Common Problems in RF Circuit Design

btty038

Common Problems in RF Circuit Design [Copy link]

Common problems in RF circuit design
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 separately. 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 120dB. Obviously, if the digital signal cannot be well separated from the RF signal, 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. Noise interference of the power supply
RF circuits are quite sensitive to power supply noise, especially to glitch voltages and other high-frequency harmonics. Microcontrollers will suddenly absorb most of the current in a short time during 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 lMHz, 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, 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 design, most digital circuit functions perform well even without a ground wire 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 l nH, and the inductive reactance of a 10toni PCB line at 433 MHz is about 27Ω. If a ground wire layer is not used, most ground wires will be longer and the circuit will not have the designed characteristics.
4. Radiated interference of antennas 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 RF signal of the ADC. If the processing at the ADC input is not reasonable, the RF signal may self-excite in the ESD diode at the ADC input, thereby causing ADC deviation.