High frequency board
High frequency board ・ Overview
A high-frequency board is a printed circuit board used for high-frequency signal transmission (frequency of 1 GHz or higher).
Signal speed is important for high frequency boards (circuits). The reason for increasing the frequency is to transmit the signal faster, and as shown in Equation 1, the lower the dielectric constant ε, the faster the signal speed.
When dealing with high frequency bands, it is best to deal with substrates with a low dielectric constant ε.
Equation 2 shows how the impedance of the wiring affects the line width, thickness, and relative permittivity of the circuit when designing a high-frequency board.
Equation 1
Equation 2
About the material of the high frequency board
In high-frequency circuits, the material of the printed circuit board often affects the circuit characteristics, so it is necessary to carefully select the material.
The characteristics of typical board materials used for high-frequency boards are described below.
[FR-4]
FR-4 is excellent in terms of cost, but the frequency can be used up to about 1GHz. Adoption may be difficult in cases where higher frequencies or severe characteristics are required.
[Teflon board]
Teflon substrate is a substrate material that has excellent high frequency characteristics and is often used in high frequency applications. As for the Teflon base material itself, companies such as Rogers, Chukoh Chemical Industries, and Nippon Pillar supply Teflon with various characteristics, so it is possible to select a high-frequency substrate that matches the frequency band and characteristics.
An example of the relative permittivity comparison of each base material is shown below as a selection factor.
Teflon material: about 1.95 to 3.5 (a wide range of characteristic materials)
FR-4: 4.3-4.7
Panasonic material "MEGTRON6": 3.7
Panasonic material "MEGTRON6 (N)": 3.4
Toshimasa Industrial Material "PPE": 3.1-3.5
Hitachi Material "MCL-LW-900G": 3.2-3.5
[Ceramic substrate]
In addition to high heat dissipation, the dielectric constant is stable and the dielectric loss tangent is low, so it can be expected that the loss when used as a high-frequency substrate will be kept low. However, because it is so expensive, its applications are often used in some limited areas, such as the aerospace field.
Hybrid board
[High frequency hybrid board] is one of the options for creating a high frequency board.
By stacking high frequency material and general material (FR-4), the antenna and control / power supply board can be integrated. The hybrid board reduces the cost of high-frequency boards, supports multiple layers, and improves the degree of freedom in wiring.
High frequency board design (an example of noise countermeasures)
The GND guard enhances the noise reduction effect by providing a GND via to secure the return. Moreover, the GND guard leads to further reduction of noise by putting GND not only in the same layer but also in the upper and lower layers.
Does not consider the return path of GND
Return is ensured by placing a GND via near the signal line
Since there is a slit in the GND plane, it affects the increase in circuit inductance, etc.
In high frequency circuits, RF lines are always designed to secure a return path to prevent noise. The idea of always ensuring this return path is important not only from the viewpoint of high frequency circuits / high frequency signals but also from the viewpoint of SI.
The GND plane should not be provided with slits that divide the GND plane.
There is wiring under the RF line
When it is unavoidable to cross the lower layer (not the layer directly below) of the RF line, it is usually crossed at a right angle, but when crossing another line, even if it is a little detour, it can be passed directly under the parts to reduce interference as much as possible. increase.
Even if it makes a detour, it reduces interference with the RF line by passing directly under the parts.
There is an extra protrusion on the solid GND and it becomes an antenna
A high-frequency circuit that is resistant to noise can be made smooth by eliminating unnecessary sharp edges so that patterns and GND solids do not become antennas.
Eliminates unnecessary sharpness of patterns and solid surfaces and finishes smoothly
Only GND guard wire, no GND via
It is not enough to just apply the GND guard, but it is necessary to take measures such as hitting the GND via together with the GND guard line.
Along with applying a GND guard, the GND via is placed within 15 mm.
The GND on the outer circumference is cut off at the edge of the board.
As for noise countermeasures, it is important to take care of both making noise out of your own circuit and making it less susceptible to external noise.
Surround the outer circumference of the board with GND (if possible, place VIA as much as possible with about 1 mm)
The above is an example of noise countermeasures.
You can check the concept and achievements of our high frequency board design in more detail.