First, the following some of the system to pay special attention to anti-electromagnetic interference:
1, the microcontroller clock frequency is particularly high, the bus cycle is particularly fast system.
2, the system contains high-power, high current drive circuit, such as the spark of the relay, high current switch.
3, with weak analog signal circuit and high-precision A / D converter circuit system.
Second, in order to increase the system’s anti-electromagnetic interference ability to take the following measures:
1, the choice of low frequency microcontrollers
The use of low external clock frequency microcontrollers can effectively reduce noise and improve the system’s anti-jamming capability. The same frequency of the square wave and sine wave, square wave in the high-frequency components than the sine wave much more. Although the amplitude of the wave of the high frequency component of the square wave is smaller than that of the fundamental wave, the higher frequency is easier to emit as a noise source, the most influential high frequency noise generated by the microcontroller is about three times the clock frequency.
2, to reduce the distortion in the signal transmission
Microcontrollers are mainly manufactured using high-speed CMOS technology. Signal input side of the static input current of about 1mA, the input capacitor 10PF or so, the input impedance is very high, high-speed CMOS circuit output has a considerable load capacity, that is a considerable output value, a door output through a very Long lines lead to a very high input impedance input, the reflection problem is very serious, it will cause signal distortion, increase system noise. When Tpd> Tr, it becomes a transmission line problem, must consider the signal reflection, impedance matching and other issues.
The delay time of the signal on the printed circuit board is related to the characteristic impedance of the lead, that is, the dielectric constant of the printed circuit board material. It can be roughly assumed that the transmission speed of the signal in the printed circuit board is about 1/3 to 1/2 of the speed of light. The Tr (standard delay time) of the commonly used logical telephone elements in the system constituted by the microcontrollers is between 3 and 18 ns.
On a printed circuit board, the signal passes through a 7W resistor and a 625px lead, and the line delay time is approximately between 4 and 20ns. In other words, the signal in the printed circuit on the lead as short as possible, the longest should not exceed 625px. And the number of vias should be as little as possible, preferably no more than two.
When the rise time of the signal is faster than the signal delay time, it is necessary to follow the fast electronically. At this time to consider the transmission line impedance matching, for a printed circuit board on the integrated block between the signal transmission, to avoid the emergence of Td> Trd situation, the greater the speed of the printed circuit board system can not be too fast.
Use the following conclusions to summarize a rule for printed circuit board design:
The signal is transmitted on the printed board and its delay time should not be greater than the nominal delay time of the device used.
3, to reduce the cross-interference between the signal lines
A step signal with a rise time Tr is passed through the lead AB to the B terminal. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the A-point signal, the signal reflection after the arrival of point B and the delay of the AB line, the Td time will induce a page pulse signal with a width Tr. At point C, due to the transmission and reflection of the signal on the AB, it will sense a width of twice the delay time of the signal on the AB line, that is, 2Td positive pulse signal. This is the cross-interference between signals. The intensity of the interfering signal is related to the di / at of the C-point signal, which is related to the distance between lines. When the two signal lines are not very long, AB is actually seen on the two pulse superposition.
CMOS technology manufacturing micro-control by the input impedance is high, high noise, noise margin is also high, the digital circuit is superimposed 100 ~ 200mv noise does not affect its work. If the AB line is an analog signal, this interference becomes intolerable. If the printed circuit board is a four-layer board, one of which is a large area of ​​the ground, or double-sided, the signal line is a large area of ​​the ground, the cross-interference between these signals will become smaller. The reason is that a large area to reduce the characteristic impedance of the signal line, the signal at the D-side reflection is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the signal line to the ground, which is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, to avoid the digital circuit signal line CD on the AB interference, AB line below a large area to the AB line to the CD line distance is greater than the AB line and distance of 2 to 3 times. Can be used to shield the local, in the lead side of the left and right sides of the cloth to the ground line.
4, reduce the noise from the power supply
The power supply also supplies its noise to the power supply while supplying energy to the system. Circuit in the microcontroller reset line, interrupt line, and some other control lines are most susceptible to external noise interference. Strong interference on the grid through the power into the circuit, even if the battery-powered system, the battery itself also has high-frequency noise. Analog circuits in the analog signal can not withstand the interference from the power supply.
5, pay attention to the printed circuit board and components of the high-frequency characteristics
In the case of high frequency, the printed circuit board on the lead, through the hole, resistance, capacitance, connector distribution inductance and capacitance can not be ignored. Capacitance distribution inductance can not be ignored, the inductive distributed capacitance can not be ignored. The resistance produces a reflection of the high frequency signal, and the distributed capacitance of the lead will work. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect is generated and the noise is emitted through the lead.
The vias on the printed circuit board cause approximately 0.6 pf of capacitance.
An integrated circuit itself encapsulates the material into a 2 ~ 6pf capacitor.
A connector on a circuit board has a 520nH distributed inductance. A double-stranded 24-pin integrated circuit block, the introduction of 4 ~ 18nH distributed inductance.
These small distribution parameters are negligible for microcontrollers in this lower frequency system, and special attention must be paid to high speed systems.
