Develop methods for doing dielectric sprectroscopy (DS) measurements of human physiology. The task was split in two main phases: demonstrate ability to do DS on common calibration liquids and exploring how the technique can be applied to human measurements.
This is the simplest electrode design. It has a 1mm gap between the two measurement pads.
The Wheatstone bridge1 uses the balancing behaviour of the two legs of a bridge to make differential measurements and determine the value of one unknown element. There are different configurations of a Wheatstone bridge that allow you to measure capacitance, inductance and frequency2.
This measurement was done uing the Neil's transmission step response board 3. It works by sending a square wave using the microcontroller's pin (sending pin), which is in turn sampled by the ADC at a precise time ∂t using a second (receiving) receiving. This is used to evaluate the slope of the transmitted wave after it went through the dielectric.
|500µM NaCl in 20µL H2O||34400|
In order to have more flexibility about the frequency of the wave being sent, one can use a signal generator + oscilloscope setup to achieve the same measurement as above. Using the Tintin electrode, sending at 2MHz, we obtain the following results for the differential conduction of the wave in a transmission measurement.
Note that the scale is at 1V/div in this picture
Note that the scale is at 2V/div in this picture
While using the oscilloscope and signal generator in the setup decribed above is informative, it remains a time domain measurement. In order to move to a frequency domain measurement, we use a network analyzer. Note that the higher in frequency you get, the more careful you need to be about setup. For that reason, we limited our use of the analyzer to the GHz order of magnitude.
The electrodes were strips of copper tape soldered to a SMA cable
What is that valley at ~3.2 MHz?