Common problems in RF (Radio Frequency) circuit design

April 30, 2024

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 individually. However, once they 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 swing frequently between ground and positive power supply (> 3 V), and the cycle is particularly short, often at the nanosecond level. Due to the large amplitude and short switching time, these digital signals contain a large amount of high-frequency components independent of the switching frequency. In the analog part, the signal transmitted from the radio tuning loop to the receiving part of the radio device is generally less than 1 μV. 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 well separated, the weak RF signal may be damaged, resulting in deterioration of the performance of the wireless device, or even complete failure to work.

2. Noise interference from the power supply

RF circuits are quite sensitive to power supply noise, especially to voltage spikes and other high-frequency harmonics. The microcontroller will suddenly draw most of the current for a short time in each internal clock cycle, 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 adopted, it is bound to cause voltage spikes on the power line. If these voltage spikes reach the power pin of the RF part of the circuit, it may cause 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, even without a ground plane, most digital circuit functions perform well. At the RF frequency band, even a short ground wire can act like an inductor. Roughly calculated, the inductance per millimeter length is about 1 nH, and the inductance at 433 MHz for a 10-toni PCB line is about 27 Ω. If a ground plane is not used, most ground wires will be relatively long, and the circuit will not have the designed characteristics.

4. Radiation interference of the antenna 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 conversion (ADC) or digital-to-analog conversion (DAC). The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog input end of the ADC. Because any circuit line may emit or receive RF signals like an antenna. If the processing at the ADC input end is unreasonable, the RF signal may self-excite in the ESD diode at the ADC input end, thereby causing ADC deviation.