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Pcb schematics and diagram of printed circuit board
The PCB board is a printed circuit board on which electronic components are placed and has wiring. The anti-corrosion line is printed on the copper-plated substrate, and the line is etched and washed away. I believe everyone understands the working principle of the circuit board well: the insulating material of the substrate is used to isolate the conductive layer of the surface copper foil, so that the current can propagate in each component along the designed route, thereby realizing the functions of work, amplification, attenuation, modulation, demodulation, encoding, etc.
Schematic design
The pcb schematics is the first step in the entire circuit board design. Its main task is to graphically represent the functions and connection relationships of the circuit. In the process of schematic design, the designer needs to clarify the input and output of the circuit and the connection between each module, taking into account the transmission and processing of the signal, as well as the selection and parameter setting of various devices.
1. Functional analysis
Before designing the circuit board, it is necessary to analyze the function of the circuit to determine the functions to be implemented and the required functional modules. If a module will appear in the circuit many times, you can first design a module graphic and then use it many times in the schematic diagram to improve the reuse rate of the circuit.
2. Symbol selection
During the schematic design process, standard electrical symbols need to be used to represent components, circuits, and signals in the circuit. Different types of electrical components have different symbols, such as diodes, transistors, relays, and capacitors. When choosing symbols, pay attention to the standard symbol type and symbol naming to ensure the accuracy and readability of the symbols.
3. Line connection
The basic elements of the schematic are various devices and circuits. Devices are short for electronic components, and circuits are responsible for connecting them correctly. In the schematic, the pins and connection methods of each device need to be clearly identified. At the same time, the transmission direction and path of the signal in the circuit should be considered to ensure the correctness and accuracy of the transmission.
4. Parameter setting
During the circuit design process, the parameters of each device, such as resistance, capacitance, inductance, etc., need to be set. Correctly setting parameters is the basis for ensuring the normal operation of the circuit. Incorrect parameter settings may cause the circuit to fail to work properly.
5. Electrical design rules
Before designing the schematic, it is necessary to clarify the design rules and set basic parameters such as the maximum voltage and maximum current of components and circuits. At the same time, it is also necessary to pay attention to timing design and synchronous detection to ensure the correctness and stability of the circuit.
*Capacitors are the most common and commonly used devices in circuit design. They are one of the passive components. Active devices are simply devices that require energy (electricity) power. Active devices are passive devices that do not require energy (electricity). Capacitors also often play an important role in high-speed circuits.
There are generally many functions and uses of capacitors. For example: in bypassing, decoupling, filtering, energy storage; in completing oscillation, synchronization and time constants...
Let's analyze it in detail:
1. DC isolation: The function is to prevent DC from passing and allow AC to pass.
2. Bypass (decoupling): Provide a low-impedance path for certain parallel components in the AC circuit.
3. Coupling: As a connection between two circuits, it allows AC signals to pass and be transmitted to the next level circuit.
4. Filtering: This is very important for the circuit. The capacitors behind the CPU basically have this function.
That is, the larger the frequency f, the smaller the impedance Z of the capacitor. When the frequency is low, the useful signal can pass smoothly because the impedance Z of capacitor C is relatively large; when the frequency is high, the impedance Z of capacitor C is already very small, which is equivalent to short-circuiting the high-frequency noise to GND.
5. Temperature compensation: Compensate for the impact of other components' insufficient adaptability to temperature and improve the stability of the circuit.
6. Timing: Capacitors and resistors are used together to determine the time constant of the circuit.
7. Tuning: System tuning of frequency-related circuits, such as mobile phones, radios, and televisions.
8. Rectification: Open or close semi-closed conductor switching elements at a predetermined time.
9. Energy storage: Store electrical energy for release when necessary.
*Inductor related knowledge:
The main characteristics of inductors in circuit design are: filtering high-frequency harmonics, passing DC and blocking AC; hindering current changes and maintaining the stability of the device's working current.
The inductor parameters that need to be checked when selecting an inductor include inductance value, DC resistance, rated current and self-resonant frequency (the frequency with the largest Q value)
Generally, the larger the inductance value, the larger the corresponding DC resistance; the larger the inductance value, the smaller the corresponding resonant frequency; the larger the inductance value, the smaller the corresponding rated current.
*Magnetic bead knowledge:
Magnetic beads are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses.
Below the turning point frequency, the magnetic beads are inductive and reflect noise; above the turning point frequency, the magnetic beads are resistive, and the magnetic beads absorb noise and convert it into heat energy.
*Electrostatic discharge
When designing PCB, ESD protection should be considered. Routing should follow the horizontal and vertical routing directions. Routing should be as thick as possible if space allows. Do not arrange noise-sensitive signals such as clock signals and reset signals at the edge of the PCB. When the PCB consists of multiple layers, sensitive routing should have a good reference ground plane as much as possible. For filters, optical couplers, weak signal routing, etc., the routing spacing should be increased as much as possible. Long-distance traces need to be filtered. According to ESD protection, shielding covers should be added appropriately.
For board design, if the middle two layers are power layers and ground layers, set indentations to reduce electromagnetic radiation.
In actual PCB design, there are two main routing models: microstrip and stripline. Microstrip is a signal line running on the top or bottom layer of the circuit board, and stripline is a signal line running on the inner layer of the circuit board.
Several suggestions for handling serpentine lines:
(1) Try to increase the distance (S) between parallel segments to at least 3H, where H refers to the distance from the signal routing to the reference plane. In layman's terms, it means taking a big detour. As long as S is large enough, the mutual coupling effect can be almost completely avoided.
(2) Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the generated crosstalk will reach saturation.
(3) The signal transmission delay caused by the serpentine line of the stripline or buried microstrip line is smaller than that of the microstrip line. In theory, the stripline will not affect the transmission rate due to differential mode crosstalk.
(4) For signal lines with high speed and strict timing requirements, try not to use serpentine lines, especially in a small range.
(5) If space permits, serpentine wiring at any angle can be used to effectively reduce mutual coupling.
(6) In high-speed PCB design, serpentine lines have no so-called filtering or anti-interference capabilities and may only reduce signal quality. Therefore, they are only used for timing matching and have no other purpose.
(7) Sometimes you can consider spiral routing for winding. Simulation shows that its effect is better than ordinary serpentine routing.
(8) The corners of the serpentine line use 45° corners or rounded corners.
This article explores in detail the design process, key elements, and importance of pcb schematics in modern electronic devices. By analyzing the basic steps of schematic design, common problems, and their solutions, we emphasize the role of standardized design and tool use in improving design efficiency and reliability.