Common problems with RF circuits and digital circuits on the same PCB

　　Insufficient isolation of sensitive lines and noise signal lines is a common problem. As mentioned above, PCB circuit boards have high swing and contain a large number of high-frequency harmonics in digital signals. If the digital signal wiring on the PCB board is adjacent to sensitive analog signals, high-frequency harmonics may couple over. The most sensitive nodes of RF devices are usually the loop filtering circuit of the phase-locked loop (PLL), the external voltage controlled oscillator (VCO) inductor, the crystal oscillator reference signal, and the antenna terminal. These parts of the circuit should be handled with special care.

　　(1) Power supply noise

　　Due to the fluctuation of input/output signals by several volts, digital circuits are generally acceptable for power supply noise (less than 50 mV). However, analog circuits are quite sensitive to power noise, especially to spike voltage and other high-frequency harmonics. Therefore, the wiring of power lines on PCB boards containing RF (or other analog) circuits must be more careful than on ordinary digital circuit boards, and automatic wiring should be avoided. It should also be noted that microcontrollers (or other digital circuits) will suddenly draw in most of the current for a short period of time within each internal clock cycle, as modern microcontrollers are designed using CMOS technology. Therefore, assuming a microcontroller operates at an internal clock frequency of 1 MHz, it will extract (pulse) current from the power supply at this frequency. If appropriate power decoupling is not taken, it will inevitably cause voltage spikes on the power supply lines. If these voltage spikes reach the power pins of the RF section of the circuit, it may seriously cause operational failure, so it is necessary to ensure that the analog power lines are separated from the digital circuit area.

　　(2) Unreasonable ground wire

　　The RF circuit board should always have a ground layer connected to the negative pole of the power supply. If not handled properly, some strange phenomena may occur. For a digital circuit designer, this may be difficult to understand because even without a ground layer, most digital circuit functions well. In the RF frequency band, even a short line can act like an inductor. Roughly calculated, the inductance per mm length is about 1 nH, and the inductance of a 10 mm PCB circuit at 434 MHz is about 27 Ω. If a ground layer is not used, most ground wires will be longer and the circuit will not be able to guarantee design characteristics.

　　(3) Radiation from the antenna to other simulated parts

　　This is often overlooked in circuits that include radio frequency and other components. In addition to the RF part, there are usually other analog circuits on the board. For example, many microcontrollers have built-in analog-to-digital converters (ADCs) for measuring analog inputs as well as battery voltage or other parameters. If the antenna of the RF transmitter is located near (or on) this PCB, the high-frequency signal emitted may reach the analog input of the ADC. Don't forget that any circuit can emit or receive RF signals like an antenna. If the input processing of the ADC is not reasonable, the RF signal may self excite within the ESD diode of the ADC input, causing deviation in the ADC.