Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Based on the AD7741 chip and 74HC04 NOT gate, this paper designs two passive crystal oscillator circuits that can be applied to downhole tools and provide clock sources for them. According to the working environment of downhole tools, the experimental pair at room temperature and high temperature of 175℃ is designed. The stability of the circuit was tested. According to the comparison of experimental results, a better CPLD-based passive crystal oscillator circuit design was proposed on the basis of the 74HC04 chip NOT gate, and the prototype test proved that the circuit is reasonable and feasible.

Abstract: Based on the AD7741 chip and 74HC04 NOT gate, this article designs two passive crystal oscillator circuits that can be applied to downhole tools and provide clock sources for them. According to the working environment of downhole tools, design the normal temperature and high temperature 175℃ The experiment tests the stability of the circuit. According to the comparison of the experimental results, a better CPLD-based passive crystal oscillator circuit design is proposed on the basis of the 74HC04 chip NOT gate, and the prototype test proves that the circuit is reasonable and feasible.

0 Preface

With the development of the world economy, the demand for oil and natural gas resources in various countries is also increasing. The demand for equipment for downhole operations is increasing rapidly. However, due to the harsh working environment of downhole high temperature, high pressure, vibration and shock, and mud, the various equipment and equipment The requirements are higher. When the instrument structure and other modules are confirmed, the design and realization of the hardware circuit becomes the key to the reliable and stable operation of the instrument.However, in actual applications, the components in the hardware circuit are prone to failure methods such as burnout, open circuit, and electrical parameter drift under high temperature, high pressure, and strong impact.[4], Which has an impact on the normal operation and safety production of the instrument, so the design and development of high temperature, high pressure, and impact-resistant circuits that can be applied to downhole instruments greatly promotes the development of downhole operations. Circuit design applied to downhole instruments and equipment. In the design of Electronic circuits and the application of processor chips, the clock circuit is an indispensable part of high-speed processing circuits and equipment, so the stability and reliability of the clock circuit is very important. This paper conducts a specific research on the design of crystal oscillator circuit used in downhole instruments. By comprehensively comparing the crystal oscillator circuit size of different design schemes, the characteristics of electronic components and working reliability and other influencing factors, the crystal oscillator configuration suitable for this design is selected. And carried out experimental verification.

1 Principle design

The function of the crystal oscillator in the circuit is to provide a reference frequency for each component of the system. Generally, a quartz crystal resonator can be used as a passive crystal crystal (crystal) in the application, or a quartz crystal clock oscillator can be selected as an active crystal oscillator (oscillator)[1].

Passive crystal oscillator is an excellent frequency selection and control device due to its high quality factor (Q value), high stability, small size and low cost. Orientation development[4]. Therefore, in view of the high-temperature and high-pressure working environment of downhole logging tools and the applicability of the harsh environment where the tools work, the use of SMD electronic components is reduced as much as possible, and the space occupied by the circuit is saved. Source crystal oscillator circuit.

2 hardware circuit design

Electronic circuit is the heart of various instruments and equipment, it determines the function and purpose of electrical equipment, especially the reliability of electrical equipment performance is determined by the reliability of its electronic circuit[3]. Circuit form, component selection, design scheme, etc. also determine the reliability to a large extent. For this reason, two design schemes are proposed.

2.1 AD7741AR starting circuit design scheme

AD7741 is a single-channel 8-pin synchronous voltage-frequency converter that converts voltage into frequency signals. It has a single buffer input. Among its 8 pins, VDD is the power input, GND is the ground reference point of all circuits on the part, and CLKINOUT is the external clock output. When the main clock of the device is a crystal, it is between CLKIN and CLKOUT Connection. When an external clock is applied to CLKIN, the CLKOUT pin provides an inverted clock signal, CLKIN is the external clock input, and FOUT is the frequency output. According to the functional characteristics of the circuit board, the voltage-frequency conversion chip on the circuit board has the function and pins to make the passive crystal oscillator oscillate. Therefore, the starting circuit for this circuit board is shown in Figure 1.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 1 Passive crystal oscillator starting circuit based on AD7741YR

2.2 The design scheme of the non-gate starting oscillation circuit

Combining the characteristics of the CPLD timing control circuit itself, a passive crystal oscillator starting circuit based on the NOT gate 74HC04 is first designed to verify the feasibility of the design of the NOT gate circuit based on the CPLD board.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 2 Based on the inverter 74HC04 start-up circuit

74HC04 is a six inverter, which has the function of high and low level conversion, and increases the driving ability of the signal; R23 is the feedback resistor, which can make the inverter in the linear working area at the beginning of the oscillation. C25 and C26 are load capacitors, which have a slight influence on the oscillation frequency. From the perspective of the parallel resonant circuit, that is, the two ends of the crystal oscillator, a positive feedback is formed to ensure that the circuit continues to oscillate. as shown in picture 2.

