Today's cellular phone design in various ways to integrate the all things together, this is very bad for RF circuit design. Now the industry is very competitive, everyone is looking for a way to use the smallest size and the smallest cost integrated most functions. Analog, digital and RF circuit are tightly packed together and used to separate each problem areas of space is very small, and considering the cost factor, layer number of circuit board and often reduced to a minimum. Feel incredibly, multi-purpose chip can integrate a variety of functions in a very small on the bare chip, and the connection between external pins are packed very tightly, so the RF, the IF, analog and digital signals are very close, but they are usually on the electric is irrelevant. Power distribution may be a nightmare for designers, in order to extend battery life, different parts of the circuit is according to the need and time sharing, and by the software to control the conversion. This means that you may need to provide your cell phone power supply 5 to 6 kinds of work.
In the design of the RF layout, there are a few general principles must be preferred to meet:
As far as possible the high RF power amplifier (HPA) and low noise amplifier (LNA) apart, in a nutshell, is to make high power RF transmission circuit from low power RF receiving circuit. If you have a lot of physical space on the PCB, so you can easily do this, but often many components, PCB space is small, so it is usually not possible. You can put them on both sides of the PCB, or let them work alternately, rather than work at the same time. Sometimes also can include RF high power circuit buffer and voltage control oscillator (VCO).
To ensure high power PCB area at least a whole piece, there is no best hole, of course, the more copper, the better. Later, we will discuss how to break the design principles according to need, and how to avoid and may cause problems.
Chip and power supply decoupling is also very important, the principle of several kinds of method to realize are discussed later.
RF input RF output usually need to get away, we will discuss in more detail later.
The sensitive analog signal should as far as possible away from the high speed digital signal and RF signal.
How to partition?
Design can be decomposed into physical partitioning and electrical partition. Physical partitioning involves components layout, orientation and shielding, etc.; Electrical partition can continue into the power distribution, the RF circuit and signal line, sensitivity, earthing and other partitions.
First we discuss physical partitions. Components layout is the key to achieve a good RF design, the most effective technique is first fixed in the RF path components, and adjust its towards to the RF path to minimize the length of the input from the output, and as far as possible the separation of high power and low power circuit.
The most effective circuit board stack method is to connect the main ground (main) arrangement under the surface of the second layer, and as far as possible to walk on the surface of the RF line. The RF path to minimize the size of hole can not only reduce the path inductance, but also can reduce the empty spot on the ground, and can reduce the RF energy leakage to opportunities in other areas of the laminated board.
In physical space, such as the multistage amplifier linear circuit is usually enough to multiple RF area between separate from each other, but the duplexer, mixer, and intermediate frequency amplifier/mixer always have multiple RF/IF signal interference, so we must carefully to minimize the impact. RF and IF should go as far as possible get the line cross, and every other field between them as much as possible. Proper RF path is very important to the performance of the whole piece of PCB board, which is why components layout in cellular phone PCB design usually account for the cause of most of the time.
On the cellular phone PCB, usually low noise amplifier circuit can be placed on one side of the PCB, and high power amplifier on the other side, and finally through the duplexer put them in the same plane antenna connected to RF and baseband processor. Need some skills to ensure that straight through hole will not put the RF energy from side of the plate to the other side, the common technique is use blind holes in both sides. Can pass straight through hole arranged on both sides of PCB without RF interference region to be straight through the hole to minimize the adverse impact.
Sometimes is unlikely to ensure enough isolation between multiple circuit block, in this case it is necessary to consider using metal shield in the RF shielding the RF energy area, but the metal shield also exist problems, such as: its cost and assembly cost is very expensive;
Irregular shape of the metal shield in the manufacturing is difficult to guarantee the high precision, rectangular or square metal shield and component layout is limited by some; Metal shield against component replacement and fault location; Because the metal shield must be welded on the ground, must keep an appropriate distance with components, so take up valuable space of PCB.
As far as possible to ensure integrity of the shield is very important, into the metal shield layer of digital signal lines should go as far as possible, and the best go below the line layer is a layer of PCB formation. RF signal lines can be from small gap at the bottom of metal shield and wiring layer to gaps in go out, but the gaps around to some cloth to as much as possible, different level land through multiple hole together.
Despite the above problems, but the metal shield is very effective, and often the only solution or isolate the key circuit.
In addition, appropriate and effective chip supply decoupling is also very important. Many integrated linear circuit of RF chip is sensitive to the noise from the power supply, each chip generally need to use up to four capacitor and a separate inductance to ensure filter out all of the power supply noise).
