Single chip RF device greatly facilitates the application of wireless communication in a certain range. A complete wireless communication link can be formed by using appropriate microcontroller and antenna combined with this transceiver. They can be integrated on a small circuit board and applied in many fields, such as wireless digital audio and video data transmission system, wireless remote control and telemetry system, wireless data acquisition system, wireless network and wireless security system.
Potential contradiction between digital circuit and analog circuit
If analog circuit (RF) and digital circuit (microcontroller) work separately, they may work well, but once they are put 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 oscillates between the ground and the positive power supply (size 3 V), and the period is very short, often ns level. Due to the large amplitude and small switching time, these digital signals contain a large number of high frequency components 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 μ v. Therefore, the difference between digital signal and RF signal will reach 10-6 (120 dB). Obviously, if the digital signal and RF signal can not be well separated, the weak RF signal may be damaged. In this way, the working performance of wireless devices will deteriorate, or even fail to work at all.
Common problems of RF circuit and digital circuit on the same PCB
It is a common problem that sensitive line and noise signal line can not be isolated sufficiently. 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 wired close to the sensitive analog signal, high frequency harmonics may be coupled. The most sensitive nodes of RF devices are usually PLL loop filter circuit, external VCO inductor, crystal oscillator reference signal and antenna terminal. These parts of the circuit should be handled carefully.
(1) Noise of power supply
Because the input / output signal has several V swing, the digital circuit is generally acceptable for power supply noise (less than 50 MV). However, analog circuits are very sensitive to power supply noise, especially to burr voltage and other high frequency harmonics. Therefore, the wiring of power line on PCB containing RF (or other analog) circuit must be more careful than that on ordinary digital circuit board, and automatic wiring should be avoided. At the same time, it should be noted that the microcontroller (or other digital circuits) will suddenly inhale most of the current in a short time in each internal clock cycle, because modern microcontrollers are designed in CMOS technology. Therefore, if a microcontroller operates at an internal clock frequency of 1 MHz, it will extract (pulse) current from the power supply at this frequency. If proper power decoupling is not adopted, it will cause voltage glitch on the power line. If these voltage spikes reach the power supply pin of RF part of the circuit, it may lead to work failure seriously. Therefore, it must be ensured that the analog power line is separated from the digital circuit area.
(2) Unreasonable ground wire
The RF circuit board should always be equipped with a ground 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 perform well even without a ground layer. In RF band, even a very short wire will act as an inductor. Roughly, the inductance per mm length is about 1 NH, and the inductive reactance of 10 mm PCB is about 27 Ω at 434 MHz. If the ground 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) Antenna radiation to other analog parts
This is often overlooked in circuits containing RF and other components. 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 the ADC input is not reasonable, the RF signal may self excite in the ESD diode of the ADC input, resulting in the deviation of the ADC.
The solution of RF circuit and digital circuit on the same PCB
Here are some general design and cabling strategies for most RF applications. 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 layer 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 decoupled nodes. It is very important that the amplitudes of multiple signals on the board may differ by 120 dB. On the surface mount 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 wire layer 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 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 grounding point will cause the change of inductance characteristics nearby, so the selection of inductance value and arrangement of inductance must be carefully considered.
(2) Shorten the connection distance with the ground layer
All connections to the ground layer must be as short as possible, and the ground via should be placed at (or very close to) the pad of the component. Never let two ground signals share one ground via, which may cause crosstalk between two pads due to via connection impedance.
(3) RF decoupling
The decoupling capacitor should be placed as close to the pin as possible, and the capacitive decoupling should be used at each pin that needs decoupling. Using high quality ceramic capacitor, the best dielectric type is “NPO”, “X7R” can work well in most applications. The ideal choice 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 Ω, 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, the 33 pf capacitor is an ideal choice. In addition to the small value capacitance for RF decoupling, a large value capacitance should also be placed on the power line to couple the low frequency. A 2.2 μ f ceramic or a 10 μ f tantalum capacitor can be selected.
(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 line from the common power supply point. In this case, the 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 close to the IC. This is a number
An effective way to reduce the power supply noise from the RF part. If the modules with serious noise are placed on the same circuit board, inductors (magnetic beads) or small resistance (10 Ω) can be connected in series between the power line and the module, and tantalum capacitors of at least 10 μ f must be used for power decoupling of these modules. Such modules as RS 232 driver or switching power supply regulator.
(5) Reasonable layout of PCB
In order to reduce the interference from the noise module and the surrounding analog part, the layout of each circuit module on the board is important. Sensitive modules (RF part and antenna) should always be kept away from noise modules (microcontroller and RS 232 driver) to avoid interference.
(6) Influence of Shielding RF signal on other analog parts
As mentioned above, RF signal will cause interference to other sensitive analog circuit modules such as ADC. Most of the problems occur in the lower operating frequency band (such as 27 MHz) and high power output level. It is a good design habit to connect RF decoupling capacitor (100pF) to ground to decouple sensitive points.
(7) Special consideration of on board loop antenna
The antenna can be made on PCB as a whole. Compared with the traditional whip antenna, it not only saves space and production cost, but also has more stable and reliable mechanism. Traditionally, the loop antenna design is applied to a relatively narrow bandwidth, which helps to suppress unwanted strong signals so as not to interfere with the receiver. It should be noted that the ring antenna (like all other antennas) may receive noise capacitively coupled by nearby noise signal lines. It interferes with the receiver and may also affect the modulation of the transmitter. Therefore, the digital signal line must not be laid 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 transmitting and receiving radiation range (distance). For all kinds of antennas, it is necessary to pay attention to the fact that the enclosure of the circuit board may also affect the antenna tuning. At the same time, attention should be paid to remove the ground plane at the antenna area, otherwise the antenna can not work effectively.
(8) Connection of circuit board
If the RF circuit board is cabled to an external digital circuit, twisted pair cables should be used. Each signal line must be twisted with GND line (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 power supply line for RF circuit board must also be twisted with GND (VDD / GND).
The rapid development of radio frequency integrated circuit provides the greatest possibility for engineers and technicians engaged in the design of wireless digital audio and video data transmission system, wireless remote control and telemetry system, wireless data acquisition system, wireless network and wireless security system to solve the bottleneck of wireless application. 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.
Source: EDN electronic technology design