Summary of general steps in electronic circuit design

　　1. Basic steps in designing electronic circuits:

　　1. Clarify the design task requirements:

　　Fully understand the specific requirements of the design task such as performance indicators, content and requirements, and clarify the design task.

　　2. Solution selection:

　　Based on the knowledge and information mastered, design a reasonable, reliable, economical and feasible design framework according to the tasks, requirements and conditions proposed by the design, analyze its advantages and disadvantages, and have a clear idea of ​​it.

　　3. Design circuit units, calculate parameters and select devices according to the design framework:

　　When designing specifically, you can imitate mature circuits to make improvements and innovations, paying attention to the relationships and limitations between signals; then, estimate and calculate the parameters based on the circuit working principle and analysis method; when selecting devices, the operating, voltage, frequency, power consumption and other parameters of the components should meet the circuit index requirements, and the limit parameters of the components must have sufficient margin, which should generally be greater than 1.5 times the rated value. The parameters of resistance and capacitance should select nominal values ​​close to the calculated values.

　　4. Drawing of circuit schematic:

　　The circuit schematic is the basis for assembly, welding, debugging and maintenance. When drawing a circuit diagram, the layout must be reasonable, evenly arranged, clear, easy to see and read. The signal flow is generally drawn from the input end or signal source, from left to right or from top to bottom according to the signal flow. The service unit circuit is drawn in sequence, and the signal flow of the feedback path is the opposite. Graphic symbols and standards are used, and appropriate annotations are added. The connecting lines should be straight lines, and there should be minimal crossings and bends. Interconnected intersections are represented by dots, and ground wires are represented by grounding symbols.

　　2. Assembly of electronic circuits

　　Circuit assembly usually adopts two methods: general printed circuit board welding and plug-in on the experimental box. No matter which method is used, please pay attention to the following:

　　1. Integrated Circuits:

　　Be clear about the direction and find the first pin. Do not insert it upside down. The insertion direction of all ICs should generally be consistent, and the pins should not be bent or broken.

　　2. Insertion and installation of components:

　　Remove the oxide layer on the pins of the components, determine the location of the components according to the circuit diagram, and connect the components in sequence according to the flow of the signal;

　　3. Selection and connection of wires:

　　The diameter of the wire should be equivalent to the via (or jack). It is not good if it is too large or too thin. To facilitate circuit inspection, wires of different colors should be selected according to different uses. The general practice is to use red wire for positive power, blue wire for negative power, black wire for ground, and wires of other colors for signal lines. The connecting wires are required to be close to the board and have good welding or contact. The connecting wires are not allowed to cross ICs or other devices. They should be as horizontal and vertical as possible to facilitate wire inspection and device replacement. However, the connections of high-frequency circuits should be as short as possible. There should be a common ground between circuits.

　　4. Test space and terminals should be reserved at the input, output and test ends of the circuit to facilitate measurement and debugging;

　　5. A circuit with reasonable layout and correct assembly not only makes the circuit neat and beautiful, but also improves the reliability of the circuit operation and facilitates the inspection and troubleshooting of faults.

　　3. Electronic circuit debugging

　　The commonly used instruments for experiments and debugging are: multimeter, voltage regulator, oscilloscope, signal generator, etc. The main steps of debugging.

　　1. Check without power supply before debugging

　　Compare the circuit diagram and the actual circuit to check whether the connection is correct, including wrong connection, insufficient connection, excessive connection, etc.; use the resistance range of the multimeter to check whether the welding and connection are good; whether there is a short circuit between the pins of the components, whether there is a poor contact at the connection, whether the polarity of the diode, transistor, integrated circuit and electrolytic capacitor is correct; whether the power supply, including the polarity and signal source connection, is correct; whether there is a short circuit between the power supply end and the ground (use a multimeter to measure the resistance).

　　If the circuit has passed the above inspection and is confirmed to be correct, it can be transferred to static testing and debugging.

　　2. Static detection and debugging

　　Disconnect the signal source, connect the accurately measured power supply to the circuit, use the voltage range of the multimeter to monitor the power supply voltage, and observe whether there are any abnormal phenomena: such as smoke, abnormal smell, hot components when touched, power supply short circuit, etc. If any abnormal situation is found, immediately cut off the power supply and eliminate the fault;

　　If there is no abnormality, measure the DC voltage of each key point, such as the static working point, the high and low level values ​​and logical relationship of each input and output of the digital circuit, the DC voltage of the input and output of the amplifier circuit, etc. to see if they are in normal working state. If not, adjust the parameters of circuit components, replace components, etc., so that the circuit finally works in a suitable working state;

　　For the amplifier circuit, an oscilloscope should be used to observe whether self-excitation occurs.

