Common Problems in Radio Frequency (RF) Circuit Design

RF PCB design is fraught with uncertainties in currently published theories and is often described as a "black art". Generally speaking, for circuits below the microwave frequency band (including low-frequency and low-frequency digital circuits), careful planning with a comprehensive grasp of various design principles guarantees a successful one-time design. For circuits above the microwave frequency band and high-frequency PC digital circuits, 2 to 3 versions of the PCB are required to ensure circuit quality. For RF circuits above the microwave frequency band, however, more PCB design iterations and continuous improvements are often needed, even with considerable design experience. This underscores the difficulties of RF circuit design.

Analog (RF) and digital circuits may function well independently, but when integrated on the same circuit board and powered by a single power supply, the entire system is likely to become unstable. This is mainly because digital signals switch frequently between ground and the positive power supply (＞3 V) with an extremely short period, often on the nanosecond scale. Due to their large amplitude and short switching time, these digital signals contain a large number of high-frequency components independent of the switching frequency. In the analog section, the signal transmitted from the wireless tuning circuit to the receiving part of the wireless device is generally less than 1 μV. Thus, the difference between digital and RF signals can reach 120 dB. Obviously, if digital and RF signals are not well isolated, the weak RF signal may be corrupted, degrading the performance of the wireless device or even rendering it completely inoperable.

RF circuits are quite sensitive to power supply noise, especially glitches and other high-frequency harmonics. Microcontrollers draw most of their current in short bursts during each internal clock cycle, a characteristic of modern CMOS-based microcontrollers. For example, a microcontroller operating at an internal clock frequency of 1 MHz will draw current from the power supply at this frequency. Without proper power supply decoupling, voltage glitches on the power line are inevitable. If these glitches reach the power pins of the RF section of the circuit, it may lead to operational failure in severe cases.

Poor grounding in RF circuits can cause anomalous phenomena. For digital circuit design, most digital circuits function well even without a ground plane. However, in the RF frequency band, even a short ground wire acts as an inductor. A rough calculation shows an inductance of approximately 1 nH per millimeter of length, with a reactance of about 27 Ω for a 10 mm PCB trace at 433 MHz. Without a ground plane, most ground wires will be long, and the circuit will fail to perform as designed.

PCB circuit designs typically include other analog circuits, such as analog-to-digital converters (ADCs) or digital-to-analog converters (DACs) in many circuits. High-frequency signals emitted by the antenna of an RF transmitter may reach the analog input of the ADC, as any circuit trace can act as an antenna to emit or receive RF signals. Improper handling of the ADC input can cause RF signals to oscillate in the ESD diodes at the ADC input, leading to ADC offset.

When designing an RF layout, the following general principles must be prioritized:

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

Component placement principles Component layout is the key to an excellent 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 separate high-power and low-power circuits as much as possible.

PCB stack-up design principles The most effective PCB stack-up method is to place the main ground plane on the second layer below the top layer and arrange RF traces on the top layer as much as possible. Minimize the size of vias on the RF path—this not only reduces path inductance but also reduces cold solder joints on the main ground plane and the chance of RF energy leaking to other areas in the stacked board.

RF device and RF trace layout principles In physical space, linear circuits such as multi-stage amplifiers are usually sufficient to isolate multiple RF regions from each other. However, duplexers, mixers and IF amplifier/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully minimized. RF and IF traces should cross at right angles as much as possible, with a ground plane between them if possible. A proper RF path is critical to the performance of the entire PCB, which is why component layout usually takes up most of the time in cellular phone PCB design.

Design principles for reducing interference coupling between high/low power devices On cellular phone PCBs, the low noise amplifier circuit can usually be placed on one side of the PCB and the high power amplifier on the other, eventually connecting them to the antenna on the same side for the RF and baseband processor ends via a duplexer. Techniques must be used to ensure vias do not transfer RF energy from one side of the board to the other; a common technique is to use blind vias on both sides. The adverse effects of vias can be minimized by placing them in areas on both sides of the PCB that are not subject to RF interference.

Power transmission principles The DC current of most circuits in cellular phones is quite small, so trace width is usually not an issue. However, a dedicated, as wide as possible high-current trace must be provided for the power supply of the high power amplifier to minimize transmission voltage drop. To avoid excessive current loss, multiple vias are needed to transfer current from one layer to another.

Power supply decoupling for high-power devices Insufficient decoupling at the power pins of the high power amplifier will cause high-power noise to radiate across the entire board and cause various problems. Grounding of the high power amplifier is critical, and a metal shield is often required for it.

RF input and output isolation principles In most cases, it is equally critical to ensure RF outputs are kept away from RF inputs—this also applies to amplifiers, buffers and filters. In the worst case, amplifiers and buffers may oscillate if their outputs are fed back to their inputs with the appropriate phase and amplitude. In the best case, they will operate stably under all temperature and voltage conditions. In practice, they may become unstable and add noise and intermodulation signals to the RF signal.

Filter input and output isolation principles If RF signal traces have to loop back from the filter input to the output, this can seriously degrade the filter's bandpass characteristics. For good input-output isolation, a ground ring must first be placed around the filter. Second, a ground plane should be placed in the area under the filter and connected to the main ground around the filter. It is also a good practice to keep signal traces that need to pass through the filter as far as possible from the filter pins. In addition, grounding must be done carefully everywhere on the board, otherwise an unwanted coupling path may be introduced unknowingly.

Isolation of digital and analog circuits Keeping digital circuits as far as possible from analog circuits is a general principle in all PCB designs, and it also applies to RF PCB design. The common analog ground and the ground used for shielding and separating signal traces are usually equally important; a negligent design change may result in a near-complete design having to be redone. RF traces should also be kept away from analog traces and some critical digital signals. As much grounded copper as possible should be filled around all RF traces, pads and components, and connected to the main ground as much as possible. If an RF trace must cross a signal trace, a ground plane connected to the main ground should be placed between them along the RF trace if possible. If not, they must cross at right angles—this minimizes capacitive coupling. At the same time, as much ground as possible should be placed around each RF trace and connected to the main ground. In addition, minimizing the distance between parallel RF traces minimizes inductive coupling.

The rapidly developing RF integrated circuits provide broad prospects for engineers and technicians engaged in various wireless communications. At the same time, RF circuit design requires designers to have certain practical experience and engineering design capabilities. The experience summarized in this paper can help RF integrated circuit developers shorten the development cycle, avoid unnecessary detours, and save human and material resources.

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