RF Circuit PCB Design Processing Techniques
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This post was last edited by Eason爱RF on 2022-4-22 13:50
How to weigh the pros and cons to find a suitable compromise point in the PCB design process, reduce these interferences as much as possible, and even avoid interference of some circuits is the key to the success of RF circuit PCB design. This article provides some processing techniques from the perspective of PCB LAYOUT, which is very useful for improving the anti-interference ability of RF circuits. Since the radio frequency (RF) circuit is a distributed parameter circuit, it is easy to produce skin effect and coupling effect in the actual operation of the circuit. Therefore, in the actual PCB design, it will be found that the interference radiation in the circuit is difficult to control, such as: mutual interference between digital circuits and analog circuits, noise interference of power supply, interference caused by unreasonable ground wires, etc. Because of this, how to weigh the pros and cons to find a suitable compromise point in the PCB design process, reduce these interferences as much as possible, and even avoid interference of some circuits is the key to the success of RF circuit PCB design. This article provides some processing techniques from the perspective of PCB LAYOUT, which is very useful for improving the anti-interference ability of RF circuits.
1. RF layout The main discussion here is the component position layout of the multilayer board.
The key to component location layout is to fix the components on the RF path, minimize the length of the RF path by adjusting its direction, keep the input away from the output, separate high-power circuits and low-power circuits as far as possible, and keep sensitive analog signals away from high-speed digital signals and RF signals. The following techniques are often used in layout.
1.1 Linear layout The components of the RF main signal should be arranged in a straight line as much as possible, as shown in Figure 1. However, due to the limitations of the PCB board and cavity space, they cannot be arranged in a straight line in many cases. In this case, an L-shape can be used. It is best not to use a U-shaped layout (as shown in Figure 2). Sometimes, when it is unavoidable, try to increase the distance between the input and output as much as possible, at least 1.5 cm.
Figure 1 Linear layout
Figure 2 L-shaped and U-shaped layouts In addition, when using an L-shaped or U-shaped layout, it is best not to turn the turning point just after entering the interface, as shown in the left figure of Figure 3, but turn it after a slightly straight line, as shown in the right figure of Figure 3.
Figure 3 Two solutions
1.2 Same or symmetrical layout Same modules should be arranged in the same or symmetrical layout as much as possible, as shown in Figures 4 and 5.
Figure 4 Same layout diagram
5 Symmetrical layout
1.3 Cross-shaped layout The feed inductor of the bias circuit is placed vertically to the RF channel, as shown in Figure 6, mainly to avoid mutual inductance between inductive components. Figure 6 Cross-shaped layout 1.4 45-degree layout To reasonably utilize space, the components can be arranged at a 45-degree angle to make the RF line as short as possible, as shown in Figure 7.
Figure 7 45-degree layout
2. RF wiring The overall requirements for wiring are: the RF signal line should be short and straight, reduce line mutations, make fewer vias, do not intersect with other signal lines, and add as many vias as possible around the RF signal line. The following are some common optimization methods:
2.1 Gradient Line Processing When the RF line width is much larger than the width of the IC device pin, the line width of the contact chip is gradient, as shown in Figure 8.
Figure 8 Gradient Line
2.2 Arc Line Processing When the RF line cannot be straight, it can be processed into an arc line, which can reduce the external radiation and mutual coupling of the RF signal. Experiments have shown that the corners of the transmission line are curved at right angles to minimize the return loss. As shown in Figure 9. Figure 9 Arc Line
2.3 The ground wire and power ground wire should be as thick as possible. If conditions permit, each layer of the PCB should be laid as much as possible, and the ground should be connected to the main ground. More ground vias should be drilled to minimize the ground impedance. The power supply of the RF circuit should not be divided by plane as much as possible. The whole power plane not only increases the radiation of the power plane to the RF signal, but is also easily interfered by the RF signal. Therefore, the power line or plane is generally in the shape of a long strip, and is processed according to the size of the current. It is as thick as possible while meeting the current capacity, but it cannot be widened indefinitely. When processing the power line, be sure to avoid forming a loop. The direction of the power line and the ground line should be parallel to the direction of the RF signal but not overlap. In places where there is an intersection, it is best to use a vertical cross-crossing method.
