How to place the two masters of RF circuit and digital circuit on a PCB board?
Single-chip radio frequency devices greatly facilitate the application in the field of wireless communications within a certain range. Using appropriate microcontrollers and antennas combined with this transceiver device can form a complete wireless communication link. They can be integrated on a very small circuit board and used in many fields such as wireless digital audio and digital video data transmission systems, wireless remote control and telemetry systems, wireless data acquisition systems, wireless networks and wireless security systems.
1. Potential conflicts between digital circuits and analog circuits
If analog circuits (radio frequency) and digital circuits (microcontrollers) work alone, they may work well, but once they are put 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 ground and positive power (3 V), and the period is extremely short, often on the 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 transmitted from the antenna tuning loop to the receiving part of the wireless device is generally less than 1μV. Therefore the difference between the digital signal and the RF signal will be 10-6 (120 dB). Obviously, if the digital signal and the radio frequency signal cannot be well separated, the weak radio frequency signal may be destroyed. As a result, the performance of the wireless device will deteriorate or even fail to work at all.
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2 Common problems when RF circuits and digital circuits are installed on the same PCB
Failure to adequately isolate sensitive lines and noisy signal lines is a common problem. As mentioned above, digital signals have high swings and contain a large number of high-frequency harmonics. If digital signal traces on a PCB are placed in close proximity to sensitive analog signals, high-frequency harmonics may couple through. The most 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 terminals. These parts of the circuit should be handled with special care.
(1) Power supply noise
Since the input/output signals have a swing of several V, digital circuits are generally acceptable for power supply noise (less than 50 mV). Analog circuits are quite sensitive to power supply noise, especially glitch voltages and other high-frequency harmonics. Therefore, routing power lines on PCBs containing RF (or other analog) circuits must be more careful than routing on ordinary digital circuit boards, and automatic routing should be avoided. It should also be noted that a microcontroller (or other digital circuit) will suddenly draw most of the current for a short period of time during each internal clock cycle. This is because modern microcontrollers are designed using a CMOS process. So, assuming a microcontroller is running at an internal clock frequency of 1 MHz, it will draw (pulse) current from the power supply at this frequency, which will inevitably cause voltage glitches on the power supply line if proper power supply decoupling is not taken. If these voltage glitches reach the power pins of the RF part of the circuit, serious work failures may occur, so it is necessary to ensure that the analog power lines are separated from the digital circuit area.
(2) Unreasonable ground wire
RF circuit boards should always have a ground layer connected to the negative pole of the power supply. If not handled properly, some strange phenomena may occur. This may be difficult for a digital circuit designer to understand because most digital circuit functions perform well even without a ground plane. In the RF band, even a short wire can act like an inductor. A rough calculation shows that 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 layer is not used, most ground wires will be long and the circuit will not be able to guarantee the design characteristics.
(3) Radiation from the antenna to other analog parts
This is often overlooked in circuits that contain RF and other parts. In addition to the RF section, 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’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 trace may act like an antenna, emitting or receiving RF signals. If the ADC input is not processed properly, the RF signal may self-excite in the ESD diode of the ADC input, causing ADC deviation.
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3 Solutions for RF circuits and digital circuits to be installed on the same PCB
The following are some common design and layout strategies found in most RF applications. However, it is more important to follow the layout recommendations for RF devices in actual applications.
(1) A reliable ground layer
When designing a PCB with RF components, a reliable ground layer should always be used. Its purpose is to establish an effective 0 V potential point in the circuit so that all devices can be easily decoupled. The 0 V terminal of the power supply should be connected directly to this ground plane. Due to the low impedance of the ground plane, there will be no signal coupling between the two nodes that have been decoupled. This is important for multiple signals on the board whose amplitudes may differ by 120 dB. On a surface mount PCB, all signal routing is on the same side as the component mounting surface, and the ground plane is on the opposite side. Ideally the ground plane should cover the entire PCB (except under the antenna PCB). If a PCB with more than two layers is used, the ground layer should be placed on the layer adjacent to the signal layer (such as the layer below the component surface). Another good approach is to also fill the empty parts of the signal routing layer with ground planes, which must be connected to the main ground plane through multiple vias. It should be noted that since the existence of a ground point will cause the characteristics of the nearby inductor to change, the selection of the inductor value and the placement of the inductor must be carefully considered.
