Publication: A study and loss modeling of rf to dc converter
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Date
2010-04-01
Authors
Mohd Naim, Nor ‘Adilah
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Abstract
Sensors without battery are one of the essential requirements in passive sensor. A rectifier circuit is used to convert the coupled AC electro-magnetic waveforms to a DC voltage and serve as a power supply for the rest parts of the sensor circuits. The RF transmitter transmits sufficient energy to power up the sensor at large distances. However, the rectifier output is limited on the amount of power that can be transmitted by transmitter and the path loss associated with electromagnetic propagation. Apparently, there is a little power that reaches the sensor. Therefore, the power harvesting circuit design must maximize the operating distance in order to activate the sensor by converting the limited incoming RF power to DC power with a sufficient voltage. The initial design of the rectifier was based on a full-wave regulator voltage doubler and later modified to the cascade doubler circuit. Some simulations are done by using Multisim simulation tools during the design of the rectifier. The high frequency rectifier has been designed using Eagle layout tools and the layout has been fabricated. There are another designs needed in this project which is feeding board of antenna. Computer Simulation Technology (CST) software was used to draw the schematic of feeding board in order to get the layout drawing for the design. Some calculations are performed during MAX757 design to determine the value of resistor that should be used. When the circuit board has been fabricated measurement part started. Measurement part includes of transmitter and receiver antenna, rectifier and voltage regulator. From measurement, both antennas working perfectly in the range of UHF and the voltage regulator is working in stand alone with the output is 3.489 V. The minimum operating for Ceramic antenna is 725 MHz while LPRS antenna is 624 MHz. For the rectifier part, it was working but the desired output cannot be obtained due to some loss efficiency such as impedance mismatch, path loss, threshold voltage and sensitivity of Schottky diode and losses in the circuit. It is obtained when comparing the input power (25 dBm = 316 mW) with the output power of rectifier (1.2V x 0.012μA = 0.144 μW). When the rectifier connected with voltage regulator, the output is lower than 1.8 V due to the loading effect problem. After taking note the causes of loading effect, a test is done to verify the problem. The test result shows the loading effect occur with the combination of rectifier and voltage regulator.