With the development of communication technology , use of hand-held radio frequency circuit technology more widely , such as: radio pager , mobile phone , a wireless PDA, wherein the RF circuit performance directly affects the quality of the product . A most important feature of these products is smaller handheld , meaning the density miniaturized components is large, which makes the components ( including SMD, SMC, die , etc. ) interfere with each other very prominent . Electromagnetic interference signal if handled improperly , could cause the entire electrical system does not work, therefore , how to prevent and suppress electromagnetic interference, improve electromagnetic compatibility, has become a very important when designing RF PCB issue . The same circuit , different PCB design structure , its performance will vary greatly. This discussion uses Protel99 SE software for handheld products, RF PCB design, if the circuit to maximize performance indicators to achieve electromagnetic compatibility requirements.
1 . Select the sheet
A printed circuit board comprising a substrate of organic and inorganic class two categories . The most important properties of the substrate is a dielectric constant εr, the dissipation factor (or dielectric loss ) tanδ, and the thermal expansion coefficient of moisture absorption CET . Where εr affect the circuit impedance and signal transmission rate. For high-frequency circuits , the dielectric constant of tolerance is more crucial primary consideration should be chosen substrate dielectric constant tolerances .
2 . PCB design flow
Since Protel99 SE software and other software using Protel 98 different, so , first briefly discuss the use of Protel99 SE software for PCB design process .
① Since Protel99
SE uses a project (PROJECT) database schema management, under Windows 99 is implicit , it should first establish a key database file for managing the design of the circuit schematic and PCB layout .
② schematic design . To be able to achieve a network connection between the conducting principles of design, the components used in the component library must exist , and shall make the necessary components coexist in SCHLIB the storage file . Then , simply call the desired components from the component library, and connect to a circuit diagram of the design.
③ After the completion of the schematic design , can form a network table in preparation for the use of PCB design.
④ PCB design.
OK a.PCB shape and size. PCB in accordance with the designed location of the product , the size of the space , and the shape and the other components used to determine the shape and size of the PCB . In MECHANICAL LAYER PCB layer to draw the shape with PLACE TRACK command.
b. according to the requirements of SMT , making the positioning hole on the PCB , as the eyes , the reference points.
c. components production. If the database needs to use some components do not exist in the particular components in the layout must first be produced before the components . In the process of making components in a relatively simple Protel99 SE , select "DESIGN" menu "MAKE LIBRARY" command after entering the components making window , then select "TOOL" menu "NEW COMPONENT" command element can be carried out design of the device. Then only part of the actual shape and size of the layer PLACE PAD TOP LAYER commands such as shown at a predetermined position corresponding to a desired edit pad and a pad ( pad including shape, size, diameter size and angle, etc. , should also mark the corresponding pin pad name ) , then PLACE TRACK command to draw the largest components in the TOP OVERLAYER shape layer, taking the name of a component into the component library can be.
d. After finished components , layout and wiring , the two parts are discussed in detail below .
must be checked after e. above process is completed. This aspect includes a circuit schematic checks, on the other hand must also check the match and assembly problems between each other. Schematic manual inspection can be checked , may be used ( network formed by a network schematic and PCB can be compared formation ) to check the network automatically .
f. After the check , the file archive output. In Protel99 SE must use the "FILE" option in the "EXPORT" command , the files stored in the specified path and file ("IMPORT" command to a file is transferred to Protel99 SE in ) . Note : After Protel99 SE in "FILE" option in the "SAVE COPY AS ..." command is executed, the selected file name in Windows 98 is not visible , so the Explorer can not see the file. This 98 in Protel "SAVE AS ..." function is not exactly the same.
3. Layout components
Since SMT generally use infrared heaters to heat welding the welding components , the layout of components and thus affect the quality of solder joints , thereby affecting the yield of the product. For RF PCB design , the electromagnetic compatibility requirements of each circuit module try not produce electromagnetic radiation, and has some immunity to electromagnetic interference , so the layout of components but also directly affects the interference and noise immunity circuit itself it is also directly related to the performance of the designed circuit. Therefore, when RF PCB design in addition to considering the general layout of the PCB design , the principal must consider how to reduce radio frequency interference between the various parts of the circuit , the circuit itself how to reduce interference to other circuits and the circuit itself the anti-jamming capability . According to experience, the effect is good or bad for the RF circuit board itself depends not only on the RF performance metrics , it also depends on the interaction with the majority of CPU processing plates , therefore, making PCB design, reasonable layout is very important.
