Design of signal modulation amplifier in dual laterolog tool

　　In dual lateral logging, the deep and shallow screen current source circuits first provide and transmit 32Hz and 128Hz shield currents to the formation through the screen current electrodes. The screen current flowing into the formation generates a potential difference between the monitoring electrodes 1M and 1'M, 2M and 2'M. Obviously, this potential difference contains the current frequency of the deep and shallow laterals. It is amplified by the balanced amplifier hybrid circuit to control the main current generator to generate the main current containing the above two frequencies. Therefore, the main current always tracks the polarity and phase changes of the screen current. For this reason, the generation of the main current makes the potential difference between the monitoring electrodes tend to zero, and the main current is focused accordingly, as shown in Figure 1. The deep lateral screen current source circuit is essentially a current source with a controlled frequency of 32Hz. Its control signal is DU2, which comes from the deep lateral voltage detection voltage detector and is proportional to the deep lateral measurement voltage. The output current of the deep lateral screen current source is added to the electrode 1A ('1A), and its screen current flows out from 1A ('1A) and returns to the ground electrode B. The deep lateral screen current source circuit is composed of a signal modulation amplifier (including a differential amplifier, a modulation amplifier, a deep bandpass filter) and a power amplifier.

　　The main function of the differential amplifier is to control the amplitude of the deep screen current with U2D output by the additional phase-sensitive detector of the deep lateral voltage measurement channel. The amplitude of the deep screen current output by the deep screen current source circuit is proportional to (U2-2U2D), and U2 is the reference signal. When the formation resistivity increases, if there is no U2D control, the deep lateral screen current will decrease very little. Because the deep lateral main current and the screen current have a following effect, the decline is also small, and accordingly, the deep lateral voltage UD rises more. Because U2D is proportional to UD, U2D follows UD to rise. When there is U2D control, the rise of U2D will cause UD to rise more appropriately and UD to rise less. As a result, the variation range of the deep lateral measurement voltage and current is relatively moderate, which is conducive to expanding the dynamic range of the deep lateral measurement apparent resistivity.

　　The modulation amplifier converts the slowly varying DC proportional to (U2-2U2D) into 32Hz AC, and then passes it through a bandpass filter to convert it into a sine wave with an amplitude proportional to (U2-2U2D) and a frequency of 32Hz, and finally after power amplification, it is added to the shielding electrode A1 or A2.

　　The composition of the shallow screen current source circuit is similar to that of the deep screen current source circuit, and is also composed of a signal modulation amplifier (including a differential amplifier, a modulation amplifier, and a shallow bandpass filter) and a power amplifier. The role of the differential amplifier is to use U2D to control the amplitude change of the shallow screen current source voltage, thereby making the change range of the shallow lateral measurement voltage US and the measurement current IS more moderate, so as to expand the dynamic range of the shallow lateral measurement of the formation resistivity.

　　It can be seen that in lateral logging, the shield current should be in phase with the main current, and the magnitude of the shield current is automatically controlled by the potential difference between the monitoring electrodes. The signal output by the phase-sensitive detector is a slow-varying signal. In order to control the shield current, it needs to be converted into a sinusoidal signal with the same frequency as the main current. This function can be achieved by using a signal modulation amplifier.

　　Principle and simulation of differential amplifier

　　This study takes the shallow screen current source circuit as an example to design a chopper-type modulation amplifier, which consists of a square wave signal generating circuit, a differential amplifier, a modulation amplifier and a multi-channel negative feedback bandpass active filter. It overcomes these shortcomings, has relatively simple control, stable and accurate output, and can effectively realize signal modulation amplification.

　　The signal modulation amplifier circuit is divided into four modules: differential amplifier, modulation amplifier, square wave generation and inversion circuit, and multi-channel negative feedback active bandpass filter. The differential amplifier is a signal preprocessing circuit, the square wave generation and inversion circuit is the control signal generation circuit of the modulation amplifier, and the modulation amplifier outputs a modulated square wave signal. After passing through a multi-channel negative feedback active bandpass filter, a sine wave with an amplitude following the initial input signal is obtained. The relationship between the various parts of the designed signal modulation amplifier is shown in Figure 2. The

        chopper modulation amplifier is the main part of the entire signal modulation amplifier, which consists of a first-stage pre-differential amplifier and a second-stage chopper modulator.

　　Preamplifier design

　　The main function of the first-stage pre-differential amplifier is to use the shallow lateral voltage measurement channel to add the phase-sensitive detector output to control the amplitude change of the shallow screen current, thereby making the variation range of the shallow lateral measurement voltage and measurement current more moderate, so as to expand the dynamic range of the shallow lateral measurement of formation resistivity.

