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The increasing functionalities of mobile phones require more advanced PCB board designs. With the advent of Bluetooth devices, cellular phones, and 3G, engineers are focusing more on RF circuit design skills. RF board design is often referred to as a “black art” due to theoretical uncertainties, though this view is only partly accurate. There are numerous guidelines and principles in RF board design that should be followed and respected. However, in practical design, the most valuable technique is knowing how to compromise these principles due to various design constraints. Of course, there are many critical topics in RF design worth discussing, such as impedance matching, insulating layer materials, laminates, wavelength, and standing wave, all of which significantly impact the EMC and EMI of mobile phones. Here is a summary of the essential conditions for designing the RF layout of a mobile phone PCB:

1. Whenever possible, isolate high-power RF amplifiers (HPA) from low-noise amplifiers (LNA). Essentially, keep the high-power RF transmitting circuitry away from the low-power RF receiving circuitry. With numerous components packed into a small PCB space, and considering wiring limitations during the design process, high-level design skills are required. At times, it may be beneficial to design PCB layers to function alternately rather than simultaneously, typically involving four to six layers. High-power circuits may include RF buffers and voltage-controlled oscillators (VCO). Ensure there is at least one entire layer dedicated solely to high-power areas on the PCB without any perforations. Additionally, more copper layers generally enhance performance. It’s crucial to keep sensitive analog signals as distant as possible from high-speed digital and RF signals.

2. Design partitioning can be categorized into physical and electrical partitions. Physical partitioning encompasses component layout, orientation, shielding, and more. Electrical partitioning further divides into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.

2.1 Discussing physical partitioning: Component layout is pivotal in RF design implementation. A practical approach involves fixing components along the RF path, orienting them to minimize RF path length, ensuring input separation from output, and maximizing distance between high-power and low-power circuits. An effective method is stacking circuit boards with the main ground layer on the second layer from the surface, utilizing RF lines on the surface layer. Reducing through-hole sizes in RF paths not only cuts down on path inductance but also minimizes virtual solder joints on the main ground, reducing RF energy leakage within the laminate. Linear circuits like multistage amplifiers typically suffice for isolating various RF regions, whereas diplexers, mixers, and IF amplifiers/mixers necessitate careful reduction of mutual RF/IF signal interference.

2.2 RF and IF paths should intersect and separate as far as possible. Proper RF pathing significantly influences overall PCB performance, making component layout a time-intensive aspect of mobile PCB design. Ideally, in cell phone PCB designs, positioning the low-noise amplifier circuit on one side and the high-power amplifier on the other, and subsequently linking them to the RF and baseband processor antenna via a diplexer, is feasible. Techniques involving blind holes on both sides are commonly employed to ensure minimal RF energy transfer between board sides. Metal shielding might be necessary to contain RF energy within the RF region, ensuring it’s adequately grounded and positioned at a suitable distance from components, thereby consuming valuable PCB space. Maintaining shield integrity is crucial; digital signal lines entering the metal shield should preferably traverse inner layers, with the PCB layer beneath acting as the base. RF signal lines can exit from gaps at the metal shield’s bottom and wiring layer gaps, with surrounding gaps preferably encased in ground. Inter-layer grounding can be achieved via multiple holes.

2.3 Proper and effective chip power decoupling is also crucial. Many RF chips with integrated linear circuits are highly sensitive to power source noise, often requiring up to four capacitors and an isolating inductor per chip to effectively filter out all power source noise. Integrated circuits or amplifiers typically feature open drain outputs, necessitating pull-up inductors to provide high impedance RF loads and low impedance DC power supplies. The same principle applies to decoupling power supplies at inductor ends. Some chips require additional decoupling capacitance and inductance sets, ensuring these are sufficiently spaced apart to prevent mutual inductance interference signals, with spacing equal to at least one device height, or placed at right angles to each other.

