RF PCB

RF circuits (RF circuit) the number of special features , it is difficult to explain briefly a few words , you can not use the traditional simulation software to analyze , for example, SPICE. However, there are some EDA software currently on the market with a harmonic balance (harmonic balance), the projection method (shooting method) ... and so on complex algorithms that can quickly and accurately simulate RF circuits. But before learning the EDA software , you must first understand the characteristics of the RF circuit , in particular, to understand the meaning of some terminology and physical phenomena , because it is the basis of knowledge of RF engineering .

RF interface

Wireless transmitter and receiver conceptually can be divided into two parts, the baseband and RF . The output signal of the frequency range of the frequency range of the input baseband signal includes a transmitter , and also contains the receiver. Determines the bandwidth of the baseband data rate of the system substantially flowable . The fundamental frequency is used to improve the reliability of the data stream , and in a particular data transmission rate , the transmitter is applied to reduce the transmission medium (transmission medium) load. Therefore, the baseband circuit PCB design , signal processing requires a lot of engineering knowledge . The RF transmitter circuit can already processed baseband signal conversion , frequency up to a specified channel , and the injection of the signal to the transmission medium . Instead , the receiver circuit to obtain the RF signal from the transmission medium , and convert into a baseband frequency down .

The transmitter has two main pcb design goals: First, they must be the least possible power consumption in the case of transmission of specific power . The second is that they can not interfere with the normal operation of the transceiver within the adjacent channel . The receiver is concerned, there are three main PCB design goals: First, they must be accurately restored small signal ; Second, they must be able to remove unwanted signals outside the desired channel ; final point as the transmitter , they consume power must small.

Small desired signal

The receiver must be very sensitive to detect small input signal. Generally , the receiver's input power can be as small as 1 μV. The receiver sensitivity is limited by its noise generated by the input circuit . Therefore, the noise is an important consideration when the receiver PCB design . Moreover, in order to have the ability to predict noise simulation tool is essential . One drawing of a typical superheterodyne (superheterodyne) receiver. The received signal is first filtered , and then to a low noise amplifier (LNA) to amplify the input signal . Then use the first local oscillator (LO) signal is mixed with this , so that this signal is converted into an intermediate frequency (IF). The front end (front-end) noise performance of the circuit depends on the LNA, mixer (mixer) and LO. Although the use of conventional SPICE noise analysis can find LNA noise , but for the purposes of the mixer and LO , it is useless because of the noise in these blocks will be severely affected large LO signal.

Small input signal requires a receiver must have a great zoom function usually requires such a high gain 120 dB . In such a high -gain , any custom output coupled (couple) back to the input signal are likely to cause problems. Important reasons to use a superheterodyne receiver architecture is that it can be distributed in several frequency gain in order to reduce the probability of coupling . It also makes the frequency of the first LO frequency and the input signal type , the interference signal can prevent large "pollution" of the small input signal.

For different reasons , in some wireless communication systems, direct conversion (direct conversion) or in the difference (homodyne) superheterodyne architecture structure may be substituted . In this architecture , the RF input signal is directly converted in a single step into a base frequency , and therefore , most of the gain in the fundamental frequency and the LO frequency is the same as the input signal. In this case , you must understand the influence of a small amount of coupling and must establish a " spurious signal path (stray signal path)" detailed models , such as: the coupling through the substrate (substrate) , the package pins and wire coupling, and the coupling through the power supply line (bondwire) between .

Large interference signals

The receiver must be very sensitive to small signal , if there is a large interfering signal ( barrier ) exists . This occurs when the attempt to receive a weak signal or the transmission distance , and near a strong adjacent channel transmitters in the broadcast . The interference signal may be large than expected 60 ~ 70 dB signal and the receiver may be a large number of input stage cover means or the receiver excessive amount of noise in the input stage , to block the normal signal reception . If the receiver input stage, the interference source is driven into the nonlinear region, the problem occurs that two . To avoid these problems , the front end of the receiver must be very linear.

Therefore, the " linear" PCB is also an important consideration when designing receivers . Because the receiver is a narrow circuit, based on the measurement of nonlinear " intermodulation distortion (intermodulation distortion)" to the statistics. This involves the use of two frequencies are similar , and in the center of the band (in band) sine or cosine waves to drive the input signal , and then measuring the intermodulation product . Generally speaking , SPICE is a time-consuming and cost simulation software as many times since it must perform the calculation cycle to obtain the desired frequency resolution for the case of distortion .

Interference from adjacent channels.

