Wiring is the key to the design of high frequency circuits

- Dec 15, 2017-

If the frequency of digital logic circuit reaches or exceeds 45MHZ~50MHZ, and the circuit that works on this frequency has occupied a certain part of the whole electronic system (for example, 1/3), it is usually called the high frequency circuit. The design of high frequency circuit is a very complicated design process, and its wiring is very important to the whole design.

[first recruit] multi-layer board wiring

High frequency circuits often have high integration and large wiring density. Using multilayer boards is not only necessary for wiring, but also an effective means to reduce interference. In the PCB Layout stage, printed board dimensions reasonable choice of certain layers, can make full use of the intermediate layer shielding, better realize the nearest ground, and effectively reduce the parasitic inductance and shorten the length of the signal transmission, but also greatly reduce the cross interference signal, all of these methods are reliability of high frequency the favorable circuit.

[Second] the less the lead bend between the pins of the high speed electronic devices, the better the better

Lead high frequency circuit wiring the best use of the whole line, turning to 45 degrees, the available line or arc turning, the requirements in the low-frequency circuit only used to improve the fixation strength of copper foil, and in high frequency circuit, meet this requirement can reduce the coupling emission of high frequency signals and the foreign.

[Third] high frequency circuit devices lead foot between the shorter the better

The radiation intensity signal and the signal line is the line length is proportional to the high frequency signal lead longer, the more likely it is coupled to the components near it up, so the clock oscillator, DDR, such as signal data, LVDS line, USB line, HDMI line high frequency signal line are required to do may go line as short as possible.

The lower the interlayer of the lead between the pins of the high frequency circuit device, the better the better, the better the fourth

The less the better the interlayer alternation of the lead is, the better the less the Via used in the element connection. According to the side, an over hole can bring about 0.5pF distribution capacitance, reducing the number of holes can significantly increase the speed and reduce the possibility of data error.

"Fifth recruit" attention to the "crosstalk" introduced by the close parallel line of the signal line

High frequency circuit wiring should pay attention to the crosstalk introduced by signal line and short distance parallel line. Crosstalk is the coupling phenomenon between signal lines without direct connection. Due to the high frequency signal along the transmission line is transmitted in the form of electromagnetic waves, the signal line will play the role of an antenna electromagnetic energy around the transmission line emission, noise signal is not expected due to the mutual coupling between the electromagnetic signal generated is called crosstalk (Crosstalk). The parameters of the PCB layer, the distance between the signal lines, the electrical characteristics of the driving and receiving ends and the end connection mode of the signal line have a certain influence on the crosstalk. So in order to reduce the crosstalk of high frequency signal, we need to do the following points as much as possible when wiring.

When the wiring space is allowed, a ground wire or ground plane is inserted between two lines which are more serious crosstalk, which can play an isolation role and reduce crosstalk.

If there is a time-varying electromagnetic field in the space around the signal line, if the parallel distribution can not be avoided, a large area of ground can be arranged on the opposite side of the parallel signal line to reduce interference greatly.

Under the premise of the permission of the wiring space, we should increase the distance between adjacent signal lines, reduce the parallel length of the signal line, and try to keep the clock line as vertical as possible with the key signal lines.

[] sixth move power pins of integrated circuit block high frequency decoupling capacitor

The power pin of each integrated circuit block is nearly added to a high frequency decoupling capacitor. Increasing the high frequency decoupling capacitance of the power pin can effectively suppress the high frequency harmonics on the power pin to form interference.

(seventh) isolation of ground lines from high frequency digital signals and analog signal ground lines

The analog ground line, the digital ground line and the like are connected to the common ground wire with high frequency choke beads or directly isolated and select the appropriate place single point interconnection. The high frequency digital signal to the ground potential is generally not consistent, they often have the direct voltage difference, and ground harmonic components of high frequency digital signal with high frequency signal is often very rich, when directly connected to digital signal and analog signal to the ground line, the harmonic will high frequency signal interference simulation the signal through the wire coupling way.

(eighth) a loop to avoid the formation of a line

All kinds of high frequency signals do not form the loop as far as possible. If you can't avoid it, the loop area should be as small as possible.

[ninth] good signal impedance matching must be guaranteed

In the process of transmission, when the impedance is not matched, the signal will reflect the signal in the transmission channel. The reflection will make the synthetic signal overshoot, resulting in the fluctuation of the signal near the logic threshold.

The fundamental way to eliminate the reflection is to make the impedance of the transmission signal match well. Because the larger the difference between the load impedance and the characteristic impedance of the transmission line is, the greater the reflection is, so we should make the characteristic impedance of the signal transmission line equal to the load impedance as far as possible. At the same time, we must also notice that the transmission line of PCB can't change abrupt or corner, and try to keep the point impedance of the transmission line continuously. Otherwise, there will also be reflection between the segments of the transmission line.

[tenth] to keep the integrity of the signal transmission, and to prevent the "ground" phenomenon caused by the segmentation of the ground line.

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