Since the current trend is to develop in the wireless direction, there is a RF circuit in each wireless device. RF PCB design is a very complex and tricky concept that requires a lot of RF base knowledge to design a radio circuit's layout because of the map design, the map design is more restrictions and challenges compared to the simulation / digital PCB layout design.
The purpose of this tutorial is to provide some guidance for RF (RF) PCB layout design.
(Topic photo provided by Radio Communication Lab)
RF base
The RF circuit operates at 500 MHz to several thousand megabo band.
The RF signal includes a frequency, or a frequency band that modulates on a very high frequency carrier. The energy contained in the RF current can be radiated from the conductor to the space, which is a so-called electromagnetic wave. RF current is often concentrated on the surface of the conductor, and it is very deep. This is the so-called skin effect.
The basic components of any radio frequency circuit are amplifiers, inverters, and filters.
The wavelength of a signal is equal to the physical distance that will pass through one cycle. Any orbit on the PCB, if its length is greater than or equal to 1/4, it should be considered as an electric length. TH its wavelength. In this case, in order to avoid reflection, the characteristic impedance of the track must match the load impedance. That is to say, in order to ensure maximum power transmission, source impedance and load impedance must match. The matching network is used to convert any impedance to the feature impedance of RF tracking. Parameter returns loss and insertion loss indicates the validity of the network. The retransmission loss is defined as the ratio of incident power and reflection power. The insertion loss is defined as the score of the power lost before the next phase. In the ideal matching circuit, zero echo loss and unlimited insertion loss. The critical length of any signal line is a signal propagated to the load and returned, equal to the length of the rise time. It is after this length, we need to start considering the design and end technology of the transmission line.
Fig. Transmission line equivalent circuit
The characteristic impedance (ZO) of the transmission line is an attribute indicating the ratio of the distributed series inductance and distribution parallel capacitance. In most applications, the characteristic impedance of the radio channel is 50W. The ZO depends on the PCB medium material, the substrate thickness, the magnetic circuit width, and the distance between the radiochannel and the ground.
The transmission line can have a variety of types, but the two most suitable transmission lines of the RF circuit are microstrip and coplanar waveguides.
The microstrip consists of a trajectory separated from the substrate and the ground. Microstrip characteristic impedance depends on the following parameters
Trace width (w)
Copper thickness
Substrate dielectric constant (XR) and
Distance between trajectories and ground (h)
Common waveguides are similar to the microstrip, but there are copper on both sides of the radio channel. In addition to the above parameters, the CPWG characteristic impedance also depends on the distance (g) between the radio channel and the two-sided copper. Copper on both sides is also ground.
PCB overlay
It is highly recommended that all RF PCB layouts use 4 layers of PCB overlapping because it provides a complete surface and power supply planes, thus allowing simpler signal lines. All unused regions of the signal layer will also be covered by the ground. As compared with the 2nd floor design, the 4-layer design has the advantage of that the 4-layer design can make a radio frequency decoupling that uniformly distributed between the two main ground planes. Ground plane on both sides of the power plane. This arrangement greatly increases electromagnetic compatibility. In addition, the ground plane splicing holes will be used for the edge of the board to avoid radiation emission on the edge of the board. A continuous ground plane also ensures that the returns of the sensitive RF path can take the shortest path to avoid extended ground loops. The reflow current uses the path to the smallest inductor. Therefore, there should be no radio tracking under discontinuous ground, all roads, from the driver to the receiver. Continuous ground also allows distributed microstrip, so that the impedance of the track (usually 50W) can be strictly controlled.
For those strict budgets, a 2-layer design can also be successfully implemented, and the underlying coverage is mainly grill casting. In addition, the PCB thickness should also be strictly controlled because the thickness of the PCB should not exceed 0.8 mm to 1.0 mm, otherwise the width of the transmission line will become too large.
The ground plane should be as close as possible to the signal route.
Tracking layout guide
The characteristic impedance of each radio channel is 50W. When calculating 50W characteristic impedance, the appropriate transmission line type should be selected. There are many supporting applications, such as Saturn, PCB Toolkit, and AppCAD, which can be used to calculate the surface of the tracking width and distance to stack the specific PCB. The calculated tracking width should be kept within the entire tracking length, for the CPWG, the distance between tracking and on both sides must always be maintained.
Do not run RF tracking in parallel. If this situation is inevitable, be sure to follow the 3W rule, that is, the center to the center to the center distance between the parallel trajectory should be equal to or greater than 3 times the tracking width to avoid crosstalk and noise coupling.
Since it introduces parasitic inductance and capacitance, the number of through holes on the aircraft will be as low as possible. For PCBs having a typical thickness of 1.6 mm, the single pass hole can increase the inductance of 1.2 NH and 0.5 pF capacitance according to the size of the through hole and the dielectric material. A VIA should not share between multiple pins of the component.
Make sure that the RF trajectory is a route that is not curved. When the bend is inevitable, turn it into a curved bend rather than sharp curvature. This will help keep a uniform width.
Do not create a stub on RF tracking. Placing test points on RF trajectories is also avoided because they form a stub. Storage will affect impedance matching, which will act as antenna to produce radiation launch problems.
In any possible place, the sensitive radio frequency circuit around the surface guards. Sew slits on these guards, sew them onto the ground. Such a device isolates the sensitive circuit from the other portions of the circuit.
Power decoupling
The power decoupling is to ensure that noise in the power is filtered, not to other sensitive devices. Be sure to provide a sufficient decoupling capacitor. Several parallel capacitors may be required to filter out noise of different frequencies. The frequency of the self-harmonic frequency of the capacitor is close to the frequency to be filtered will be most effective. Each decoupling capacitor should have its own by connecting to the ground. According to standard practice, the decoupled capacitor should be as close as possible to the power port of the integrated circuit. In addition, a large-capacity capacitor can be provided to meet the burst of sudden surge in IC. The power supply should be connected to the power pin of the IC, so that the power supply flows through the decoupling capacitor to the IC. Do not place any through holes between the capacitor and IC power pins.
This tutorial aims to introduce the principle of RF PCB design technology. The reasonable PCB design and layout is the key to the success of the RF circuit.
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