RF Layout and Optimization Techniques in Mobile PCB Design

The increasing functionality of mobile phones places higher demands on PCB design. With the arrival of Bluetooth devices, cellular phones, and the 3G era, engineers are focusing more on the design techniques for RF circuits. RF (Radio Frequency) circuit board design is often described as a “black art” due to the many uncertainties in theory. However, this view is only partially correct. RF circuit board design also follows many guidelines and rules that should not be ignored. The real challenge in practical design is how to compromise when these guidelines and rules cannot be precisely implemented due to various design constraints.

Many important RF design topics are worth discussing, such as impedance and impedance matching, dielectric materials and stacked boards, as well as wavelength and standing waves. These have significant effects on the EMC and EMI of mobile phones. The following summarizes the conditions that must be met when designing RF layouts for mobile phone PCBs:

1. Isolate High Power RF Amplifiers (HPA) and Low Noise Amplifiers (LNA)

Whenever possible, isolate high-power RF transmit circuits from low-power RF receive circuits. Mobile phones have many functions, numerous components, and limited PCB space. Considering the wiring design constraints, the demand for design techniques becomes high. This may require designing four- to six-layer PCBs for alternating operation rather than simultaneous operation. High-power circuits can sometimes include RF buffers and voltage-controlled oscillators (VCO). Ensure that the high-power region of the PCB has a solid ground plane, ideally without vias, and the more copper planes the better. Sensitive analog signals should be kept as far away as possible from high-speed digital signals and RF signals.

2. Design Partitioning: Physical and Electrical

Design partitioning can be broken down into physical partitioning and electrical partitioning. Physical partitioning mainly deals with component layout, orientation, and shielding, while electrical partitioning can be further divided into power distribution, RF traces, sensitive circuits and signals, and grounding.

a. Physical Partitioning:

Component layout is key to achieving an excellent RF design. The most effective technique is to fix components on the RF path and adjust their orientation to minimize the length of the RF path, separating the input from the output and keeping high-power and low-power circuits as far apart as possible.

The best PCB stacking method is to arrange the main ground plane (primary ground) in the second layer under the surface layer and route RF traces on the surface layer. Minimizing via size on the RF path not only reduces path inductance but also reduces the chance of soldering defects on the main ground and prevents RF energy from leaking to other areas of the stacked board. Linear circuits like multi-stage amplifiers are often enough to isolate multiple RF regions, but duplexers, mixers, and IF amplifiers/mixers often have multiple RF/IF signals that interfere with each other, requiring careful isolation.

b. RF and IF Trace Routing:

RF and IF traces should cross each other as much as possible, ideally with a ground plane separating them. Proper RF routing is crucial for the overall performance of the PCB, which is why component layout typically takes up a large portion of mobile phone PCB design. For mobile PCB design, the low-noise amplifier circuit can often be placed on one side of the PCB, and the high-power amplifier on the other side, eventually connecting them through a duplexer on the same side to the RF end and baseband processor end of the antenna. Special care is needed to ensure that vias do not transfer RF energy from one side of the board to the other. A common technique is to use blind vias. The adverse impact of vias can be minimized by arranging them in areas on both sides of the PCB that are not exposed to RF interference.

Sometimes, it may not be possible to guarantee sufficient isolation between multiple circuit blocks, and in such cases, metal shields are often used to contain RF energy within the RF region. These shields must be soldered to the ground and kept at an appropriate distance from the components, which requires occupying valuable PCB space. Ensuring the integrity of the shield is critical, and digital signal lines entering the metal shield should ideally be routed to inner layers, with the layer below being ground. RF signal lines can exit the shield through small gaps at the bottom of the shield, but additional ground should be placed around these gaps, and different layers of ground can be connected through multiple vias.

c. Proper and Effective Chip Power Decoupling:

Proper decoupling is critical, as many RF chips that integrate linear circuits are very sensitive to power supply noise. Each chip may require up to four capacitors and an isolation inductor to filter out all power supply noise. An integrated circuit or amplifier with an open-drain output typically requires a pull-up inductor to provide a high-impedance RF load and a low-impedance DC power supply. The same principle applies to decoupling the power supply at this inductor. Some chips require multiple power supplies to operate, and in such cases, two to three sets of capacitors and inductors may be needed for separate decoupling. Inductors should not be placed in parallel too close together, as this can form a coreless transformer that induces interference. The distance between them should be at least the height of one of the components or arranged at right angles to minimize mutual inductance.

d. Electrical Partitioning:

Electrical partitioning follows similar principles to physical partitioning but also includes additional factors. Some sections of the mobile phone operate at different voltages, which are controlled by software to extend battery life. This requires running multiple power supplies, which introduces more isolation issues. The power typically enters from the connector, is immediately decoupled to filter out any external noise, and then distributed through a set of switches or regulators. Most circuits on the mobile PCB have relatively small DC currents, so trace width is usually not an issue, but high-power amplifiers need dedicated wide current traces to minimize voltage drop. To avoid excessive current loss, multiple vias should be used to transfer current from one layer to another.

If the power supply pin of a high-power amplifier is not adequately decoupled, high-power noise will radiate throughout the entire PCB, causing various issues. Proper grounding of high-power amplifiers is crucial, and often a metal shield needs to be designed for it. In most cases, ensuring that the RF output is separated from the RF input is equally critical. This also applies to amplifiers, buffers, and filters. In the worst case, if the output of amplifiers and buffers feeds back into their inputs with the correct phase and amplitude, self-oscillation could occur, leading to instability and interference.

3. Key Considerations in Mobile PCB Design:

a. Power and Grounding:

Even with good routing across the entire PCB, improper power and ground layout can cause interference that degrades the product’s performance and sometimes even its success rate. Therefore, power and ground noise should be minimized to ensure product quality.

b. Grounding for Digital and Analog Circuits:

Mobile PCBs are often mixed digital and analog, so it’s necessary to consider interference issues between them, especially noise on the ground. High-frequency digital circuits should be kept as far away as possible from sensitive analog components. The digital and analog grounds are usually separate within the PCB, but must be connected at the external interface.

c. Signal Routing on Power (Ground) Layers:

In multi-layer boards, if signal traces are already close to completion, adding more layers may waste space and increase production complexity. To solve this, large copper planes can be used for the ground and power, ensuring noise is minimized.