The transmitter distortion also plays an important role. In the output circuit of the transmitter generated by a nonlinear , so that the bandwidth of the transmission signal may be spread to adjacent channels . This phenomenon is called " spectral regrowth (spectral regrowth)". Before the signal reaches the transmitter power amplifier (PA), with which the bandwidth is limited ; but in the PA " crosstalk " causes the bandwidth increases again . If too much bandwidth increases , the transmitter will be unable to meet the power requirements of its adjacent channels. When sending the digital modulation signal , in fact, can not be predicted spectral re-growth with the SPICE. Since about 1000 the number sign (symbol) transfer operations must be simulated in order to obtain a representative of the spectrum , and also requires a combination of high-frequency carrier , which will enable SPICE transient analysis becomes impractical.

RF PCB technology

RF PCB design

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 .

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 .

Design Tips

Successful RF design must be careful attention throughout the design process every step and every detail , this means that we must be thorough , careful planning at the beginning of the design stage , and the progress of each design step of a comprehensive ongoing * estimates . And this is fine most of the domestic electronics design skills lacking in the corporate culture .

In recent years, due to the Bluetooth devices, wireless local area network (WLAN) equipment , and mobile phone demand and growth , prompting industry increasingly concerned about RF circuit design techniques . From the past to the present , RF board design as electromagnetic interference (EMI) , like the title , has been the most difficult to control the part of the engineers , even a nightmare . If you want to succeed once the design must be carefully planned in advance and attention to detail can be effective.

Radio frequency (RF) circuit board design due in theory , there are many uncertainties, it is often described as a ' black art' (black art). But this is just a generalization of view , RF board design still has a lot to follow the rules. However , in the actual design, the real trick is useful when these rules can not be implemented due to various constraints , how they are compromises . Important RF design issues include : impedance and impedance matching , the best materials and the insulating layer laminated plate , wavelength and harmonic ... and so on.

Types of micro- vias

The different nature of the circuit board must be separated , but also in the best case a connection does not produce electromagnetic interference , which need to use micro-vias (microvia). Typically micro vias diameter of 0.05mm to 0.20mm, these vias are generally divided into three categories, namely blind hole (blind via), buried vias (bury via) and through-hole (through via). Blind holes located on the top and bottom surface of the printed wiring board having a certain depth , for connecting the inner surface of the line and below the line , the depth of the hole is usually not more than a certain percentage ( pore diameter ) . Buried in the connection hole is the inner bore of the printed circuit board , it does not extend to the surface of the board . These two types of holes are located in the inner layer circuit board , the through holes prior to lamination molding processes using the completion of the formation of the via holes may also do some overlapping layer.

The third is called through-hole , this hole through the entire circuit board can be used as an adhesive for internal interconnection or positioning hole components.

Adopt zoning techniques

In the design of RF circuit board , as far as possible the high-power RF amplifier (HPA) and low noise amplifier (LNA) to isolate. Simply put RF connection , is to make high-power low-power RF transmitter circuit after an absence of closed circuit . If the PCB has a lot of space , you can easily do it. But many Typically components , PCB space will become very small, so it is difficult to achieve. You can put them on both sides of the PCB , or let them work alternately , not simultaneously. High -power RF circuit may also include a buffer (buffer) and a voltage controlled oscillator (VCO).

Design partitions can be divided into physical partitions (physical partitioning) and electrical partition (Electrical partitioning). The main components involved in physical partition layout , orientation and shielding such topics ; partition can continue into electrical power distribution , RF traces , sensitive circuits and signals, grounding partition.

Physical partitions

Component layout is designed to implement a superior RF critical and most effective technique is to first fixed component located on the RF path and adjust its position, so that to minimize the length of the RF path . And to stay away from RF input RF output and high power circuits as possible after an absence and low power circuits.

The most effective method is to stack the board arranged on the second floor of the main ground subsurface , and the RF line to walk as much as possible on the surface . The RF path through the pore size can not only reduce the path to minimize the inductance , but also can reduce the soldered points on the main ground , and reduce leakage of RF energy into the other areas of the laminated sheet opportunities .

In the physical space , multi- linear circuit such as the amplifier is usually sufficient RF isolation between a plurality of open area , but the duplexer , mixer and IF amplifier always has a plurality of RF / IF signals interfere with each other , Therefore, care must be taken to minimize this effect . RF and IF traces should go as far as possible the cross , and they are spaced as far as possible in the area of ​​a piece of ground . RF path on the performance of the correct piece of PCB board is very important , which is the reason why the component layout is usually accounted for most of the time in the mobile phone PCB board design.

PCB board in a mobile phone , can usually be placed in a low-noise amplifier circuit side of the PCB board , and the high power amplifier on the other side , and, ultimately, by the same surface of the duplexer to connect them to one end of the RF antenna and the other end of the baseband processor . It requires some skill to ensure that RF energy does not by vias passing from the side of the board to the other side , a common technique is to use both sides of the blind hole . The blind hole can be arranged on both sides of the PCB board from RF interference area to the hole through minimize adverse effects .