2.4 Cross-over processing RF signal and IF signal routing cross, and try to place a ground between them. When RF signal crosses other signal routing, try to arrange a layer of ground connected to the main ground along the RF routing between them. If it is not possible, make sure they are cross-over. Other signal routing here also includes power lines.
2.5 Ground Envelope Processing Ground envelope processing is performed on RF signals, interference sources, sensitive signals and other important signals. This can not only improve the anti-interference ability of the signal, but also reduce the interference of the signal to other signals. As shown in Figure 10.
Figure 10 Ground Envelope Processing
2.6 Copper foil processing Copper foil processing requires smoothness and flatness, and no long lines or sharp corners are allowed. If it cannot be avoided, add a few ground vias at the sharp corners, slender copper foil or the edge of the copper foil.
2.7 Spacing The RF line should be at least 3W away from the edge of the adjacent ground plane, and there should be no non-ground vias within the 3W range.
Figure 11 Spacing The RF lines on the same layer should be grounded, and ground vias should be added on the ground copper sheet. The hole spacing should be less than 1/20 of the wavelength (λ) corresponding to the signal frequency, and should be evenly arranged. The edge of the ground copper sheet should be 2W away from the RF line, or 3H in height, where H represents the total thickness of the adjacent dielectric layer.
3. Cavity treatment For the entire RF circuit, the RF units of different modules should be isolated by cavities, especially between sensitive circuits and strong radiation sources. In high-power multi-stage amplifiers, isolation between stages should also be ensured. After the entire circuit branch is placed, the shielding cavity is processed. The following precautions should be taken for the processing of the shielding cavity: The entire shielding cavity should be made into a regular shape as much as possible to facilitate molding. For each shielding cavity, try to make it rectangular to avoid square shielding cavities. The corners of the shielding cavity are arc-shaped. The shielding metal cavity is generally cast. The arc-shaped corners are convenient for demolding during casting. As shown in Figure 12. Figure 12 Cavity shielding The periphery of the cavity is sealed. The interface line is generally introduced into the cavity using stripline or microstrip line, while different modules inside the cavity use microstrip line. The connection between different cavities is slotted. The width of the slot is 3mm, and the microstrip line runs in the middle. 3mm metallized holes are placed at the corners of the cavity to fix the shielding shell. The same metallized holes should also be evenly placed on each long cavity to strengthen the support. The cavity is usually windowed to facilitate welding of the shielding shell. The cavity is usually more than 2 mm thick and has two rows of windowed via screens. The vias are staggered, and the spacing between vias in the same row is 150MIL.
4. Conclusion The key to the success of RF circuit PCB design lies in how to reduce circuit radiation and thus improve anti-interference capabilities. However, in the actual layout and wiring, the handling of some issues is conflicting. Therefore, how to find a compromise point to optimize the overall performance of the entire RF circuit is a problem that designers must consider. All of these require designers to have certain practical experience and engineering design capabilities, but to have these capabilities, every designer cannot achieve them overnight. Only by learning from others' experience and adding their own constant exploration and thinking can they make continuous progress. This article summarizes some design experiences in the work, which is conducive to improving the anti-interference capability of RF circuit PCB and helping beginners in RF circuit design avoid unnecessary detours.
Four basic characteristics of PCB RF circuits
Here we will interpret the four basic characteristics of RF circuits from four aspects: RF interface, small desired signal, large interference signal, and interference from adjacent channels, and give important factors that need special attention in the PCB design process.
RF circuit simulation RF interface
Wireless transmitters and receivers can be conceptually divided into two parts: baseband and RF. The baseband includes the frequency range of the transmitter's input signal and the frequency range of the receiver's output signal. The bandwidth of the baseband determines the basic rate at which data can flow in the system. The baseband is used to improve the reliability of the data stream and reduce the load imposed by the transmitter on the transmission medium at a specific data rate. Therefore, a lot of signal processing engineering knowledge is required when designing the baseband circuit on the PCB. The transmitter's RF circuit can convert and upconvert the processed baseband signal to a specified channel and inject this signal into the transmission medium. Conversely, the receiver's RF circuit can obtain the signal from the transmission medium and convert and downconvert it to the baseband.