3 performance test

According to the design principles of circuit design, after the finished product is manufactured, the corresponding circuit debugging should be performed on it, including the troubleshooting of system faults, and system performance testing and functional testing to ensure the performance and overall performance of the overall electronic circuit quality[7].The extreme working conditions of domestic logging tools are 175 ℃ and 140 MPa, but they are mainly used in the temperature range of 150 ℃[2].Therefore, refer to the basic environmental test method of SY/T5134-1993 petroleum exploration and development instruments[5]In order to determine whether it can be applied to the design of downhole petroleum instruments, such related standards, circuits applied to petroleum instruments need to be verified by environmental tests such as high temperature test, vibration test, and shock test during the prototype design process. Therefore, normal temperature debugging and 175℃ high temperature debugging are carried out according to the design requirements and use conditions.

3.1 AD7741AR start-up circuit design scheme test

3.1.1 Experimental test at room temperature

After the circuit is connected as shown in Figure 1, R25 (resistance value of 10K) is not added. At this time, the crystal oscillator is difficult to start and the output frequency value is unstable. Tap the test point with the probe of the oscilloscope, and the crystal oscillator will vibrate. According to the information, there are about 10PF capacitance and resistance on the oscilloscope probe, so if C15 is replaced with 30PF, the crystal oscillator is still difficult to oscillate and the waveform is distorted.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 3 Frequency output waveform when R25=10K (137.5KHz)

After analysis, the impedance on the oscilloscope probe is compensated, so R25 is connected and the resistance value is 10K. The crystal oscillator can start to vibrate at the moment of power-on, and the output is a more regular sine wave. The waveform is shown in Figure 3.

3.1.2 AD7741AR frequency output linearity test

The main function of AD7741AR in the circuit board is to realize the voltage-to-frequency conversion of signals, so the linearity of AD7741AR frequency output and signal acquisition are very important. The comparison between the theoretical value and the measured value of the frequency after the AD7741AR VIN pin inputs the current is shown in Table 1. It can be found that the deviation between the theoretical value and the measured value is not large. Figures 4, 5, and 6 are the frequency output waveforms when the VIN pin input DC voltage is 0V, 1.25V, and 2.50V, respectively.

Table 1 AD7741AR output linearity experimental data

(F=137.5KHz in the table)

AD 7741YR VIN pin input DC (V)

0

1.25

2.50

The waveform is shown in Figure 5

Theoretical value (KHz)

6.875 (0.05f)

34.375(0.25f)

61.875 (0.45f)

The waveform is shown in Figure 6

Measured value (KHz)

6.59181

34.6379

62.0513

The waveform is shown in Figure 7

Fig. 4 Frequency output waveform when VIN pin inputs DC 0V

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 5 Frequency output waveform when VIN pin inputs 1.25V DC

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Fig. 6 Frequency output waveform when VIN pin input DC 2.5V

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 7 Normal temperature crystal oscillator frequency output waveform

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 8 Frequency output waveform after 1 hour of constant temperature at 175℃

After a 1 hour monitoring frequency experiment at room temperature and a constant temperature of 175°C, the waveform of the output frequency is smooth and smooth, and the frequency waveform changes very little. It can be seen that the stable output frequency of the circuit shows that it can meet the design requirements and working conditions. The output frequency waveform diagram is shown in Figure 7 and 8.