General principles layout: components are arranged in the same direction as far as possible , by selecting the PCB into the direction of the molten tin system to reduce or avoid adverse welding phenomenon ; according to experience at least between 0.5mm pitch components have to meet the components of molten tin request if the PCB board space allowed , the pitch components should be as wide as possible . For dual side panel should generally be designed for SMD and SMC components, discrete components , compared with the other side .
Layout should be noted:
* First determine the interface components or systems other PCB board PCB board position , care must be taken with the problem of the interface between components ( such as the direction of components , etc. ) .
* Because of the volume of palm products are small, very compact arrangement between the components , so for larger components, priority must be given to determine the appropriate location , and to consider the issue with each other.
* Careful analysis of the circuit configuration of the circuit block processing (e.g., high-frequency amplifier circuit, a mixer circuit and a demodulation circuit, etc. ) , as will be strong and weak electrical signal separate circuit and the digital signal separated from the analog signal circuit , perform the same function in the circuit should be arranged within a certain range, thereby reducing the signal loop area ; filtering network for each part of the circuit must be connected to the nearest , so not only can reduce the radiation, and can reduce the chance of interference , according to the circuit the anti-jamming capability .
* Based on unit circuit in use with different sensitivities to electromagnetic compatibility group . For the portion of the circuit components susceptible to interference in the layout should also try to avoid interference sources ( such as data processing from the CPU board interference, etc. ) .
4. wiring
After completion of the basic layout of components , you can start wiring up . The basic principles of cabling for : packing density licensed under the circumstances, try to use low-density wiring design, and signal traces consistent thickness , is conducive to impedance matching.
For unreasonable RF circuit design , signal line direction, width , line spacing , which may cause cross interference signal between the signal transmission line ; addition, the system itself but also the presence of noise power , they must be taken into account when designing RF PCB reasonable cabling.
When wiring , all traces away from the PCB border ( about 2mm), so as to avoid breakage or disconnection hidden when the PCB production. To make the power cord can be wide to reduce loop resistance , while the power lines in the same direction toward the ground , and data transfer to improve anti-jamming capability ; the cloth signal lines should be as short as possible and to minimize over- hole number ; connections between the various components as short as possible to minimize electromagnetic interference and mutual distribution parameters ; for the amount of signal lines should mutually incompatible away , but try to avoid parallel lines , but both sides in the forward vertical signal line application interoperability ; wiring in the corner of the address side should need to 135 ° angle appropriate to avoid a right angle turn .
When wiring lines and pads directly connected should not be too wide , the alignment should be left unconnected components to avoid short circuit ; through-hole components are not painted on FIRE , and should be kept away from unconnected components, in order to avoid the production of appeared in weld, even welding , short circuit and so on.
In RF circuit PCB design, proper power and ground wiring is particularly important , reasonable design is the most important means to overcome electromagnetic interference. A considerable number of sources of interference on the power supply and ground through the PCB generated , wherein the ground noise caused max.
Ground easily formed mainly the presence of electromagnetic interference on the ground impedance . When a current flows through the ground , it will produce a voltage at the ground line , resulting in ground loop currents formed in the ground loop interference . When multiple circuits share some ground , it will form a common impedance coupling , resulting in the so-called ground noise. Therefore , when an RF PCB ground wiring should be done:
* First, the circuit block processing , RF circuitry can basically be divided into high-frequency amplifier , mixer , demodulation, and other parts of the vibration , the potential to provide a common reference point that each module ground circuits for each respective circuit module , so that the signal can be transmitted between the different circuit modules. Then , summarized in RF PCB ground access to places that are summarized in the total ground . Since there is only one reference point , there is no common impedance coupling exists, so there is no mutual interference.
* Digital and analog zone area as ground isolation , and digital and analog ground to be separated , and finally connected to power ground.
* Ground in various parts of the circuit should also pay attention to the internal single-point grounding principles , to minimize signal loop area , and with the address of the nearest appropriate phase filter circuit .
* In the space allows, among the best of each module can be isolated to the ground to prevent signal coupling effects between them .
5. Conclusions
The key RF PCB design is how to reduce the radiation and how to improve anti-jamming capability , reasonable layout and design of RF PCB layout is guaranteed . In this paper the method will help improve the reliability of RF circuit PCB design , to solve the problem of electromagnetic interference , and thus achieve the purpose of electromagnetic compatibility .
