Common problems and solutions in the design of the

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Common problems and solutions in RF circuit design

single chip RF devices greatly facilitate the application in the field of wireless communication in a certain range. Using a suitable microcontroller and antenna combined with this transceiver device can form a complete wireless communication link. They can be integrated on a small circuit board and applied to 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 contradiction between digital circuit and analog circuit

if the analog circuit (RF) and digital circuit (microcontroller) work separately, they may work well, but once they are placed on the same circuit board and use the same power supply to work together, 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 (size 3 V), and the period is particularly short, often ns level. Due to large amplitude and small switching time, these digital signals contain a large number of high-frequency components independent of switching frequency. In the analog part, the signal transmitted from the antenna tuning loop to the wireless device receiving part is generally less than 1 v. Therefore, the difference between digital signal and RF signal will reach (120 dB). Obviously, if the digital signal and RF signal cannot be well separated, the weak RF signal may be destroyed. In this way, the working performance of wireless devices will deteriorate, or even completely unable to work

2 RF circuit and digital circuit are made on the same PCB "This design can reduce the common problem of white pollution.

it is often a problem that sensitive lines and noise signal lines cannot be fully isolated. As mentioned above, digital signals have high swing and contain a large number of high-frequency harmonics. If the digital signal wiring on the PCB is adjacent to sensitive analog signals, high-frequency harmonics may be coupled. The most sensitive node of RF devices is usually phase locked loop (PLL) Loop filter circuit, external voltage controlled oscillator (VCO) inductance, crystal oscillator reference signal and antenna terminal, these parts of the circuit should be handled with special care

(1) power supply noise

because the input/output signal has a swing of several V, the digital circuit is generally acceptable for power supply noise (less than 50 MV). Analog circuits are quite sensitive to power supply noise, especially to burr voltage and other high-frequency harmonics. Therefore, the wiring of power lines on PCB boards containing RF (or other analog) circuits must be more careful than that on ordinary digital circuit boards, and automatic wiring should be avoided. At the same time, it should also be noted that microcontrollers (or other digital circuits) will suddenly absorb most of the current in a short time in each internal clock cycle, because modern microcontrollers are designed using CMOS technology. Therefore, suppose a microcontroller operates at an internal clock frequency of 1 MHz, and it will extract (pulse) current from the power supply at this frequency. If proper power decoupling is not taken, voltage burr on the power line will be caused. If these voltage burrs reach the power pin of the RF part of the circuit, it may seriously lead to work failure. Therefore, it must be ensured that the analog power line is separated from the digital circuit area

(2) unreasonable ground wire

rf circuit board should always be equipped with a ground wire layer connected to the negative pole of the power supply. If it is not handled properly, some strange phenomena may occur. This may be difficult for a digital circuit designer to understand, because most digital circuits function well even without a ground layer. In the RF band, even a very short line will act like an inductance. Roughly, the inductance per mm length is about 1 NH, and the inductive reactance of 10 mmpcb line is about 27 at 434 MHz. If the ground wire layer is not used, most of the ground wires will be long, and the circuit will not be able to guarantee the design characteristics

(3) the radiation of antenna to other analog parts

is often ignored in circuits containing RF and other parts. 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 (ADCs) for measuring analog inputs as well as battery voltage or other parameters. If the antenna of the RF transmitter is located near (or on) this PCB, the high-frequency signal may reach the analog input of the ADC. Don't forget that any circuit line can send or receive RF signals like an antenna. If the processing of ADC input is unreasonable, the RF signal may self excite in the ESD diode of ADC input, resulting in ADC deviation

3 solutions for RF circuits and digital circuits on the same PCB

the following gives some connections in most RF applications, which leads to the emergence of the operating stress range with design and wiring strategies. However, it is more important to follow the wiring recommendations of RF devices in practical applications

