The Design Of Ultra-low Frequency And High Power Signal Source

The high-power and ultra-low frequency excitation signal source is one of the maincomponent of casing resistivity logging model machine. by launching Extra ultra-lowFrequency excitation signal into the downhole metal casing, The stratum is voltaic respond tothe tiny voltage descend that the signal shows on metal casing measurement electrode,Detecting the leakage of the current so as to achieve the purpose of formation resistivity.Inorder to obtain better measurement accuracy,Incentives should be increased as much aspossible of the current signal.Due to the resistance of cable logging can not beignored,Therefore requested the source of excitation signals with max current value is6A.Seeing from the relationship of excitation signal frequency and the skin depth,the excitingsignal source should be kept below10Hz.Because the range of voltage drop on the cablelogging is-150V~+150V, requested the source of excitation signals with the output voltagevalue will be from-300V to+300V. To sum up, the total requested the source of excitationsignals with frequency no more than10Hz and high voltage output capacity,and a higherdegree of stability.This artcle introduces the development and principle of the cased hole resistivity loggingtechnology.According to the frequency characteristics and voltage amplitude feature of theexciting signal, the design scheme of ultra-low frequency and high power exciting signalsource is proposed. The hardware design of the exciting signal source consists of two parts--the generating section and the power amplifier section. Using the latest frequency synthesistechnology which is named direct digital frequency synthesis (DDS),the first part generatesultra-low frequency sine signal which will be treated by differential amplifier、lowpass filterand voltage amplier. The second part is the design of the high power voltage source, whosecore module is single phase H-bridge inverter circuit. Then designing a new type of the Boostconverter supplies enough voltage amplitude for the H-bridge circuit’s input.The ultra-low frequency sine signal being from the first part compared with trianglewave to generate SPWM waveform which controls H-bridge circuit.Analyzing the harmonicsof the output SPWM waveform,the two-order Butterworth filter is designed to remove thehigh frequency signal.By experimental verification,the system can output the signal frequency stability with 0.01%and output the current stability is less than0.01%.The output voltage amplitude’srange can reache300Vand the current value can reach6A when the inductive loadresistance is50Ω.So the system can meet the requirements of design acquisition.