With the rise of Internet of Things technology, it is more and more common for electronic products to be equipped with wireless communication function, and wireless communication technology is realized by RF circuit on PCB. Unfortunately, even PCB designers in Chengdu are often discouraged from RF circuit, because it will bring great design challenges and require professional design and simulation analysis tools. Because of this, for many years, the RF part of PCB has been designed by independent designers with RF design expertise.
The RF circuit design engineer moved out of 18 martial arts skills, and after a fierce operation, he designed the RF circuit layout below, and exported DXF format to PCB Layout for copying. Wouldn’t it be cool?
After introducing DXF format file of RF circuit into Chengdu PCB design siege lion, it was found that the routing had both right angles and sharp corners. I thought to myself, emmm, this RF is really water, the salary is higher than that of labor, and I don’t know how to avoid sharp chamfer arc transition. Then I re-optimized the routing of RF circuit.
In order to avoid misunderstanding in the future, the RF bacteria called the layout bacteria after work, closed the door and guided some key points of RF PCB design.
According to the theory of RF circuit, when the wavelength of the signal transmitted on the signal connection line can be compared with the geometric size of the discrete circuit elements, the pads of RF IC pins, the transmission lines of RF signals on PCB, RF passive devices, vias and even the grounding copper are all important factors that seriously affect the performance of RF signals.
Microstrip line is an ideal choice for transmitting high-frequency signals on PCB. Unless the connection distance between IC and antenna is very short, please use coaxial cable or transmission line with characteristic impedance matching. On the printed circuit board, it is best to use the microstrip transmission line with the structure shown in the following figure.
Microstrip transmission lines include fixed width metal traces (conductors) and the grounding area directly below the adjacent layers. For example, a trace on the first layer (top metal) requires a solid grounding area on the second layer. The width of the trace, the thickness of the dielectric layer and the type of dielectric determine the characteristic impedance (usually 50 ω or 75 ω).
Of course, besides microstrip lines, there is another common transmission line, namely stripline, as shown in the following figure.
The stripline includes a fixed width trace in the inner layer, and grounding areas above and below it. The conductor can be located in the middle of the grounding area or have a certain offset. This method is suitable for RF wiring in the inner layer.
Since stripline is also suitable for RF wiring, why does old wu say that microstrip line is an ideal choice for transmitting high-frequency signals on PCB?
Whether it is microstrip line or stripline, both of them have excellent performance in transmitting millimeter wave frequency, but the difference lies in the manufacturing cost.
Compared with stripline circuit, microstrip circuit has fewer processing steps, and the circuit elements are easier to place, so it is easier to manufacture (lower manufacturing cost). Compared with microstrip line, stripline can provide more isolation for adjacent circuit lines and support more dense component layout. In addition, stripline circuit is also very suitable for manufacturing multilayer circuit boards, and each layer can be well isolated.
The electrical properties of microstrip line and stripline conductor are affected by dielectric constant of insulating material and proximity effect of grounding layer. A microstrip line has only one ground plane, while a stripline has two ground planes. For microstrip lines, the effective dielectric constant that affects the conductor impedance is the sum of the relative dielectric constants of the insulating material and the air above the circuit (equal to 1). The effective dielectric constant of stripline is the sum of the relative dielectric constants of the two substrates above and below the conductor.
For all high-frequency circuits, keeping impedance controlled is essential to achieve consistent amplitude and phase response electrical performance. The impedance of the conductors of the two transmission lines is a function of conductor width, conductor thickness, thickness of insulating substrate, relative permittivity or dielectric constant of substrate, among other factors. For striplines, it doesn’t matter whether the distance between the center conductor and the two grounding layers is equal, or whether the dielectric constants of the insulators above and below the conductor are the same (the same is true for microstrip lines).
The stripline has two grounding layers, so the 50ω (or any given impedance) line of the stripline is thinner than the conductor with the same impedance of the microstrip line. Although thinner wires support higher circuit density, thinner wires also require stricter manufacturing tolerance, and the dielectric constant of the substrate of the whole circuit should be very consistent. The dielectric loss (defined by the dissipation factor of the substrate) of a single-ended (unbalanced) transmission line of a microstrip line is less than that of a stripline, because some field lines of the microstrip line are in the air, and their dissipation factors are negligible.
Of course, the properties of these two kinds of transmission lines are actually almost the same as those of the insulating substrate used for their manufacture. Just as the adopted PCB material, such as FR-4, can reduce the cost, but at the same time it will limit its performance. Choosing the most suitable material according to different microstrip and stripline applications will give full play to the advantages of these two transmission lines.
Like many engineering decisions, the choice of microstrip line or stripline will be weighed. For example, the circuit density of stripline circuit is high, so under the same frequency condition, it needs more material layers, more processing time and expense, and more attention to detail processing than microstrip line circuit.
Compared with the common microstrip lines and striplines, there is another kind of RF transmission line, which is grounded coplanar waveguide, which provides better isolation between adjacent RF lines and other signal lines. This medium includes the middle conductor and the grounding areas on both sides and below as shown in the following figure:
It is recommended to install via “fences” on both sides of the grounded coplanar waveguide, as shown in the following figure. This top view provides an example of installing a row of grounding vias in the top metal grounding area on each side of the middle conductor. The loop current caused by the top layer is short-circuited to the ground layer below.
Compared with microstrip lines, the grounded coplanar waveguide has a larger grounding area because it not only has a grounding plane on the bottom of the dielectric, but also is distributed on both sides of the signal transmission line on the top of the dielectric. Coplanar waveguide achieves the stability of electrical performance by using the ground plane to surround the signal line.
The transmission modes of microstrip and grounded coplanar waveguide circuits are quasi-transverse electromagnetic modes (quasi -TEM). Due to the reinforced grounding structure of grounded coplanar waveguide circuit, its machining is more complicated to some extent. Compared with microstrip lines, the grounded coplanar waveguide circuit has the characteristics of low dispersion. When the frequency rises to millimeter wave band, the radiation loss of the grounded coplanar waveguide circuit is lower than that of microstrip lines.
Due to the enhanced grounding structure, the grounded coplanar wave conductive path has wider effective bandwidth and larger impedance range than the microstrip line circuit. However, the microstrip line circuit structure is relatively robust, and its simple bottom ground plane circuit structure is easy to process. In addition, the circuit performance of microstrip line is insensitive to circuit processing factors, and its circuit performance is less affected by conductor/gap etching difference and conductor thickness difference.
And those sharp corners of RF circuit layout are specially designed transmission line corner compensation.
When the transmission line is required to bend (change direction) due to wiring constraints, the bending radius used should be at least 3 times the width of the middle conductor. That is:
Bending radius ≥ 3x (line width).
The characteristic impedance change of this corner is minimized.
If gradual bending is not possible, the transmission line can be bent at right angles (not curves), as shown in the following figure. However, this must be compensated to reduce the sudden change of impedance caused by the increase of local effective line width when passing through the bending point.
