"A surge in demand for Bluetooth devices, cordless phones and cellular phones is prompting Chinese electronic engineers to pay more and more attention to RF circuit design skills. The design of RF circuit board is the most troublesome part for design engineers. If you want to succeed at one time, careful planning and attention to detail are two key design rules that must be paid great attention to.
Radio frequency (RF) circuit board design is often described as a "black art" because there are still many uncertainties in theory, but this view is only partially correct. RF circuit board design also has many guidelines that can be followed and rules that should not be ignored. However, in practical design, the real practical skill is how to compromise these guidelines and rules when they cannot be accurately implemented due to various design constraints.
Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminates, wavelength and standing wave. However, this paper will focus on various issues related to RF circuit board zoning design.
Today's cellular phone design integrates everything in various ways, which is very unfavorable for RF circuit board design. Now the competition in the industry is very fierce. Everyone is looking for ways to integrate the most functions with the smallest size and the smallest cost. Analog, digital and RF circuits are tightly packed together, the space used to separate their problem areas is very small, and considering the cost factor, the number of circuit board layers is often minimized. It is incredible that the multi-purpose chip can integrate multiple functions on a very small bare chip, and the pins connecting the outside world are arranged very closely. Therefore, RF, if, analog and digital signals are very close, but they are usually electrically independent. Power distribution can be a nightmare for designers. In order to prolong battery life, different parts of the circuit work time-sharing according to needs, and the conversion is controlled by software. This means that you may need to provide 5 to 6 working power supplies for your cellular phone.
1、 RF layout concept
When designing RF layout, there are several general principles that must be met first:
Isolate the high-power RF amplifier (HPA) from the low-noise amplifier (LNA) as much as possible, that is, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. If you have a lot of physical space on your PCB, you can easily do this, but usually there are many components and the PCB space is small, so this is usually impossible. You can put them on both sides of the PCB, or let them work alternately rather than at the same time. High power circuits may sometimes also include RF buffers and voltage controlled oscillators (VCOs).
Ensure that there is at least a whole piece of land in the high-power area on the PCB. It is best that there are no vias on it. Of course, the more copper sheets, the better. Later, we will discuss how to break this design principle as needed and how to avoid the problems that may arise.
Decoupling chip and power supply is also very important. Several methods to realize this principle will be discussed later.
RF output usually needs to be far away from RF input, which will be discussed in detail later.
Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible.
2、 How to partition?
Design zones can be decomposed into physical zones and electrical zones. Physical zoning mainly involves the layout, orientation and shielding of components; Electrical partitions can be further divided into partitions of power distribution, RF wiring, sensitive circuits and signals, grounding, etc.
First, we discuss the physical partition problem. Component layout is the key to an excellent RF design. The most effective technology is to first fix the components on the RF path and adjust its orientation to minimize the length of the RF path, keep the input away from the output, and separate the high-power circuit and low-power circuit as far as possible.
The most effective circuit board stacking method is to arrange the main ground (main ground) on the second layer below the surface layer, and walk the RF line on the surface layer as much as possible. Minimizing the via size on the RF path can not only reduce the path inductance, but also reduce the false solder joints on the main ground, and reduce the chance of RF energy leakage to other areas in the laminate.
In physical space, linear circuits such as multistage amplifiers are usually enough to isolate multiple RF regions from each other, but duplexers, mixers and if Amplifiers / mixers always have multiple RF / if signals interfering with each other, so this effect must be carefully minimized. The routing of RF and if shall cross as far as possible and be separated by a piece of land as far as possible. The correct RF path is very important for the performance of the whole PCB, which is why the component layout usually accounts for most of the time in the design of cellular telephone PCB.
On the cellular telephone PCB, it is usually possible to place the low-noise amplifier circuit on one side of the PCB and the high-power amplifier on the other side, and finally connect them to the antenna at the RF end and the baseband processor end on the same side through the duplexer. Some skills are needed to ensure that the through holes do not transfer RF energy from one side of the board to the other. The common technology is to use blind holes on both sides. The adverse effect of the through-hole can be minimized by arranging the through-hole in the area where both sides of the PCB are free from RF interference.