　　The first stage pre-differential amplifier is composed of operational amplifier U1. The input signal of the in-phase terminal of U1 is U2D, and the input of the inverting terminal is U1. U1 is obtained by dividing the +15V DC power supply through the voltage divider composed of R1 and potentiometer R2, and its size can be adjusted within 0~10V. The gain of U1 is Substituting the designed component values ​​into the above formula, we can get KP=-1, so the output of U1 is 　　Design process and simulation analysis results of chopper modulator

　　Figure 3 is the principle circuit diagram of the chopper modulation amplifier, which consists of a differential amplifier and a chopper modulator. Among them, U2D is the slow-changing signal output by the phase-sensitive detector attached to the shallow lateral voltage measurement channel, Q and -Q are the square wave generated by the 555 square wave generator and the square wave inverted by the 4049 inverter, which serve as the control signal of AD7510DI. Among them, AD7510DI has the following functions: when the 5th and 6th terminals are high level and the 3rd and 4th terminals are low level, its 10th and 12th terminals are connected with the 9th, 11th, 13th and 15th terminals, while the 14th and 16th terminals are suspended; when the 5th and 6th terminals are low level and the 3rd and 4th terminals are high level, its 14th and 16th terminals are connected with the 9th, 11th, 13th and 15th terminals, while the 10th and 12th terminals are suspended.

　　After the slowly varying signal U2D is amplified by the differential amplifier U1, Ua is added to the input of the modulation amplifier. When the switch driven by Q is turned on and the switch driven by -Q is turned off, point a is grounded, and the input signal Ua is input from the inverting terminal of U2, with an amplification factor of 1, and the output is -Ua. When the switch driven by Q is turned off and the switch driven by -Q is turned on, point b is grounded, and 1/3 of the input signal Ua is input from the in-phase terminal of U2, amplified 3 times, and the output is Ua. Therefore, the high and low levels of the square wave output by the chopper modulator are Ua and -Ua (differential amplifier output voltage), respectively, and the frequency is 128Hz (the frequency of the control signal). (Figure 4)

　　The signal output by the chopper is converted into a sine wave after bandpass filtering, and then added to the shielding electrodes A1 and A2 through power amplification to generate a shallow shielding current.

　　Design and simulation of multi-channel negative feedback active bandpass filter

　　In lateral logging, the frequency used is not high. If a passive filter is used where needed, a larger inductor and capacitor are used. In order to reduce the loss of the passive filter and improve the filtering performance, the DC resistance of the inductor must be reduced and the loss of the capacitor must be guaranteed. To improve the filtering performance, the DC resistance of the inductor must be reduced and the loss of the capacitor must be guaranteed. Such a filter is large in size, expensive, and inconvenient to use. In the low-frequency band, the use of RC active filters can completely avoid the use of large inductor components. Since the energy lost by passive components can be supplemented by active units, a good active filter circuit can be formed by using small resistors and capacitors, coupled with an operational amplifier.

　　There are many active networks with bandpass filtering functions, such as single-T, double-T bandpass active filters, multi-channel negative feedback bandpass active filters, etc., which can all play the role of bandpass filtering. The former has only one feedback path, while the latter has two feedback paths. Therefore, the latter has some obvious advantages. For example, it uses fewer components, and the characteristics are good, close to the ideal second-order bandpass filter characteristics. The principle of this filter is shown in Figure 5. The

         main parameters of this circuit are: In the design, generally according to the given 002fπω=, quality factor α1=Q and passband gain KP, the capacitance is selected as a certain standard value C, so that C1=C2=C, and the value of the resistor used is calculated. In the signal modulation amplifier circuit designed this time, the requirements for the filter are: the center frequency is 128Hz, the passband bandwidth f is 12.8Hz, and the passband gain KP is 1.

　　From the design requirements, we can calculate: quality factor Q=10, center angular frequency=256, select =, all are 220nf, and we can get: R1=56.2k, R2=200, R3=113k.

　　As shown in Figure 6, the waveform of the modulated signal output after passing through the filter shows that the linear distortion of the output sine wave is very small. The signal is added to the shielding electrodes A1 and A2 after power amplification to generate a shallow shielding current, which is then added to the shielding electrodes A1 and A2 through a transformer. This connection makes A1 and A2 the return electrodes for the shield current and the main current, making the polarity of the main current and the shield current exactly the same.

　　Signal modulation amplifier circuit overall design and experimental results

　　Connecting the above designed circuits according to the principle block diagram forms the overall signal modulation amplifier circuit. The overall circuit diagram is shown in Figure 7. In the overall circuit, the output waveforms of each circuit (555 multivibrator and inverting circuit, chopper modulator, multi-channel negative feedback active bandpass filter) can be designed to meet the requirements, and the simulation results are very similar to the above design of each circuit. The signal modulation amplifier can well realize the function of making the main electrode and the screen current electrode have the same polarity in the shallow screen current circuit, meeting the design requirements. Connect the overall circuit on the breadboard, connect the power supply, and observe the chopper modulation amplifier waveform with an oscilloscope as shown in Figure 8. After continuous experimental analysis and improvement, the output waveforms of each part of the overall circuit connected on the breadboard meet or are very close to the design requirements of the project. The purpose of converting the DC signal output by phase-sensitive detection and differential signal into a sinusoidal AC signal is achieved.