　　3. Dynamic detection and debugging

　　Dynamic debugging is carried out on the basis of static debugging. The debugging method is to add the required signal source to the input end of the circuit, and follow the signal injection to detect the waveform, parameters and performance indicators of each relevant point step by step to see if they meet the design requirements. If necessary, the circuit parameters should be further adjusted. If a problem is found, try to find out the cause, eliminate the fault, and continue. (See the general method of checking faults for details)

　　4. Debugging precautions

　　(1) Use the ground terminal of the measuring instrument correctly, and the ground terminal of the instrument and the ground terminal of the circuit must be reliably connected;

　　(2) At the input end where the signal is weak, use shielded wires as much as possible. The outer shielding layer of the shielded wire should be connected to the common ground wire. When the frequency is high, try to isolate the influence of the distributed capacitance of the connecting wire. For example, when measuring with an oscilloscope, you should use an oscilloscope probe to connect to reduce the influence of the distributed capacitance.

　　(3) The input impedance of the instrument used to measure voltage must be much larger than the equivalent impedance of the measured point.

　　(4) The bandwidth of the measuring instrument must be greater than the bandwidth of the circuit being measured.

　　(5) Correctly select the measuring point and measure

　　(6) Carefully observe and record the experimental process, including conditions, phenomena, data, waveforms, phases, etc.

　　(7) When a fault occurs, carefully find the cause.

　　4. General methods for checking electronic circuit faults

　　For newly designed and assembled circuits, common causes of failure are:

　　(1) The experimental circuit does not match the designed schematic; the components are used improperly or are damaged;

　　(2) The designed circuit itself has some serious shortcomings and cannot meet the technical requirements, resulting in short circuits and open circuits in the connections;

　　(3) The solder joints are poorly soldered, the connectors are in poor contact, the variable resistors, etc. are in poor contact;

　　(4) The power supply voltage does not meet the requirements and the performance is poor;

　　(5) Improper function of the instrument;

　　(6) Improper grounding treatment;

　　(7) Failures caused by mutual interference, etc.

　　The general methods for checking faults are: direct observation method, static inspection method, signal tracing method, comparison method, component replacement method, bypass method, short circuit method, open circuit method, exposure method, etc. The following mainly introduces the following methods:

　　1. Direct observation method and signal inspection method: Similar to the visual inspection and static inspection before debugging introduced above, but more targeted.

　　2. Signal tracing method: directly input a signal of a certain amplitude and frequency at the input end, and use an oscilloscope to observe the waveform and amplitude step by step from the front stage to the back stage. If any stage is abnormal, the fault is at that stage; for various complex circuits, the front and rear stages of each unit circuit can also be disconnected, and appropriate signals can be added to the input end of each unit to check whether the output of the output end meets the design requirements.

　　3. Comparison method: Compare the parameters of the circuit with the parameters of the working state and the same normal circuit (or the current, voltage, waveform and other parameters of theoretical analysis and simulation analysis) to determine the fault point and find out the cause.

　　4. Component replacement method: Replace the possibly faulty component with a good device of the same model.

　　5. Accelerated exposure method: Sometimes the fault is not obvious, or it appears sometimes and not sometimes, or it takes a long time to appear. The accelerated exposure method can be used, such as knocking on components or circuit boards to check for poor contact, cold solder joints, etc., using heating methods to check poor thermal stability, etc.

　　5. Electronic Circuit Design Experiment Report

　　The design experiment report mainly includes the following points:

　　1. Project Name

　　2. Summary

　　3. Design content and requirements

　　4. Compare and select design options

　　5. Unit circuit design, parameter calculation and device selection

　　6. Draw a complete circuit diagram and explain the working principle of the circuit

　　7. Assembly and debugging contents, such as the main instruments and meters used, methods and techniques for debugging circuits, test data and waveforms and comparative analysis with calculation results, faults that occur during debugging, causes and troubleshooting methods

　　8. Summarize the characteristics of the designed circuit and the advantages and disadvantages of the solution, point out the core and practical value of the subject, and put forward suggestions for improvement and prospects

　　9. Make a list of components

　　10. List references

　　11. Gains and Experiences

　　When actually writing, appropriate adjustments can be made according to the actual situation.

