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## The Difference Between db And dbm–Fmuser

The difference between db and dbm

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DB is a pure counting unit : dB = 10logX. dB meaning actually be easier , is to a large ( followed by a long string of 0 ‘s ) or small ( in front of a long list of zero ) number of relatively brief that out . Such as:

X = 1000000000000000 ( the number ) = 10lgX = 150 dB

X = 0.000000000000001 = 10lgX = -150 dB

dBm definition is miliwatt. 0 dBm = 10lg1 mw;

dBw defined watt. 0 dBw = 10lg1 W = 10lg1000 mw = 30 dBm.

DB always defined power units default to 10log of dollars . Of course, in some cases you can use the signal strength (Amplitude) to describe the functionality and power , this time with 20log of dollars . Whether or signal processing control areas are like . For example, sometimes you can see the expression dBmV .

In dB, dBm calculation, pay attention to the basic concepts. For example, said earlier 0dBw = 10log1W = 10log1000mw = 30dBm; another example, another with a minus dBm dBm , the result is dB. Such as : 30dBm – 0dBm = 30dB.

In general, in the project , between dB and dB only addition and subtraction , multiplication and division no . The most used is the subtraction : dBm dBm is actually two power reduction dividing the signal power and noise power division is the signal to noise ratio (SNR). plus dBm dBm is actually two power multiplied this has been rare ( I only know of such applications in the power spectrum of the convolution calculation ) . Take what dBm dBm is , 1mW of power of 1mW ? In addition to old classmates wrote me this is almost neck and neck and expressions Goldbach conjecture , I lived for so many years have not seen what the engineering field to play this .

dB power gain is flat , which means that a relative value. When computing power compared to B A large or small number of dB when , according to the formula 10 lg A / B calculation . For example : A big power doubled power than B , then 10 lg A / B = 10 lg 2 = 3dB. That is, the ratio B of the power A large power 3dB; if the power A is 46dBm, the power B of 40dBm, it can be said , A than B- 6dB; if the antenna A is 12dBd, B antenna 14dBd, it can be said A smaller than B 2dB.

dBm is an absolute value of said power unit , is calculated as : 10lg power value / 1mW. For example : If the transmit power is 1mW, the value after conversion by dBm units should be : 10 lg 1mW/1mW = 0dBm; For 40W of power , the 10 lg (40W/1mW) = 46dBm.

dB (Decibel, dB ) is a pure counts , was intended to represent the ratio of the size of two quantities , no units.

In engineering applications often see seemingly different definitions way ( just look different ) . For power , dB = 10 * lg (A / B). For voltage or current , dB = 20 * lg (A / B). Here A, B represented in comparing the current value or the power voltage values ??.

dB meaning actually be easier , is to a large ( followed by a long string of 0 ‘s ) or small ( in front of a long list of zero ) number of relatively brief that out . If ( here to power , for example) :

X = 100000 = 10 ^ 5

X (dB) = 10 * lg (X) dB = 10 * lg (10 ^ 5) dB = 50 dB

X = 0.000000000000001 = 10 ^ -15

X (dB) = 10 * log (X) dB = 10 * log (10 ^ -15) dB = -150 dB

In general, in the project , between dB and dB only addition and subtraction , multiplication and division no . The most used is the subtraction : dBm dBm is actually two power reduction dividing the signal power and noise power division is the signal to noise ratio (SNR). For example : 30dBm – 0dBm = 1000mW/1mW = 1000 = 30dB. plus dBm dBm is actually two power multiplied , there is no actual physical meaning.

In the field of electronic engineering , the use of the amplifier gain is dB ( decibels ) . The ratio of the amplifier output and input the magnification , the unit is ” X “, such as 10 times the amplifier , the amplifier 100 times . When the switch to ” db ” doing unit, called the gain on the magnification , which is a concept called the two .

Electrical conversion relationship decibels and magnification is:

A (V) (dB) = 20lg (Vo / Vi); voltage gain

A (I) (dB) = 20lg (Io / Ii); current gain

Ap (dB) = 10lg (Po / Pi); power gain

When decibel defined voltage ( current ) gain and power gain formula is different, but we all know the relationship between power and voltage, current is P = V ^ 2 / R = I ^ 2 * R. After using this formula , both on the same gain value :

10lg [Po / Pi] = 10lg [(Vo ^ 2 / R) / (Vi ^ 2 / R)] = 20lg (Vo / Vi).