Metal shield

Sometimes , it is unlikely to save enough segments between a plurality of circuit blocks , in this case it is necessary to consider the metal shield to shield the RF energy in the RF area , but the metal shield also has side effects , for example: Manufacturer costs and assembly costs are high.

Irregular shape of the metal shield in the manufacture of hard to ensure high precision, rectangular or square metal shield component layout and make some restrictions apply ; metal shield is not conducive component replacement and fault displacement ; due to the metal shield must solder the ground surface, and must maintain a proper spacing and components , and therefore need to take up valuable PCB board space.

As far as possible to ensure full metal shield is very important, so enter the metal shield of digital signal lines should go inside as much as possible , and preferably the lower layer signal line ground plane layer set . RF signal line can go from the bottom of the metal shield cover small gaps and ground wiring layer alignment notch , but around the notch to be surrounded by the majority of the ground area as much as possible , on the ground of different signals can be multiple layers over holes together.

Despite these shortcomings, but the metal shield is still very effective , and often critical circuit isolation only solution .

Power to the decoupling circuit

In addition, appropriate and effective power to the chip decoupling (decouple) circuit is also very important. Many linear integrated circuit chip RF noise on the power supply is very sensitive to each chip usually requires the use of up to four capacitors and an inductor to isolate the power supply filter out all the noise . ( Figure I ).

Figure a chip supply to the decoupling circuit

Usually depends on the minimum capacitance value of the capacitance of the resonant frequency itself, and pin inductance , the value of C4 is accordingly chosen . C3 and C2 value because of its relationship pin inductance is relatively large, so RF coupling effects to be worse , but they are more suited to filter out low frequency noise signals. RF coupled to the inductor L1 is completed , it is not of the RF signal from the power line coupled to the chip . Since both receive all traces are also a latent RF signal transmitting antenna , so the RF signal and the critical path , component isolation is necessary.

Coupled to the physical location of these components is often critical. Several important components of the layout of this principle is : C4 to be as close to the IC pin and ground , C3 must be closest to the C4, C2 must be closest to C3, and IC pin connected to C4 traces as short as possible , these ground components (especially C4) board should normally be connected to a ground layer and the die Dir grounding pin . The vias connected to the ground layer of the component should be as close to the pads on the PCB assembly , is preferably used to play in the blind bore on the connection pads to minimize the inductance , the inductor L1 should be close to C1.

An integrated circuit or amplifier often has an open collector (open collector) output , and therefore requires a pullup inductor (pullup inductor) to provide a high RF impedance load and a low impedance DC power supply , the same principle applies to this inductor coupled to the power supply terminal conduct . Some chips require multiple power to work , so it may take two to three sets of capacitance and inductance coupled to deal with them separately , if that is not enough space around the chip , so the effect may be coupled to the poor .

In particular, special attention is needed : a parallel inductor little close together , as this will form an air-core transformer and the mutual induction interference signal, the interval between them corresponds to the height of at least one of , or a right angle arranged so as to minimize the mutual inductance .

Electrical partition

Electrical and physical partition partition principle the same, but also includes several other factors . Some parts of modern mobile phones with different operating voltage, and with the software to control it , in order to prolong battery life. This means that mobile phones need to run a variety of power , which generates more isolated problems. Power is usually introduced by the cable (connector), and immediately treated to be coupled to filter any noise from the external circuit board , and then through a switch or regulator, Thereafter, the power distribution .

In the mobile phone, most of the direct current circuits are quite small, so the trace width is usually not a problem , however , must be high power amplifier design a single high-current line as wide as possible , so that the pressure at launch drop (voltage drop) can be minimized . In order to avoid too much current losses, need to use multiple vias current passes from one layer to another . In addition, if the pin is not in the high- end power amplifier coupled to it adequately , then the noise will radiate high power to block the board and brings a wide variety of topics. Ground high power amplifier is very important , and often need to design a metal shield .

RF output must be an absence of RF input

In most cases , must be away from the RF output RF input . This principle also applies to amplifiers, buffers and filters. In the worst case , if the output of the buffer amplifier and appropriate feedback to the phase and amplitude of the input side thereof , so they are likely to produce self-oscillation . They may become unstable , and the multiplied signal and intermodulation noise (intermodulation products) is added to the RF signal.

If the RF signal from the input end of the line around the back output of the filter , which could seriously damage the band-pass filter characteristics . In order to get a good input and output isolation , first in the filter must be a main ground around the area , followed by the lower area of ​​the filter must also be a piece of ground area , and this area must be linked with the ground around the filter main ground . The need to pass through the filter of the signal line is also a good way to stay away as much as possible the filter pin . In addition, various parts of the whole earth circuit board should be very careful , otherwise you may unknowingly introduce a coupling channel do not want to see happen . ( Figure II ) specifically illustrate this earth approach.

