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A frequency ___domain sensor (FDS) converts changes in physical variables, into frequency changes.
'''Frequency ___domain''' ('''FD''') sensor is an instrument developed for measuring [[soil]] moisture content. The instrument has an oscillating circuit, the sensing part of the sensor is embedded in the soil, and the operating frequency will depend on the value of soil's [[dielectric constant]].
<ref>Murrieta-Rico, F. N., Hernandez-Balbuena, D., Petranovskii, V., Nieto Hipolito, J. I., Pestryakov, A., Sergiyenko, O., ... & Tyrsa, V. (2014, June). Acceleration measurement improvement by application of novel frequency measurement technique for FDS based INS. In Industrial Electronics (ISIE), 2014 IEEE 23rd International Symposium on (pp. 1920-1925). IEEE.</ref>
There are two types of sensors:
* ''Capacitance probe'', or fringe capacitance sensor. Capacitance probes use capacitance to measure the dielectric permittivity of the soil. The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric of water (80) is much greater than the other constituents of the soil (mineral soil: 4, organic matter: 4, air: 1). Thus, when the amount of water changes in the soil, the probe will measure a change in capacitance (from the change in dielectric permittivity) that can be directly correlated with a change in water content. Circuitry inside some commercial probes change the capacitance measurement into a proportional millivolt output. Other configuration are like the [[neutron probe]] where an access tube made of [[PVC]] is installed in the soil. The probe consists of sensing head at fixed depth. The sensing head consists of an [[oscillator]] circuit, the frequency is determined by an annular electrode, fringe-effect [[capacitor]], and the [[dielectric constant]] of the soil.
* ''Electrical [[Electrical impedance|impedance]] sensor'', which consists of soil probes and using electrical impedance measurement. The most common configuration is based on the [[standing wave]] principle (Gaskin & Miller, 1996). The device comprises a 100 MHz sinusoidal [[oscillator]], a fixed impedance [[coaxial]] [[transmission line]], and probe wires which is buried in the soil. The oscillator signal is propagated along the transmission line into the soil probe, and if the probe's impedance differs from that of the transmission line, a proportion of the incident signal is reflected back along the line towards the signal source.
Compared to [[time ___domain reflectometer]] (TDR), FD sensors are cheaper to build and have a faster response time. However because of the complex electrical field around the probe, the sensor needs to be calibrated for different soil types. Some commercial sensors have been able to remove the soil type sensitivity by using a high frequency.
==References==
* Gaskin G.J., Miller J.D. 1996. Measurement of soil water content using a simplified impedance measuring technique. Journal of Agricultural Engineering Research 63, 153-160.
*Campbell C.S., Campbell G.S., Cobos D.R.2004. Response of Low Cost Dielectric Moisture Sensor to Temperature Variation. Eos Trans. AGU, 85(17), Jt. Assem. Suppl. Abstract NS44A-05.
[[Category:Soil physics]]
[[Category:Hydrology]]
[[Category:Physical chemistry]]
[[Category:Soil mechanics]]
[[Category:Measuring instruments]]
[[Category:Impedance measurements]]
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