Use decibel unit has three major advantages to do .

( 1 ) the value becomes smaller , easy to read and write . Total magnification electron systems are often thousands , tens of thousands or even hundreds of thousands, a radio signal received from the antenna to put into the speaker sound output , a total of about 2 times to enlarge . Decibels first take logarithmic , the value will be much smaller.

( 2 ) operational convenience . When the amplifier cascade , is multiplied by the total magnification levels . When done with the decibel units , the total gain is added. If a pre- amplifier 100 times (20dB), the level is 20 times (13dB), then the total power of magnification is 100 × 20 = 2000 times , the total gain of 20dB +13 dB = 33dB. ( 3 ) comply with the sense of hearing , estimates and convenient. The loudness of sound is heard and the relative growth was positively related to power . For example , when the growth of electric power from 0.1 watts to 1.1 watts when he heard voices rang a lot ; enhanced from 1 watt to 2 watts, not much loudness backward ; further enhanced from 10 watts to 11 watts , no one can hear the difference in loudness to . If you are using the power of the absolute value of 1 watt , but with the gain represented were 10.4dB, 3dB and 0.4dB, which can be more consistent loudness differences reflect the human ear to hear . If you note that you will find the volume knob on the scale Hi-Fi amplifier are subject dB , allowing you to change the volume when some intuitive .

Value in decibels ,-3dB and 0dB two points must be understood . Also known as semi -3dB cutoff frequency power point or points . Then half the power , voltage, or current is normal normal 1/ 2 . In the electro-acoustic system, ± 3dB difference is considered not affect the overall characteristics. So a variety of devices indicators such as frequency range, the output power of equality, without explanation , then may have ± 3dB discrepancy . For example , the previously mentioned frequency response 10Hz ~ 40kHz, is expressed in this frequency , the output amplitude does not exceed ± 3dB, 10Hz and 40kHz to say on these two endpoints frequency, output voltage amplitude is only an intermediate frequency band 0.707 ( 1 / ) times the . 0dB indicates the input or output as large as the two comparison signals. A decibel is a relative size of the volume , not absolute values. But you can also see how much value measured in dB level meter on the street or on the noise meter , it is because people give 0dB first set a benchmark . Such as sound level meter 0dB is 2 × 10-4μb ( Barye ) , so the road noise is 50dB, 60dB Qing Xiang had an absolute concept . 0dB commonly used benchmarks are the following categories : dBFS – to full scale magnitude of 0dB, commonly used in a variety of characteristic curves ; dBm – generate 1mW power ( or 0.775V voltage ) to 0dB on 600Ω load , commonly used in the AC level measurement instrumentation ; dBV – 1 volt to 0dB; dBW – 1 watt is 0dB. After reading how many dB general , there is no longer as voltage , the magnitude of the sound pressure and other physical , professionals can understand . Only in very rare occasions only to be equivalent . Then just into the formula : 10A/20 ( or A/10) × D0 can be calculated . A numerical readout in decibels , D0 is the reference value , voltage, current, or sound pressure 0dB when using A/20, electric power , sound power or sound intensity is used A/10. Now you can answer the question since the beginning of this article . Only the second speaker in the same input than the first speaker sound only twice as loud speakers if kept two words , as long as only the second half of the input power to the speaker . The first amplifier only just very ordinary varieties, only the second amplifier is very Hi-Fi, over the entire frequency range of the output voltage is only ± 2.3% of the difference !

Signal to noise ratio , i.e. SNR (Signal to Noise Ratio) is also known as the signal to noise ratio, i.e. the voltage amplifier and the output signal while the output voltage noise ratio is often expressed in decibels . The higher the SNR of the device , the less noise it generates . Generally , the larger the SNR , the smaller the description of the mixed signals in noise , the higher the sound quality of the sound playback , otherwise the opposite. SNR generally should not be less than 70dB, high-fidelity speakers should reach 110dB SNR over .

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