Understand RF power in one article

Dec 24,2024
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RF power is a key parameter in the design of RF circuits for wireless communication, radar systems, and satellite communication. It directly affects the quality of signal transmission and the stability of communication distance. In this article, we will delve into the concept, calculation methods, and importance of RF power in practical applications, from theory to practice. Whether you are a beginner or an experienced engineer, I believe that by reading this article, you will have a clearer understanding of RF power and be able to better apply and optimize it in RF circuit design.

1、 Definition of RF power

In low-frequency circuits, the signal size is usually represented by voltage or current, while in RF circuits, due to the presence of standing waves on the transmission line, voltage and current lose their uniqueness, so the RF signal size is generally represented by power.

Power is defined as the energy flow per unit time, and the internationally recognized power unit is W (watts), which is defined as J/s (joules/s). Under traveling wave conditions, RF power can also be expressed in a similar way to low-frequency circuits:

In the above formula, P is power (W), I is current (A), V is voltage (V), and Z0 is the characteristic impedance of the lossless transmission line.

2、 The unit of measurement for power level - dB (decibels)

In different transmission and reception systems, the power levels encountered vary greatly, and even within the same system, there can be power levels that differ by trillions of times. For example, in cellular systems, the amplitude of the carrier frequency and intermodulation products are 20W and 10e-14W, respectively. To avoid the simultaneous occurrence of too large and too small values, and also to allow for direct addition and subtraction, logarithmic units dB are usually used to describe the magnitude of power. Logarithmic units can describe both the relative magnitude of power levels and the magnitude of absolute values.

Taking the system shown in the following figure as an example,

The change in power after passing through the attenuator is:

S21=10lg(P2/P1)

The change in power after passing through the amplifier is:

S32=10lg(P3/P2)

S21 and S32 are the power changes in dB, which can also be expressed in dBc. Please note that after passing through the attenuator, the power decreases, so S21 is negative, while after passing through the amplifier, the power increases, so S32 is positive.

The power change in dB can be directly added or subtracted. In the example above, the change from P1 to P3 is:

S31=10lg(P3/P1)

It can also be expressed as:

S31=S31+S32

3、 Absolute value expression of power

P1=1mW, which means 1mW is used as a reference level and 1mW is used as a reference level. Compared with P1, the absolute value of P2 can be expressed as:

10lg(P2/1mW）

Its unit is dBm. If P2 is 1mW, it can be expressed as 0dBm, and if P2 is 100mW, it can be expressed as 20dBm.

In Figure 1-1, if P1 is 0dBm, the attenuation of the attenuator is 3dB, and the gain of the amplifier is 25dB, then:

P2=0dBm-3dB=-3dBm

P3=0dBm-3dB+25dBm=22dBm

Note that positive values are commonly used to describe attenuation, but negative values should be used in calculations.

After using dBm as the unit, we will find that various measurements and calculations between powers become very convenient. Taking passive intermodulation in cellular mobile communication systems as an example, for a carrier frequency power of 20W and a passive intermodulation product of 10e-14, using dBm as the unit, it can be described as: relative to a carrier frequency of+43dBm, the passive intermodulation product generated by a certain device is -110dBm.

4、 3dB cut-off frequency

-3dB is a very important concept, also known as the half power point or cutoff power point. The power of the modified point is half of its total power, as shown in the above figure:

10lg(P1/P2)=10lg（1/2）=-3dB

RF power, as a key parameter in RF circuit design, is crucial for achieving efficient wireless communication and radar systems. By gaining a deep understanding of the concept, calculation methods, and optimization and protection measures in RF power applications, we can better design and optimize RF circuits. I hope this article can provide you with comprehensive and in-depth knowledge of RF power, helping you achieve better results in practical applications. May you fully leverage the advantages of RF power and make more contributions to the development of wireless communication.