"How to weigh the perspective of PCB, weigh the perspective, to reduce these interference, and can even avoid interference of partial circuits, is the key to the success or failure of the radio frequency circuit PCB. This article is from the Layout angle of PCB, Some techniques have been provided, and there is a large use of improving the anti-interference ability of the RF circuit.
Since the radio frequency (RF) circuit is a distributed parameter circuit, it is easy to generate a skin effect and coupling effect in the actual operation of the circuit, so in the actual PCB design, it is found that the interference radiation in the circuit is difficult to control, such as digital circuitry and simulation. Interference between the circuits, noise interference, and unreasonable interference from the power supply power supply. Because of this, how to weigh the advantages and disadvantages in the design process of the PCB, to reduce these interferences, and can even avoid the interference of partial circuits, and is the key to the success or failure of the RF circuit PCB design. From the perspective of the PCB's layout perspective, there is a significant use of the anti-interference ability to improve the radio frequency circuit.
1, RF layout
It is mainly discussed here to be the layout of the components of the multi-layer board. The key to the component position layout is to secure the components located on the RF path. By adjusting the direction, the length of the RF path is minimized, and the input is remote from the output, and the high power circuit and low power circuit are separated as much as possible. The signal is remote from the high speed digital signal and the RF signal.
The following techniques are often used in the layout.
1.1 word layout
The components of the RF main signal use a zigzag layout as shown in Figure 1. However, due to the limitations of the PCB board and cavity space, it is not possible to use a shape. At this time, it is best not to use a U-shaped layout (as shown in Figure 2), sometimes it is not possible to avoid it. It may be larger than the distance between the input and output, at least 1.5 cm or more.
Figure 1 word layout
Figure 2 L-shaped and U-shaped layout
In addition, when the L-shaped or U-shaped layout is used, the turning point is best not to enter the interface. As shown in Figure 3, it is repeated after a slightly segment line, as shown in Figure 3.
Figure 3 two programs
1.2 identical or symmetric layout
The same modules are as good as the same layout or symmetrical layout, as shown in FIG. 4, and 5.
Figure 4 identical layout
Figure 5 Symmetrical layout
1.3 cross layout
The feed inductance of the bias circuit is placed vertically, as shown in Figure 6, mainly to avoid mutual inductance between the sensing device.
Figure 6 Cross layout
1.4 45 degree layout
For reasonable utilization space, the device 45 degree direction can be laid out, so that the RF line is as short as possible, as shown in Figure 7.
Figure 7 45 degree layout
2, RF wiring
The overall requirements of the wiring are: the RF signal is short and straight, reduce the mutation of the line, and the holes are small, not intersecting other signal lines, and the RF signal line is perbided as much as possible.
Here are some common ways:
2.1 Gradient line processing
In the case where the radio frequency line width is larger than the width of the IC device pin, the line width of the contact chip uses a gradation method, as shown in FIG.
Figure 8 Gradient line
2.2 arc line processing
In the case where the RF line cannot be straight, the arc process is made so that the RF signal external radiation and the coupling of mutual questions can be reduced. Experiments have shown that the corners of the transmission line use the right angle of variable band, which can minimize the loss of retraction. As shown in Figure 9.
Figure 9 Circular arc
2.3 Ground wires and power supplies
The ground wire is as thick as possible. In the case of condition, each layer of the PCB is as possible, and the ground is connected to the main place, and the via hole is made, and the ground impedance is reduced as much as possible.
The power supply of the RF circuitry should not use a plane segmentation. The entire power supply plane not only increases the radiation of the RF signal, but also easily interfered with the RF signal. Therefore, the power cord or plane generally uses the strip shape, according to the size of the current, as thick as possible in the premise of satisfying current power, but does not limit unlimited. When processing the power cord, it must be avoided to form a loop.
The direction of the power cord and the ground wire is maintained in parallel with the direction of the RF signal but cannot overlap, and it is best to use vertical cross-crossing in the crossed place.