Sometimes it is impossible to ensure sufficient isolation between multiple circuit blocks. In this case, it must be considered to use metal shield to shield RF energy in the RF area, but the metal shield also has problems, for example, its own cost and assembly cost are very expensive;
The irregular metal shield is difficult to ensure high precision in manufacturing, and the rectangular or square metal shield limits the layout of components; Metal shield is not conducive to component replacement and fault location; Since the metal shield must be welded to the ground and an appropriate distance must be maintained from the components, it needs to occupy valuable PCB space.
It is very important to ensure the integrity of the shield as much as possible. The digital signal line entering the metal shield should go through the inner layer as much as possible, and the PCB layer below the wiring layer is the layer. The RF signal line can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but more ground should be distributed around the gap as much as possible, and the ground on different layers can be connected together through multiple vias.
Despite the above problems, metal shields are very effective and often the only solution to isolate critical circuits.
In addition, proper and effective chip power decoupling is also very important. Many RF chips integrated with linear lines are very sensitive to power supply noise. Usually, each chip needs up to four capacitors and an isolation inductor to ensure that all power supply noise is filtered (see Figure 1).
The minimum capacitance value usually depends on its self resonant frequency and low pin inductance, and the value of C4 is selected accordingly. The values of C3 and C2 are relatively large due to their own pin inductance, so the RF decoupling effect is worse, but they are more suitable for filtering low-frequency noise signals. The inductance L1 prevents the RF signal from being coupled to the chip from the power line. Remember: all cabling is a potential antenna that can receive and transmit RF signals. In addition, it is necessary to isolate the induced RF signals from the critical lines.
The physical location of these decoupling elements is usually also critical, and Figure 2 shows a typical layout method. The layout principle of these important components is: C4 should be as close to the IC pin and grounded as possible, C3 must be closest to C4, C2 must be closest to C3, and the connection route between IC pin and C4 should be as short as possible. The grounding terminals of these components (especially C4) should usually be connected to the grounding pin of the chip through the next layer. The via connecting the components to the ground should be as close to the component pad on the PCB as possible. It is best to use the blind hole punched on the pad to minimize the inductance of the connecting line, and the inductance should be close to C1.
An integrated circuit or amplifier often has an open drain output, so a pull-up inductor is required to provide a high impedance RF load and a low impedance DC power supply. The same principle also applies to decoupling the power supply at the inductor end. Some chips need multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them respectively. If there is not enough space around the chip, you may encounter some problems.
Remember that inductors rarely lean together in parallel, because this will form a hollow core transformer and induce interference signals with each other, so the distance between them should be at least equal to the height of one of the devices, or arranged at right angles to minimize mutual inductance.
The principle of electrical zoning is basically the same as that of physical zoning, but it also includes some other factors. Some parts of modern cellular phones use different working voltages and are controlled by software to prolong the service life of the battery. This means that cellular phones need to run multiple power supplies, which creates more problems for isolation. The power supply is usually introduced from the connector and immediately decoupled to filter out any noise from the outside of the circuit board, and then distributed after passing through a group of switches or regulators.
The DC current of most circuits in cellular phones is quite small, so the routing width is usually not a problem. However, it is necessary to route a large current line as wide as possible for the power supply of the high-power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, multiple vias need to be used to transfer current from one layer to another. In addition, if it cannot be fully decoupled at the power pin end of the high-power amplifier, the high-power noise will radiate to the whole board and bring all kinds of problems. The grounding of high power amplifier is very important, and it is often necessary to design a metal shield for it.
In most cases, it is also critical to ensure that the RF output is away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the outputs of amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may produce self-excited oscillation. At best, they will be able to operate stably at any temperature and voltage. In fact, they may become unstable and add noise and intermodulation signals to the RF signal.
