dB commonly known as decibel, deci can also indicate to one tenth.
It is a unit of Power Gain, in the units of 10lg, it shows a relative value.
10 lg A/B is a unit for counting, it can be used in calculations, for solve how many dB has power A gain or loss than power B.
For voltage or electron current, dB = 20*lg(A/B).
In other words, a dB is to facilitate an enormous number (positive or negative numbers) into a brief form.
e.g.
1: X=1000000000000000
10lgX=150dB
2: X=0.000000000000001
10lgX=-150 dB
3: A is twice as power as B, in formula: 10lg A/B = 10 lg 2 = 3dB.
Which is A has 3dB power more than B.
4: A has 46dBm power, B has 40dBm power, and therefore, A has 6dB power more than B.
If antennae A has 12dBd power, and antennae B has 14dBd, then antennae A is 2dBd less than antennae B.
Under the circumstances of the control or signal processing field, the signal strength (Amplitude) can be used to describe the work or power and use 20lg in units. Such as dBmV is form of expression people see very often.
dBm indicates the units of absolute power value, in formula (Power Value/ 1mW).
e.g.
1: If the output power(P) is 1mw, which will be 0dBm after converted.
2: To turn a power of 40W into the dBm units, in formula:
10lg (40W/1mw) =10lg (40000) =10lg (4*10^4) =40+10*lg4=46dBm.
dBm stands for dB-milliwatt, that is read by a number which compared to milliwatt.
0 dBm=10lg1mw
dBw is defined as watt. 0 dBw = 10lg1 W = 10lg1000 mw = 30 dBm
dBc has the exact same calculation as dB, it is also a relative value of power.
Mostly, dBc can be represent in formular :10 lg A/B relative to carrier power.
In principle, dBc can also substitute by dB to measure the relative value of carrier power in many circumstances. For example, measure the relative in interference measurement (co-channel interference, intermodulation interference, adjacent channel interference, out-of-band interference etc.) multi-coupling and stray current, where dB can be used instead of dBc.
dBi and dBd
dBi and dBd is an examination of the Power Gain, them both are relative value in different refence.
dBi can be reference to omnidirectional antenna. For the dBd, it can be reference to dipole antenna.
Generally, dBi is greater than dBd by 2.15 to indicate gain.
e.g.
1: For an antenna with a gain of 16dBd, when its gain is converted into dBi, it is 18.15dBi (usually written as 18dBi)
2: 0dBd=2.15dBi.
3: GSM900 antenna gain 13dBd (15dBi), GSM1800 antenna gain 15dBd (17dBi).
Notice:
- Knowledge the principle of dB and dBm calculations.
e.g.
0dBw=10lg1W=10lg1000mw=30dBm;
Another example, when dBm minus a dBm, it can only get answers in dB: 30dBm-0dBm=30db.
- Only has addition and subtraction between dB’s
Generally speaking, multiplication and division cannot be used in dB’s calculation.
Engineering mostly uses subtraction to calculate the signal-noise ratio (SNR): dBm minus dBm is actually doing a division. Take SNR for example. which is the band power divided by noise equivalent power.
On the other hand, dBm plus dBm is actually on a multiplication, but it is not a commonly used method.
