General electronic products are the most likely out of the question there is: RE- radiation, CE- conduction, ESD- static electricity.
Communication electronics comprise more than only three: RE, CE, ESD, there Surge- surge (lightning, thunder)
Medical Devices easiest question that arises is: ESD- electrostatic, EFT- transient pulse interference, CS- conducted immunity, RS- radiation interference.
For dry areas in the north, ESD- electrostatic requirements of the product to be very high.
For as in Sichuan, and some Xinanduolei areas, EFT lightning protection requirements to be high.
How to improve noise immunity and electromagnetic compatibility of electronic products:
1, some of the following systems pay special attention to anti-electromagnetic interference:
(1) the clock frequency of the microcontroller is particularly high, particularly fast system bus cycle.
(2) The system contains a high power, high current drive circuit of the relay as sparks, high current switch.
(3) containing a weak analog signal circuit system, and high-precision A / D conversion circuit.
2, the following measures to increase the electromagnetic interference capability of the system:
(1) selection of a low frequency for the microcontroller
The selection of low external clock frequency microcontroller may effectively reduce the noise and improve the anti-interference ability of the system. The same frequency square wave and sine wave, square wave high-frequency component much more than the sine wave. Although the high-frequency component of amplitude of the square wave, the fundamental wave is small, but the higher the frequency the more likely the source of noise emission, generated by the microcontroller of the most influential high-frequency noise is about three times the clock frequency.
(2) reducing the distortion in the signal transmission
Main microcontroller high-speed CMOS technology. Static input current signal input terminal is about 1mA, about 10PF input capacitance, very high input impedance, the output terminal of the CMOS circuit has a relatively high speed of the load capacity, i.e. a large output value, the output of a gate by a very long lead to a relatively high input impedance input, reflection problem is very serious, it causes signal distortion, increase system noise. When Tpd> Tr, the transmission line has become a problem, the signal to be reflected, the impedance matching and other issues to consider.
Characteristic signal delay time in the lead on the PCB impedance related, i.e. related to the dielectric constant printed wiring board material. Can be roughly considered, the signal transmission speed between the printed circuit board leads, about 1/3 to 1/2 the speed of light. The system configuration of the microcontroller telephone custom logic element Tr (standard delay time) between 3 to 18ns.
On the printed wiring board, the signal delay time through a resistor and 7W 25cm long lead period, a line generally between 4 ~ 20ns. In other words, the signal leads on a printed line as short as possible, up to not more than 25cm. And the number of vias also should be minimal, preferably no more than two.
When the signal rise time is faster than the signal delay time, we must follow the fast processing electronics. At this time, to consider the impedance of the transmission line for signal transmission between the manifold on a printed circuit board, to avoid Td> Trd is the case, the larger the system the more the speed of the printed wiring board can not be too fast.
A printed circuit board design rules are summarized in the following conclusions:
Signal transmitting printing plate, which should not exceed the nominal delay time means the delay time used.
(3) reducing the cross-interference between the signal lines
A point a rise time Tr of step signal transmitted by wire to the terminal B AB. The signal delay time on the line AB is Td. The point D, since the point A onward transmission of the signal, the reflected signal arrival delay line AB and the point B, after time Td will induce a pulse width Tr page signal. At point C, since the transmission and reflection of an AB signal, will induce a signal of twice the delay time of the line AB, i.e. a positive pulse signal 2Td. This is the cross-interference between signals. Di C setpoint signal intensity of the interference signal / at related, related to the distance between lines. When two signal lines are not very long, it is actually seen on AB superposition of two pulses.
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microcontroller manufactured by a CMOS process, high input impedance, high noise, noise margin is also high, the digital circuit is superposition 100 ~ 200mv noise does not affect their work. If a line AB in FIG analog signal, such interference becomes intolerable. When the printed wiring board is a four-layer board, where there is a large area of the layer, or double sided, negative signal line is large area, the cross interference between such signal becomes small. The reason is that a large area reduces the characteristic impedance of the signal line, the signal is greatly reduced in the reflection of the D terminal. To the characteristic impedance of the signal line between the dielectric constant is inversely proportional to the square, and the natural logarithm is proportional to the dielectric thickness. If the line AB is an analog signal, the digital circuit to prevent interference of signal lines AB, CD, a large area must be below line AB, the line AB to line CD from 2 to 3 times greater than the distance of the line AB. Available local mask, the cloth on both sides to the ground with a lead wire around one side of the junction.
(4) reduce the noise from the power supply
Power while providing energy to the system, which will be the noise to the power supply. The reset line of the microcontroller circuit, interrupt and other control lines most likely to suffer interference from external noise. Strong interference into the circuit by the power supply line, even if the battery powered systems, battery itself also has high-frequency noise. An analog signal in analog circuit withstand more interference from the power source.
High-frequency characteristics (5) Note that the printed wiring board and the component
At high frequencies, leads on the printed wiring board, vias, resistors distributed inductance, capacitance, and the capacitance of the connector can not be ignored. Distributed inductance and capacitance can not be ignored, distributed capacitance and inductance can not be ignored. Resistance of the high-frequency signal reflection, distributed capacitance of the leads will act, when the length is greater than the noise frequency corresponding to 1/20 wavelength will act as antennas, noise is emitted outward through the leads.
Through-hole printed circuit board about the cause 0.6pf capacitance.
An integrated circuit packaging material itself is introduced into the 2 ~ 6pf capacitance.