3.2 Design scheme of non-gate starting oscillation circuit

3.2.1 Experimental test at room temperature

74HC04 non-gate starting circuit, after debugging, it is observed that the output frequency of the crystal oscillator is 4.4MHz, but the waveform distortion is more serious. The analysis shows that there should be no impedance matching at the output end or inappropriate capacitance values ​​of C25 and C26. After changing the capacitance value of C26 to 100PF, the results of normal temperature and high temperature testing and circuit diagrams are shown in Figures 9 and 10 after debugging.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 9 Output waveform

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 10 Frequency output waveform at a high temperature of 175℃ and constant temperature for one hour

(C1=22PF, C2=100PF, R0=268Ω)

After experimental testing, the passive crystal oscillator starting circuit based on the inverter 74HC04 meets the design requirements after modification, which shows that the crystal oscillator starting scheme based on the inverter is feasible. Based on the experiment of the external inverter crystal oscillator starting scheme, we combine CPLD The characteristics of the timing control circuit itself[3], Use a NOT gate inside CPLD EPM7256AETI100 to design the starting circuit, the schematic diagram is shown in Figure 11. Figures 12 and 13 show the output waveforms of normal temperature and high temperature detection.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 11 Schematic diagram of the oscillation circuit based on the internal NOT gate of the CPLD

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole InstrumentsDesign and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 12 Normal temperature frequency output waveform Figure 13 High temperature 175℃ frequency output waveform

4 Results and analysis

⑴ The accuracy of the starting circuit of the AD7741AR design scheme is calculated as follows:

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments(Where μ is the maximum deviation; f is the standard frequency output)

⑵ AD7741AR voltage-frequency conversion output linearity error analysis

Using the above experimental data, using OriginPro 7.5 data analysis software to do the following analysis of the data, the fitting curve equation is Y=6.875+22X, which is the same as the one provided in the AD7741AR chip data, and the error is less than 0.0001. AD7741AR voltage-frequency conversion output linearity fitting curve is shown in Figure 14.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 14 AD7741AR voltage-frequency conversion output linearity fitting curve

The accuracy of the design scheme of the non-gate starting oscillation circuit is calculated as follows:Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

The data involved in the difference calculation in the above formula are all experimental record data, the former is normal temperature data, and the latter is high temperature data. Through calculation, the accuracy of the designed circuit is within a reasonable error range, which can meet the design requirements and achieve stable operation.

5 Conclusion analysis

(1) AD7741AR oscillating circuit design scheme. According to the experimental results at room temperature and high temperature, the output of the circuit is a sine wave signal of 137.5KHz with an amplitude of 4V. The voltage-to-frequency conversion function has good linearity and can meet the conditions of use of the circuit.

(2) The design scheme of the non-gate starting circuit. The frequency output of the passive crystal starting circuit based on the non-gate 74HC04 is relatively stable, but the waveform is distorted. The distortion of the waveform output can be improved by adjusting the circuit capacitance and resistance parameters. .

(3) The waveform distortion after debugging with the start-up circuit of the inverter design inside the CPLD chip is smaller and closer to a square wave. However, the output waveform of the two at high temperature has a small amount of distortion compared with normal temperature, which can be compensated by matching resistance, and the amplitude also meets the input requirements of CPLD resistance, and its performance is better than the circuit design of 74HC04. Combining the characteristics and resources of the circuit itself, a starting circuit scheme based on the internal NOT gate of the CPLD is adopted. The circuit package sample is shown in Figure 15.

Design and Realization of Passive Crystal Oscillator Circuit Used in Downhole Instruments

Figure 15 Real object of the oscillation circuit based on the internal NOT gate of the CPLD

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Xia Angran,Huang Ping,Liu Xiuping,Design and experimental research of the crystal oscillator circuit in capsule endoscopy,[J], Modern Manufacturing Engineering,2014,(10):

60-65

[2]. Chi Wei, Li Cheng, Research on the Method of High Temperature Resistance of High Logging Tools, Journal of Transducer Technology, 2007,08(8):1903-1906

Chi Wei,Li Cheng,Methods for Improving High-Temperature Resistance Performances of Logging Tools,Chinese Journal of Sensors and Actuators,2007,08(8):1903-1906

[3]. Pang Bingqiang, Research on Downhole Circuits of Induction Logging While Drilling[D], Zhejiang University, 2014, 1

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About the Author

Yuan Hongfang (1984-), female, graduate student, Xi’an Stan Instrument Co., Ltd., electronic R&D engineer, engaged in the development of petroleum instruments.

The Links:   AT070TN07-VA DMC20434

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