Common Problems and Solutions in RF Printed Circuit Board Design
The problem of RF circuits and digital circuits be made in the same printed circuit borad
Not adequately isolating sensitive lines and noise signal lines are often problems. The digital signal has a high swing and contains a large number of high frequency harmonics. If the digital signal on the Printed Circuit Board is routed close to the sensitive analog signal, the high frequency harmonics may be coupled in the past. The most sensitive node of an RF device is usually a loop filter circuit of a phase-locked loop (PLL), an external voltage-controlled oscillator (VCO) inductor, a crystal reference signal and an antenna terminal, and these parts of the circuit should be handled with particular care.
(1) power supply noise
Since the input / output signal has a few V swing, the digital circuit is generally acceptable for power supply noise (less than 50 mV). The analog circuit is very sensitive to power supply noise, especially for burr voltage and other high-frequency harmonics. Therefore, the power cord wiring on the Printed Circuit Board containing the RF (or other analog) circuit must be more carefully routed than on an ordinary digital circuit board, and automatic routing
should be avoided. It should also be noted that microcontrollers (or other digital circuits) will inhaled most of the current for a short period of time in each internal clock cycle, due to the use of CMOS microprocessors for modern microcontrollers. Therefore, assuming that a microcontroller operates at an internal clock frequency of 1 MHz, it will extract (pulsed) the current from the power supply at this frequency, and if it does not take the appropriate power decoupling, it will cause a voltage glitch on the power line. If these voltage burrs reach the power supply pin of the RF part of the circuit, it may cause malfunctions, and it is necessary to ensure that the analog power cord is separated from the digital circuit area.
(2) unreasonable ground
RF circuit board should always be connected with the power supply connected to the ground layer, if handled properly, may produce some strange phenomenon. This may be difficult to understand for a digital circuit designer,since most digital circuit functions perform well even without a ground layer. In the RF band, even a very short line will act like an inductor. Roughly calculated, the inductance per mm length is about 1 nH, and the inductance of the 10 mm Printed Circuit Board line at 434 MHz is about 27 Ω. If you do not use the ground layer, most of the ground will be longer, the circuit will not be able to guarantee design features.
(3) Radiation of the antenna to other analog parts
This is often overlooked in circuits that contain radio frequency and other parts. In addition to the RF section,there are usually other analog circuits on the board. For example, many microcontrollers built-in analog-to-digital converters (ADCs) are used to measure analog inputs as well as battery voltages or other parameters. If the antenna of the RF transmitter is located near the Printed Circuit Board (or on this Printed Circuit Board ), the high frequency signal may arrive at the analog input of the ADC. Do not forget that any circuit lines may emit or receive RF signals as an antenna. If the ADC input is unreasonable, the RF signal may be self-excited within the ESD diodes of the ADC input, causing ADC bias.
RF circuits and digital circuits are available on the same block of Printed Circuit Board solutions
Here are some general design and routing strategies in most RF applications. However, it is more important to follow the wiring recommendations for RF devices in practical applications.
(1) a reliable ground level
When designing Printed Circuit Boards with RF components, a reliable ground layer should always be used. The goal is to establish an effective 0 V potential in the circuit, making all the components easy to decoupling. The 0 V terminal of the power supply should be connected directly to this ground layer. Due to the low impedance of the ground layer, there will be no signal coupling between the two nodes that have been decoupled. This is important for the fact that the amplitude of multiple signals on the board may differ by 120 dB. On the surface-mount Printed Circuit Board, 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 layer should cover the entire Printed Circuit Board (except the antenna Printed Circuit Board below). If two or more Printed Circuit Boards are used, the ground layer should be placed on a layer adjacent to the signal layer (such as the next layer of the element surface). Another good way is to fill the remainder of the signal wiring layer with a ground plane that must be connected to the main ground plane through multiple vias. It should be noted that: due to the presence of the grounding point will cause the inductance characteristics of the next change, so select the inductance and layout inductance must be carefully considered.
