Hello,

Today, I would like to share with you this topic which is dB & dBm discussion (basic concepts and calculation methods).

People often see these two concepts on the network, which may be considered very simple, but they are very important; if they are not understood in optical fiber communication, they may not be able to open the business for a long time, and will seriously burn the board or even the equipment.

In the calculation of dB and dBm, we should pay attention to the basic concepts. For example, the aforementioned 0dBw = 10lg1W = 10lg1000mw = 30dBm; for example, when one dBm is subtracted from another dBm, the result is dB. For example: 30dBm-0dBm = 30dB.

*dB and dB only add and subtract*

Generally speaking, in engineering, there is only addition and subtraction between dB and dB, and there is no multiplication and division. The most commonly used is subtraction: dBm minus dBm is actually the division of two powers. The division of signal power and noise power is the signal-to-noise ratio (SNR). dBm plus dBm is actually the multiplication of two powers, this is rare (I only know that there are such applications in power spectrum convolution calculations).

*Unit*

db, indicating a relative value. When calculating how many dB the power of A is larger or smaller than that of B, it can be calculated according to the formula 10lg A / B. For example: A power is twice as large as B power, then 10lg A / B = 10 lg 2 = 3dB. In other words, the power of A is 3dB greater than that of B; if the power of A is 46dBm and the power of B is 40dBm, it can be said that A is 6dB larger than B; if the A antenna is 12dBm and the B antenna is 14dBm, it can be said A is 2dB smaller than B.

dBm is a unit that represents the absolute value of power, the calculation formula is: 10 lg power value / 1mW. For example: if the transmit power is 1mW, the value converted in dBm should be: 10 lg 1mW / 1mW = 0dBm.

*dBm*

dBm is a value that evaluates the absolute value of power, and the calculation formula is: 10lg (power value / 1mw).

**[Example 1]** If the transmit power P is 1mw, it is converted to dBm and then 0dBm.

**[Example 2]** For 40W power, the value after conversion in dBm units should be:

10lg (40W / 1mw) = 10lg (40000) = 10lg (4 * 10 ^ 4) = 40 + 10 * lg4 = 46dBm.

*dBi and dBd*

dBi and dBd are the values of the gain (power gain). Both are relative values, but the reference is different. The reference reference of dBi is an omnidirectional antenna, and the reference reference of dBd is a dipole, so the two are slightly different. 15 It is generally considered that the same gain is expressed in dBi, which is 2.15 larger than expressed in dBd.

**[Example 3]** For an antenna with a gain of 16dBd, when the gain is converted into dBi, it is 18.15dBi (the decimal place is generally ignored and is 18dBi).

**[Example 4]** 0dBd = 2.15dBi.

**[Example 5]** The GSM900 antenna gain can be 13dBd (15dBi), and the GSM1800 antenna gain can be 15dBd (17dBi).

*dB*

dB is a value representing the relative value. When considering how many dB the power of A is compared with the power of B, the following calculation formula is used: 10lg (power of A / power of B). When considering how much dB A voltage (or current) is compared with B voltage (or current), according to the following calculation formula, 20lg (A voltage (or current) / B voltage (or current)) [1].

**[Example 6]** A power is twice as large as B power, then 10lg (A power / B power) = 10lg2 = 3dB.

That is, the power of A is 3 dB greater than the power of B.

**[Example 7]** The transmission loss of 100 meters of 7/8 inch GSM900 feeder is about 3.9dB.

**[Example 8]** If the power of A is 46 dBm and the power of B is 40 dBm, it can be said that A is 6 dB larger than B.

**[Example 9]** If the antenna of A is 12dBd and the antenna of B is 14dBd, it can be said that A is 2 dB smaller than B.

**[Example 10]** A voltage is twice as large as B voltage, then 20lg (A voltage / B voltage) = 20lg2 = 6dB.

Welcome to leave a message and exchange in the comment area.

Thanks!