What is the relationship between the source internal resistance and the characteristic impedance of the transmission line required for driving the transmission line?

乱世煮酒论天下

What is the relationship between the source internal resistance and the characteristic impedance of the transmission line required for driving the transmission line? [Copy link]

 

In order to apply the driver's source voltage to the transmission line as much as possible, the driver's source internal resistance is required to be as small as possible. What is the specific quantitative relationship between this source internal resistance and the characteristic impedance of the transmission line? Within what percentage of the characteristic impedance of the transmission line does the source internal resistance need to be?

When a signal is added to a transmission line for transmission, is the signal transmitted simultaneously on the signal path and the return path, or is it first transmitted on the signal path and then returned to the signal end on the return path?

Jacktang

When the instantaneous impedance changes suddenly during the transmission process, the input impedance is no longer equal to the characteristic impedance.

Characteristic impedance mainly refers to the impedance of uniform transmission lines.

gmchen

Generally speaking, a transmission line refers to a conductor that transmits radio frequency signals, such as a coaxial cable, a microstrip line on a printed circuit board, etc. The characteristic impedance of this type of transmission line is most commonly 50 ohms.

In RF transmission, the output impedance of the excitation source should be consistent with the characteristic impedance of the transmission line, and the input impedance of the receiving end should also be equal to it. If the impedance does not match, signal reflection will occur during the transmission process, resulting in a decrease in the signal power received by the receiving end.

gmchen

If all wires that transmit signals are broadly called transmission lines, then the impedance matching problem can be ignored in low-frequency conditions. At that time, the output impedance of the excitation source should of course be as low as possible, such as the output of a normal op amp.

乱世煮酒论天下

gmchen published on 2024-5-11 08:30 Generally speaking, a transmission line refers to a conductor that transmits radio frequency signals, such as a coaxial cable or a microstrip line on a printed circuit board. The characteristic impedance of this transmission line is ...

I saw in the literature that the lower the output impedance of the excitation source, the better, but don’t we need to consider impedance matching at this time? According to the literature, the output impedance of the general excitation source is only 80-130Ω, and the output impedance of the excitation source with the latest technology is less than 10Ω.

gmchen

乱世煮酒论天下Published on 2024-5-11 16:05 I saw in the data that the lower the output impedance of the excitation source, the better, but don't we need to consider impedance matching at this time? The data shows that the output impedance of the excitation source is generally...

Whether impedance matching is required has a lot to do with the signal frequency. Don’t know the frequency geometry of your excitation source? What is the application environment?

乱世煮酒论天下

gmchen posted on 2024-5-14 13:12 Whether impedance matching is required has a lot to do with the signal frequency. I don’t know the frequency geometry of your excitation source? What is the application environment?

Various fieldbuses

maychang

Talking about the world in a chaotic world published on 2024-5-14 13:58 Various fieldbuses

Various fieldbuses

No matter which field bus it is, it is still far from the transmission line.

乱世煮酒论天下

maychang posted on 2024-5-14 18:09 [Various fieldbuses] No matter what kind of fieldbus, it is still far from the transmission line.

What if it is a parallel bus in a computer? How many Gigabits is its frequency?

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-5-14 18:22 What if it is a parallel bus in a computer? How many Gs is the frequency?

[What if it is a parallel bus in a computer? How many Gigabits is its frequency? ]

The parallel bus in a computer has a frequency of only a few GHz. It's not that scary.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-5-14 18:22 What if it is a parallel bus in a computer? How many Gs is the frequency?

When we say frequency is several G, that is the frequency of the CPU. The propagation speed of electromagnetic waves is 3^8m/s. Assuming the CPU frequency is 3GHz, the wavelength is 0.1m, or 10cm. If you look inside a desktop computer, is the CPU package as large as 10cm? The actual size of the silicon chip is less than 2cm. Therefore, the internal wiring of the CPU is not even as large as a transmission line.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-5-14 18:22 What if it is a parallel bus in a computer? How many Gs is the frequency?

Not all conductors can be called transmission lines. For example, high-voltage transmission lines in the power industry are tens or even hundreds of kilometers long, but no one has ever considered them transmission lines, and no one requires power plants or substations to do impedance matching. This is because the wavelength of the power frequency is too long (50Hz wavelength is 6,000 kilometers, and China is only 5,000 kilometers from west to east). Even a 600-kilometer-long high-voltage transmission line is only one-tenth of the wavelength.

乱世煮酒论天下

maychang posted on 2024-5-14 19:06 Not all wires can be called transmission lines. For example, high-voltage transmission lines in the power industry are tens or even hundreds of kilometers long, but no one has ever thought that...

So in which situations do we need to consider impedance matching? Or is impedance matching required?

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-5-14 21:08 So in which situations do we need to consider impedance matching? Or is impedance matching required?

Impedance matching is overused. In many cases, it is not actually impedance matching, but just a requirement for a certain impedance.

When it comes to impedance matching of transmission lines, first of all, it must be a transmission line. Those that are not transmission lines are not considered. Secondly, the transmission line is required not to reflect the signal. If the transmission line is required to reflect, then there is no need for impedance matching.

gmchen

Again, impedance matching is required for transmission lines that are specifically used to transmit RF signals, a concept that only exists when the signal wavelength and the length of the transmission line are comparable. Impedance matching is required because in the impedance matching state, the signal has the smallest reflection, and the load can obtain the maximum RF power and the smallest transmission distortion.

In other cases, the conductors that transmit signals can also be called transmission lines, but that is a transmission line in a broad sense, which is completely different from the transmission line that transmits radio frequency signals.

gmchen

For example, in high-speed digital circuits, the signal transmission lines on the printed circuit board also need to consider reducing signal reflection (reducing ringing), but usually it is not considered impedance matching, but the method of adding absorption resistance is adopted. For another example, in the transmission of differential signals, the synchronization of the two signals must be considered, and usually the length of the two signals is designed to be the same (snake line), etc.