A circuit board connector with distributed inductance of 520nH. One dual-pin integrated circuit straight stick the stick holder 24, the introduction of distributed inductance 4 ~ 18nH.
These small microcontroller system for the distribution parameters in the lower frequency line is negligible; for high-speed system must be given special attention.
(6) the partition element is arranged to be reasonable
Elements arranged on the printed wiring board position to fully consider the problem of electromagnetic interference, one of the principle of a lead between the components to be as short as possible. In the arrangement, should the analog signal portions, a high-speed digital circuit portion, a noise source part (e.g., relays, high current switches, etc.) reasonable three parts separated, so that a minimum of signal coupling between each other.
(7) for the grounding wire
A printed circuit board, the power and ground lines of the most important. Overcome electromagnetic interference, the most important means is grounded.
For double-sided, special attention ground arrangement, by a single point ground method, the power and ground are connected to both ends of the printed wiring board from a power up, a power point, to a point. The printed circuit board, there must be more than a ground return, these will gather on that point back to power, the so-called single-point ground. The so-called analog ground, digital ground, to open sub-power devices, wiring means separately, and finally to the ground point are gathered up. When connected to a signal other than the printed wiring board, usually shielded cable. And high-frequency digital signals, shielded cables are grounded at both ends. Low frequency analog signals shielded cable with one end grounded as well.
Very sensitive to noise and interference in high frequency circuits or circuit noise should be particularly severe shielded with a metal cover.
(8) with a good decoupling capacitor
Good high-frequency decoupling capacitors can be removed to a high-frequency component of 1GHZ. Ceramic chip capacitors or high frequency characteristics of the multilayer ceramic capacitor better. When designing a printed wiring board, the power of each integrated circuit, to be applied between a decoupling capacitor. Decoupling capacitor has two functions: one is the storage capacitors of this integrated circuit, provided the integrated circuit and absorb door opening and closing moments of the charging and discharging energy; hand bypass high frequency noise of the device. A typical digital circuit decoupling capacitance decoupling capacitor 0.1uf 5nH are distributed inductance, its parallel resonance frequency in about 7MHz, say 10MHz or less for better noise decoupling, of more than 40MHz noise almost does not work.
1uf, 10uf capacitor, the parallel resonance frequency above 20MHz, the high frequency noise removing effect is better. Where the power enters the printing plate and to a high-frequency capacitors or 10uf 1uf often advantageous, even with a battery powered systems in need of such capacitors.
Each integrated circuit 10 to increase about the charging and discharging a capacitor, or called storage discharge capacity, the size of optional capacitor 10uf. Preferably no electrolytic capacitors, electrolytic capacitors are two Pu film roll up, rolled up configuration such as the performance at high frequencies the inductor is preferable to use a polycarbonate or gall capacitance capacitor brew.
Select decoupling capacitance value is not critical and may be C = 1 / f is calculated; 0.1uF taking i.e. 10MHz, the system configuration of the microcontroller, can take between 0.1 ~ 0.01uf.
3, to reduce noise and electromagnetic interference of some experience
(1) do not use low-speed chip high-speed, high-speed chip used in key areas.
(2) the approach can be used a resistor string, the vertical reduction control circuit in the transition rate.
(3) try to provide some form of damping of relays.
(4) use of the lowest frequency clock to meet system requirements.
(5) The clock generator as close as possible to the device using the clock. Ground quartz crystal oscillator to the housing.
(6) the land area circled clock line, clock line as short as possible.
(7) I / O driver circuit as close to the edge of the printing plate, the printing plate allowed to leave as soon as possible. Signals into the printed circuit board to add filtering to the signal from the high noise areas also add filtering, while terminating resistor string by way of signal reflection is reduced.
(8) MCD unnecessary to pick high end, or ground, or defined as the output terminal, the ground terminal connected to the power supply on the integrated circuit must be connected, not floating.
(9) idle input gates not suspended, the idle positive input terminal of the operational amplifier, the negative input terminal of the output terminal. (10) make use of printed circuit board 90 without the fold line 45 and fold line so as to reduce the wiring emit high-frequency signal coupled to the external.
(11) and the printed circuit board according to the frequency characteristics of the switching current partition, noise and non-noise element from the element to be a little further away.
(12) a single panel and double panel to the power and the single-point ground using a single site, power lines, ground coarse as possible, it is economically sustainable with plywood to reduce the power capacity of the raw inductively.
(13) clock, the bus, the chip select signal for a remote I / O lines and connectors.
(14) an analog voltage input line, a reference voltage terminal to the signal line far away from the digital circuit, in particular a clock.
(15) do not intersect on the Class A / D devices, digital and analog portions of the lower portion rather uniform.
(16) clock lines are perpendicular to the line of small interfering I O / ratio of parallel I / O lines, a clock pin element remote from I / O cable.
(17) the pin element as short as possible, as short pin decoupling capacitor.
(18) of the line to try to rough key, and surrounded with protected. High-speed lines to be shorter straight.
(19) sensitive to noise and high current line not parallel, high-speed switching line.
(20) The following quartz and noise-sensitive devices do not go below the line.
(21) a weak signal circuit, low frequency circuit not formed around the current loop.
(22) Any signal not to form a loop, as is inevitable, so the loop area as small as possible.
(23) each integrated circuit a decoupling capacitor. Each side of the electrolytic capacitor will be added a small high frequency bypass capacitor.
(24) with a large-capacity tantalum capacitors or poly cool without electrolytic capacitor circuit for charging and discharging the storage capacitor. When a tubular capacitor to ground the housing.