(2) Shorten the connection distance to the ground layer
All connections to the ground plane must be kept as short as possible, and ground vias should be placed at (or very close to) the component pads. Never allow two ground signals to share a ground via, as this may cause crosstalk between the two pads due to the via connection impedance.
(3) RF decoupling
Decoupling capacitors should be placed as close to the pins as possible, and capacitor decoupling should be used at each pin that needs decoupling. Using high quality ceramic capacitors, preferably “NPO” dielectric type, “X7R” will work well in most applications. Ideally the capacitor value should be chosen so that its series resonance is equal to the signal frequency. For example, at 434 MHz, an 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 via is also in the same range. The series connected capacitors and vias form a notch filter for the signal frequency, enabling effective decoupling. At 868 MHz, a 33 pF capacitor is an ideal choice. In addition to the small value capacitor for RF decoupling, a large value capacitor should also be placed on the power line to decouple low frequencies. You can choose a 2. 2 μF ceramic or 10 μF tantalum capacitor.
(4) Star wiring of power supply
Star wiring is a well-known technique in analog circuit design (shown in Figure 1). Star wiring – Each module on the circuit board has its own power line from a common power supply point. In this case, star wiring means that the digital and RF parts of the circuit should have their own power lines, which should be separately decoupled close to the IC. This is a separation from numbers
Efficient method for summing up power supply noise from the RF section. If a module with severe noise is placed on the same circuit board, an inductor (magnetic bead) or a small value resistor (10 Ω) can be connected in series between the power line and the module, and a tantalum capacitor of at least 10 μF must be used for these. Decoupling of the module’s power supply. Such modules are RS 232 drivers or switching power supply regulators.
(5) Reasonably arrange the PCB layout
In order to reduce interference from the noise module and surrounding analog parts, the layout of each circuit module on the board is important. Sensitive modules (RF section and antenna) should always be kept away from noisy modules (microcontroller and RS 232 driver) to avoid interference.
(6) Shield the impact of RF signals on other analog parts
As mentioned above, RF signals can cause interference to other sensitive analog circuit modules such as ADCs when being sent. Most problems occur at lower operating frequency bands (such as 27 MHz) and at high power output levels. It is a good design practice to decouple sensitive points with RF decoupling capacitors (100pF) connected to ground.
(7) Special considerations for on-board loop antennas
The antenna can be built entirely on the PCB. Compared with traditional whip antennas, it not only saves space and production costs, but is also more stable and reliable in structure. Conventionally, loop antennas are designed for relatively narrow bandwidths, which help suppress unwanted strong signals from interfering with the receiver. It should be noted that loop antennas (like all other antennas) may receive noise capacitively coupled from nearby noisy signal lines. It can interfere with the receiver and may also affect the modulation of the transmitter. Therefore, be sure not to lay digital signal lines near the antenna, and it is recommended to maintain free space around the antenna. Any object close to the antenna will form part of the tuning network, causing the antenna tuning to deviate from the expected frequency point, reducing the transmitting and receiving radiation range (distance). As with all types of antennas one must be aware of the fact that the circuit board enclosure (outer packaging) may also affect antenna tuning. At the same time, attention should be paid to removing the ground layer at the antenna area, otherwise the antenna will not work effectively.
(8) Circuit board connection
If you use cables to connect the RF circuit board to external digital circuits, use twisted pair cables. Each signal line must be twisted together with the GND line (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 line of the twisted pair cable, and the cable length should be as short as possible. The lines supplying power to the RF circuit board must also be twisted with GND (VDD/GND).
4 Conclusion
The rapid development of radio frequency integrated circuits provides the greatest opportunity for engineers and technicians engaged in the design of wireless digital audio and video data transmission systems, wireless remote control and telemetry systems, wireless data acquisition systems, wireless networks and wireless security systems to solve the bottlenecks of wireless applications. possible. At the same time, the design of radio frequency circuits requires designers to have certain practical experience and engineering design capabilities. This article is the author’s experience summarized in actual development. I hope it can help many RF integrated circuit developers shorten the development cycle, avoid unnecessary detours, and save manpower and financial resources.
