Typically, for circuits (including low frequency and low frequency digital circuits) of microwaves (including low frequency and low frequency digital circuits), carefully planning is a guarantee for successful design under the premise of various design principles. For microwave or higher band and high-frequency PC type digital circuit, it takes 2 to 3 versions of PCB to ensure circuit quality.
Frequently Asked Questions in RF Wireless RF Circuit Design
RF (RF) PCB design has many uncertainties in the theory of publication, often described as a "black art". Typically, for circuits (including low frequency and low frequency digital circuits) of microwaves (including low frequency and low frequency digital circuits), carefully planning is a guarantee for successful design under the premise of various design principles. For microwave or higher band and high-frequency PC type digital circuit, it takes 2 to 3 versions of PCB to ensure circuit quality. For the RF circuit of the microwave or more frequency band, more versions of PCB design are often required and constantly improved, and it is prequel with considerable experience. This can be seen that there is difficulties in RF design.
Interference between digital circuit modules and analog circuit modules
If the analog circuit (RF) and the digital circuit work separately, it may work well. However, once the two is placed on the same power supply to work with the same power supply, the entire system is likely to be unstable. This is mainly because digital signals are frequently swung between ground and positive power (greater than 3 V), and the cycle is particularly short, often nanosecond. Due to a large amplitude and shorter switching time, these digital signals contain a large amount and independent of the high frequency component of the switching frequency. In the analog portion, the signal transmitted from the wireless tuning circuit to the wireless device receiving portion is generally less than 1 μV. Therefore, the difference between the digital signal and the RF signal reaches 120 dB. Obviously, if the digital signal cannot be made well with the RF signal well, the weak radio frequency signal may be destroyed, so that the operating performance of wireless devices will deteriorate, even completely.
Noise interference from power supply
RF circuits are quite sensitive to power supply noise, especially for glitch voltages and other high-frequency harmonics. The microcontroller will suddenly suck most of the current in a short period of time during each internal clock cycle, because the modern microcontroller is manufactured by CMOS process. Therefore, it is assumed that a microcontroller operates at an internal clock frequency of 1 MHz, which will extract current from the power source with this frequency. If a suitable power supply is not taken, it will cause voltage burrs on the power line. If these voltage burrs arrive at the power pins of the circuit RF part, it may cause the work to fail when it is severe.
Unreasonable ground
If the groundwork of the RF circuit is improper, some strange phenomena may occur. For digital circuit design, most digital circuit functions are good even if there is no ground layer. In the RF band, even if a short ground line is like in the inductor. Roughly calculated, the inductance of about 1 NH and 433 MHz per mMPCB is about 27 Ω. If the ground layer is not used, most of the grounds will be longer, and the circuit will not be designed.
Antenna radiation interference to other analog circuit parts
In the PCB circuit design, there are usually other analog circuits on the board. For example, there are many modes / digital or digital / analog converters (DACs) on many circuits. The high frequency signal emitted by the radio transmitter antenna may reach the analog input of the ADC. Because any circuit lines may be issued or received as antennas. If the processing of the ADC input is unreasonable, the RF signal may be self-excited in the ESD diode input to the ADC, thereby causing the ADC deviation.
RF circuit design principle and program
RF layout concept
When designing the RF layout, you must give priority to the following general principles:
(1) Separate high-power RF amplifier (HPA) and low noise amplifier (LNA) as much as possible, simply, the high power RF transmit circuitry away from low power RF receiving circuits;
(2) Make sure there is at least one of the high-power regions on the PCB board, and there is no overhead, of course, the larger the copper foil area is, the better;
(3) The circuit and power supply are also extremely important;
(4) RF output usually needs to be kept away from RF;
(5) Sensitive analog signals should be as close as possible to high-speed digital signals and RF signals.
Physical partition and electrical partition design principles
The design partition can be broken down into physical partitions and electrical partitions. The physical partition mainly involves components layout, direction and shielding. The electrical partition can continue to decompose into partitions of power distribution, RF traces, sensitive circuits, and grounding.
