Although Protel has the function of automatic layout,it cannot fully meet the working requirements of high-frequency circuits.Generally,according to the designer's experience,according to the specific situation,the position of some components should be optimized by manual layout first,and then the overall design of the PCB should be completed according to the automatic layout.The rationality of the layout directly affects the life,stability,EMC(electromagnetic compatibility)of the product,etc.It must be based on the overall layout of the substrate,the communication of the wiring and the manufacturability of the PCB,mechanical structure,heat dissipation,EMI(electromagnetic interference),and reliability.Comprehensive considerations such as performance and signal integrity.
Generally speaking,first place the fixed-position parts related to the mechanical size,then place the special large parts,and place the small parts.At the same time,the wiring requirements should be considered,the placement of high-frequency components should be as compact as possible,and the wiring of signal lines should be as short as possible to reduce cross interference of signal lines.
1.1 The placement of the positioning plug-in related to the mechanical size.
Power sockets,switches,interfaces between PCBs,indicator lights,etc.are all positioning plug-ins related to mechanical dimensions.Usually,the interface between the power supply and the PCB is placed on the edge of the PCB,indicating that there is a 3mm~5mm interval from the edge of the PCB.Switches and fine-tuning components that must be placed correctly when necessary,such as adjustable inductors,adjustable resistors,etc.In the position close to the edge of the PCB,the parts that need to be replaced frequently must be placed in a position with few parts,which is easy to replace.
1.2 Placement of special parts.
High-power tubes,transformers,rectifier tubes and other heating devices generate more heat when they work in high-frequency conditions.Therefore,ventilation and heat dissipation should be fully considered in the layout,and such devices should be placed in a place where the PCB can easily circulate.High-power rectifier tubes and adjustment tubes must be equipped with radiators,away from the transformer.Electrolytic capacitors and other parts that are afraid of heat must also be kept away from heat-generating parts.Otherwise,the electrolyte will dry,the resistance will increase,and the performance will be poor,which will affect the stability of the circuit.
The parts that are prone to failure,such as adjustment tubes,electrolytic capacitors,relays,etc.,should be placed in consideration for easy maintenance.For the test points that often need to be measured,please note that the test rod is easy to touch when configuring the components.
Because the 50Hz leakage magnetic field is generated inside the power supply device,when it is cross-linked with a part of the low-frequency power amplifier,it will cause interference to the low-frequency power amplifier.Therefore,it must be isolated or shielded.All levels of the power amplifier are arranged in a straight line according to the schematic diagram.The advantage of this arrangement is that the ground current of each level is closed and flows at this level,and does not affect the operation of other circuits.The input stage and output stage should be as far away as possible to reduce parasitic coupling interference.
Considering the signal transfer relationship between the functional circuits of each unit,low-frequency circuits and high-frequency circuits,analog circuits and digital circuits should also be separated.The integrated circuit should be placed in the center of the PCB to facilitate the wiring connection of each pin with other devices.
Inductors,transformers and other equipment have magnetic coupling,and they should be placed orthogonally to each other to reduce magnetic coupling.In addition,they all have a strong magnetic field,and there are appropriate large spaces or magnetic shields around them to reduce the impact on other circuits.
The important part of the PCB is equipped with an appropriate high-frequency bonding capacity,for example,an electrolytic capacity of 10μF~100μF is connected to the input terminal of the PCB power supply,and a porcelain capacity of about 0.01pF is connected to the power supply pin of the integrated circuit.In order to reduce the influence between high and low frequency circuits,some circuits must be equipped with appropriate high frequency or low frequency electric coils.This should be considered when designing and drawing the schematic,otherwise it will also affect the performance of the circuit.
The spacing of the components must be appropriate,and the spacing must be considered whether it is damaged or ignited.
Amplifiers including push-pull circuits and bridge circuits should pay attention to the symmetry of the electrical parameters of the components and the symmetry of the structure when configuring,and try to make the distribution parameters of the symmetrical components consistent.
After completing the manual layout of the main components,please use the method of component locking to avoid moving these components during automatic layout.In other words,execute the Editchange command,or select Locked in the properties of the component,you can lock it without moving.