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How do RF circuits and digital circuits live in harmony on the same PCB?

Jun 15, 2018

Monolithic RF devices greatly facilitate the application of wireless communications in a range of applications. The use of a suitable microcontroller and antenna combined with the transceiver can form a complete wireless communications link. They can be integrated on a small circuit board and used in wireless digital audio, 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 and analog circuits

If analog circuits (radio frequency) and digital circuits (microcontrollers) work individually, they may all work well, but once the two are placed on the same circuit board, using the same power supply to work together, the entire system is likely to be unstable. . This is mainly because the digital signal swings frequently between ground and a positive supply (3 V in size) and has a particularly short period, often ns. Due to the larger amplitude and the smaller 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 1 μV. Therefore, the difference between the digital signal and the RF signal will reach 10-6 (120 dB). Obviously, if the digital signal and the RF signal are not well separated, the weak RF signal may be damaged. As a result, the performance of the wireless device may deteriorate or even fail at all.

2. Frequently Asked Questions about RF Circuits and Digital Circuits on the Same PCB

Insufficient isolation of sensitive lines and noise signal lines is a frequent problem. As mentioned above, the digital signal has a high swing and contains a large number of high frequency harmonics. If the digital signal on the PCB is routed close to a sensitive analog signal, high frequency harmonics may be coupled. The most sensitive node of an RF device is usually a phase-locked loop (PLL) loop filter circuit, an external voltage-controlled oscillator (VCO) inductor, a crystal reference signal, and an antenna terminal. These parts of the circuit should be handled with special care.

(1) Power supply noise

Digital circuits are generally acceptable for power supply noise (less than 50 mV) because of several V swings in the input/output signal. Analog circuits are quite sensitive to power supply noise, especially for glitch voltages and other high-frequency harmonics. Therefore, the power line layout on PCB boards containing RF (or other analog) circuits must be more careful than on ordinary digital circuit boards and automatic routing should be avoided. It should also be noted that the microcontroller (or other digital circuit) will suddenly draw most of the current for a short period of time during each internal clock cycle because modern microcontrollers are designed using a CMOS process. Therefore, suppose a microcontroller runs at an internal clock frequency of 1 MHz. It will extract (pulse) current from the power supply at this frequency. If proper power supply decoupling is not used, voltage glitches on the power supply line will be caused. If these voltage glitches reach the power supply pins of the RF section of the circuit, they may be seriously destructive. Therefore, it is necessary to ensure that the analog power line is isolated from the digital circuit area.

(2) Unreasonable grounding

The RF board should always be wired with a ground plane connected to the negative side of the power supply. If handled improperly, it may cause some strange phenomena. This may be difficult for a digital circuit designer to understand because most digital circuits perform well even without a ground plane. In the RF band, even a short line will act like an inductor. For rough calculations, the inductance per mm length is about 1 nH, and the inductance of a 10 mm PCB line at 434 MHz is about 27 Ω. If you do not use a ground plane, most ground lines will be long and the circuit will not guarantee design features.

(3) Radiation from the antenna to other analog parts

This is often overlooked in circuits that contain radio frequencies 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 antenna of the RF transmitter is located near this PCB (or just on this PCB), the emitted high-frequency signal may reach the analog input of the ADC. Do not forget that any circuit line may send or receive RF signals like an antenna. If the ADC input is not handled properly, the RF signal may self-excite within the ESD diode of the ADC input, causing an ADC bias.

3. RF circuit and digital circuit solution on the same PCB

The following gives some general design and routing strategies in most RF applications. However, it is more important to follow the wiring recommendations for RF devices in practical applications.

(1) A reliable ground level

When designing a PCB with RF components, a reliable ground plane should always be used. Its purpose is to establish an effective 0 V point in the circuit, making all devices easy to decouple. 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, no signal coupling will occur between the two nodes that have been decoupled. It is important that the amplitudes of multiple signals on the board may differ by 120 dB. On surface-mounted PCBs, all signal wiring is on the same side of the component mounting surface, and the ground plane is on the opposite side. The ideal ground plane should cover the entire PCB (except under the antenna PCB). If more than two layers of PCB are used, the ground layer should be placed on the layer adjacent to the signal layer (such as the next layer of the component plane). Another good method is to fill the empty portions of the signal wiring layer with ground planes. These ground planes must be connected to the main ground plane through multiple vias. It should be noted that since the presence of a grounding point will cause the inductance characteristics of the side to change, the choice of inductance and the layout of the inductance must be carefully considered.

(2) Shorten the connection distance with the ground layer

All connections to the ground plane must be 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 one ground via, which may result in crosstalk between the two pads due to via connection resistance.

(3) RF decoupling

The decoupling capacitors should be placed as close as possible to the pins. Capacitor decoupling should be used for each pin that needs to be decoupled. With high quality ceramic capacitors, the dielectric type is preferably "NPO" and "X7R" works well in most applications. The ideal value of the selected capacitor should be such that the 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 approximately 4 Ω, and the inductance of the via is within the same range. The capacitors and vias in series form a notch filter for the signal frequency, allowing it to be effectively decoupled. The 33 pF capacitor is an ideal choice at 868 MHz. In addition to RF decoupling small value capacitors, a large value capacitor should also be placed on the power line to decouple low frequencies. A 2.2 μF ceramic or 10 μF tantalum capacitor can be selected.

(4) Star wiring for power supply

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

Partially and effectively from the power supply noise of the RF section. If a severely noisy module is placed on the same board, an inductor (beads) 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. Module power supply decoupling. Such modules are RS 232 drivers or switching power regulators.

(5) Reasonably arrange PCB layout

In order to reduce the interference from the noise module and the surrounding analog parts, the layout of the various circuit modules on the board is important. Always keep sensitive modules (RF sections and antennas) away from noise modules (microcontrollers and RS 232 drivers) to avoid interference.

(6) The effect of shielding RF signals on other analog parts

As mentioned above, RF signals can interfere with other sensitive analog circuit blocks, such as ADCs. Most problems occur in the lower operating frequency bands (eg 27 MHz) and 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 board loop antennas

The antenna can be integrated on the PCB. Compared with the traditional whip antenna, it not only saves space and production costs, but is also more stable and reliable. In practice, a loop antenna design is applied to a relatively narrow bandwidth, which helps 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 by nearby noise signal lines. It can interfere with the receiver and it can also affect the transmitter's modulation. Therefore, no digital signal lines must be placed near the antenna and it is recommended to keep free space around the antenna. Any object close to the antenna will form part of the tuning network, which will cause the antenna to tune away from the desired frequency, reducing the transmit and receive radiation range (distance). This fact must be taken into account for all types of antennas. The circuit board housing (outside package) may also affect antenna tuning. At the same time, attention should be paid to removing the ground plane at the antenna area, otherwise the antenna cannot work effectively.

(8) Connection of the circuit board

If the RF circuit board is connected to an external digital circuit with a cable, a twisted pair cable should be used. 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 board and the digital application board with the GND wire of the twisted pair cable. The cable length should be as short as possible. The line supplying the RF circuit board must also be twisted with GND (VDD/GND).

4 Conclusion

Rapidly developing radio frequency integrated circuits provide the largest range of engineering and technical personnel who design wireless digital audio and video data transmission systems, wireless remote control, telemetry systems, wireless data acquisition systems, wireless networks, and wireless security systems to solve bottlenecks in wireless applications. may. At the same time, the RF circuit design requires the designer to have certain practical experience and engineering design capabilities. This article is the author's experience in actual development, hoping to help many RF integrated circuit developers shorten the development cycle, avoid unnecessary detours, save manpower and financial resources.


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