Physical partition principle
(1) The principle of component location layout. Component layout is a key to implementing an RF design. The effective technology is to first secure components on the RF path and adjust their direction to reduce the length of the RF path to make the input away from the output, and separated as far as possible Power circuit and low power circuit.
(2) PCB stacking design principles. The effective circuit board stacking method is to arrange the main ground (primary) in the second layer under the surface layer and arrange the RF line on the surface layer as much as possible. Reduce the vias size on the RF path to reduce the path inductance, but also reduce the solder spots on the main ground, but also reduce the opportunity of RF energy leaks to other regions within the laminate.
(3) RF device and its RF wiring layout principle. On physical space, linear circuits such as multi-stage amplifiers are typically sufficient to isolate multiple RF regions, but duplexers, mixers, and intermediate frequency amplifiers / mixers always have multiple RF / IF. The signals are interfered with each other, so this effect should be carefully reduced. RF and IF traces should be as cross as much as possible and as much as possible between them. The correct RF path is very important to the performance of the entire PCB, which is the reason for the component layout usually accounts for most of the time in cellular phone PCB design.
(4) Reduce design principles of high / low power devices interference coupling. On the cellular phone PCB, the low noise amplifier circuit can be placed on a side of the PCB, and the high power amplifier is placed on the other side and the duplexer is connected to the RF terminal and the baseband processor on the same surface through the duplexer. On the antenna of the terminal. To ensure that the through hole does not pass the RF energy from the side of the plate to the other, the commonly used technology is the blind hole in the second side. The adverse effects of the through hole can be reduced by disconnecting the through hole in the PCB board two regions that are not subject to RF interference.
Electrical partition principle
(1) Principle of power transmission. The DC current of most of the circuits in the cellular phone is quite small, so the wiring width is usually not a problem. However, a high-power amplifier must be set separately as possible to reduce the transmission pressure to. In order to avoid too much current loss, multiple through holes need to be passed to pass the current from a certain layer to another.
(2) Power supply decoupling of high power devices. If it is not possible to adequate decoupling on the power-on power pin end of the high power amplifier, high power noise will radiate to the entire board and bring a variety of problems. The high-power amplifier is relatively critical, often designed a metal shield.
(3) RF input / output isolation principle. In most cases, the same critical is to ensure that the RF output is remote from the RF input. This also applies to amplifiers, buffers, and filters. In bad cases, if the output of the amplifier and the buffer feeds back to their input at an appropriate phase and amplitude, they may generate self-excitation oscillation. In case, they will be able to operate stably at any temperature and voltage conditions. In fact, they may become unstable and add noise and intermodulated signals to the RF signal.
(4) Filter input / output isolation principle. If the RF signal line has to wind back the output from the input of the filter, this may seriously impair the band pass characteristics of the filter. In order to make the input and output good isolation, it is first necessary to arrange a circle around the filter, and the lower layer region of the second filter is also arranged and connected to the main land around the filter. It is also a good way to keep the signal lines that need to pass through the filter. In addition, the grounding of each place on the entire board is very careful, otherwise it may introduce a unwanted coupling channel in an unknown.
(5) Digital circuit and analog circuit isolation. In all PCB designs, the digital circuit is kept away from the analog circuit as a general principle, which is equally applicable to the RF PCB design. Public simulation and use in the ground and spaced apart signal lines are usually equally important. Design changes caused by negligence will result in the upcoming design. It is also possible to keep the RF line away from the analog lines and some critical digital signals, all RF traces, pads and components should be filled with as much as possible as much as possible and connected to the main place. If the RF trace must pass through the signal line, it is as possible to arrange a layer along the RF trace along the RF trace. If it is impossible, be sure to ensure that they are cross-crossing, which can be coupted, while as much as possible around each RF trace, and connect them to the main ground. In addition, the distance between parallel RF traces will be reduced to enable sensitive coupling to.
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Source: Wiku Electronic Market Network
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