Usually depends on its minimum capacitance value since the resonant frequency and low pin inductance, the value of C4 is selected accordingly. C2 and C3 inductance value due to its own pin and relatively large, the relation of RF decoupling effect thereby is less, but they are more suited to filter out low frequency noise signal. Inductance L1 to RF signal from the power cord coupled to the chip. Remember: all walk line is a potential can also can launch RF signal receiving antenna, in addition to the induction of RF signal isolated from the key line is also very necessary.
These decoupling components of the physical location is also key, usually several important components layout principle is: the C4 as far as possible close to the IC pins and grounding, C3 must be the most close to the C4, C2 must be the most close to C3, and IC pin connection with C4 go line should be as short as possible, these a few elements of the grounding end (especially C4) should be the next formation usually connected to the GND pin of the chip. The components are connected to the formation of holes should be as close to the PCB board welding plate element, it is best to use to play in the blind holes on the bonding pad to the connector to minimize inductance, inductance should be close to C1.
An integrated circuit or amplifier with an open drain output, therefore need a pull-up inductance to provide a high impedance RF load and a low impedance dc power supply, the same principle applies to the inductance of the power supply decoupling. Some chip need more power to work, so you may need two or three sets of capacitance and inductance to them respectively decoupled, if there is not enough space around the chip, so may run into some trouble.
Remember few inductance parallel together, because this will form a tubular transformer and mutual induction interference signal, so the distance between them must be at least equal the height of one of the devices, or a right Angle are lined up in the mutual inductance to a minimum.
Electrical partition principle of roughly the same as the physical partitions, but also contains some other factors. The modern cell phone parts with different working voltage, and with the aid of software to control, so as to prolong the working life of the battery. This means that the cell phone needs to run a variety of power supply, which bring more problems to isolation. Power from the connector in normally, and immediately to be decoupled filter out any noise from external circuit board, and then after a set of switch or stabilizer for distribution.
Most of the cellular phone circuit dc current is quite small, so walk line width usually is not a problem, however, must go to the power of the high power amplifier alone a large current line as wide as possible, to reduce the transmission pressure drop to the lowest. In order to avoid too much current loss, need to use more than one hole to transfer current from one layer to another layer. In addition, if not in the power supply pin end of the high power amplifier for full decoupling, the high power noise would be radiation to the whole board, and lead to all sorts of problems. High power amplifier grounding is the key, and often need to design a metal shield.
In most cases, the key is to ensure that the same RF output from the RF input. This also applies to the amplifier, buffer and filter. In the worst case, if the amplifier and the output of the buffer with appropriate feedback to their input phase and amplitude, so they are likely to produce self-excited oscillation. In the best case, they will be able to at any temperature and voltage under the condition of steady work. In fact, they may become unstable and add noise and intermodulation signals to the RF signal.
If the radio frequency signal lines have to filter the input end of the round back to the output side, it could seriously damage the filter band-pass characteristics. Get good isolation, to make the input and output must first be a enclosure around the filter cloth, the lower second filter area also want a piece of land in cloth, and connected to the main land surrounding the filter. Signal lines as far as possible away from the need for through the filter filter pin is also a good way. In addition, in every region of the whole board earthing should be very careful, or you may be in imperceptible in introducing a you don't want to happen the coupling channel. Figure 3 illustrates the grounding measures in detail.
Sometimes can choose to go single-ended or balance the RF signal lines, the cross interference and EMC/EMI principle applies here. Balance the RF signal lines if it is right, can reduce the noise interference and cross, but they are usually of impedance is higher, and to maintain a reasonable line width in order to get a matching source, go line and the impedance of the load, the actual wiring may have some difficulties.
Buffer can be used to improve the effect of isolation, because it can put the same signal is divided into two parts, and is used to drive a different circuit, especially the local oscillator may need buffer to drive multiple mixer. When the mixer in the RF frequency reaches the common-mode isolation, it will not be able to work properly. Buffer is a good way to separate different frequency impedance variations, thus will not interfere with each other between circuit.
Buffer is a big help for the design, they can keep up with the need to be at the back of the drive circuit, so that higher power output line is very short, due to buffer the input signal level is lower, so it is not easy to cause interference to other circuits on the board.
There are a lot of very sensitive signals and control line need to pay special attention to, but they are beyond the scope of this paper, so this paper only slightly, is no longer in detail.
