Sunday, January 16, 2011

【 Weak current College 】 RF wireless RF circuit design problems and design principles 】

Summary: details the RF circuit design for common problems and their solutions.
Keywords: PCB; radio frequency RF Circuit Design;;

1 Introduction
Radio frequency (RF) PCB design, in the current open publication of theory has a lot of uncertainty, often portrayed as a "black art". Typically, the following frequency bands for Microwave circuits (including low-frequency and low frequency digital circuit), the comprehensive knowledge of all kinds of design principle-based planning is the guarantee of successful design at once. For microwave frequency and high frequency of more than PC class digital circuits. You will need 2 ~ 3 version of the PCB can guarantee the quality of the circuit. For microwave band RF Circuit above. tend to need more versions: PCB design and continuous improvement, and has considerable experience of the premise. Donate and RF power design problems.

2RF design FAQ
2.1 digital circuit module and analog circuit interference between modules
If analog circuit (RF) and digital circuits work alone, may their work well. However, once they are placed on the Board in the same block, use the same power to work together, the entire system is likely to become unstable. This is mainly because the digital signal frequently to the ground and is power (> 3V), and cycle between swing are very short, often nanosecond. Due to the large amplitude and short switching time. Make these digital signal contains a large and independent of the switching frequency of high frequency components. In the analog portion, from wireless tuned circuit to the wireless device to receive part of the signal generally less than l μ V. Therefore digital signal and RF signal difference will reach 120dB. Obviously, if you cannot enable digital signal and RF signal well separated. Faint radio signals may be damaged, this wireless devices work performance deterioration or even completely unable to work.

2.2 powered noise interference
RF circuits forpower supply noise quite sensitive, especially for high-frequency voltage spikes and other harmonic. Micro-controller on each internal clock cycle short sudden inhalation most current, this is due to the modern microcontroller using CMOS process. As a result. Suppose a microcontroller to lMHz internal clock frequency is running, it will draw the extraction from power frequency electric current. If you do not take the appropriate power supply decoupling. caused by power lines will be the voltage Burr. If the voltage circuit RF part Burr reach the power PIN, serious and may result in failure of work.

2.3 unreasonable ground wire
If the RF Circuit ground mishandled, can produce some strange phenomenon. For digital circuit design, even if no ground wire layer, most digital circuit function is also good. While in RF band, even if a very short period of ground will be the same as the inductor. A rough calculation, length per mm of inductance about lnH, 433MHz 10toniPCB lines when they are about 27 Ω resistance. If you do not use the ground floor, most of the Earth will be longer, the circuit will not be able to have the design characteristics.

2.4 antenna on other analog circuits section of the radiated interference
PCB circuit design, the Board usually have other analog circuit. For example, many of the circuits on the die, number conversion (ADC) or number/die converter (DAC). RF transmitter antenna emits high frequency signal could reach the ADC analog input terminal. Because any circuit lines are possible such as antennas, RF signal sent or received. If the ADC input processing is not reasonable, RF signal possible in ADC input of ESD diode from stress. Thus giving rise to ADC.

3RF circuit design principles and programmes
3.1RF layout concept
In the design of RF layout, you must first meet the following general principles:
(1) to the extent possible, the high-power RF amplifiers (HPA) and low noise amplifier (LNA) separates, simply, is to let the high power RF transmitter away from low-power RF receiver circuit:
(2) ensure that the PCB Board high power district has at least one block, the best free hole, of course, the larger the better foil area;
(3) of the circuit and power decoupling is also extremely important;
(4) RF output from the RF input is usually required;
(5) sensitive analog signal should as far as possible away from the high-speed digital signal and RF signals.

3.2 physical partitions and electrical partitioning design principles
Designing partitions can be decomposed into physical partitions and electrical Division. Physical partitioning is primarily concerned with components layout, orientation, and shielding; electrical Division can continue into power distribution, RF alignment, sensitive circuits and signals as well as grounding, etc.

