PCB Technical - RF circuit board partition design

When designing the RF layout, there are several general principles that must be met first:

Separate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitter circuit away from the low-power RF receiver circuit. If you have a lot of physical space on your PCB, you can do this easily, 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 board, or let them work alternately instead of working at the same time. High-power circuits sometimes include RF buffers and voltage controlled oscillators (VCO).

Make sure that there is at least a whole piece of ground in the high-power area of the PCB, preferably without vias. Of course, the more copper, the better. Later, we will discuss how to break this design principle as needed, and how to avoid the problems that may be caused by this.

The decoupling of the chip and the power supply is also extremely important, and several ways to implement this principle will be discussed later.

How to partition?

Design partitions can be decomposed into physical partitions and electrical partitions. Physical partitioning mainly involves issues such as component layout, orientation, and shielding; electrical partitioning can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.

First we discuss the issue of physical partitioning. Component layout is the key to achieving a good RF design. The most effective technique is to first fix the components on the RF path and adjust their orientation to minimize the length of the RF path, keep the input away from the output, and as far as possible Ground separation of high-power circuits and low-power circuits.

The most effective circuit board stacking method is to arrange the main ground plane (main ground) on the second layer below the surface layer, and route the RF lines on the surface layer as much as possible. Minimizing the size of the vias on the RF path can not only reduce the path inductance, but also reduce the virtual solder joints on the main ground and reduce the chance of RF energy leaking to other areas in the laminate.

In physical space, linear circuits like multi-stage amplifiers are usually sufficient to isolate multiple RF zones from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IFs. The signals interfere with each other, so care must be taken to minimize this effect. The RF and IF traces should be crossed as much as possible, and a ground should be placed between them as much as possible. The correct RF path is very important to the performance of the entire PCB board, which is why the component layout usually takes up most of the time in the cellular phone PCB board design.

It is difficult to guarantee high precision when manufacturing irregularly shaped metal shields. Rectangular or square metal shields impose some restrictions on the layout of components; metal shields are not conducive to component replacement and fault location; because metal shields must be welded on On the ground, a proper distance must be kept from the components, so it takes up valuable PCB board space.

It is very important to ensure the integrity of the shielding cover as much as possible. The digital signal lines entering the metal shielding cover should be routed to the inner layer as much as possible, and it is best that the PCB layer below the wiring layer is the ground layer. RF signal lines can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but as much ground as possible around the gap, the ground on different layers can be connected together through multiple vias .

Despite the above problems, metal shields are very effective and are often the only solution to isolate critical circuits.

The minimum capacitance value usually depends on its self-resonant frequency and low pin inductance, and the value of C4 is chosen 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 lower frequency noise signals. The inductance L1 prevents the RF signal from coupling into the chip from the power line. Remember: all traces are a potential antenna that can both receive and transmit RF signals, and it is also necessary to isolate the induced RF signals from the critical lines.

The physical location of these decoupling components is usually also critical. The layout principle of these important components is: C4 must be as close to the IC pin as possible and grounded, C3 must be the closest to C4, C2 must be the closest to C3, and the IC pin and The connection traces of C4 should be as short as possible. The ground terminals of these components (especially C4) should usually be connected to the ground pin of the chip through the next ground layer. The vias connecting the components to the ground layer should be as close as possible to the component pads on the PCB. It is best to use blind holes punched on the pads to minimize the inductance of the connecting wires. The inductance should be close to C1.

The principle of electrical zoning is roughly the same as that of physical zoning, but it also contains some other factors. Some parts of modern cellular phones use different operating voltages and are controlled by software to extend battery life. This means that cellular phones need to run multiple power sources, and this brings more problems to isolation. The power is usually introduced from the connector, and is immediately decoupled to filter out any noise from the outside of the circuit board, and then distributed after passing through a set of switches or regulators.

If the RF signal line has to be looped from the input end of the filter back to the output end, this may seriously damage the bandpass characteristics of the filter. In order to get a good isolation between the input and output, a ground must be laid around the filter first, and then a ground must be laid in the lower layer area of the filter and connected to the main ground surrounding the filter. It is also a good way to keep the signal lines that need to pass through the filter as far away as possible from the filter pins. In addition, be very careful about the grounding of various places on the entire board, otherwise you may unknowingly introduce a coupling channel that you do not want to occur.

The buffer can be used to improve the isolation effect, because it can divide the same signal into two parts and used to drive different circuits, especially the local oscillator may need a buffer to drive multiple mixers. When the mixer reaches the common mode isolation state at the 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 follow the circuit that needs to be driven, so that the high-power output traces are very short. Because the input signal level of the buffer is relatively low, they are not easy to interfere with other on the board. The circuit is causing interference.

The resonant circuit (one for the transmitter and the other for the receiver) is related to the VCO, but it also has its own characteristics. Simply put, 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.

Designing AGC circuits must comply with good analog circuit design techniques, which are related to the short op amp input pins and short feedback paths, both of which must be far away from RF, IF, or high-speed digital signal traces. Similarly, good grounding is also essential, and the chip's power supply must be well decoupled. If it is necessary to run a long wire at the input or output end, it is best to go at the output end. Usually, the impedance of the output end 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 much as possible, and it also applies to RFPCB design. The common analog ground and the ground used to shield and separate signal lines are usually equally important. The problem is that without foresight and careful planning in advance, there is very little you can do in this area each time. Therefore, in the early stages of design, careful planning, well-thought-out component layout and thorough layout evaluation are very important. Design changes caused by negligence may lead to a design that is about to be completed and must be rebuilt. This serious consequence caused by negligence, in any case, is not a good thing for your personal career development.