People always have a process of understanding things, and they are generally from concrete to abstract. Understanding the characteristic impedance is also the same. Before we recognize the characteristic impedance, we must first understand the physical quantity-resistance that is related to the characteristic impedance.
The resistance is a real physical component. We can know the relationship between voltage, current and resistance through Ohm's law, U=I*R.
We use a specific circuit to analyze the specific relationship between the three. Please see the simplest circuit diagram below. This circuit diagram consists of only one power supply, one resistor, and some wires.
Of course, the resistance of this resistor can also be measured directly by using a multimeter.
The characteristic impedance is not the same. When a multimeter is used to measure a 50-ohm characteristic impedance, it will be short-circuited. This requires that we conceptually distinguish between resistance (even if it is a resistor that is just 50 ohms) and characteristic impedance. Just like the degree (degrees Celsius) and the degree above the temperature, it is not a thing.
The physical quantity of resistance is understood by everyone and will not be explained here. Let's analyze the sacredness of this characteristic impedance, and under what conditions will we use this thing.
In fact, the characteristic impedance is a physical quantity that is closely spaced from the radio frequency. Before you understand the characteristic impedance, you must first understand the radio frequency. We know that radios, cell phone communication signals, wifi, etc. are devices that send signal energy to the outside world. That is, energy is emitted from the antenna. The energy is no longer returned to the antenna. It can be imagined that the bullet fired just like the machine gun fired outside. Going out will not come back.
Well, after understanding this thing with radio frequency, we come to the specific wire that transmits the radio frequency energy. The radio signal transmitted over the wire is also the same, and it is hoped that it will not be transmitted back in the past. If there is energy transmitted back, it means that the transmission is not effective.
In order to more specifically illustrate the characteristic impedance of this thing I make an analogy here:
There are two wires on the same board (assuming they are two long wires, you can imagine how long it is), because the same board, then the copper thickness of the two wires is the same . The two conductors, the length (infinity) and the thickness are the same, only the only difference is the width, assuming that the width of the No. 1 wire is 1 (units) and that of the No. 2 wire is 2 (units). In other words, the width of Line 2 is twice that of Line 1.
The following figure can be specifically seen in the schematic diagram of the two wires.
As shown in the above figure, if we are all connected to the same RF emitter at the same time, for the same short time T, then we can see what the difference between these two wires is. With the same source, the output RF voltages of the two lines are the same, and the RF transmission distance is the same (assuming all the speeds of light are actually less than the speed of light).
The only difference is the line width. Line 2 is twice as wide as Line 1. Then Line 2 needs twice the power of Line 1 to fill up the extra line area (in fact, the copper wire and the bottom of the wire) The resulting capacitive effect). In other words: Q2 = twice Q1
Since i = Q/T (RF current = power/time), it can be seen that the RF current of line 2 is twice that of line 1 (because the time is the same, line 2 is twice the power of line 1). .
Well, we know that i2=double i1
Here, we find a mysterious characteristic impedance is not far, why, because we know resistance = voltage / current. In fact, the characteristic impedance also has this relationship: Characteristic impedance = RF voltage / RF current.
From the above we know that the same RF voltage, the current relationship is i2 = twice the i1
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.
The above is an example of the difference between the characteristic impedance and the resistance, and why the same board, the characteristic impedance and the line width, has nothing to do with the length.
There are many factors that actually affect the characteristic impedance, including the material, the distance between the lead and the floor, and so on.
The characteristic impedance of a wire is described in plain language (but metaphorically), and it is the magnitude of the resistance of the wire to the transmitted RF energy.
Understand the reflection of the transmission line
Above we assume that the wire is infinitely long and the actual wire length is limited. When the RF signal reaches the end of the wire, there is no way to release the energy and it will be sent back along the wire. Just as we cried against the wall, the sound came back to the wall and echoed back. In other words, the fact that we don't imagine that the radio signals are sent out without reflection back in reality does not exist.
As shown in the figure above, if we connect a resistor at the end of the line to consume (or receive) the RF energy transmitted over the line.
Some people may ask, why the resistance of the characteristic impedance of the wire does not consume energy, have to take a resistor to consume it? Actually, a wire only transmits energy, and the wire itself does not consume energy or does not consume energy (a bit like the properties of a capacitor or an inductor). Resistance is an energy-consuming element.
We have found three special cases:
When R = RO, the transmitted energy is just absorbed by the end resistor R, and no energy is reflected back. It can be seen that this wire is wireless long.
When R = ∞ (open circuit), all energy is reflected back, and the end point of the line will produce twice the voltage of the source.
When R=0, the end point will produce a -1 times the source voltage reflected back.
Understanding impedance matching
Impedance matching is a working state in which the load impedance and the internal impedance of the excitation source are matched to each other to obtain the maximum power output.
Impedance matching is for radio frequency etc. It is not suitable for power circuits, otherwise it will burn things.
We often hear that the characteristic impedance is 50 ohms, 75 ohms, etc.. How does this 50 ohm come from, why is 50 ohm instead of 51 ohm, or 45 ohms?
This is an agreed-upon, 50 ohms should say that for general radio frequency circuit transmission effect is better. In other words, our wire and cable have to be 50 ohms because the circuit load is already equivalent to a 50 ohm resistor. You do other impedance wires and do not match the load. The farther it deviates, the worse the transmission will be!
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