Many senior engineers, often walking the basis of the wrong design of the road. This is the electromagnetic compatibility (EMC) design of high-frequency thinking.

Electromagnetic compatibility (EMC) problems often occur in high frequency conditions, except for individual problems (voltage drop and instantaneous interruption, etc.). High-frequency thinking, is the characteristics of the device, the circuit characteristics, in the case of high frequency and conventional low-frequency state is not the same, if still in accordance with the ordinary control of thinking to determine the analysis, it will go into the design errors. For example: capacitance, in the case of low-frequency or DC, is an energy storage components, only the performance of a capacitor characteristics; in the case of high-frequency, it is not just a capacitor, it has an ideal capacitor characteristics, The current (expressed as R on the high frequency equivalent circuit) has a lead inductance and an ESR (equivalent series resistance) that generates heat in the event of a voltage pulse fluctuation (as shown below).
Electromagnetic compatibility (EMC) _ High frequency equivalent circuit diagram
Figure 1: Electromagnetic compatibility (EMC) _ capacitance of the high-frequency equivalent circuit diagram

From the above figure to see, designers in the electromagnetic compatibility (EMC) design can play a great help.

According to conventional ideas, 1 / 2πfc is the capacitive reactance of the capacitor, the higher the frequency, the smaller the capacitive reactance, the better the filtering effect, that is, the more high frequency clutter is easier to be vented, but this is not the case, The existence of an inductor, a capacitor only in its 1 / 2πfc = 2πf L equation is established when the overall impedance is the smallest time, the filter effect is the best, low frequency will be low filter effect, which can Analysis of the conclusion, why in the IC VCC side will add two capacitors, an electrolytic, a tiles, and the value of the general difference of more than 100 times more than a little more. It is two different capacitance of the resonant frequency point divergence for some distance, both for the slightly higher frequency filtering, but also conducive to the lower frequency of the filter.
Electromagnetic compatibility (EMC) _ High frequency equivalent circuit diagram
Figure 2: Electromagnetic compatibility (EMC) _ high-frequency equivalent circuit wire diagram

Cable or PCB wiring of the high-frequency equivalent characteristics (as shown above), regardless of high and low frequency, the line resistance is an objective reality, but for the line inductance, it is only in the high frequency when it can be revealed. There is also a distribution of the existence of a capacitor, but there is no conductor in the vicinity of the conductor, the distribution capacitor is also no good, must have two conductors can play a role.

Inductance and resistance characteristics are easier to understand, do not explain. But the magnetic ring and magnetic beads of the high-frequency equivalent characteristics have to mention, because the magnetic ring on the high-frequency pulsating wave effect, and the performance of the inductive somewhat similar, it is often considered to be inductive characteristics, but in fact wrong The The magnetic ring is a resistance characteristic, but this resistance is a bit special, its resistance is the size of the function of the frequency R (f), so, in a high frequency fluctuations with the signal through the beads when the high frequency fluctuations due to I2R the role of heat, the fluctuations will be intertwined by the energy - magnetic - thermal energy conversion process, so the wave is relatively strong fluctuations in the time, the magnetic ring will feel warm.

The above is the electromagnetic compatibility (EMC) professional high-frequency thinking of the basic knowledge, to understand the electromagnetic compatibility (EMC) knowledge, a lot of design problems are known to the answer. such as:

1, IC VCC side why the installation of two capacitors, an electrolytic capacitor, a ceramic capacitor, because the high-frequency capacitor equivalent characteristics, lead inductance and capacitance of the series lead to its integrated impedance changes with frequency, and At the frequency point of WL = (1 / WC), it is the point with the smallest impedance (as shown below). And the two capacitors have their own minimum impedance points, respectively, corresponding to the different frequency points, in order to IC for different frequency range of power supply needs to provide current.
Electromagnetic compatibility (EMC) _ Impedance - Frequency characteristics
Figure 3: Electromagnetic compatibility (EMC) _IC decoupling capacitance impedance - frequency characteristics of the map

2, the electrostatic table grounding wire with a wide copper belt and wire mesh snake leather tube, rather than yellow and green circular grounding cable, rounded grounding cable alignment inductance is too large, is not conducive to high-frequency static Charge release.

3, the distance between the cable and the cable should not be too close, otherwise there will be distributed due to the existence of wire capacitance between the signal cable crosstalk, of course, the signal line on the ground coupling that is best near, In this way, the fluctuation of the signal line can easily discharge to the ground up.

For electronic engineers, the electronic components are treated as a high-frequency equivalent combination circuit, the electromagnetic compatibility (EMC) design will be more perfect.