my question is whether there is any features that I can use that will give me information about how the conductivity is changing irrespective of what distance R I measure from. Ideally, the solution shouldn't involve any major changes in the hardware of the reader or tag (they are just plain commercial ones). I have though about using two tags with one being a reference but I still don't know how I will process the features so that I have some invariance with respect to variations in R.
Many thanks for reading.
I'm am trying to measure the effect metals of different conductivity has on the tags response. To do this I place a tag on the surface of a metal and then I read the tags response using the reader coil from a height (R) above the tag. One of the features I used to distinguish and differentiate the different metals is the peak value of the tags waveform (i.e. the average peak value of the pulses). This works well provided I keep the height R constant for all measurements. Even slight variations in the read distance R can cause big changes in the peak value.
I have also looked at different features such as the amplitude in the frequency domain but that too seems to be susceptible to variations in R.
So, my question is whether there is any features that I can use that will give me information about how the conductivity is changing irrespective of what distance R I measure from. Ideally, the solution shouldn't involve any major changes in the hardware of the reader or tag (they are just plain commercial ones). I have though about using two tags with one being a reference but I still don't know how I will process the features so that I have some invariance with respect to variations in R.
Many thanks for reading.
