PCB News - Design RF Circuit Board and Knowhow

Design RF Circuit Board and Knowhow

Successful RF PCB design must pay careful attention to each step and detail in the whole design process, which means that thorough and careful planning must be carried out at the beginning of the design, and the progress of each design step must be comprehensively and continuously evaluated. This meticulous design skill is what most domestic electronic enterprise cultures lack.

In recent years, due to the demand and growth of Bluetooth devices, wireless local area network (WLAN) devices and mobile phones, operators pay more and more attention to the skills of RF circuit design. From the past to the present, RF circuit board design, like electromagnetic interference (EMI), has always been the most difficult part for engineers to control, even a nightmare. If you want to design successfully at one time, you must plan carefully and pay attention to details in advance to work.

Radio frequency (RF) circuit board design is often described as a "black art" because there are many uncertainties in theory. However, this is only a partial coverage point of view. There are still many rules that can be followed in RF circuit board design. However, in practical design, the real practical skill is how to compromise these laws when they cannot be implemented due to various restrictions. Important RF design topics include impedance and impedance matching, insulating layer materials and laminates, wavelength and harmonics, etc. This paper will focus on various problems related to RF circuit board zoning design.

Types of Micro Vias

Circuits with different properties on the Printed Circuit Boardmust be separated, but they must be connected under the best condition without electromagnetic interference, which requires the use of micro vias. Generally, the diameter of micro vias is 0.05mm to 0.20mm. These vias are generally divided into three categories, namely blind via, bury via and through via. The blind hole is located on the top and bottom surfaces of the printed circuit board and has a certain depth. It is used for the connection between the surface circuit and the inner circuit below. The depth of the hole usually does not exceed a certain ratio (aperture). Buried hole refers to the connection hole located in the inner layer of the printed circuit board, which will not extend to the surface of the printed circuit board. The above two types of holes are located in the inner layer of the circuit board. They are completed by the through-hole forming process before lamination. Several inner layers may be overlapped during the formation of vias. The third is called through hole, which passes through the whole circuit board and can be used to realize internal interconnection or as adhesive positioning hole of components.

Using zoning techniques

When designing RF circuit board, high-power RF amplifier (HPA) and low-noise amplifier (LNA) should be isolated as far as possible. In short, high-power RF transmitting circuit should be kept away from low-noise receiving circuit. If there is a lot of space on the PCB, this can be easily done. However, when there are many components, the 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 rather than at the same time. High power circuits may also include RF buffers and voltage controlled oscillators (VCOs).

Design zoning can be divided into physical partitioning and electrical partitioning. Physical zoning mainly involves the layout, orientation and shielding of parts and components; Electrical zoning can continue to be divided into power distribution, RF wiring, sensitive circuit and signal, grounding and other zoning.

Entity partition

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 their orientation to minimize the length of the RF path. And keep the RF input away from the RF output, and away from high-power circuits and low-noise circuits as far as possible.

The most effective circuit board stack method is to arrange the main grounding 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 grounding, 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 always have multiple RF / if signals interfering with each other, so this effect must be carefully minimized. RF and if wiring shall cross as far as possible, and a grounding area shall be separated between them 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 takes up most of the time in the design of mobile phone PCB.

On the mobile phone 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 one end of the RF antenna and the other end of the baseband processor on the same side by a duplexer. This requires some skills to ensure that RF energy is not transferred from one side of the board to the other through vias. The common technology is to use blind holes on both sides. By arranging the blind hole in the area where both sides of the PCB are free from RF interference, the adverse effect of the via can be minimized.

Metal shield

Sometimes, it is impossible to keep enough separation 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 side effects, such as high PCB manufacturing cost and PCB assembly cost.

The irregular metal shield is difficult to ensure high precision in manufacturing, and the rectangular or square metal shield limits the layout of parts and components; The metal shield is not conducive to component replacement and fault displacement; Since the metal shield must be welded on 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 metal shield as much as possible, so the digital signal line entering the metal shield should go through the inner layer as much as possible, and it is best to set the next layer of the signal line layer as the grounding layer. The RF signal line can be routed from the small gap at the bottom of the metal shield and the wiring layer at the grounding gap, but the periphery of the gap should be surrounded by a large grounding area as much as possible. The grounding on different signal layers can be connected together through multiple via s.

Despite these disadvantages, metal shields are still very effective and often the only solution to isolate critical circuits.

Power decoupling circuit

In addition, an appropriate and effective chip power decoupling circuit 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 filter out all power supply noise.

The minimum capacitance usually depends on the resonant frequency and pin inductance of the capacitor itself, 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. RF decoupling is completed by inductor L1, which makes the RF signal unable to be coupled from the power line to the chip. Because all wiring is a potential antenna that can receive and transmit RF signals, it is necessary to isolate RF signals from key lines and components.

The physical location of these decoupling components is often also critical. 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 the IC pin and C4 should be as short as possible. The grounding terminals of these components (especially C4) should usually be connected to the chip grounding pin through the first grounding layer under the board. The via connecting the component to the ground plane 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 L1 should be close to C1.

An integrated circuit or amplifier often has an open collector 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 end of this inductor. Some chips require multiple power supplies to work, so two or three sets of capacitors and inductors may be required to decouple them respectively. If there is not enough space around the chip, the decoupling effect may be poor.

In particular, it should be noted that the inductors are rarely close together in parallel, because this will form an air core transformer and induce interference signals with each other. Therefore, the distance between them should be at least equal to the height of one of them, or arranged at right angles to minimize mutual inductance.

Electrical zoning

Electrical zoning is the same as physical zoning in principle, but it also includes some other factors. Some parts of modern mobile phones use different working voltages and are controlled by software to prolong the service life of the battery. This means that mobile phones need to have multiple power supplies, which creates more isolation problems. The power supply is usually introduced by 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 linear regulators.

In mobile phones, the DC current of most circuits is quite small, so the routing width is usually not a problem. However, a large current line as wide as possible must be designed separately for the power supply of high-power amplifier to minimize the transient voltage drop during transmission. 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 of the high-power amplifier, the high-power noise will radiate to the whole printed circuit 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.