3.2.1 physical partition principles
(1) where the layout of components. Components layout is to achieve an excellent key RF design. most effective technology was first fixed in RF path components and adjust its orientation, so that the length of the RF path to minimize the input from the output. And as far as separation of high power circuit and low power circuits.
(2) PCB stacked design principles. The most effective method of circuit boards stacked is the primary ground (main) in the surface of the second layer, and where possible will RF line layout in surface. The RF path through-hole size reduced to a minimum, this not only reduces the path inductance, and can also reduce the main ground of solder joints, and can reduce RF energy leakage to other regions in the stack Panel.

(3) radio frequency devices and RF cabling layout principles. In physical space, such as multistage amplifier is usually sufficient to linear circuits will be multiple RF zone separation between, diplexers, mixers and intermediate frequency amplifier/mixer often have multiple RF/IF signals interfere. therefore must be carefully this impact is minimized. RF and IF trace should be crossed, and the interval as possible in their land. Correct RF path on the whole block of the PCB performance is important, this is usually in components layout cell phone PCB design covers most of the time.
(4) reduce the high/low power devices interfere with coupling design principles. In cellular phone, PCB, can often be low noise amplifier circuit on one side of a PCB, and high power amplifier on the other side, and eventually through the duplexer put them on the same surface is connected to the RF and baseband processor-end of the antenna. To use techniques to ensure that via will not make RF energy from one side of the plate is passed to the other side, common technique is to use the blind side. Can be arranged through holes in the PCB Board second surface are not subject to RF interference zone to the adverse effects of through-hole to minimize.

3.2.2 electrical Division principles
(1) the principle of power transmission. Cellular phone most of the circuit of the direct current are quite small, so the wiring width is usually not a problem. However, must be a high-power amplifier power separate set one as the current line width to be transferred to minimize pressure drop. In order to avoid too much current consumption, need to use more than one hole to the electric current is passed from one layer to another layer.
(2) high power devices for power decoupling. If you are not in high-power amplifier power pins side to full decoupling, high power radiation noise will be to block Board and bring a variety of issues. High power amplifier is quite critical, ground often need to design a metal shielding enclosures.
(3) RF input to output isolation principle. In most cases, the same key is to ensure that RF output from the RF input. This also applies to amps/buffers/filters. In the worst case, if the amplifier and the output buffer to the appropriate phase and amplitude feedback to their input, then they may have self-excited oscillation. In the best case, they will be able at any temperature and voltage conditions and stable work. As a matter of fact. They may become unstable, and noise and intermodulation signal added to RF signals.
(4) to filter the input and output isolation principle. If the RF signal line had to filter input around back to the output, so this may severely damage the filter band pass characteristic. In order to make the input and output a good isolation. First of all must be arranged in a circle around the filter. Second filter lower regions to lay out a piece of land, and land surrounding the primary filter. The need to pass through the filter of signal lines as far as possible away from the filter PIN is also a good way. In addition, the block on the various parts of the Earth have to be very careful, or you may introduce in the perception of an undesired coupled channel.
(5) digital and analog circuit isolation. All PCB design in the digital circuit from the analog circuit is a general principle, it also applies to RFPCB design. Public analog and used to mask and separated from the signal line and typically is equally as important, due to the negligence of design changes will likely cause nearly completed design also must forklift. The same should make RF line away from the analog circuit and some very critical digital signal. all RF alignment, pads and components around should fill in the ground as much as possible tongpi. as far as possible, and connected to the main land. If the RF alignment must be crossed the signal line, then as far as they go along RF alignment layout layer and the main land-attached. If it is not possible, make sure they are crossed this to capacitive coupling is minimized, while at the same time as possible in every RF alignment around some more, and get them connected to the main land. In addition. The parallel between the RF alignment to minimize distance makes sense to minimize coupling.
4 closing
The rapid development of RF IC is engaged in various wireless communication engineers provides broad prospects. But at the same time, radio frequency circuit design requires designers to have a certain experience and engineering design capability. This article summarizes some experience can help RF IC developers shorten the development cycle, avoid taking unnecessary detours, saving manpower and resources.

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