2.4 Cross Treatment
The RF signal is cross-crossing with the IF signal line and is as close as possible to them.
When the RF signal is interspersed with other signal traces, it is as best as possible to arrange a layer along the RF trace along the RF trace. If it is impossible, be sure that they are cross cross. Other signal traces here also include a power cord.
2.5 pack processing
The radio frequency signal, interference source, sensitive signal, and other important signals are treated, so that the anti-interference ability of the signal can be improved, and the signal can be reduced to other signals. As shown in Figure 10.
Figure 10
2.6 copper foil treatment
The copper foil treatment requires a round and smooth, and there is no longline or sharp corner. If it is not avoided, a few perforations are added to the edge of the sharp corner, the elongated copper foil or the copper foil.
2.7 Pitch Processing
The radio frequency line is at least 3 W width from the edge edge edge of adjacent ground, and there is no non-ground vias within the 3 W range.
Figure 11 spacing
The same radio frequency line should be treated, and the via holes are added on the ground, and the pores should be less than 1/20 of the wavelength (λ) corresponding to the signal frequency, uniformly arranged neat. The height of the width or 3 h of the width of the radiocide edge of the copper-skinned edge, H represents the total thickness of the adjacent dielectric layer.
3, cavity treatment
For the entire RF circuit, the radio frequency unit of the different modules should be separated from the cavity, especially between the sensitive circuit and the strong radiation source, and in a high-power multi-stage amplifier, isolation between the stage should also be guaranteed. After the entire circuit tributary is placed, it is the processing of the shielding chamber, and the processing of the shield cavity has the following precautions:
The entire shield cavity is as a rule shape as possible for mold. For each shield, the mass is performed, avoiding the square shielding chamber.
The corner of the shielding chamber uses a curved shape, and the shielded metal cavity is generally used, and the curved corner is easy to cast when molded. As shown in Figure 12.
Figure 12 cavity
The periphery of the shield cavity is sealed, the line of the interface is introduced into the cavity generally uses a stripline or microstrip line, and the different modules in the cavity are microstrip line, and the different cavity is connected to the groove treatment, the width of the groove 3mm, the microstrip line is in the middle.
The corner of the cavity is placed for 3 mm metal holes for fixing the shield case, and the equivalent metallized hole is also placed evenly on each long chamber to reinforce the support.
The cavity is generally opened, facilitating the welding shield case, and the cavity is generally thick 2 mm or more, and the cavity is added 2 rows the window through the hole screen, and the vias are displaced, and the same row of via holes are 150 mils.
4 Conclusion
The key to the implementation failure of the radio frequency circuit PCB is how to reduce circuit radiation, thereby improving anti-interference ability, but in the actual layout and processing of some problems in the wiring, how to seek a compromise, so that the whole RF circuit is integrated Performance is optimal, the problem that the designer must consider. All of this requires designers to have certain practical experience and engineering design capabilities, but must have these capabilities, each designer is impossible, only from others to learn from experience, plus their own non-stop exploration and thinking, Can constantly progress. This article summarizes some design experience in the work, which is conducive to improving the anti-interference ability of RF circuit PCB to help radio frequency circuit design beginner less unnecessary detours.
Four basic features of PCB RF circuit
Here, four aspects of the radio frequency interface, small desired signal, large interference signal, adjacent channel interference, and give an important factor in the PCB design process requires special attention.
Radio circuit simulation radio interface
The wireless transmitter and receiver can be conceptually divided into two parts: baseband and radio frequency. The fundamental frequency includes the frequency range of the input signal of the transmitter, and also includes the frequency range of the output signal of the receiver. The frequency band of the fundamental frequency determines the basic rate of data in the system. The base frequency is used to improve the reliability of the data stream, and under a particular data transfer rate, reduce the load applied to the transport medium (Transmission Medium). Therefore, when the PCB designs the fundamental frequency circuit, a large amount of signal processing is required. The radio frequency circuit of the transmitter can convert the processed fundamental frequency signal to the specified channel, and inject this signal into the transmission medium. Conversely, the radio frequency circuit of the receiver can acquire signals in the transmission medium, conversion, and reduce frequency.