Transmitters have two main PCB design goals: first, they must transmit a specific power while consuming the least power possible. Second, they must not interfere with the normal operation of transceivers in adjacent channels. For receivers, there are three main PCB design goals: first, they must accurately restore small signals; second, they must be able to remove interfering signals outside the desired channel; and finally, like transmitters, they must consume very little power.
Large interference signal in RF circuit simulation
The receiver must be sensitive to small signals even in the presence of large interfering signals (obstructions). This situation occurs when trying to receive a weak or distant transmitter when there is a powerful transmitter broadcasting in an adjacent channel nearby. Interfering signals may be 60-70 dB larger than the desired signal and can block the reception of the normal signal by flooding the receiver input stage with large amounts of noise or by causing the receiver to generate excessive amounts of noise at the input stage. The two problems mentioned above can occur if the receiver is driven into a nonlinear region at the input stage by the interferer. To avoid these problems, the receiver front end must be very linear.
Therefore, "linearity" is also an important consideration when designing a receiver on a PCB. Since the receiver is a narrowband circuit, nonlinearity is measured by measuring "intermodulation distortion". This involves using two sine or cosine waves with similar frequencies and located in the center band to drive the input signal, and then measuring the product of their intermodulation. Generally speaking, SPICE is a time-consuming and costly simulation software because it must perform many cycles of calculations to obtain the required frequency resolution to understand the distortion situation.
RF circuit simulation of small desired signals
The receiver must be sensitive enough to detect small input signals. Generally speaking, the input power of the receiver can be as low as 1 μV. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, noise is an important consideration when designing a receiver on a PCB. Moreover, the ability to predict noise with simulation tools is essential. Figure 1 shows a typical superheterodyne receiver. The received signal is filtered and then amplified by a low noise amplifier (LNA). The signal is then mixed with the first local oscillator (LO) to convert it to an intermediate frequency (IF). The noise performance of the front-end circuit mainly depends on the LNA, mixer, and LO. Although the noise of the LNA can be found using traditional SPICE noise analysis, it is useless for the mixer and LO because the noise in these blocks will be severely affected by the large LO signal.
Small input signals require a lot of amplification in the receiver, often requiring as much as 120 dB of gain. At such high gain, any signal that couples from the output back to the input can cause problems. An important reason to use a superheterodyne receiver architecture is that it spreads the gain across several frequencies to reduce the chance of coupling. This also allows the first LO to be at a different frequency than the input signal, preventing large interfering signals from "contaminating" the small input signal.
For different reasons, in some wireless communication systems, the direct conversion or homodyne architecture can replace the superheterodyne architecture. In this architecture, the RF input signal is directly converted to baseband in a single step, so most of the gain is at baseband and the LO has the same frequency as the input signal. In this case, the influence of small amounts of coupling must be understood and detailed models of the "stray signal path" must be established, such as coupling through the substrate, coupling between package pins and bondwires, and coupling through power lines.
Adjacent channel interference in RF circuit simulation
Distortion also plays a role in transmitters. Nonlinearities in the transmitter output circuitry can cause the bandwidth of the transmitted signal to spread across adjacent channels. This phenomenon is called "spectral regrowth." Before the signal reaches the transmitter's power amplifier (PA), its bandwidth is limited; but "intermodulation distortion" within the PA causes the bandwidth to increase again. If the bandwidth increases too much, the transmitter will not be able to meet the power requirements of its adjacent channels. When transmitting digitally modulated signals, it is actually impossible to predict spectral regrowth using SPICE. This is because the transmission of approximately 1,000 digital symbols must be simulated to obtain a representative spectrum, and this requires the combination of high-frequency carriers, which makes SPICE transient analysis impractical.
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