The specific circuit of the wiring board is used to analyze the relationship between voltage, current and resistance

    Resistance is a real physical component, we can know the relationship between voltage, current and resistance by Ohm's law, U=I*R Let's analyze the specific relationship between the three through a specific circuit, see the simplest circuit diagram below. The circuit diagram consists of a single power supply, a resistor and some wires. Of course, the resistance of this resistance can also be directly measured by using a multimeter. The characteristic impedance is different. When measuring a characteristic impedance of 50 ohm with a multimeter, it will be found to be short-circuited. This requires us to conceptually distinguish between resistance (even a resistance of exactly 50 ohms) and characteristic impedance, which are different things. Like degrees above temperature (Celsius) and degrees in Angle, they are not the same thing. Resistance is a physical quantity we all understand, so I won't explain it here. Let's analyze where this characteristic impedance is and under what conditions it will be used. In fact, characteristic impedance is closely separated from the radio frequency of a physical quantity, before understanding the characteristic impedance to understand the radio frequency. We know that radio stations, cell phone signals, wifi, etc. are all devices that emit signal energy to the outside, that is to say, the energy comes out of the antenna, the energy doesn't come back to the antenna, you can imagine it's like a machine gun strafing the outside, the bullet goes out and doesn't come back. Ok, so with radio frequency out of the way, let's go to the wire that transmits radio frequency energy. The same is true of the RADIO frequency signal transmitted above the wire. If there is energy transmitted back, it means that the transmission effect is poor. To illustrate the characteristic impedance, I will use an analogy: there are two wires on the same circuit board (assuming they are very long, you can imagine it as long as possible), because of the same board, then the copper skin thickness of the two wires is the same. Two wires, the length (infinite length) and the thickness are the same, the only difference is the width, let's say wire 1 is 1, wire 2 is 2. In other words, line 2 is twice as wide as line 1. The diagram below illustrates the two wires in detail. As shown in the figure above, if both wires are connected to the same RF source at the same time, for the same short period of time T, then let's see what the difference will be between these two wires. For the same source, the output RF voltage of the two wires is the same, and the distance of rf transmission is the same (assuming both are light speed, which is actually less than the speed of light). The only difference is the line width, and the line width of Line 2 is twice that of line 1, so line 2 needs 2 times of power of line 1 to fill the extra line width area (in fact, it is the capacitance effect caused by the copper skin and the bottom surface of the wire). In other words: Q2 is equal to twice Q1 because? I = Q/T (RF current = charge/time), then it can be known that the RF current of line 2 is twice that of line 1 (since the time is the same, the charge of line 2 is twice that of line 1). Well, we know that i2 is equal to twice i1 and when we get here, we find a mysterious property and the impedance is not far off, and why is that? Because we know that resistance is equal to voltage over current. In fact, the characteristic impedance has the same relationship: characteristic impedance = RF voltage/RF current. We know from above that the rf voltage is the same, the current relationship is i2= twice i1, right? The characteristic impedance of line 2 is only half of line 1! This is what we call the wider the line, the smaller the characteristic impedance. Above is my example to illustrate the difference between characteristic impedance and resistance, and why the same board, characteristic impedance is related to line width, not length. In fact, there are many factors that affect the characteristic impedance, including material, wire and floor space, and many other factors. The characteristic impedance of a wire is described (metaphorically) in colloquial terms as the amount of resistance a wire has to the rf energy transmitted above it. In thinking about the reflection of a transmission line we assumed that the wire was infinite in length, when in fact it is finite in length. When the RF signal reaches the end of the wire, there is no way to release the energy, and it comes back down the wire. Just like we yell at the wall, and it bounces back. That is to say, the situation that we imagine rf signals going out and not being reflected back is not real. As shown above, suppose we attach a resistor to the end of the wire to consume (or receive) the rf energy transmitted from the wire. Some people may ask why the characteristic resistance of the wire does not consume energy, but must be connected to a resistance to consume? In fact, wires only transmit energy. Wires themselves do not consume or almost do not consume energy (similar to the properties of capacitors or inductors). A resistor is a device that loses energy. We find three special cases: when R=RO, the transmitted energy is just absorbed by the resistance R at the end, and no energy is reflected back. You can view this wire as a wireless length. When R=∞ (open), all the energy is reflected back, and the end point of the line generates a voltage twice that of the source. When R=0, the end point will generate a reflection of -1 times the source voltage. Recognition impedance matching Impedance matching refers to the load impedance and excitation source internal impedance match each other to obtain the maximum power output of a working state. Impedance matching is for rf, etc., not for power circuits, otherwise it burns things up. We often hear about characteristic impedance 50 ohms, 75 ohms and so on, how did this 50 ohms come from, why is it 50 ohms and not 51 ohms, or 45 ohms? This is by convention, 50 ohms should be better for general RF circuits. In other words, our wire, our cable, has to do 50 ohms, because the circuit load is already equal to 50 ohms of resistance. You make other impedance wires, they don't match the load. The farther the deviation, the worse the transmission will be!