The transmitter has two major PCB design objectives: the first is that they must transmit specific power when consuming to minimize power. The second is that they cannot interfere with the normal operation of transceivers within adjacent channels. In terms of receiver, there are three major PCB design objectives: First, they must accurately reduce small signals; second, they must remove interference signals outside the expected channel; the last point is the same as the transmitter, they consume the power required Small.
Radio circuit simulation large interference signal
The receiver must be very sensitive to small signals, even if there is a large interference signal (blocking material) exists. This happens at an attempt to receive a weak or long-distance transmit signal, and there is a powerful transmitter near the adjacent channel. The interference signal may be more than 60 to 70 dB of the expected signal, and can be overwise in a large amount of noise at the input phase of the receiver, or cause excessive noise amount in the input phase to generate excessive noise to block the reception of the normal signal. If the receiver is driven in the input phase, the two problems described above will occur during the input phase. To avoid these issues, the front end of the receiver must be very linear.
Therefore, "linear" is also an important consideration when the PCB design receiver is designed. Since the receiver is a narrowband circuit, the nonlinearity is statistically statistically determined. This involves driving the input signal with two frequencies, and in the central band (IN BAN) sine wave or cosine wave, and then measure the product of its interaction modulation. Generally speaking, SPICE is a time-consuming emulation software because it must perform many cyclic calculations to get the required frequency resolution to understand the distortion.
Small expected signal of radio frequency circuit simulation
The receiver must be very sensitive to detect a small input signal. In general, the input power of the receiver can be small to 1 μV. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, the noise is an important consideration when the PCB design receiver is designed. Moreover, it is indispensable with the ability to predict noise in simulation tools. The drawings are a typical superheterodyne receiver. The received signal is first filtered, and the input signal is enlarged by a low noise amplifier (LNA). The first local oscillator (LO) is then mixed with this signal to convert this signal to an intermediate frequency (IF). The noise effect of the Front-end circuit depends primarily on the LNA, mixer (Mixer) and LO. While using conventional SPICE noise analysis, the noise of LNA can be found, but for mixers and LO, it is useless because noise in these blocks will seriously affect the LO signal.
Small input signals require that the receiver must have great enlargement, usually require 120 dB as high gains. Under such a high gain, any signal from which to return to the input of the input is possible. An important reason for using an ultra-unity receiver architecture is that it can distribute the gain in several frequencies to reduce the chance of coupling. This also makes the frequency of the first LO are different from the frequency of the input signal, and the large interference signal "contamination" to a small input signal.
For different reasons, in some wireless communication systems, direct conversion (Homodyne) architectures can replace the ultra-horizontal architecture. In this architecture, the radio frequency input signal is directly converted to a base frequency under a single step, and therefore, most of the gains are in the base frequency, and the LO is the same as the frequency of the input signal. In this case, a small amount of coupling must be understood, and the detailed model of "striped signal path" is established, such as the coupling of the substrate, package foot and welding wires. (Bondwire) coupling between and passing through the power cord.
Interference of adjacent channels of radio frequency circuit simulation
Distortion also plays an important role in the transmitter. The transmitter is non-linear generated by the output circuit, which may cause the bandwidth of the transfer signal to spread in the adjacent channel. This phenomenon is called "Spectral RegroWTH". Before the signal reaches the power amplifier (PA) of the transmitter, its bandwidth is limited; but "intercalation distortion" within the Pa will result in frequency width again. If the bandwidth increases too much, the transmitter will not conform to the power requirements of its adjacent channels. When transmitting a digital modulation signal, actually, it is unable to predict the reappearance of the spectrum with SPICE. Because about 1000 digital symbols (Symbol) must be simulated to seek representative spectrum, and also need to bind to high frequencies, which will make Spice's transient analysis unreal.
Original from: Electronic Engineering Album "
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