How to place both RF circuit and digital circuit on PCB board?

If the analog circuit (RF) and the digital circuit (microcontroller) work well individually, but once you put the two on the same circuit board and use the same power supply to work together, the entire system is likely to be unstable. This is mainly because the digital signal frequently swings between the ground and the positive power supply (size 3 V), and the period is particularly short, often ns level. Due to the large amplitude and small 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 from the antenna tuning loop to the receiving part of the wireless device is generally less than 1μV.

Inadequate isolation of sensitive lines and noisy signal lines is a frequent problem. As mentioned above, digital signals have a high swing and contain a large number of high-frequency harmonics. If the digital signal wiring on the PCB is adjacent to sensitive analog signals, high-frequency harmonics may be coupled past. The sensitive nodes of RF devices are usually the loop filter circuit of the phase-locked loop (PLL), the external voltage controlled oscillator (VCO) inductor, the crystal reference signal and the antenna terminal, and these parts of the circuit should be treated with special care.

Since the input/output signal has a swing of several V, digital circuits are generally acceptable for power supply noise (less than 50 mV). Analog circuits are sensitive to power supply noise, especially to burr voltages and other high frequency harmonics. Therefore, the power line routing on the PCB board containing RF (or other analog) circuits must be more careful than the wiring on the ordinary digital circuit board, and automatic routing should be avoided. It should also be noted that a microcontroller (or other digital circuit) will suddenly suck in most of the current for a short period of time during each internal clock cycle, due to the CMOS process design of modern microcontrollers.

The RF circuit board should always have a ground line layer connected to the negative electrode of the power supply, which may produce some strange phenomena if not handled properly. This may be difficult for a digital circuit designer to understand, because most digital circuits function well even without the grounding layer. In the RF band, even a short wire acts like an inductor. Roughly calculated, the inductance per mm length is about 1 nH, and the inductive reactance of a 10 mm PCB line at 434 MHz is about 27 Ω. If the ground line layer is not used, most ground lines will be longer and the circuit will not guarantee the design characteristics.

This is often overlooked in circuits that contain the radio frequency and other parts. In addition to the RF portion, there are usually other analog circuits on the board. For example, many microcontrollers have built-in analog-to-digital converters (ADCs) to measure analog inputs as well as battery voltage or other parameters. If the RF transmitter’s antenna is located near (or on) this PCB, the emitted high-frequency signal may reach the analog input of the ADC. Don’t forget that any circuit line can send or receive RF signals like an antenna. If the ADC input is not properly processed, the RF signal may self-excite in the ESD diode input to the ADC, causing ADC deviation.

All connections to the ground layer must be as short as possible, and the ground through-hole should be placed (or very close to) the pad of the component. Never allow two ground signals to share a ground through-hole, which can cause crosstalk between the two pads due to the through-hole connection impedance. The decoupling capacitor should be placed as close to the pin as possible, and capacitor decoupling should be used at each pin that needs to be decoupled. Using high-quality ceramic capacitors, the dielectric type is “NPO”, “X7R” also works well in most applications. The ideal value of the selected capacitance should be such that its series resonance is equal to the signal frequency.

For example, at 434 MHz, the SMD-mounted 100 pF capacitor will work well, at this frequency, the capacitive reactance of the capacitor is about 4 Ω, and the inductive reactance of the hole is in the same range. The capacitor and the hole in series form a notch filter for the signal frequency, allowing it to be effectively decoupled. At 868 MHz, 33 p F capacitors are an ideal choice. In addition to the RF decoupled small value capacitor, a large value capacitor should also be placed on the power line to decouple the low frequency, can choose a 2.2 μF ceramic or 10μF tantalum capacitor.

Star wiring is a well-known technique in analog circuit design. Star wiring – Each module on the board has its own power line from the common power supply power point. In this case, the star wiring means that the digital and RF parts of the circuit should have their own power lines, and these power lines should be decoupled separately near the IC. This is a separation from the numbers

An effective method for partial and power supply noise from the RF portion. If the modules with severe noise are placed on the same board, the inductor (magnetic bead) or the small resistance resistance (10 Ω) can be connected in series between the power line and the module, and the tantalum capacitor of at least 10 μF must be used as the power supply decoupling of these modules. Such modules are RS 232 drivers or switching power supply regulators.

In order to reduce the interference from the noise module and the surrounding analog part, the layout of each circuit module on the board is important. Sensitive modules (RF parts and antennas) should always be kept away from noisy modules (microcontrollers and RS 232 drivers) to avoid interference. As mentioned above, RF signals can cause interference to other sensitive analog circuit modules such as ADCs when they are sent. Most problems occur in lower operating bands (such as 27 MHz) as well as high power output levels. It is a good design practice to decouple sensitive points with an RF decoupling capacitor (100p F) connected to the ground.

If you are using cables to connect the RF board to an external digital circuit, use twisted-pair cables. Each signal cable must be twinned with the GND cable (DIN/ GND, DOUT/ GND, CS/ GND, PWR _ UP/ GND). Remember to connect the RF circuit board and the digital application circuit board with the GND cable of the twisted-pair cable, and the cable length should be as short as possible. The wiring that powers the RF board must also be twisted-with GND (VDD/ GND).