Voltage-controlled oscillator (VCO) can change the voltage can be converted into the frequency of change, this feature is used for high-speed channel switch, but they will also trace the noise on the control voltage can be converted into the frequency of small change, and this will give the RF signal increases the noise. After this level, on the whole, you no longer have no way to RF output signal from the noise is removed. So where is the difficulty? First of all, the line of control range of expected bandwidth may from DC until 2 MHZ, and through a filter to remove such a wide-band noise is almost impossible; Second, the frequency of the VCO control line is usually a control part of the feedback loop, it could introduce noise in many places, so must be very careful to deal with VCO control line.
To ensure that the RF go offline layer is solid, and all the components are firmly connected to the main floor, and from the other may bring noise go line. In addition, to ensure that the power of the VCO is properly decoupling, due to the RF VCO output is often a relatively high level, the VCO output signal are easily confused with other circuits, so the VCO must be under special attention. In fact, the VCO often cloth at the end of the RF area, sometimes it also need a metal shield.
Resonant circuit (one for the transmitter, and the other one for the receiver) is associated with VCO, but also has its own characteristics. Simply speaking, the resonant circuit is a parallel resonant circuit with diode somewhere to sex, it will help to set the working frequency of VCO and voice or data modulation on the RF signal.
All of the VCO design principle also applies to the resonant circuit. Due to the resonance circuit contains quite a lot of number of components, wide distribution area in the plate and the usually run under a high frequency of RF, so resonance circuit usually is sensitive to noise. Signals are usually arranged in the chip of the adjacent feet, but the signal pin and cooperate with relatively large inductance and capacitance is needed to work, which in turn requires the inductance and capacitance position must be in close proximity, and even back to a control loop is sensitive to noise. To do this is not easy.
Automatic gain control (AGC) amplifier is also a vulnerable place, whether it is transmitting or receiving circuit can have AGC amplifier. AGC amplifier is usually can effectively filter out noise, but because of the cellular phone with processing transmit and receive signal strength the ability of rapid change, so the AGC circuit is a very wide bandwidth, which makes some of the key circuit is easy to introduce noise AGC amplifier.
AGC circuit design must follow the good analog circuit design techniques, which with short op-amp input pins and short feedback path, the two must be far away from the RF, the IF, or high speed digital signal lines. Also, a good grounding is necessary, and the power of the chip must be good decoupling. If you must walk a long string in the input or output end, then it is better on the output side, usually the output impedance is much lower, and is not easily induced noise. Usually signal level is higher, the easier it is to introduce the noise to other circuits.
Of (among) all the PCB design, as far as possible away from the analog circuit is a digital circuit is the general rule, it also applies to RFPCB design. Public to simulate and used for shielding and separated signal lines are usually equally important, the problem is that if did not foresee and careful planning in advance, every time you can do in this respect are few. In the early design stage, therefore, careful planning and thoughtful components layout and complete the layout of the evaluation is very important, due to the negligence of design change could lead to a finishing design must be re-evaluated again. The serious consequences caused by negligence, in any case for your personal career development is not a good thing.
Also should make the RF circuit away from analog circuit and some essential digital signal, and all of the RF cables, welding plate and components should be around as much as possible to fill the grounding copper, and connected to the main ground as soon as possible. Like bread plate of the micro hole tectonic plate in RF circuit development phase is very useful, if you chose tectonic plate, so you do not need to spend any costs can be free to use a lot of holes, or on the common PCB drilling will be increase the cost of development, which will increase cost in mass production.
If RF cables must be through the line, so try to walk along the RF line between them cloth layer and the main ground connected to the ground. If not possible, make sure they are of the cross, the capacitive coupling can be reduced to a minimum, at the same time as much as possible in each RF go line around the fabric some more, and connect them to the Lord. In addition, to leave the parallel RF line to minimize the distance between the inductive coupling can be reduced to a minimum.
A solid blocks connect directly on the surface of the first floor, the isolation effect is best, in spite of careful design the practice of other work. I tried to connect the ground into several blocks to isolate analog, digital and RF circuit, but I never satisfied with the result, because eventually there is always some high speed signal lines to cross these separate, this is not a good thing. In each layer of the PCB board, cloth should be as much as possible, and connect them to the main floor. Walk line together as much as possible in order to increase the internal signal and power distribution layer, the number of plots and adjust line so that you can connect to via decorate go up to the surface of the isolation block. Should be avoided in the PCB layers generated on free land, because they can pick up like a small antenna or injection of noise. In most cases, if you can't connect them to the Lord, so you'd better put them removed.
