Self-Adjusted of Direct Digital Synthesizers Signal Based on Automatic Gain Control for Soil Impedance Measurement

Soil sensors are important for understanding of soil variability and the output should provide repeatable measurements, rapid, inexpensive, robust and a stable signal over measurement. This paper presents the development of signal source for soil impedance measurement based on Direct Digital Synthesizer (DDS). Using DDS as signal source, it could control the frequency of the signal easily. Since it is digital device, it can be controlled using microprocessor and a PC at the end. Moreover, the amplitude of the signal can be controlled using closed loop path to maintain flat response of amplitude for overall desired frequency. The spectrum signal with flat amplitude is required for impedance measurement in order to eliminate the amplitude variation effects. Due to loading effect, the output of DDS varies over all frequencies that applied. This requires an automatic gain controller (AGC) to make it constant and flat for all frequencies applied. As the result, the signal source frequency is easy to be controlled and the error due to amplitude variations can be eliminated. The result shows flat response of the output at frequency range of 300 kHz-10 MHz with error about 3%.