With the development of communication technology, radio frequency circuit technology is used more and more widely. The performance index of the radio frequency circuit directly affects the quality of the whole product, and the anti-interference design of the printed circuit board (PCB) of the radio frequency circuit is of great significance for reducing the electromagnetic information radiation of the system.
There are still many uncertainties in RF circuit board design in theory. In actual design, the real practical skill is how to compromise these guidelines and laws when they cannot be accurately implemented due to various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminated boards, wavelengths and standing waves, etc. Careful planning under the premise of a comprehensive grasp of various design principles is the guarantee for a successful design at the first time . This article will list and answer frequently asked questions about RF circuit design.
1. Interference between digital circuit modules and analog circuit modules
If the analog circuit (RF) and the digital circuit work separately, each may work fine. However, once you put the two on the same board and work together from the same power supply, the whole system is likely to be unstable. This is primarily because digital signals frequently swing between ground and the positive supply (>3 V), and with extremely short periods, often on the order of nanoseconds. Due to the larger amplitude and shorter switching times. These digital signals contain a large number of high-frequency components 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 1 μV. Therefore, the difference between the digital signal and the RF signal can reach 120 dB. Obviously. If you can't separate the digital signal from the RF signal well. Weak radio frequency signals can be corrupted, and as a result, the wireless device's performance will deteriorate, or even not work at all.
2. Noise interference from power supply
RF circuits are quite sensitive to power supply noise, especially to glitch voltage and other high frequency harmonics. Microcontrollers will suddenly draw most of the current for a short period of time during each internal clock cycle, because modern microcontrollers are manufactured in a CMOS process. therefore. Suppose a microcontroller runs at an internal clock frequency of 1MHz, at which frequency it will draw current from the power supply. If proper decoupling of the power supply is not taken, it will inevitably cause voltage glitches on the power line. If these voltage spikes reach the power pins of the RF part of the circuit, it may cause work failure in severe cases.
3. Unreasonable ground wire
If the ground wire of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit designs, most digital circuits function well even without a ground plane. And at RF frequencies, even a very short ground lead acts like an inductor. Roughly calculated, the inductance per millimeter of length is about lnH, and the inductance of a 10 toni PCB line at 433 MHz is about 27Ω. Without a ground plane, most ground traces would be long and the circuit would not behave as designed.
4. Radiation interference from the antenna to other analog circuit parts
In PCB circuit design, there are usually other analog circuits on the board. For example, analog-to-digital conversion (ADC) or digital-to-analog converter (DAC) is found on many circuits. The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog signal of the ADC. If the processing at the ADC input is not done properly, the RF signal may self-excite within the ESD diodes at the ADC input. Thus causing ADC deviation.
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