Key experiences of High-frequency circuit design

Multilayer PCB wiring

High-frequency circuits are often highly integrated, wiring density, the use of multi-layer wiring is both necessary, but also reduce the effective means of interference. In the Layout stage, a reasonable choice of a certain number of PCB size, can make full use of the middle layer to set the shield, to achieve closer to the ground, and effectively reduce the parasitic inductance and shorten the signal transmission length, but can also significantly reduce Signal interference and so on. Data show that the same kind of material, four-layer board than the double-panel noise 20dB lower. However, the higher the PCB layer, the more complex the manufacturing process, the higher the unit cost, which requires us to select the appropriate number of layers in addition to the PCB, but also need to plan the layout of components and reasonable use of the correct wiring Rules to complete the design.

High-speed electronic devices pin lead less bending

High-frequency circuit wiring leads the best use of all straight lines, the need for turning, the available 45-degree polylines or arcs turn, this requirement is used in low-frequency circuit only to improve the strength of copper foil, and in high-frequency circuits to meet this A requirement can reduce the launch of high-frequency signals and the coupling between each other.

High-frequency circuit device pin lead short

The radiation intensity of the signal is proportional to the length of the signal line. The longer the signal frequency of the high frequency signal, the easier it is to couple to components close to it. Therefore, for such signals as the clock, crystal oscillator, and DDR data, LVDS lines, USB lines, HDMI lines and other high-frequency signal lines are required as short as possible, the better the alignment.

High-frequency circuit device pins alternately between layers less

The so-called "lead alternating between the layers as possible" means that the vias (Via) used in the connection of components are as small as possible. It has been measured that a via can deliver about 0.5pF of distributed capacitance, reducing the number of vias can significantly increase the speed and reduce the possibility of data errors.

Note that "crosstalk"

High-frequency circuit wiring should pay attention to the signal line close parallel lines introduced by the "crosstalk," crosstalk is not directly connected between the signal line coupling phenomenon. Since the high-frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line functions as an antenna and the energy of the electromagnetic field is transmitted around the transmission line. As a result, undesired noise signals due to the mutual coupling of the electromagnetic fields Called Crosstalk. PCB board parameters, the spacing of the signal lines, the electrical characteristics of the driver and receiver, as well as the signal line termination have a certain impact on the crosstalk.

Integrated circuit block power pin to increase high-frequency decoupling capacitor

The power pin of each IC block adds a high-frequency decoupling capacitor nearby. Increase the power pin high-frequency decoupling capacitor, can effectively inhibit the power supply pin on the high-frequency harmonic interference.

High-frequency digital signal ground and analog signal ground isolation

Analog ground, digital ground, etc. When connected to the public ground to use high-frequency choke beads to connect or directly isolated and select the appropriate place single-point interconnection. Ground potential of high-frequency digital signal ground is generally inconsistent, the two often there is a certain voltage difference, and high-frequency digital signal ground often also has a very rich high-frequency signal harmonic components, when the direct When connecting the digital signal ground and the analog signal ground, harmonics of the high frequency signal interfere with the analog signal through the ground coupling. Therefore, usually, the high-frequency digital signal ground and analog signal ground is to be isolated, you can use a single point in the appropriate location of interconnection, or the use of high-frequency choke bead interconnection.

Good signal impedance matching must be guaranteed

During the signal transmission process, when the impedance does not match, the signal will occur in the transmission channel signal reflection, reflection will make the composite signal overshoot, resulting in signal fluctuations in the vicinity of the logic threshold.

The basic method to eliminate the reflection is to make the impedance of the transmission signal well matched. Since the larger the difference between the load impedance and the characteristic impedance of the transmission line, the larger the reflection is. Therefore, the characteristic impedance of the signal transmission line should be equal to the load impedance. At the same time also pay attention to the transmission line on the PCB can not be abrupt changes or corners, as far as possible to maintain the impedance of the transmission line continuous, or in the transmission line will also appear between the paragraphs reflect. This requires that high-speed PCB layout, you must comply with the following wiring rules:

USB routing rules: USB signal differential routing requirements, line width 10mil, line spacing 6mil, ground and signal line spacing 6mil.

HDMI routing rules: HDMI signal differential routing requirements, line width 10mil, line spacing 6mil, every two HDMI differential signal spacing of more than 20mil.

LVDS wiring rules: Requires LVDS signal differential routing, line width 7mil, line spacing 6mil, the purpose is to control the HDMI differential signal impedance of 100 + -15% ohms.

DDR Cabling Rules: DDR1 traces require that the signal not pass through the hole as much as possible, the signal line is wide, the line and the line are equidistant, and the trace must meet the 2W principle to reduce crosstalk between signals. For DDR2 and above high-speed devices, High-frequency data trace length, in order to ensure the signal impedance matching.