Analysis of RF circuit design issues

1 Introduction

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.

2 Common Problems in RF Circuit Design

2.1 Interference between digital circuit modules and analog circuit modules

If the analog circuit (RF) and the digital circuit work separately, they may work well on their own. 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 particularly 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 lμV. Therefore, the difference between the digital signal and the RF signal can reach 120 dB. Obviously, if the digital signal cannot be separated from the RF signal 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.

2.2 Noise interference from power supply

RF circuits are very sensitive to power supply noise, especially glitch voltage and other high-frequency harmonics. Microcontrollers will suddenly absorb most of the current in a short period of time in each internal clock cycle. This is because modern microcontrollers are manufactured using CMOS technology. Therefore, assuming that a microcontroller runs at an internal clock frequency of 1MHz, 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.

2.3 Unreasonable

If the ground line 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 line will act like an inductor. Roughly calculated, the inductance per millimeter length is about l nH, and the inductive reactance of a 10 toni PCB line at 433 MHz is about 27Ω. If a ground layer is not used, most ground lines will be long and the circuit will not have the designed characteristics.

2.4 Antenna radiation interference to other analog circuit parts

In PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have analog-to-digital converters (ADCs) or digital-to-analog converters (DACs). 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. This will cause ADC deviation.

3 RF circuit design principles and solutions

3.1 RF Layout Concept

When designing RF layout, the following general principles must be met first:

(1) Isolate the high-power RF amplifier (HPA) and low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit:

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

(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.

3.2 Design principles for physical and electrical partitions

Design partitions can be decomposed into physical partitions and electrical partitions. Physical partitions mainly involve component layout, orientation, and shielding, etc. Electrical partitions can be further decomposed into power distribution, RF routing, sensitive circuits and signals, and grounding.

3.2.1 Physical partitioning principles

(1) Component layout principle. 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 and keep the input away from the output. Also, separate the high-power circuit and the low-power circuit as far as possible.

(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.

(3) RF devices and RF wiring layout principles. In physical space, linear circuits such as multi-stage amplifiers are usually sufficient to isolate multiple RF areas from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IF signals interfering with each other. Therefore, 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.

(4) Design principles to reduce interference coupling between high/low power devices. On a cellular phone PCB, the low noise amplifier circuit can usually be placed on one side of the PCB, and the high power amplifier on the other side, and finally connected to the antennas at the RF end and the baseband processor end on the same side through a duplexer. Skills should be used to ensure that the through hole does not transfer RF energy from one side of the board to the other side. A common technique is to use blind holes on both sides. The adverse effects of through holes can be minimized by arranging the through holes in areas on both sides of the PCB that are not affected by RF interference.