PCB News - How to place RF circuit and digital circuit on PCB at the same time

Analog (radio frequency) and digital (microcontroller) circuits may work well individually, but once the two are on the same printed circuit board and working together using the same power supply, the whole system is likely to be unstable. This is mainly because the digital signal oscillates frequently between the ground and the positive power supply (size 3 V), and the period is extremely short, often ns class. Due to the large amplitude and small switching time, these digital sig

nals contain a large number of high frequency components independent of switching frequency. In the analog part, the signal transmitted 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 common problem. As mentioned above, digital signals have high swing amplitude 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 the RF device 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 terminals. These parts of the circuit should be handled with special care.

Since input/output signals have a swing of several V, digital circuits are generally acceptable to power noise (less than 50 mV). Analog circuits are very sensitive to power noise, especially to burr voltage and other high-frequency harmonics. Therefore, power lines on PCBS containing RF(or other analog) circuits must be routed more carefully than on ordinary digital boards, and automatic wiring should be avoided. It should also be noted that microcontrollers (or other digital circuits) will suddenly suck in most of the current for a short period of time within each internal clock cycle, because modern microcontrollers are designed using CMOS processes.

RF boards should always have a grounding layer connected to the negative power supply, which can produce some strange phenomena if not handled properly. This may be difficult for a digital circuit designer to understand, since most digital circuits function well even without ground layers. In RF, even a very short wire acts like inductance. By rough calculation, the inductance per mm length is about 1 nH, and the inductive reactance of 10 mmPCB lines at 434 MHz is about 27 ω. If ground wires are not used, most ground wires will be long and the design characteristics of the circuit will not be guaranteed.

This is often overlooked in circuits that contain rf 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 rf transmitter antenna is located near (or on) the PCB, the high frequency signal emitted may reach the analog input of the ADC. Don't forget that any circuit can send or receive RF signals just like an antenna. If the ADC input is not properly processed, the RF signal may be self-excited within the ESD diode of the ADC input, resulting in ADC deviation.

All connections to the ground layer must be as short as possible and the ground hole should be placed at (or very close to) the pad of the element. Never allow two ground signals to share a single ground through hole. This may cause crosstalk between pads due to the resistance of the hole connection. The decoupling capacitor should be placed as close to the pins as possible and should be used at each pin where decoupling is required. High quality ceramic capacitors, dielectric type "NPO", and "X7R" work well in most applications. The ideal value of selected capacitance should be such that its series resonance equals the signal frequency.

For example, the SMD mounted 100 p F capacitor will work well at 434 MHz, at which capacitive reactance is about 4 ω, and through hole reactance is in the same range. The capacitors and holes in series form a notch filter for signal frequency, which can be effectively decoupled. At 868 MHz, 33 p F capacitor is an ideal choice. In addition to RF decoupling of the low value capacitor, a high value capacitor should also be placed on the power line to decouple the low frequency, choose a 2. 2 μF ceramic or 10μF tantalum capacitor.

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

An effective method of partial and partial power supply noise from RF. If heavy noise modules are placed on the same circuit board, inductors (beads) or small resistance (10 ω) can be connected in series between the power cord and modules, and tantalum capacitors of at least 10 μF must be used to decouple the power supply of these modules. Such modules as RS 232 drivers or switching power 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 with other sensitive analog circuit modules such as ADC when transmitted. Most problems occur at lower operating bands (e.g. 27 MHz) as well as at high power output levels. It is a good design practice to use RF decoupling capacitance (100p F) connected to ground to decouple sensitive points.

If using cable to connect RF board to external digital circuit, use twisted-pair cable. Each signal cable must be twin-twisted with the GND cable (DIN/ GND, DOUT/ GND, CS/ GND, PWR _ UP/ GND). Connect the RF circuit board to the digital application circuit board using the GND cable of the twisted-pair cable. The cable length should be as short as possible. The line supplying the RF board must also be connected to GND twisted-pair (VDD/ GND).