The resistor is a real physical component, and we can know between voltage, current and resistance, u = i * r.
We analyze the specific relationships between the three through a specific circuit, please see the simplest circuit diagram below. This circuit diagram has only one power supply and some wires consisting.
Of course, the resistance of this resistor can also be directly measured by a multimeter.
The characteristic impedance is different. When measuring a 50 ohm characteristic impedance with a multimeter, it will be found to be short-circuited. This requires us to distinguish between resistance from concept (even if it is just 50 ohms resistance) and characteristic impedance is two yards. Just like the degree of the temperature (degrees Celsius) and the angle, it is not a thing.
Resistance This physical quantity knows everyone, this is not explained here. Let's analyze this feature impedance, where is the sacred, which will use this thing under what conditions.
In fact, the characteristic impedance is a physical quantity that is closely separated by radiofrequency, and knows the radio frequency before the understanding of the characteristic impedance. We know that the radio, mobile phone communication signal, WiFi, etc. are devices that transmit signal energy to the outside, which means that energy is shot from the antenna, and the energy is no longer coming back to the antenna. It can be imagined like a machine gun to take the outside. I don't come back.
Ok, after I understand the radio, we come to the specific transmission of radio frequency energy. The radio signal transmitted above the wire is also the same. I hope that it will not be reversed back, if there is an energy reverse pass, it means that the effect is poor.
For more specific description characteristics impedance this thing I am playing one more:
There are 2 wires on the same board (assuming is a long two lines, can you think of how long it is, because the same board, then the two wire copper thickness is the same . The two wires, long (unlimited length) and thickness are the same, only the only difference is the width, assuming that the width of the No. 1 wire is 1 (unit), the 2nd wire is 2 (unit). That is, the width of the second line is twice the line 1.
The following figure can be specifically seen by two wires.
As shown in the figure above, if it is connected, it is the same RF emission source, the same short period T, then we look at the difference between the two wires. The same launch source, then the output RF voltage of the two lines is the same, the distance between the radio frequency transmission is the same (assuming is light speed, the actual is less than the light speed).
The only difference is the line width, and the lines of Line 2 are twice as large lines in Line 1. So 20 times the line 2 needs to fill more out of the line width area (in fact, wire copper and bottom surface) The generated capacitance effect). That is, Q1 is twice the Q1.
Because i = q / t (radio frequency current = electricity / time), then you can know that the RF current of Line 2 is twice the line 1 (because time is the same, the Line 2 is twice the line 1) .
Ok, we know that I2 = twice the I1.
Here, we have to find a mysterious feature impedance is not far away, why, because we know the resistance = voltage / current. In fact, characteristic impedance also has this relationship: characteristic impedance = RF voltage / RF current.
From the above, we know that the RF voltage is the same, the current relationship is i2 = twice the I1.
Then the characteristic impedance of Line 2 is half of the line!
This is what we said is, the smaller the characteristic impedance.
The above is the difference between the characteristic impedance and the resistance, and why the same board, the characteristic impedance is related to the line width, and there is no relationship with the length.
There are many factors that actually affect characteristic impedance, including materials, wires and bottom ground spacing, etc. Many factors.
The characteristic impedance of the wire is described (just a metaphor), which is the size of the radio energy that is transmitted above.
Recognize 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 arrives at the end of the wire, the energy is not released, and it will be back along the wire. Just shouting with us, the sound came back to the wall back to generate echo.
That is to say, the situation in which the RF signal in our imagination is not reflected back, it does not exist in reality.
As shown in the figure above, if the end of our online end is connected to a resistor to consume (or receiving) the radio frequency energy transferred over the line.
Some people will ask, why the resistance of the characteristic impedance of the wire does not consume energy, can you consume a resistor? In fact, the wire is only transmitted, and the wire itself does not consume energy or approximately no loss energy (a bit like a capacitor or an inductor attribute). The resistor is a component of loss energy.
We found three special circumstances:
When R = R0, the transmitted energy is just better than the resistance R of the end, and there is no energy reflection back. It can be seen as this wire is a wireless length.
When R = ∞ (open), the energy is reflected back, and the on-line endpoint generates a voltage of 2 times the emitting source.
When R = 0, the end point generates a -1 times the source voltage reflects back.
Know impedance match
Impedance matching refers to a working state of load impedance and an excitation source internal impedance to obtain a working state of maximum power output.
Impedance matching is for RF or the like, which is not applicable for power circuits, otherwise it will burn.
We often listen to the characteristic impedance 50 ohms, 75 ohms, etc., how did this 50 ohm come, why is 50 ohms instead of 51 ohms, or 45 ohms?
This is agreed, 50 ohms should be said to be better for general RF circuits.
That is to say, our wires, cables should be 50 ohms because the circuit load is equivalent to 50 ohms resistance. You do other impedance lines, just do not match the load. The farther the deviation, the worse the effect of transmission!
Reprinted from the network.
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