2.4 Electrical zoning principles are generally similar to physical zoning, but additional factors come into play. Parts of the phone operating at varying voltages, software-controlled for extended battery life, pose isolation challenges. Power typically enters via connectors, immediately filtered through decoupling to eliminate external board noise, and then distributed via switches or regulators. Cell phone PCBs generally handle low DC currents, minimizing wiring width issues, though a separate wide high-current line is advisable for high-power amplifier power supplies to reduce voltage drop during transmission, with multiple holes facilitating current transfer between layers. Inadequate decoupling at the high-power amplifier’s power pin can propagate high-power noise across the board, causing various issues. Proper grounding of high-power amplifiers is critical, often necessitating metal shielding. Ensuring RF output remains isolated from RF input applies to amplifiers, buffers, and filters. In unfavorable scenarios, amplifiers and buffers might generate self-induced oscillations if their outputs feed back into inputs at the correct phase and amplitude, leading to instability and introducing noise and intermodulation signals into RF signals. Routing RF signal lines from filter inputs to outputs can severely compromise filter bandpass characteristics. To ensure input-output isolation, a surrounding field is first laid around the filter, with a lower region field subsequently laid, both connected to the main ground encompassing the filter. Routing signal lines needing filter passage as far as possible from filter pins is advisable. Careful overall board grounding is essential to prevent introducing coupling paths. Options between single-ended or balanced RF signal line routes apply, with cross interference principles and EMC/EMI considerations equally applicable. Balanced RF signal lines can reduce noise and cross interference with proper routing but typically feature high impedance, requiring careful width adjustment for source, routing, and load impedance matching. Buffers prove beneficial for isolation, capable of splitting signals for driving different circuits, particularly useful when a local oscillator requires multiple mixer drives. Buffers ensure stable operation across temperature and voltage ranges, minimizing noise and intermodulation signal addition to RF signals.

2.5 To mitigate noise escalation, consider the following: The control line’s expected bandwidth range may extend from DC to 2MHz, making comprehensive noise removal via filtering almost impractical. The control line, typically part of a feedback loop regulating frequency, introduces noise at multiple points, demanding meticulous handling. A solid RF floor and secure component grounding are crucial, ensuring VCO power supply decoupling meets stringent standards, especially since VCO RF outputs tend towards higher levels, potentially interfering with other circuits. VCOs are often placed towards the RF region’s end, occasionally necessitating metal shielding. Resonant circuits related to VCOs (one for transmission, another for reception) feature specific characteristics. Essentially, a resonant circuit incorporates capacitive diodes aiding VCO operating frequency setup and voice/data modulation into RF signals. Design principles for VCOs equally apply to resonant circuits, which are sensitive to noise due to numerous components and extensive board coverage at high RF frequencies. Signals typically align across adjacent chip pins, necessitating sizeable inductors and capacitors in close proximity, ideally interconnected within a noise-sensitive control loop. Automatic Gain Control (AGC) amplifiers represent another problematic area, especially in transmitting and receiving circuits. AGC amplifiers effectively filter noise but require wide bandwidth for managing rapid signal intensity changes in mobile phones, potentially introducing noise into crucial circuits. AGC line designs emphasize optimal analog circuit principles, featuring short op-amp inputs and feedback paths distant from RF, IF, or high-speed digital signal wiring. Effective grounding and well-decoupled chip power supplies are imperative. Lengthy lines are best routed towards outputs with lower impedance, minimizing noise susceptibility. Generally, digital circuits should be spatially separated from analog circuits as much as possible, a principle equally applicable to RF PCB designs. Shielding simulations and isolated signal lines hold similar importance during early design stages, necessitating meticulous component layout review and completion, ensuring RF circuitry remains distant from essential analog and digital signals, with ample grounding, copper sheet filling, and connectivity to the main ground.

3. Significant attention should be paid to the following aspects in mobile phone PCB board design:

3.1 Power supply and ground wire handling: Even with well-completed PCB board wiring, poor consideration of power supply and ground wire interference can degrade product performance, potentially affecting product success rates. Thus, serious attention to power supply and ground wire routing is crucial to minimize electrical noise interference and ensure product quality. For any electronics design engineer, understanding the source of noise between ground wires and power lines is fundamental. Current noise suppression methods include adding decoupling capacitors between power supply and ground wires. Ideally, ground wires should be wider than power lines, following this hierarchy: ground wire > power line > signal line. Typically, signal line widths range from 0.2 to 0.3mm, with finer widths down to 0.05 to 0.07mm, while power lines measure 1.2 to 2.5mm. Digital circuit PCBs can adopt a wide ground conductor circuit approach, forming a ground network, which is impractical for analog grounds.

3.2 Common ground management of digital and analog circuits: Modern PCBs often integrate digital and analog functionalities. Consequently, during wiring, careful consideration must be given to mitigating interference between them, especially noise interference on ground lines. Due to external PCB connections, grounding is simplified to a single node, necessitating internal PCB handling for digital-analog separation. Only