1. Isolate high-power RF amplifiers (HPA) and low-noise amplifiers (LNA) as far as possible. In short, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. Mobile phones are feature-rich with numerous components, yet the PCB board space is limited. Considering the constraints of the wiring design process, these demands elevate the requirements for design skills. At this juncture, opting for a design utilizing four to six PCB layers that function alternately rather than concurrently may be beneficial. High-power circuits may also occasionally include RF buffers and voltage-controlled oscillators (VCOs). Ensure that there is at least one whole floor dedicated to the high-power area on the PCB without any perforations. Naturally, more copper surface area is preferable. Sensitive analog signals should be maintained as distant as possible from high-speed digital signals and RF signals.
2. Design zones can be classified into physical and electrical zones. Physical partitioning mainly encompasses component layout, orientation, and shielding. Electrical partitioning can be further subdivided into sections for power distribution, RF wiring, sensitive circuits and signals, and grounding.
2.1 Let’s discuss physical partitioning. Component layout is pivotal in implementing RF design. A recommended technique is to first fix components along the RF path and orient them to minimize the RF path length, ensuring that inputs are distant from outputs and that high-power and low-power circuits are segregated as much as possible. A productive method for stacking circuit boards involves situating the primary ground layer (main ground) as the second layer beneath the surface, with RF lines predominantly on the surface. Reducing the diameter of through-holes in the RF path not only lessens path inductance but also diminishes virtual solder joints on the main ground and the likelihood of RF energy leakage into other regions of the laminate. In physical space, linear circuits such as multistage amplifiers typically suffice to isolate multiple RF regions from each other. However, diplexers, mixers, and IF amplifiers/mixers invariably contend with multiple RF/IF signals interfering with each other, necessitating meticulous mitigation.
2.2 RF and IF should be crossed as infrequently as feasible, with the widest feasible separation between them. Correct RF pathing is of paramount importance to overall PCB performance, which is why component layout generally consumes the majority of the time in mobile PCB design. In cell phone PCB designs, it is often viable to situate the low-noise amplifier circuit on one side of the PCB and the high-power amplifier on the opposite side, eventually connecting them to the RF and baseband processor antenna on the same side via a diplexer. Ensuring that straight through-holes do not transmit RF energy from one side of the board to the other requires certain tricks, with a prevalent technique being the application of blind holes on both sides. The adverse effects of straight through-holes can be minimized significantly by situating them in regions on both sides of the PCB that are free from RF interference. Occasionally, ensuring adequate isolation between multiple circuit blocks proves unfeasible, necessitating consideration of a metal shield to contain RF energy within the RF region. Metal shielding must be thoroughly grounded and maintained at a suitable distance from components, thereby consuming valuable PCB space. Safeguarding the integrity of the shielding cover as extensively as possible is exceedingly crucial. Digital signal lines entering the metal shielding cover should predominantly traverse inner layers, while the wiring layer beneath the PCB board constitutes the stratum. RF signal lines can exit through the narrow gaps at the base of the metal shield cover and the wiring layer, while surrounding the gap with ample grounding connections; grounds across various layers can be interconnected via multiple apertures.
2.3 Proper and effective decoupling of chip power proves crucial. Numerous RF chips housing integrated linear circuits are exceptionally susceptible to power supply noise, typically necessitating up to four capacitors and an isolating inductor per chip to ensure thorough power supply noise filtration. Integrated circuits or amplifiers often feature an open drain output, requiring a pull-up inductor to furnish a high-impedance RF load and a low-impedance DC power supply. The same principle extends to decoupling the power supply at the inductor’s end. Some chips necessitate additional power to function optimally, warranting two or three sets of capacitors and inductors respectively for their decoupling; inductors grouped together in close proximity can form a tubular transformer and induce mutual interference signals, necessitating a spatial separation equating to at least one device height, or an orthogonal placement to mitigate mutual inductance.