If the RF signal line has to be wound back from the input to the output of the filter, it may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, a circle of ground must be arranged around the filter, and then a piece of ground must be arranged in the lower area of the filter and connected with the main ground around the filter. It is also a good way to keep the signal line that needs to pass through the filter as far away from the filter pin as possible. In addition, the grounding of all parts of the whole board should be very careful, otherwise you may unknowingly introduce a coupling channel that you don't want to happen. Figure 3 illustrates this grounding method in detail.
Sometimes single ended or balanced RF signal lines can be selected. The principles of cross interference and EMC / EMI also apply here. Balancing RF signal lines can reduce noise and cross interference if they are routed correctly, but their impedance is usually relatively high, and it may be difficult to maintain a reasonable linewidth to obtain an impedance matching signal source, routing and load.
The buffer can be used to improve the isolation effect because it can divide the same signal into two parts and drive different circuits. In particular, the local oscillator may need a buffer to drive multiple mixers. When the mixer reaches the common mode isolation state at RF frequency, it will not work properly. The buffer can well isolate the impedance changes at different frequencies, so that the circuits will not interfere with each other.
Buffers are very helpful to the design. They can closely follow the circuit to be driven, so that the high-power output line is very short. Because the input signal level of buffers is relatively low, they are not easy to cause interference to other circuits on the board.
There are many very sensitive signals and control lines that need special attention, but they are beyond the scope of this paper. Therefore, this paper only discusses them briefly and will not describe them in detail.
Voltage controlled oscillators (VCOs) can convert changing voltage into changing frequency, which is used for high-speed channel switching, but they also convert trace noise on the control voltage into small frequency change, which adds noise to the RF signal. Generally speaking, after this level, you can no longer remove the noise from the RF output signal. So where is the difficulty? First, the desired bandwidth of the control line may range from DC to 2MHz, and it is almost impossible to remove such a wide-band noise by filtering; Secondly, the VCO control line is usually part of a feedback loop that controls the frequency. It may introduce noise in many places. Therefore, the VCO control line must be handled very carefully.
Ensure that the ground under the RF wiring is solid, and all components are firmly connected to the main ground and isolated from other wiring that may cause noise. In addition, to ensure that the power supply of the VCO has been fully decoupled, because the RF output of the VCO is often a relatively high level, the VCO output signal is easy to interfere with other circuits, so special attention must be paid to the VCO. In fact, VCO is often placed at the end of the RF area, and sometimes it needs a metal shield.
The resonant circuit (one for the transmitter and the other for the receiver) is related to VCO, but it also has its own characteristics. In short, the resonant circuit is a parallel resonant circuit with capacitive diodes, which helps to set the VCO operating frequency and modulate voice or data to the RF signal.
The design principles of all VCOs also apply to resonant circuits. Because the resonant circuit contains a considerable number of components, a wide distribution area on the board and usually operates at a high RF frequency, the resonant circuit is usually very sensitive to noise. The signals are usually arranged on the adjacent pins of the chip, but these signal pins need to cooperate with relatively large inductors and capacitors to work, which in turn requires that the positions of these inductors and capacitors must be close and connected back to a control loop sensitive to noise. It is not easy to do this.
Automatic gain control (AGC) amplifier is also an easy problem. There will be AGC amplifier in both transmitting and receiving circuits. AGC amplifiers usually can effectively filter out noise. However, because cellular phones have the ability to deal with the rapid change of transmitted and received signal strength, AGC circuits are required to have a fairly wide bandwidth, which makes it easy for AGC amplifiers in some key circuits to introduce noise.
The design of AGC circuit must comply with good analog circuit design technology, which is related to the short input pin of operational amplifier and short feedback path, both of which must be far away from RF, if or high-speed digital signal routing. Similarly, good grounding is also essential, and the power supply of the chip must be well decoupled. If it is necessary to take a long line at the input or output, it is better to take it at the output. Usually, the impedance of the output is much lower and it is not easy to induce noise. Generally, the higher the signal level, the easier it is to introduce noise into other circuits.
In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as far as possible, and it also applies
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