(2) shorten the connection distance with the ground layer
All connections to the ground plane must be as short as possible and the ground vias should be placed at (or very close) to the pads of the components. Never let the two ground signals 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 as possible to the pin, and each capacitor that needs decoupling should be decoupled with a capacitor. The use of high-quality ceramic capacitors, the type of dielectric is best "NPO", "X7R" in most applications can also work better. The ideal choice of capacitance should be such that the series resonant is equal to the signal frequency. For example, at 434 MHz, the SMD-mount 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 also in the same range. The series capacitance and vias form a notch filter for the signal frequency to enable efficient decoupling. At 868 MHz, the 33 p F capacitor is an ideal choice. In addition to the RF decoupling of the small capacitor, a large capacitor should also be placed on the power supply line to decouple low frequency, select a 2. 2 μF ceramic or 10μF tantalum capacitor.
(4) Star wire for power supply
Star wiring is a well-known technique in analog circuit design (see Figure 1). Star modules. Each module on the circuit board has its own power supply line from a common power supply point. In this case, the star wiring means that the digital and RF parts of the circuit should have their own power lines, which should be decoupled separately at the IC. This is a separate number from the number
Part and an effective method from the RF part of the power supply noise. If a module with severe noise is placed on the same circuit board, the inductance (magnetic bead) or the small resistance resistor (10 Ω) can be connected in series between the power supply line and the module, and at least 10 μF of tantalum capacitors must be used The power supply of the module is decoupled. Such modules as RS 232 drives or switching power regulators.
(5) reasonable arrangements for Printed Circuit Board layout
In order to reduce the interference from the noise module and the surrounding analog part, the layout of the circuit modules on the board is important. Always turn sensitive modules (RF sections and antennas) away from noise modules (microcontrollers and RS 232 drives) to avoid interference.
(6) shielding RF signal on the impact of other analog parts
As mentioned above, the RF signal will cause interference to other sensitive analog circuit modules such as ADC. Most problems occur in lower operating frequency bands (eg 27 MHz) and high power output levels. Using RF decoupling capacitors (100p F) to connect to ground decoupling sensitive points is a good design habit.
(7) Special consideration in plate loop antenna
Antenna can be done on the Printed Circuit Board as a whole. Compared to the traditional whip antenna, not only save space and production costs, the organization is also more solid and reliable. In practice, the loop antenna design is applied to relatively narrow bandwidth, which helps to suppress unwanted strong signals to avoid interfering with the receiver. It should be noted that the loop antenna (as with all other antennas) may receive capacitive coupling noise from nearby noise signal lines. It can interfere with the receiver and may also affect the transmitter's modulation. So the antenna must not be near the digital signal lines, and suggested that the antenna around the free space. Any object close to the antenna will form part of the tuning network, causing the antenna tuning to deviate from the expected frequency, reducing the transmit and receive radiation range (distance). For all types of antennas it must be noted that the fact that the circuit board's housing (peripheral package) may also affect the antenna tuning. At the same time should pay attention to remove the antenna area at the ground level, otherwise the antenna can not work effectively.
(8) circuit board connection
If you connect an RF circuit board to an external digital circuit with a cable, use a twisted pair cable. Each signal line must be twisted together with the GND line (DIN / GND, DOUT / GND, CS / GND, PWR _ UP / GND). Remember to connect the RF circuit board and the digital application board with the GND cable of the twisted pair cable. The cable length should be as short as possible. The line that supplies power to the RF board must also be twisted with GND (VDD / GND).
Rapidly Developed RF ICs provide the largest bottlenecks for wireless applications to meet the bottlenecks of wireless applications such as wireless digital audio, video data transmission systems, wireless remote control, telemetry systems, wireless data acquisition systems, wireless networks and wireless security systems. may. At the same time, the RF circuit design requires designers to have a certain practical experience and engineering design capabilities. This article is the author in the actual development of the summed up the experience, hoping to help many RF integrated circuit developers to shorten the development cycle, to avoid unnecessary detours, saving manpower and financial resources.