PCB Blog - How to place RF circuits and digital circuits on a PCB board

They can be integrated on a small PCB board and are 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 and many other fields.The single-chip radio frequency device greatly facilitates the application in the field of wireless communication within a certain range, and a complete wireless communication link can be formed by using a suitable microcontroller and antenna combined with this transceiver device.

1. Potential conflicts between digital circuits and analog circuits
If the analog circuit (RF) and the digital circuit (microcontroller) work separately, they may work fine, but once they are placed on the same board and run from the same power supply, the whole system is likely to be unstable . This is mainly because the digital signal frequently swings between the ground and the positive power supply (3 V 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. So the difference between the digital signal and the RF signal will be 10-6 (120 dB). Obviously, if the digital signal is not well separated from the radio frequency signal, the weak radio frequency signal may be damaged, so that the working performance of the wireless device will be deteriorated, or even completely unable to work.

2. Common problems with RF circuits and digital circuits on the same PCB
Insufficient isolation of sensitive and noisy signal lines is a common problem. As mentioned above, digital signals have high swings and contain a lot of high frequency harmonics. If digital signal routing on a PCB is adjacent to sensitive analog signals, high frequency harmonics may couple through. Sensitive nodes of RF devices are usually the phase-locked loop (PLL) loop filter circuit, external voltage-controlled oscillator (VCO) inductance, crystal reference signal and antenna terminals, and these parts of the circuit should be handled with special care.

(1) Power supply noise
Since the input/output signals have swings of several volts, digital circuits are generally acceptable for power supply noise (less than 50 mV). Analog circuits, on the other hand, are quite sensitive to power supply noise, especially glitch voltages and other high frequency harmonics. Therefore, the routing of power lines on PCB containing RF (or other analog) circuits must be done more carefully than on ordinary digital 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, since modern microcontrollers are designed on a CMOS process. Therefore, assuming a microcontroller 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 supply decoupling is not taken. If these voltage glitches reach the power pins of the RF part of the circuit, it may seriously lead to failure of work, so it must be ensured that the analog power lines are separated from the digital circuit area.

(2) Unreasonable ground wire
RF circuit boards should always have a ground plane connected to the negative side of the power supply, which can cause some strange behavior if not handled properly. This may be difficult for a digital circuit designer to understand because most digital circuits function well even without a ground plane. In the RF frequency band, even a very short wire can act like an inductance. Roughly calculated, the inductance per mm of length is about 1 nH, and the inductance of a 10 mm PCB line at 434 MHz is about 27 Ω. Without a ground plane, most ground lines will be long and the circuit will not be able to guarantee the design characteristics.

(3) Radiation from the antenna to other analog parts
In circuits containing RF and other parts, this is often overlooked. Besides 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 ADC's analog input. Don't forget that any circuit line may 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 of the ADC input, causing the ADC to drift.

3. Solution A with RF circuit and digital circuit on the same PCB
Some general design and routing strategies in most RF applications are given below. However, it is more important to follow the routing recommendations for RF devices in real-world 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 0 V potential point in the circuit, allowing easy decoupling of all devices. The 0 V 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. This is very important as the amplitudes of multiple signals on the board may differ by 120dB. On a surface-mount PCB, all signal routing 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 on the component side). Another good approach is to also fill the empty parts of the signal routing layers with ground planes, which must be connected to the main ground plane through multiple vias. It should be noted that since the existence of the ground point will cause the change of the inductance characteristics next to it, the selection of the inductance value and the arrangement of the inductance must be carefully considered.

(2) Shorten the connection distance to the ground layer
All connections to ground planes 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 can 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 every pin that needs to be decoupled. Using high quality ceramic capacitors, the dielectric type is "NPO", "X7R" will also work well in most applications. The ideal 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 in the same range. The capacitors and vias in series 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. Choose a 2.2 μF ceramic or 10 μF tantalum capacitor.

(4) Star wiring of power supply
Star wiring is a well-known trick in analog circuit design. Star Wiring - Each module on the board has its own power line from a common power supply point. In this case, star routing means that the digital and RF parts of the circuit should have their own power supply lines that should be decoupled separately close to the IC. This is a spacer from a number
Partial and efficient approach to power supply noise from the RF part. 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. Power supply decoupling for the module. Such modules are RS 232 drivers or switching power regulators.

(5) Reasonable arrangement of PCB board layout
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. 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 influence of RF signal on other analog parts
As mentioned above, RF signals can interfere with other sensitive analog circuit blocks such as ADCs when transmitted. Most problems occur in 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 (100p F) connected to ground.

(7) Special Considerations for On-Board Loop Antennas
The antenna can be made on the PCB as a whole. Compared with the traditional whip antenna, it not only saves space and production cost, but also is more stable and reliable in mechanism. Conventionally, loop antenna designs are used 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 interferes with the receiver and may also affect the modulation of the transmitter. Therefore, digital signal lines must not be routed near the antenna, and it is recommended to keep free space around the antenna. Anything close to the antenna will form part of the tuning network, causing the antenna to tune away from the intended frequency, reducing the transmit and receive radiation range (distance). This fact must be noted for all types of antennas, the enclosure (peripheral packaging) of the circuit board may also affect the antenna tuning. At the same time, attention should be paid to removing the ground plane at the antenna area, otherwise the antenna will not work effectively.


4 Conclusion
The rapidly developing radio frequency integrated circuits provide the possibility for engineers and technicians engaged in the design of 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 the bottleneck of wireless applications. . At the same time, the design of RF circuits requires designers to have certain practical experience and engineering PCB board design capabilities.