PCB Tech - How to install RF circuit and digital circuit on a PCB board

How to install RF circuit and digital circuit on a PCB board

The single-chip radio frequency device greatly facilitates the application in the field of wireless communication within a certain scale. Selecting a suitable microcontroller and antenna and combining this transceiver device can form a complete wireless communication link.
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 collection systems, wireless networks, and wireless security systems.

Potential opposition between digital circuits and imitation circuits

Suppose that imitating the circuit (RF) and the digital circuit (microcontroller) may work independently, but once the two are placed on the same circuit board and the same power supply is used to work together, 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 (3 V), 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 lot of high-frequency components that are independent of the switching frequency.

In the imitating 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 reach 10-6 (120 dB).

Obviously, assuming that the digital signal and the radio frequency signal cannot be distinguished well, the weak radio frequency signal may be damaged. As a result, the operating function of the wireless device will deteriorate, or even fail to operate at all.

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

The inability to adequately block active lines and noise signal lines is a common problem. As mentioned above, the digital signal has a high swing and contains many high frequency harmonics.

Assuming the active analog signal near the digital signal wiring on the PCB board, high-frequency harmonics may be coupled before.

The most active node of the RF device is generally 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 volts, digital circuits can generally tolerate power supply noise (less than 50 mV). While imitating the circuit, it is suitable for power supply noise, especially for glitch voltage and other high-frequency harmonics.

Therefore, the power line wiring on the PCB board containing the RF (or other imitation) circuit must be more careful than the wiring on the general digital circuit board, and automatic wiring should be prevented.

At the same time, it should also be noted that the microcontroller (or other digital circuit) will suddenly sink most of the current in each internal clock cycle for a short time. This is because modern microcontrollers use CMOS process planning.

Therefore, 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. If proper power supply decoupling is not adopted, it will inevitably cause a voltage glitch on the power supply line.

Assuming that these voltage glitches reach the power supply pins of the RF part of the circuit, they may cause the operation to fail severely. Therefore, it is necessary to ensure that the imitating power line is separated from the digital circuit area.

(2) Unreasonable ground wire

The RF circuit board should always have a ground plane connected to the negative pole of the power supply. If it is not handled properly, some strange phenomena may occur.

For a digital circuit planner, this may be difficult to understand, because even without a ground plane, most digital circuit functions are outstanding.

In the RF frequency band, even a very short wire will have the same effect as an inductance. Rough calculation, the inductance per mm length is about 1 nH, and the inductance of a 10 mm PCB circuit at 434 MHz is about 27 Ω. Assuming that the ground wire layer is not selected, most ground wires will be longer, and the circuit will not be able to guarantee the planned characteristics.

(3) Radiation from antenna to other imitated parts

In circuits that include radio frequency and other parts, this is often overlooked. In addition to the RF part, there are generally 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.

Assuming that the antenna of the RF transmitter is located near this PCB (or on this PCB), the declared high-frequency signal may reach the analog input of the ADC. Don't forget that any circuit line may announce or receive RF signals the same as an antenna.

Assuming that the processing of the ADC input is unreasonable, the RF signal may be self-excited in the ESD diode of the ADC input, and then cause ADC errors. RF circuit and digital circuit are made on the same PCB processing scheme

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

(1) A reliable ground plane

When planning a PCB with RF components, you should always choose a reliable ground plane. Its purpose is to establish a useful 0 V potential point in the circuit, so that all devices are simply decoupled.

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.

It is very important that the amplitudes of multiple signals on the board may differ by 120 dB. On an externally mounted PCB, all signal wiring is on the same side of the component device surface, and the ground layer is not good.

The ground plane of the aspiration should cover the entire PCB (except below the antenna PCB). Assuming that a PCB with more than two layers is used, the ground layer should be placed on the layer of the nearby 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 surrounding inductance characteristics to change, so it is necessary to carefully consider the selection of the inductance value and the placement of the inductance.

(2) Shorten the connection distance with the ground layer

All connections to the ground layer must be as short as possible, and the ground vias should be placed (or very close to) the pad of the component. Never let two ground signals share a ground via, which 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.

Choose high-quality ceramic capacitors. The best dielectric type is "NPO". "X7R" can work well in most applications. Aspirational choice of capacitor value should make the series resonance equal to the signal frequency.

For example, at 434 MHz, SMD mounted 100 p F capacitors 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 on the same scale. The capacitors and vias in series form a notch filter with respect to the signal frequency, making it useful for decoupling.

At 868 MHz, 33 p F capacitance is an aspirational 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. A 2.2 μF ceramic or 10 μF tantalum capacitor can be selected.

(4) Star wiring of power supply

Star wiring is modeled after a well-known technique in circuit planning. 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 decoupled near the IC.

This is a useful way to isolate power supply noise from the digital part and from the RF part.

Assuming that a module with severe noise is placed on the same circuit board, an inductor (magnetic bead) or a small resistance resistor (10 Ω) can be connected in series between the power line and the module, and it is necessary to use a tantalum capacitor of at least 10 μF as a capacitor. The power supply of these modules is decoupled. Such modules are RS 232 drivers or switching power supply regulators.

(5) Arrange PCB layout reasonably

In order to reduce the disturbance from the noise module and the peripheral imitating part, the layout of each circuit module on the board is important. Always keep active modules (RF part and antenna) away from noise modules (microcontrollers and RS 232 drivers) to prevent annoyance.

(6) Shield the influence of RF signals on other imitated parts

As mentioned above, RF signals will disturb other active analog circuit modules such as ADCs when they are sent. Most of the problems occur in lower operating frequency bands (such as 27 MHz) and high power output levels. It is a good planning habit to use an RF decoupling capacitor (100p F) to connect to the ground and continuously couple the active point.

(7) Special considerations for on-board loop antennas

The antenna can all be built on the PCB.

Compared with the traditional whip antenna, it not only saves space and production cost, but also is more stable and reliable in organization. Conventionally, loop antennas are planned for relatively narrow bandwidth, which helps to restrain unwanted strong signals from disturbing the receiver. Be aware that loop antennas (just like all other antennas) may receive noise capacitively coupled from nearby noise signal lines.

It will annoy 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 maintain free space around the antenna.

Any object close to the antenna will constitute a part of the tuning network, which will cause the antenna tuning to deviate from the expected frequency point and reduce the scale (distance) of the transmission and reception radiation. Regarding all types of antennas, it is necessary to pay attention to this reality, and 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 be used effectively.

(8) Circuit board connection

Assuming that 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 together 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 power supply circuit for the RF circuit board must also be twisted with GND (VDD/ GND).

in conclusion

The rapid development of radio frequency integrated circuits provides the largest wireless application bottleneck for engineers and technicians who are engaged in wireless digital audio and video data transmission systems, wireless remote control, telemetry systems, wireless data collection systems, wireless networks, and wireless security and defense systems. perhaps.

At the same time, the planning of radio frequency circuits requires planners to have certain practical experience and engineering planning capabilities.