Discussion on the Dilemma and Improvement Measures of RF Circuit PCB Design

btty038

Discussion on the Dilemma and Improvement Measures of RF Circuit PCB Design [Copy link]

Radio frequency (RF) PCB design has many uncertainties in the 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 under the premise of fully mastering various design principles is the guarantee of a one-time successful design. For PC-type digital circuits above the microwave frequency band and high frequency, 2~3 versions of PCB are required to ensure the circuit quality. For RF circuits above the microwave frequency band, more versions of PCB design are often required and continuously improved, and this is based on considerable experience. This shows the difficulty of RF electrical design.

Typical RF board

Wireless Internet Module

You need to be familiar with the product architecture and signal flow before layout

1 Common Problems in RF Circuit Design

1.1 Interference between digital circuit modules and analog circuit modules
If the analog circuit (RF) and the digital circuit work separately, they may work well. However, once the two are placed on the same circuit board and work together using the same power supply, the entire system is likely to be unstable. This is mainly because digital signals frequently swing between the ground and the positive power supply (>3 V), and the cycle is extremely short, often in the nanosecond level. Due to the large amplitude and short switching time, these digital signals contain a large number of high-frequency components that are independent of the switching frequency. In the analog part, the signal transmitted from the wireless tuning loop to the receiving part of the wireless device is generally less than 1μV. Therefore, the difference between the digital signal and the RF signal can reach 120 dB. Obviously, if the digital signal and the RF signal cannot be separated well, the weak RF signal may be damaged, so that the working performance of the wireless device will deteriorate or even fail to work at all.

Common interference phenomena

Partition design of digital-analog RF hybrid circuit

1.2 Power supply noise interference
RF circuits are very sensitive to power supply noise, especially to glitch voltage and other high-frequency harmonics. Microcontrollers will suddenly absorb most of the current for a short time in each internal clock cycle. This is because modern microcontrollers are manufactured using CMOS technology. Therefore, assuming a microcontroller runs at an internal clock frequency of 1 MHz, it will extract current from the power supply at this frequency. If proper power supply decoupling is not taken, voltage glitches on the power supply line will inevitably occur. If these voltage glitches reach the power pins of the RF part of the circuit, they may cause work failure in severe cases.

Pay attention to the rules of RF chip power supply layout and capacitor arrangement

Important high-speed clock signal routing is on the inner layer with complete reference plane and wrapping protection

1.3 Unreasonable ground wire
If the ground wire of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit design, most digital circuit functions are good even without a ground layer. In the RF frequency band, even a very short ground wire will act like an inductor. Roughly calculated, the inductance per millimeter length is about 1 nH, and the inductive reactance of a 10 mm PCB line is about 27Ω at 433 MHz. If a ground layer is not used, most ground wires will be long and the circuit will not have the designed characteristics.

Irrational connection

1.4 Antenna radiated interference to other analog circuits
In PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have analog/digital converters (ADC) or digital/analog converters (DAC). The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog input of the ADC. Because any circuit line may emit or receive RF signals like an antenna. If the processing of the ADC input is not reasonable, the RF signal may self-excite in the ESD diode of the ADC input, causing ADC deviation.

Antenna prohibited areas and antennas interfere with internal circuits

2 RF Circuit Design Principles and Solutions
2.1 RF Layout Concepts
When designing RF layout, the following general principles must be met first:
(1) Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. In simple terms, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit.

Power amplifier circuit PA bias quarter wavelength design

One word arrangement principle

Amplifier bias inductor vertical arrangement

Wrong amplifier bias layout

(2) Ensure that there is at least one whole ground plane in the high-power area of the PCB board, preferably without vias. Of course, the larger the copper foil area, the better.

Sufficient GND vias to improve signal grounding and heat dissipation

RF filters and other grounding should be sufficient

(3) Circuit and power supply decoupling is also extremely important.
(4) RF output usually needs to be far away from RF input.
(5) Sensitive analog signals should be kept as far away from high-speed digital signals and RF signals as possible.

AD9361 transceiver circuit isolation and balun differential impedance matching

2.2 Design principles of physical partitioning and electrical partitioning
Design partitioning can be decomposed into physical partitioning and electrical partitioning. Physical partitioning mainly involves component layout, direction and shielding, etc.; electrical partitioning can be further decomposed into partitioning for power distribution, RF routing, sensitive circuits and signals, and grounding, etc.

RF routing and full-page punching

2.2.1 Physical Partitioning Principles
(1) Component Positioning Principles. Component layout is the key to achieving an excellent RF design. The most effective technique is to first fix the components on the RF path and adjust their direction to minimize the length of the RF path, keep the input away from the output, and separate the high-power circuit and the low-power circuit as far as possible.

RX/TX grounding or shielding isolation

(2) PCB stacking design principles. 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 place the RF line on the surface layer as much as possible. Minimize the size of the vias on the RF path, which can not only reduce the path inductance, but also reduce the number of cold solder joints on the main ground and reduce the chance of RF energy leaking to other areas in the stacked board.

Common RF plate models

Reasonable Ro4350 RF stacking and impedance line width design

(3) RF devices and RF wiring layout principles. In physical space, linear circuits such as multi-stage amplifiers are usually sufficient to isolate individual RF areas from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully minimized. RF and IF traces should be crossed as much as possible, and a ground plane should be placed between them as much as possible. The correct RF path is very important to the performance of the entire PCB, which is why component layout usually takes up most of the time in cellular phone PCB design.

Two-choice circuit design needs to pay attention to open microstrip

2.2.2 Electrical Partitioning Principles
(1) Power transmission principle. The DC current of most circuits in cellular phones is quite small, so the wiring width is usually not a problem. However, a large current line as wide as possible must be set up for the power supply of the high-power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, multiple through-holes are required to transfer the current from one layer to another.
(2) Power supply decoupling of high-power devices. If the power supply pin of the high-power amplifier cannot be adequately decoupled, the high-power noise will be radiated to the entire board and cause a variety of problems. The grounding of the high-power amplifier is quite critical, and a metal shielding cover is often required for it.
(3) RF input/output isolation principle. In most cases, it is also critical to ensure that the RF output is far away from the RF input. This also applies to amplifiers, buffers, and filters. In the worst case, if the outputs of the amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may produce self-oscillation. In the best case, they will be able to operate stably under any temperature and voltage conditions. In fact, they may become unstable and add noise and intermodulation signals to the RF signal.

Input and output need to be separated

3 Conclusion
In summary, RF circuits are distributed parameter circuits, and have skin effect and coupling effect, which are different from low-frequency circuits and DC circuits. Therefore, when designing RF circuit PCB, special attention should be paid to these issues to ensure the effectiveness and accuracy of circuit design. E-Link network data transmitter can also be used to build a new generation of Ethernet measurement and control system to transform the existing distributed control system composed of fieldbus and develop and produce network measurement and control equipment. It will promote the intelligence and networking of household appliances and bring profound changes to people's lifestyles.

annysky2012

It is written in great detail and is also suitable for non-RF series PCB layout. You can learn from it.

小白菜1号

It is written in great detail and is also suitable for non-RF series PCB layout. You can learn from it.