Common level units for electromagnetic measurement

Absolute values ​​can also be expressed by appropriate reference values, in which case
subscripts are used to define the reference values, for example, the subscript m in dBm stands for mW, indicating
dB values ​​relative to 1mW, and the subscript mV in dBmV stands for 1mV, indicating
dB values ​​relative to 1mV.
In electromagnetic measurements, the commonly used test unit is usually
expressed in dB (decibel). It is a logarithmic compression of signal levels with a large dynamic range.
Compared with linear coordinates, logarithmic display can obtain a larger relative amplitude range
. When measuring relative values ​​of signal amplitudes, including gain and loss, signal
ratios (division operations) can be simply expressed as dB differences (subtraction operations).

1 Definition of decibel (dB)
The unit decibel (dB) of electrical parameters is defined by means of power ratio:
P (dB) = 10lg (P1/P2)
When dB is used to represent the absolute value of power, it is used:
P (dB) = 10lg (P/P0)
For example, when P 0 = 1 m W,
P (dBm) = 10lg (P/1mW)
When P 0 = 1pW,
P (dBpW) = 10lg (P/1pW)

2 Other units expressed in dB

For example, when V0 = 1mV,
V (dBmV) = 10lg (V/1mV)
dB mV is a common unit for representing voltage values ​​in EMC testing.
When I0 = 1mA,
I (dBmA) = 10lg (I/1mA)
dB mA is a common unit for representing current values ​​in EMC testing.

In addition to dBm, dBmV, dBmA, and dBpW, there are other
important units, such as dBmV/m, dBmA/m, dBpT, dBm/Hz, dBc, and
dBc/Hz. dBmV/m is the unit of electric field strength; dBmA/m and
dBpT are the units of magnetic field strength. dBm/Hz is the power density within a 1Hz bandwidth; dBc refers to the difference between two dB values, indicating the relative value of two values;
dBc/Hz refers to the relative value density within a 1Hz bandwidth.
Representation of field strength:
E (dBmV/m) = V (dBmV) + AF (dBmV/m)
In the formula, E is the electric field strength, V is the receiver voltage reading, and AF is the antenna factor.
H (dBmA/m) = V (dBmV) + AF (dBmA/m)
In the formula, H is the magnetic field strength, and V is the receiver voltage reading. AF is the antenna conversion
factor, which includes the conversion relationship from voltage value to current value and field strength value.

3 dB Conversions between units

Table 1 Conversion between units of voltage, current and power (50Ω rated impedance)

         dBm dBpW dBuV dBuA

dBm 0 90 107 73

dBpW -90 0 17 -17

dBuV -107 -17 0 34

dBuA -73 17 -34 0

4 dB and percentage dB are often used to express the relative size of two signals. The smaller
of the two signals can be described as being a certain dB lower than the larger reference signal . For example, the measurement results of modulated signals, spurious signals, and harmonic distortion are often expressed in dB. In some cases, it is required to express the size of the smaller signal as a percentage of the larger signal, such as harmonic distortion. P (dB) = 10lg (P1/P2) P (%) = (P1/P2) × 100% P (dB) = 10lg (P% × 100)