6, component layout to a reasonable partition
Components in the printed circuit board on the arrangement of the location to fully consider the anti-electromagnetic interference problem, one of the principles is the lead between the components to be as short as possible. In the layout, the analog signal part, high-speed digital circuit part, the noise source part (such as relays, high current switch, etc.) three parts of the reasonable separation, so that the signal coupling between the minimum.
7, handle the ground wire
Printed circuit boards, the most important power lines and ground. To overcome the electromagnetic interference, the most important means is to ground.
For the double panel, the ground layout is particularly concerned, through the use of single-point grounding method, power and ground from the power supply at both ends connected to the printed circuit board, the power of a contact, to a contact. Print circuit board, there must be more than return to the ground, which will be gathered back to the power of that contact, is the so-called single point ground. The so-called analog ground, digital ground, high-power device to open points, refers to the wiring separated, and finally gathered to the ground point up. When connecting to signals other than printed circuit boards, shielded cables are usually used. For high frequency and digital signals, both ends of the shielded cable are grounded. Low-frequency analog signal with a shielded cable, one end of the ground as well.
Circuites that are very sensitive to noise and interference or circuits with severe high frequency noise should be shielded with metal shields.
8, good decoupling capacitors.
A good high-frequency decoupling capacitor can remove high-frequency components up to 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have high frequency characteristics. Design printed circuit board, each integrated circuit power supply, ground must be added between a decoupling capacitor. Decoupling capacitors have two effects: on the one hand is the integrated circuit of the storage capacitor, to provide and absorb the integrated circuit door closing door instantaneous charge and discharge energy; on the other hand bypass the device’s high-frequency noise. The typical decoupling capacitor with a decoupling capacitor of 0.1uf in the digital circuit has a 5nH distributed inductance, and its parallel resonant frequency is about 7MHz, which means that the noise below 10MHz has a better decoupling effect, The noise is almost ineffective.
1uf, 10uf capacitor, parallel resonance frequency above 20MHz, remove the high frequency noise effect is better. It is often advantageous to place the power supply into the printed board and a 1uf or 10uf high-frequency capacitor, which is required even for battery-powered systems.
Every 10 or so integrated circuit to add a charge and discharge capacitors, or called the storage capacitor, the capacitor size optional 10uf. It is best not to electrolytic capacitors, electrolytic capacitors are two layers of thin film rolled up, this roll up the structure at high frequencies when the performance of the inductor, it is best to use the gallium capacitor or polycarbonate capacitor.
Decoupling capacitor value selection is not strict, according to C = 1 / f calculation; that is 10MHz take 0.1uf, the system composed of micro-controller, take 0.1 ~ 0.01uf can be between.
Third, reduce the noise and some of the experience of electromagnetic interference.
Can use low-speed chips do not have high-speed, high-speed chip used in key places.
Can be a string of resistance to reduce the control circuit up and down along the jump rate.
As far as possible for the relay to provide some form of damping.
Use the lowest frequency clock that meets the system requirements.
The clock generator is as close as possible to the device using the clock. Quartz crystal oscillator housing to ground.
Use the ground wire to circle the clock area, the clock line as short as possible.
I / O drive circuit as close as possible to the printed side, let it leave the printing plate as soon as possible. The signal into the printed circuit board to filter, from the high noise zone to the signal should also add filtering, while the use of string termination resistor approach to reduce the signal reflection.
MCD useless side to be high, or ground, or defined as the output, the integrated circuit on the power supply to the end of the ground, do not vacant.
Idle Do not use the gate input do not vacant, idle unused op amp is the input terminal ground, negative input termination output.
Print the board as far as possible using 45 fold line without 90 fold line wiring to reduce the high frequency signal external launch and coupling.
Printed board by frequency and current switching characteristics of the partition, the noise components and non-noise components to distance and then far.
Single-sided and double-sided single-point power supply and single-point grounding, power lines, ground as thick as possible, the economy can withstand the use of multi-layer board to reduce power,
Clock, bus, chip select signal away from I / O lines and connectors.
Analog voltage input line, the reference voltage side to try to stay away from the digital circuit signal lines, especially the clock.
For A / D devices, the digital part and the analog part of the unified but not cross.
The clock line is perpendicular to the I / O line than the parallel I / O line interference is small and the clock component pin is far from the I / O cable.
Component pins as short as possible, decoupling capacitor pins as short as possible.
The key lines should be as thick as possible and add protected areas on both sides. High-speed line to be short to straight.
Do not be sensitive to noise lines with high current, high speed switch lines parallel.
Quartz crystal below and noise-sensitive devices are not to be routed below.
Weak signal circuit, low frequency circuit around do not form a current loop.
Any signal do not form a loop, such as the inevitable, so that loop area as small as possible.
Each integrated circuit has a decoupling capacitor. Each electrolytic capacitor edge should be added a small high frequency bypass capacitor.
With a large capacity of tantalum capacitors or cool capacitors without electrolytic capacitors for the circuit charge and discharge energy storage capacitor. When using a tubular capacitor, the housing is grounded.

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