Why use 50 ohms in RF?(2)

In general RF devices or equipment, we will find that the impedance of the input or output port is basically 50 ohms. Therefore, the RF circuit is also designed according to the impedance matching of 50 ohms.

Another point to note is that the impedance here does not mean that the resistance of the internal wire is 50 ohms. If you measure the resistance on both sides of the wire with a multimeter, you will find that it reads 0. In fact, it refers to the impedance of the entire transmission line system. We can regard the transmission line as a circuit composed of n resistors, capacitors, and inductors. We can use the following equivalent circuit to represent:

Why do you need impedance matching?
Before returning to this question, let's first look at what is impedance?

The resistance, inductance and capacitance in the circuit will hinder the current, and we call the resistance to the current in the circuit called impedance.

Impedance consists of two parts: resistance and reactance, and reactance can be divided into inductive reactance and capacitive reactance. Impedance can also be represented by a complex number, with resistance as the real part of the complex number and reactance as the imaginary part of the complex number.

Z is the impedance
X is reactance
R is the resistance
wL is the inductive reactance
1/wC is the capacitive reactance
f is the frequency of the signal
ω is the angular frequency

From the impedance formula, we can see that for an ideal pure resistance, its resistance is independent of the frequency of the signal. But in fact, any circuit has resistance, inductance and capacitance, and the magnitude of inductive and capacitive reactance is related to frequency. Therefore, for high-frequency signals, the inductive and capacitive reactance in the circuit or system cannot be ignored.

Radio frequency is a high-frequency signal, which is different from low-frequency signals. In addition to the resistance, the inductive and capacitive reactance in the circuit must also be considered. For low-frequency circuits, waveforms such as current and voltage are transmitted in the circuit, while RF circuits need to transmit power from the transmitter to the receiver. In the process of power transmission from one circuit to another circuit, it is necessary to ensure the power and quality of the signal as much as possible. How to ensure these, the most important thing is to do the impedance matching of the circuit, that is to say, to correctly match the impedance of the input circuit and the output circuit.

We can understand the relationship between water pipes and faucets. Water pipes are like output circuits, faucets can be used as input circuits, and water is the transmitted signal. During installation, we must ensure that the interfaces of the water pipe and the faucet match each other, so that the water can naturally flow from the water pipe to the faucet without leakage. If one of the interfaces is too large or too small, there will be water leakage.

The same is true for radio frequency circuits, however, the interface part of the radio frequency circuit needs impedance matching. If the interface part of the RF circuit is not properly matched, the signal will reflect back and forth at the interface, and the signal power transmitted to the other circuit will be reduced. Due to the short wavelength of the high-frequency signal, the reflected signal will also be superimposed with the original signal, which will also lead to signal distortion and poor signal quality. In addition, there will also be interference signals entering the circuit from the outside world. In this way, if there is no correct impedance matching, the RF signal cannot be transmitted correctly.

Students with amplifier debugging experience should have encountered this situation. In the amplifier circuit, if the matching of the amplifier ports is not done well, there will be a self-excitation phenomenon. This is also the reason why we often add attenuators at both ends of RF amplifiers. In addition to attenuating large signals, it can also prevent self-excitation due to impedance mismatch of amplifier ports.

In a radio frequency circuit or radio frequency system, impedance matching is a key task. The quality of the matching affects the transmission of the signal of the direct system or circuit, and has a great impact on the performance of the system or circuit. Usually, the debugging of impedance matching is a work that takes up a lot of work for RF engineers.

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1. Why use 50 ohms in RF?(1)