(1) a reliable ground plane

when designing 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, so that all devices can be easily decoupled. The 0 V terminal of the power supply should be directly connected to this line layer. Due to the low impedance of the ground layer, 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 the surface mounted PCB, all signal wiring is on the same side of the component mounting surface, and the ground wire layer is on the opposite side. The ideal ground layer should cover the entire PCB (except under the antenna PCB). If more than two layers of PCB are used, the ground wire layer should be placed on the layer adjacent to the signal layer (such as the next layer of the component surface). Another good method is to fill the spare part of the signal wiring layer with ground plane, which must be connected with the main ground plane through multiple vias. It should be noted that the existence of the grounding point will cause changes in the inductance characteristics next to it. Therefore, when choosing electroplated pink ball bottle 1, it becomes a hot trend. Selecting the inductance value and arranging the inductance must be carefully considered

(2) shorten the connection distance with the ground wire layer

all connections to the ground wire layer must be as short as possible, and the grounding via should be placed at (or very close to) the bonding pad of the element. Never allow two ground signals to share a ground via, which may cause crosstalk between the two pads due to the via connection impedance

(3) RF decoupling

the decoupling capacitor should be placed as close to the pin as possible, and capacitor decoupling should be used at each pin that needs decoupling. High quality ceramic capacitors are used, and the best dielectric type is NPO. X7R can also work well in most applications. The ideal selection of capacitance value should make its series resonance 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 about 4, and the inductive reactance of the via is in the same range. The series capacitor and via form a notch filter for the signal frequency, which can effectively decouple. At 868 MHz, a 33 pf capacitor is an ideal choice. In addition to the small value capacitance of RF decoupling, a large value capacitance should also be placed on the power line to couple the low frequency. You can choose a 2 2 F ceramic or 10 f tantalum capacitor

(4) star wiring of power supply

star wiring is a well-known skill in analog circuit design (as shown in Figure 1). Each module on the star wiring circuit board has its own power 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, which should be decoupled near the IC. This is an effective method to separate the power supply noise from the digital

part and the RF part. If modules with serious noise are placed on the same circuit board, inductors (magnetic beads) or small resistance resistors (10) can be connected in series between the power line and the modules, and tantalum capacitors of at least 10 f must be used as the power decoupling of these modules. Such modules as RS 232 driver or switching power regulator

Figure 1 star wiring of power supply

(5) reasonably arrange PCB layout

in order to reduce the interference from noise modules and 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 (microcontroller and RS 232 driver) to avoid interference

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

as mentioned above, RF signal will cause interference to other sensitive analog circuit modules such as ADC when sent. Most of the problems occur in the lower operating frequency band (such as 27 MHz) and the high power output level. It is a good design habit to use RF decoupling capacitor (100p f) connected to the ground to decouple sensitive points

(7) special consideration on the board ring antenna

the antenna can be made on the PCB as a whole. Compared with the traditional whip antenna, it not only saves space and production costs, but also has a more stable and reliable mechanism. Traditionally, the loop antenna is designed for relatively narrow bandwidth, which helps to suppress unwanted strong signals to avoid interference with the receiver. It should be noted that ring antennas (like all other antennas) may receive noise that is capacitively coupled by nearby noise signal lines. It will interfere with the receiver and may also affect the modulation of the transmitter. Therefore, digital signal lines must not be laid near the antenna, and it is recommended to maintain a 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 transmitting and receiving radiation range (distance). Attention must be paid to this fact for all kinds of antennas. The shell (peripheral packaging) of the circuit board may also affect antenna tuning. At the same time, pay attention to remove the ground plane at the antenna area, otherwise the antenna cannot work effectively

(8) connection of circuit board

if the RF circuit board is connected to the external digital circuit by cable, twisted pair cable should be used. Each signal wire must be twisted in pairs with GND wire (din/GND, dout/GND, cs/GND, PWR _ up/GND). Remember to connect RF circuit board and digital application circuit board with GND cable of twisted pair cable, and the cable length should be as short as possible. The line supplying power to the RF circuit board must also be twisted pair with GND (vdd/GND)

4 Conclusion

the rapid development of RF integrated circuits provides the greatest possibility 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 bottleneck of wireless applications. At the same time, the design of RF circuit requires the designer to have certain practical experience and engineering design ability. This paper is the author's experience in the actual development, hoping to help many RF IC developers shorten the development cycle, avoid unnecessary detours, and save manpower and financial resources

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