Sometimes you can choose to go single-ended (single-ended) or balanced RF signal line (balanced RF traces), about crosstalk (crosstalk) principles and EMC / EMI is equally applicable here . If balanced RF signal line traces correctly, can reduce noise and crosstalk , but they are usually relatively high impedance . And in order to get a signal source impedance matching , alignment and load , the need to maintain a reasonable width, which may be difficult in the actual wiring .

Figure II filter is surrounded by ground (green area) surrounded

Buffer

Buffers can be used to isolate the effect of progress, because it can be the same signal is divided into two parts, and used to drive a different circuit. Especially the local oscillator may need a buffer to drive the plurality of mixers. When the mixer to reach common mode isolation (common mode isolation) state at RF frequencies, it will not work. Buffers can be well isolated from the change in impedance at different frequencies, so as not to interfere with each other between the circuit.

Buffer design great help, they may need to be immediately behind the driver circuit, so that the high power output traces is very short, since the signal level of the input buffer is relatively low, so they are not susceptible to the board Other circuits cause interference.

VCO

A voltage controlled oscillator (VCO) can be converted to frequency variation of the voltage change, the switching characteristic is used for high-speed channel, they also convert the control voltage on the trace of noise slight frequency change, and this gives RF signal increases the noise. In short, the voltage-controlled oscillator treated later, no way to go out in the noise signal output from the RF. The difficulty is that the VCO control line (control line) of the range of possible look bandwidth from DC to 2MHz, and out through the filter between this wide band noise is almost impossible; secondly, VCO control line is usually a control frequency of part of the feedback loop, which are likely to introduce noise in many places, it must be handled very carefully VCO control line.

Resonant circuit

A resonant circuit (tank circuit) for the transmitter and receiver, with the VCO, but it also has its own characteristics. Briefly, a series resonant circuit is formed by a diode having a capacitance and inductance of the resonant circuit is connected from it and helps to set the VCO frequency of the voice or data onto an RF carrier modulation.

VCO design principles apply equally to all the resonant circuits. As the resonant circuit contains a considerable number of components, occupies a large area, usually run at a very high RF frequency, so the resonant circuit is usually very sensitive to noise. The signal is typically arranged adjacent pin of the chip, but these require the signal pin and the larger inductor and capacitor to work with, but it needs to be the location of these inductors and capacitors as close to the signal pin, and is connected back to the one pair is very sensitive to the noise control loop, but also to avoid noise interference. To do so is not easy.

AGC amplifiers

Automatic gain control (AGC) amplifier is also a place easily and the subject, whether there will transmit or receive AGC amplifier circuit. AGC amplifier can effectively filter out the noise usually, but because mobile phones have the processing capability to transmit and receive signal strength rapidly changing, thus requiring the AGC circuit has a fairly large bandwidth, which makes it easy to introduce noise AGC amplifier.

AGC circuit design must comply with the principles of analog circuit design, which is the use of very short input pin and a short feedback path, and that two have to be an absence of RF, IF, or high-speed digital signal lines. Again, good ground is also essential, and the chip's power must be good to be coupled. If you must go to design a long line input or output, then it is better to select the output side to achieve it, because usually the output impedance is really a much lower than the input side, and is not easy to introduce noise. Generally the higher the signal level, the noise introduced more easily to other circuits.

Grounding

To ensure alignment of the lower RF ground is solid, and all components must be securely connected to the main ground, and bring noise and other possible traces to isolate. In addition, to ensure that the VCO is coupled to the power supply has been fully, due to the RF output of the VCO is often a fairly high level, VCO output signal easily interfere with other circuits, and therefore must be special attention to the VCO. Indeed, VCO often placed at the end region of RF, sometimes also requires a metal shield.

In all PCB design, digital circuit as far as possible away from the analog circuit is a major principle, it also applies to RF PCB design. Public analog ground and ground for shielding and signal lines are usually spaced equally important. The same shall absence of the RF analog line and the line number of a key digital signal, all the RF traces, pads and the assembly is surrounded by the ground should be as copper, and connected to the main ground as possible. Micro vias (microvia) configured in the RF circuit board useful development stage, it does not involve any overhead can use many vias above, or in the general development of the PCB will increase the cost of drilling, which is produced in large quantities uneconomic.

When a solid piece of ground directly on the surface of the first level, isolate the best. The ground into pieces to isolate analog, digital and RF circuit when its effect is not good, because there is always some ultimate high-speed signal line to pass through these separate ground plane, which is not good design.

There are many and signal and control lines related issues requiring special attention, but they are beyond the scope of this paper.

Whether RF PCB design is not a "black art", follow some basic RF design rules and design examples refer to some excellent design work will contribute to the completion of RF. Successful RF design must be careful attention throughout the design process every step and every detail, this means that we must be thorough, careful planning at the beginning of the design stage, and the progress of each design step of a comprehensive ongoing * estimates. And this is fine most of the domestic electronics design skills lacking in the corporate culture.

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