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

How do RF circuits and digital circuits live in harmony on the same PCB?
The single-chip radio frequency device greatly facilitates the application in the field of wireless communication within a certain range. A complete wireless communication link can be formed by using a suitable microcontroller and antenna in combination with the transceiver device. They can be integrated on a 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 contradictions between digital circuits and analog circuits

If the analog circuit (RF) and the digital circuit (microcontroller) work separately, they may work well, but once the two are placed on the same circuit board and work together with the same power supply, 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 (3V in size), and the period is very short, often in the ns level. Due to the larger amplitude and 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 1μV. Therefore, the difference between the digital signal and the radio frequency signal will reach 10-6 (120dB). Obviously, if the digital signal and the radio frequency signal cannot be separated well, the weak radio frequency signal may be destroyed. In this way, the working performance of the wireless device will deteriorate or even fail to work at all.

2 Common problems of RF circuit and digital circuit on the same PCB

Insufficient isolation of sensitive lines and noisy signal lines is a common problem. As mentioned above, the digital signal has a high swing and contains a large number of high frequency harmonics. If the digital signal wiring on the PCB board is adjacent to the sensitive analog signal, high-frequency harmonics may be coupled 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 terminal. These parts of the circuit should be handled with special care.

(1) Power supply noise

Since the input/output signal has a swing of several V, the digital circuit is generally acceptable for power supply noise (less than 50mV). The analog circuit is very sensitive to power supply noise, especially to glitch voltage and other high frequency harmonics. Therefore, the power line wiring on the PCB containing the RF (or other analog) circuit must be more careful than the wiring on the ordinary digital circuit board, and automatic wiring should be avoided. At the same time, it should be noted that the microcontroller (or other digital circuit) will suddenly draw most of the current in a short period of time within each internal clock cycle. This is because modern microcontrollers are designed with CMOS technology. Therefore, assuming a microcontroller runs at an internal clock frequency of 1MHz, it will draw (pulse) current from the power supply at this frequency. If proper power supply decoupling is not taken, it will inevitably cause a voltage glitch on the power line. If these voltage burrs reach the power supply pins of the RF part of the circuit, they may cause work failure in serious cases. Therefore, it is necessary to ensure that the analog power line is separated from the digital circuit area.

(2) Unreasonable ground wire

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

(3) Radiation of the antenna to other analog parts

In circuits that include radio frequency and other parts, this is often overlooked. 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 (ADC) to measure analog inputs and battery voltage or other parameters. If the antenna of the RF transmitter is located near this PCB (or on this PCB), the high-frequency signal sent out may reach the analog input of the ADC. Don't forget that any circuit line may emit or receive RF signals like an antenna. If the ADC input terminal is not processed properly, the RF signal may self-excited in the ESD diode input by the ADC, causing ADC deviation.

3 The solution of RF circuit and digital circuit on the same PCB

Some general design and wiring strategies in most RF applications are given below. However, it is more important to follow the routing recommendations for RF devices in actual applications.

(1) A reliable ground plane

When designing a PCB with RF components, a reliable ground plane should always be used. Its purpose is to establish an effective 0V potential point in the circuit so that all devices can be easily decoupled. The 0V terminal of the power supply should be directly connected 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. It is very important that the amplitude of multiple signals on the board may differ by 120dB. On a surface-mounted PCB, all signal wiring is on the same side of the component mounting surface, and the ground layer is on the opposite side. The ideal ground plane should cover the entire PCB (except below 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 method is to fill the vacant part 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 the existence of the grounding point will cause the nearby inductance characteristics to change, so the selection of the inductance value and the placement of the inductance must be carefully considered.

(2) Shorten the connection distance with the ground plane

All connections to the ground plane must be as short as possible, and the ground vias should be placed (or very close to) the component pads. Never let two ground signals share a ground via. 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 capacitors should be used for decoupling at each pin that needs to be decoupled. Using high-quality ceramic capacitors, the best dielectric type is "NPO". "X7R" can also work well in most applications. The ideal choice of capacitor value should make the series resonance equal to the signal frequency. For example, at 434MHz, SMD mounted 100pF capacitors will work well. At this frequency, the capacitive reactance of the capacitor is about 4Ω, and the inductive reactance of the via is also in the same range. The series capacitors and vias form a notch filter for the signal frequency, so that it can be effectively decoupled. At 868MHz, a 33pF 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 the low frequency. You can choose a 2.2μF ceramic or 10μF tantalum capacitor.

(4) Star wiring of power supply

Star wiring is a well-known technique in analog circuit design (as shown in Figure 1). Star wiring-each module on the circuit board has its own power supply line from the common power supply point. In this case, star wiring means that the digital part and RF part of the circuit should have their own power lines, and these power lines should be separately decoupled near the IC. This is an effective way to separate power supply noise from the digital part and from the RF part. If a module with serious noise is placed on the same circuit board, an inductor (magnetic bead) or a small resistance resistor (10Ω) can be connected in series between the power line and the module, and a tantalum capacitor of at least 10μF must be used for these modules Power supply decoupling. Such modules are RS 232 drivers or switching power supply regulators.

(5) Arrange PCB layout reasonably

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

(6) Shield the influence of RF signal on other analog parts

As mentioned above, RF signals will cause interference to other sensitive analog circuit modules such as ADCs when they are sent. Most of the problems occur in lower operating frequency bands (such as 27MHz) and high power output levels. It is a good design practice to connect an RF decoupling capacitor (100pF) to ground to decouple sensitive points.

(7) Special considerations for on-board loop antennas

The antenna can be integrated on the PCB. Compared with the traditional whip antenna, it not only saves space and production cost, but is also more stable and reliable in mechanism. Conventionally, loop antenna design is applied to a relatively narrow bandwidth, which helps suppress unwanted strong signals so as not to interfere with the receiver. It should be noted that loop antennas (just like all other antennas) may receive noise capacitively coupled by nearby noise signal lines. It will interfere with the receiver and may also affect the modulation of the transmitter. Therefore, you must not lay digital signal lines near the antenna, and it is recommended to keep free space around the antenna. Any object close to the antenna will form a part of the tuning network, which will cause the antenna tuning to deviate from the expected frequency point and reduce the transmission and reception radiation range (distance). For all types of antennas, attention must be paid to the fact that the shell (outer packaging) of the circuit board may also affect the antenna tuning. At the same time, care should be taken to remove the ground plane at the antenna area, otherwise the antenna cannot work effectively.
(8) Connection of circuit board
If a cable is used to connect the RF circuit board to an external digital circuit, a twisted pair cable should be used. Each signal wire must be twisted with the GND wire (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 wire of a twisted-pair cable, and the cable length should be as short as possible. The circuit that supplies power to the RF circuit board must also be twisted with GND (VDD/GND).