2.4 The principles governing electrical zoning generally mirror those governing physical zoning, though several additional factors come into play. Various segments of the phone operate at distinct voltages, regulated by software to prolong battery life. This necessitates running the phone on multiple power sources, thereby complicating isolation. Typically, power enters via the connector, promptly decoupled to eliminate any external circuit board noise, and subsequently distributed via a set of switches or regulators. The DC current flowing through most cell phone PCBs remains relatively modest, rendering wiring width a non-issue, though a dedicated high-current line with the greatest feasible width must be established for the high-power amplifier’s power supply to minimize voltage transmission losses. To circumvent excessive current loss, multiple apertures facilitate current transfer across different layers. Moreover, inadequate decoupling at the power pin end of the high-power amplifier may lead to the pervasive propagation of high-power noise throughout the board, culminating in a plethora of complications. Proper grounding of high-power amplifiers is pivotal and frequently mandates the inclusion of a metal shield. In most scenarios, it proves equally imperative to guarantee that RF output remains well clear of RF input. This necessity extends to amplifiers, buffers, and filters, which under dire circumstances may incite self-excited oscillations should their outputs be inadvertently fed back into their inputs at a congruous phase and amplitude. Under such conditions, they will no longer operate in a stable fashion across all temperature and voltage conditions. Instead, they may become unstable and superimpose noise and intermodulation signals onto RF signals. Routing the RF signal line back from the filter’s input to its output can significantly impair the filter’s bandpass characteristics. To ensure effective input and output isolation, a field should initially encircle the filter and subsequently extend to the lowermost region, thereby linking with the main ground that envelops the filter. Placing signal lines necessitating filter traversal as far removed from the filter pins as possible further optimizes performance.
2.5 To ensure noise remains at a minimum, several considerations must be upheld. Firstly, the anticipated bandwidth span of the control line may range from DC to 2MHz, rendering the complete elimination of such a broad band noise through filtration an almost unattainable feat. Secondly, the VCO control line usually serves as a constituent of a feedback loop that governs frequency, thereby inviting potential noise induction from numerous sources; therefore, meticulous handling of the VCO control line proves imperative. Ensuring a robust RF plane and securely fastened connections to the main plane, isolated from potential sources of noise, further enhances stability. Furthermore, maintaining adequate decoupling of the VCO’s power supply warrants special attention, given its propensity for generating RF outputs that typically attain relatively high levels and are apt to interfere with other circuits. In practice, the VCO is often sited at the terminus of the RF region, sometimes necessitating a protective metal shield. Resonant circuits (one for transmission and the other for reception) exhibit direct relevance to the VCO, albeit with unique attributes. Essentially, a resonant circuit comprises a parallel resonance circuit equipped with capacitive diodes to establish the VCO operating frequency and modulate voice or data into RF signals. All design principles applicable to VCOs similarly extend to resonant circuits. Resonant circuits typically prove highly sensitive to noise, owing to their incorporation of numerous components, expansive distribution across the board, and operation at elevated RF frequencies. Normally, signals are situated adjacent to the pins of the chip, necessitating their pairing with comparably large inductors and capacitors to achieve functionality; hence, these inductors and capacitors must be positioned in close proximity and connected within a noise-sensitive control loop. Accomplishing this task is no mean feat.
3. The following aspects merit careful consideration in mobile phone PCB design:
3.1 Processing of power supply and grounding cables: Even if the PCB wiring is meticulously executed, neglecting to account for the interference arising from power supply and grounding cables can compromise product performance, potentially influencing product success rates. Hence, the wiring of power and ground cables demands meticulous handling to curtail noise interference, thereby ensuring product quality. It’s universally acknowledged among electronics design engineers that noise between ground and power lines stems from subpar decoupling capacitor addition. Regarding their relationships, ground lines should exceed power lines, and both should surpass signal line widths: typically, signal line widths range from 0.2 to 0.3mm, while finer widths may taper to 0.05 to 0.07mm, and power lines may expand from 1.2 to 2.5mm. Digital circuit PCBs can adopt a configuration of grounded copper conductors in the form of a loop network (which is infeasible for analog circuit grounding). Moreover, the implementation of large copper layer areas for grounding, where unused printed board spaces are connected as grounds, or the adoption of multilayer boards where power and grounding lines each occupy their designated layer, also proves beneficial.
3.2 Treatment of shared grounding between digital and analog circuits: Contemporary PCBs often transcend single-function circuits (digital or analog), embodying instead a hybrid blend of digital and analog circuits. Consequently, when executing wiring, the potential for interference necessitates careful consideration, particularly concerning noise interference on grounding lines. Digital circuits tend to feature higher frequencies, while analog circuits
RF PCB board design is often described as a “black art” due to theoretical uncertainties, but this view is only partially true. RF board design has many rules that can be followed and should not be ignored. However, in practical design, the most useful technique is learning how to compromise these principles and laws when precise implementation is hindered by various design constraints. There are numerous critical RF design topics worth discussing, such as impedance and impedance matching, materials for insulating layers and laminates, and wavelength and standing waves, all of which significantly impact the EMC and EMI of mobile phones.