　　6. Suppression of electronic circuit interference

　　1. Interference Source

　　When electronic circuits are working, there are often some annoying interference sources besides useful signals, some of which are generated inside the electronic circuits and some are generated outside. External interference mainly includes: high-frequency interference generated by high-frequency electrical appliances, power frequency interference generated by power supplies, and radio wave interference; internal interference mainly includes: AC noise, mutual induction and modulation between different signals, parasitic oscillation, thermal noise, and waveform distortion or oscillation caused by impedance mismatch.

　　2. Measures to reduce internal interference

　　(1) Component layout: The arrangement of components on the printed circuit board should fully consider the anti-electromagnetic interference problem. One of the principles is that the leads between components should be as short as possible. In terms of layout, the analog signal part, high-speed digital circuit part, and noise source part (such as relays, high-current switches, etc.) should be reasonably separated to minimize the signal coupling between them.

　　(2) Power line design: According to the current of the printed circuit board, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which will help enhance the anti-noise ability.

　　(3) Grounding design: In electronic equipment, grounding is an important method to control interference. If grounding and shielding are used correctly, most interference problems can be solved (see the grounding section below for detailed methods).

　　(4) Decoupling capacitor configuration One of the common practices in circuit board design is to configure appropriate decoupling capacitors at various key locations on the circuit board. The general configuration principle of decoupling capacitors is:

　　Connect a 10~100uf electrolytic capacitor across the power input terminal. If possible, it is better to connect a 100uF or larger capacitor.

　　In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the printed circuit board space is insufficient, a 1~10pF capacitor can be arranged for every 4~8 chips.

　　For devices with weak noise immunity and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and ground line of the chip.

　　The capacitor leads cannot be too long, especially high-frequency bypass capacitors cannot have leads.

　　In addition, the following two points should be noted:

　　When there are contactors, relays, buttons and other components on the printed circuit board, they will generate large spark discharges when they are operated. The RC circuit shown in the figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K, and C is 2.2 ~ 47UF.

　　The input impedance of CMOS is very high and is susceptible to induction, so the unused ends should be grounded or connected to a positive power supply when in use.

　　3. Measures to reduce external interference include:

　　(1) Keep away from interference sources or perform shielding;

　　(2) Use filters to reduce external interference.

　　7. Grounding

　　Grounding is divided into safety grounding and working grounding. What we are talking about here is working grounding. The design of grounding points is to minimize the mutual coupling interference between the currents of each branch. The main methods are: single-point grounding, series grounding, and plane grounding. In electronic equipment, grounding is an important method to control interference. If grounding and shielding can be used correctly in combination, most interference problems can be solved. The ground wire structure in electronic equipment generally includes system ground, chassis ground (shielded ground), digital ground (logic ground) and analog ground. The following points should be noted in ground wire design:

　　1. Correctly choose single-point grounding and multi-point grounding

　　In low-frequency circuits, the operating frequency of the signal is less than 1MHz, and the inductance between its wiring and devices has little effect, while the loop current formed by the grounding circuit has a greater impact on interference, so one-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible, and multi-point grounding should be used nearby. High-frequency circuits should use multi-point series grounding, the ground wire should be short and rented, and a large area of ​​grid-shaped ground foil should be used around high-frequency components as much as possible. When the operating frequency is between 1 and 10MHz, if one-point grounding is used, the ground wire length should not exceed 1/20 of the wavelength, otherwise a multi-point grounding method should be used.

　　2. Separate digital circuits from analog circuits

　　There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed. They should be connected to the ground wire of the power supply end respectively. The grounding area of ​​the linear circuit should be increased as much as possible.

　　3. Make the ground wire as thick as possible

　　If the grounding wire is too thin, the grounding potential will change with the change of current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, the grounding wire should be as thick as possible.

　　4. Make the ground wire into a closed loop

　　When designing the ground wire system of a printed circuit board composed only of digital circuits, making the ground wire into a closed loop can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power. Due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, causing the anti-noise ability to decrease. If the ground structure is made into a loop, the potential difference will be reduced, and the anti-noise ability of the electronic equipment will be improved.