With the development of electronic technology, PCB (printed circuit board) has developed rapidly. Designers engaged in high-frequency PCB must have the corresponding basic theoretical knowledge, but also have a wealth of experience in the production of high-frequency PCB. That is to say, both the drawing of schematic diagram and the design of PCB should be considered from the high-frequency working environment in which it is located, so as to design a relatively ideal PCB. Based on ProtelSE, some problems in the design of high frequency PCB are studied in this paper.
1. Layout design
Although Protel has the function of automatic layout, it cannot completely meet the work needs of high frequency circuits. Usually, based on the experience of designers, the position of some components should be optimized and adjusted by manual layout method according to specific conditions, and then the overall design of PCB should be completed by combining with automatic layout. Whether the layout is reasonable or not directly affects the life of the product, stability, EMC (electromagnetic compatibility), etc., must be from the overall layout of the circuit board, wiring connectivity and PCB manufacturability, mechanical structure, heat dissipation, EMI(electromagnetic interference), reliability, signal integrity and other aspects of comprehensive consideration.
In general, the fixed components related to the mechanical size are placed first, then the special and larger components are placed, and finally the small components are placed. At the same time, the requirements of wiring should be taken into account. The placement of high-frequency components should be as compact as possible, and the wiring of signal lines should be as short as possible, so as to reduce the cross interference of signal lines.
1.1 placement of positioning plug-ins related to mechanical dimensions
Power sockets, switches, PCB interfaces, indicators, etc. Are mechanical dimensions related to the location of the plug. Usually, the interface between the power supply and the PCB should be placed at the edge of the PCB with a spacing of 3 mm ~ 5 mm from the edge of the PCB. The indicating led should be placed exactly as required; Switches and fine-tuning components such as adjustable inductors and resistors should be placed near the edge of the PCB for easy adjustment and connection. Components that require frequent replacement must be placed in fewer locations to facilitate replacement.
1.2 placement of special components
High power tube, transformer, rectifier tube and other heating devices generate more heat when working at high frequency, so ventilation and heat dissipation should be fully considered in layout, and such components should be placed in a place where air is easy to circulate on the PCB. Radiators should be installed in high-power rectifier and regulating tubes and should be kept away from transformers. The thermophiliac elements such as electrolytic capacitors should also be kept away from the heating element, otherwise the electrolyte will be dried, resulting in an increase in its resistance, poor performance, affecting the stability of the circuit.
Malfunctions prone components, such as regulating tubes, electrolytic capacitors, relays, etc., should be placed with the convenience of maintenance in mind. For the test points that need to be measured frequently, attention should be paid to ensuring that the test bar can be conveniently contacted when arranging the components.
Because a 50 Hz leakage magnetic field is generated inside the power supply device, it can interfere with the low frequency amplifier when it is intersected with parts of the amplifier. Therefore, they must be isolated or masked. It is better to arrange all levels of the amplifier in a straight line according to the schematic diagram. The advantage of this arrangement is that the grounding current of all levels flows in the closed state of the amplifier and does not affect the work of other circuits. The input and output stages should be as far apart as possible to reduce the parasitic coupling between them.
Considering the signal transmission relationship between the functional circuits of each unit, the low frequency circuit and the high frequency circuit should be separated, and the analog circuit and the digital circuit should be separated. The IC should be placed in the center of the PCB & PCBA to facilitate wiring of each pin to other devices.
Inductors, transformers and other devices have magnetic coupling. They should be placed orthogonal to each other to reduce the magnetic coupling. In addition, they all have strong magnetic field and should be surrounded by appropriate large space or magnetic shielding, in order to reduce the impact on other circuits.
Appropriate high frequency decoupling capacitors should be provided in key parts of PCB. For example, an electrolytic capacitor of 10 F ~ 100 F should be connected at the input end of PCB power supply, and a ceramic capacitor of about 0.01 pF should be connected near the power pin of integrated circuit. Some circuits also have appropriate high or low frequency chokes to minimize the impact between high and low frequency circuits. This should be considered in schematic design and drawing, otherwise it will affect the circuit performance.
The spacing of components should be appropriate to allow for the possibility of breakdown or ignition between them.
For amplifier with push-pull circuit and bridge circuit, the symmetry of electronic parameters and structure of components should be paid attention to, so as to make the distribution parameters of symmetrical components as consistent as possible.
After the manual layout of the main components is completed, the components should be locked so that they do not move during the automatic layout. To lock a component and no longer move it, execute the Edit change command or check Locked at Properties Locked on the component.
1.3 placement of common components
For common components, such as resistance, capacitance, etc., the way of automatic layout can be adopted from several aspects such as the neat arrangement of components, the space occupied, the connectability of wiring and the convenience of welding.
2. Wiring design
Wiring is the general requirement of high frequency PCB design on the basis of reasonable layout. Wiring includes automatic wiring and manual wiring. In general, regardless of the number of critical signal lines, manual wiring of these signal lines should be carried out first. After the wiring is completed, the signal lines should be carefully checked and fixed after passing the inspection, and then the other wiring should be automatically wired. Namely USES the manual and the automatic wiring union to complete the PCB wiring.
Special attention should be paid to the following aspects in the wiring process of high frequency PCB.
2.1 wiring trend
Circuit wiring is best according to the flow of signals using the straight line, you need to turn to complete the 45 ° line or the circular arc curve, so that we can reduce the high frequency signal launch and mutual coupling. The wiring of high frequency signal lines should be as short as possible. According to the working frequency of the circuit, the length of the signal line should be selected reasonably, so that the distributed parameters can be reduced and the loss of the signal can be reduced. When making double panels, it is best to lay the wires perpendicular, oblique or curved to each other on two adjacent planes. Avoid parallelism, which reduces interference and parasitic coupling.
High frequency signal lines and low frequency signal lines should be separated as far as possible. Shielding measures should be taken when necessary to prevent mutual interference. For the weak signal input terminal, which is easy to be interfered by the external signal, the ground wire can be used to shield it or to shield the high-frequency connector. Parallel routing should be avoided at the same level, otherwise distributed parameters will be introduced and influence the circuit. If it cannot be avoided, an earthed copper foil can be introduced between two parallel wires to form an isolation wire.
In a digital circuit, differential signal lines should be paired so that they are as close to each other as possible and not too different in length.
2.2 wiring form
In the wiring process of PCB, the minimum wire width is determined by the adhesion strength between the conductor and the insulation substrate and the current strength flowing through the conductor. When the thickness of copper foil is 0.05mm and the width is 1mm ~ 1.5mm, the current can pass through 2A. Temperature is not higher than 3 ℃, besides some special go line, at the same level of other wiring width should be as consistent as possible. The spacing of wires in high frequency circuits will affect the size of distributed capacitance and inductance, thus affecting the loss of signals, circuit stability and signal interference. In high-speed switching circuits, the spacing of wires affects the transmission time of signals and the quality of waveforms. Therefore, the minimum spacing of the wiring should be greater than or equal to 0.5mm, as long as it is allowed,PCB wiring had better use a wider line.