High-frequency circuit design and wiring is the key to share ten experiences
If the digital logic circuit frequency reaches or exceeds 45MHZ ~ 50MHZ, and work in this frequency circuit has accounted for a certain amount of the entire electronic system (for example, 1/3), usually called high-frequency circuit. High-frequency circuit design is a very complex design process, the layout of the entire design is crucial!
[The first move] multilayer 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. PCB layout phase, a reasonable choice of a certain number of PCB size, can take full advantage of the middle layer to set the shield to better achieve the nearest ground, and effectively reduce the parasitic inductance and shorten the signal transmission length, but also large Amplitude reduction of signal cross-interference, etc., all of these methods are beneficial to the reliability of high-frequency circuits. Data show that the same kind of material, four-layer board than the double-panel noise 20dB lower. However, there is also a problem that the higher the PCB half-height, the more complicated the manufacturing process and the higher the unit cost. This requires that we carry out the PCB Layout, in addition to select the appropriate number of PCB board also need to be carried out Rational layout of components and layout, and use the correct wiring rules to complete the design.
【The second move】 High-speed electronic device pins between the pin bending as little as possible
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.
[The third move] High-frequency circuit device pins as short as possible
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.
[The fourth move] High-frequency circuit device pins between the layers alternately as possible
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. According to the side, a via can bring about 0.5pF distributed capacitance, reducing the number of vias can significantly improve the speed and reduce the possibility of data errors.
【Fifth move】 pay attention to the signal line close parallel routes into the “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. So in order to reduce the crosstalk of high-frequency signals, the wiring asks as much as possible to do the following:
In the wiring space allows the conditions, the crosstalk more serious between the two lines into a ground or ground plane, can play a role in isolation and reduce crosstalk.
When there is a time-varying electromagnetic field in the space around the signal line, interference can be drastically reduced by arranging a large area “ground” on the opposite side of the parallel signal line if the parallel distribution can not be avoided.
Under the premise of wiring space permission, increase the spacing between adjacent signal lines to reduce the parallel length of the signal lines, the clock lines as far as possible perpendicular to the key signal lines and not parallel.
If the same level within the parallel alignment is almost impossible to avoid, in the adjacent two layers, the alignment must be perpendicular to each other.
In digital circuits, the usual clock signal is a fast changing edge signal, large external crosstalk. Therefore, in the design, the clock line should be surrounded by ground and play more ground hole to reduce the distributed capacitance, thereby reducing crosstalk.
Try to use low-voltage differential clock signal for high-frequency signal clock and cover the way, you need to pay attention to the integrity of the package hole.
Do not leave unused input floating, but to ground or power (power in the high-frequency signal loop is also ground), because the suspension of the line may be equivalent to the transmitting antenna, grounding can inhibit the launch. Practice has proved that using this approach to eliminate crosstalk can sometimes take effect immediately.
[Sixth] IC 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.
[Seventh move] high-frequency digital signal ground and analog signal ground to do 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. The ground potential of high-frequency digital signal is generally inconsistent, there is often a certain voltage difference between the two, and often, the high frequency digital signal ground also has a very rich high-frequency signal harmonic components, when Directly connect the digital signal ground and analog signal ground, the high frequency signal harmonics will 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 beads interconnection.
Eighth move to avoid the formation of the loop alignment
As far as possible not to form a variety of high-frequency signal routing loop, if not avoid the loop area should be as small as possible.
[Ninth trick] must ensure a good signal impedance matching
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. Requirements USB signal differential alignment, line width 10mil, line spacing 6mil, ground and signal line spacing 6mil.
HDMI cabling rules. Requires HDMI signal differential alignment, line width 10mil, line spacing 6mil, every two HDMI differential signal pairs more than 20mil.
LVDS wiring rules. Requires LVDS signal differential traces, line width 7mil, line spacing 6mil, the purpose is to control the HDMI differential signal impedance of 100 + -15% ohm
DDR routing rules. DDR1 traces require signals not to pass through the holes as much as possible. The signal lines are equal in width and the lines and lines are equidistant. The traces must meet the 2W principle to reduce crosstalk between signals. High-speed data on DDR2 and above is also required Line length, in order to ensure the signal impedance matching.
【】 The tenth move to maintain the integrity of the signal transmission, to prevent the ground split due to the “bombs phenomenon.”