Calculation method for "turn" db

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DB DB (Engineering application), DB (decibel, db) is a pure counting unit, intended to represent a ratio of two to a size, without units. In engineering applications, it is common to see seemingly different definitions (just looking different).

• For power: DB = 10*LG (A/b),
• For voltage or current: DB = 20*lg (A/b).

Here A, B represents the power value or current, voltage value that participates in the comparison. The meaning of the DB is actually simple enough, that is, a large (followed by a long string of 0) or very small (preceded by a long string of 0) is a relatively short representation. For example, for power:

X = 100000 = 10^5;

X (DB) = 10*LG (x) db= 10*LG (10^5) db =-db;

X = 0.000000000000001 = 10^-15;

X (DB) = 10*log (x) db= 10*log (10^-15) db = -150 db;

In general, in engineering, there is only a plus and minus between DB and db, no multiplication. And the most used is subtraction. DBM minus dbm is actually two division of power, and the division of Signal power and noise power is the SNR. For example: 30dBm-0dBm = 1000MW/1MW = =30db. DBM Plus dbm is actually two power multiplication without physical meaning.

DB in the field of electronic engineering, the amplifier gain is used in db (decibel). The ratio of the amplifier output to the input is magnification, in units of "times", such as 10 times-amplifier, 100 times-fold amplifier. When using "decibel" as the unit, the magnification is called the gain, which is a concept of two kinds of salutation.

The conversion between decibels and magnification in electricity is:

• A (V) (DB) =20LG (VO/VI), Voltage gain
• A (I) (DB) =20LG (IO/II), current gain
• Ap (DB) =10LG (PO/PI), Power gain

The decibel-defined voltage (current) gain and power gain formulas are different, but we all know that the relationship between power and voltage and current is p=v^2/r=i^2*r. With this formula, the gain values for both are the same:

10lg[po/pi]=10lg[(VO^2/R)/(VI^2/R)]=20LG (VO/VI).

There are three major benefits to using decibels:

(1) The numerical value becomes smaller, the reading and writing is convenient. The total magnification of the electronic system is usually thousands of, tens of thousands of or even hundreds of thousands of, a radio from the antenna received from the signal to the speaker sound output, a whole to zoom in about 20,000 times times. In decibels, the logarithm is taken first, and the value is much smaller.

(2) convenient operation. When the amplifier is cascaded, the total magnification is multiplied by the levels. When a unit is made in decibels, the total gain is added. If the pre-level of a power amplifier is 100 times times (20dB) and the rear stage is 20 times times (13dB), then the total magnification is 100x20=2000 times and the total gain is 20db+13db=33db.

(3) Meet the listening sense, the estimation is convenient. The loudness of a person hears sound is positively correlated with the relative growth of power. For example, when electric power increased from 0.1 watts to 1.1 watts, the sound sounded a lot, and from 1 watts to 2 watts, the loudness was not much, and then from 10 watts to 11 watts, no one can hear the difference in loudness. If the absolute value of the power is 1 watts, and the gain is expressed as 10.4 db,3 DB and 0.4 db respectively, this can reflect the loudness difference that the human ear hears more uniformly. If you notice, you will find that the volume knob scale on the hi-Fi amplifier is the decibel level, making it intuitive to change the volume.

Of the decibel values, -3 db and 0 db two points must be understood. -3 DB is also called a half power point or cutoff frequency point. At this time the power is half normal, the voltage or current is 1/2 of normal. In electro-acoustic systems, the difference in ±3 DB is not considered to affect the overall characteristics. Therefore, various equipment indicators, such as the frequency range, the output of the equality, without explanation, there may be ±3 DB access. For example, the previously mentioned frequency response of ten Hz to nine khz means that in this frequency, the output amplitude will not exceed ±3 DB, that is, at the two endpoint frequency of the ten Hz and four khz, the output voltage amplitude is only 0.707 (1/) times the median frequency segment. For example-3 db, if the pass-band frequency magnification is 1, that is, the Aup is 1, that is, the filter circuit in the pass band without amplification voltage, then -3=20log (AU/AUP) =20log (Au), calculated au=0.707, in this case, -3 db signal attenuation is the original 7 0.7%. 0 DB indicates that the output is as large as the input or two comparison signals. The decibel is a relative size, with no absolute measure. You can also see how much db is measured on the meter or on the noise gauge on the road, because people set a benchmark for 0 db. For example, the 0 db of the sound level meter is the 2x10-4μb (micro bar), so that the noise on the road is the absolute light-ringing concept of a db and a db. Common 0 db benchmarks are available in the following categories (for reference only): dbfs--with a full scale value of 0 db, commonly used on a variety of characteristic curves; the dbm--generates 1 MW of power (or 0.775V voltage) at 0 db on 600Ω load, and if the load resistance is 50ω or 75ω, V 2 = (p*r) calculation can be used for AC level measurement instruments, dbv-to 1 kv 0 db;dbw-to 1 Watts 0 DB.

Generally read out how many DB, it will not be converted into voltage, sound pressure and other physical values, professionals can understand. Only on very few occasions is it to be converted. Simply substituting the formula: 10*A/20 (or A/10) *d0 calculation. A is a readout of the decibel value, the D0 is a reference value of 0dB, voltage, current or sound pressure with a/20, electrical power, sound power or acoustic intensity with A/10.

The more commonly used DB values are:

1. DBm

DBm is an absolute value of the test power, calculated as: 10lgP (Power value/1MW).

[Example 1] if the transmit power P is 1 MW, converted to dbm after 0 dbm.

[Example 2] for 40W power, the value of the conversion in dBm units should be: 10LG (40W/1MW) =10LG (40000) =10lg4+10lg10+10lg1000=46dbm.

2. DBi and DBd

The DBi and dBd are the value of the test gain (power gain), both of which are a relative value, but the reference datum is not the same. The reference datum of DBi is omni-directional antenna, and the reference datum of dBd is dipole, so the two are slightly different. It is generally believed that the same gain, expressed in DBi, is 2 larger than that of dBd. 15.

[Example 3] for an antenna with a gain of dBd, the gain is converted to a unit of DBI, then 18.15 DBI (generally ignore the decimal place, is the DBI).

[Example 4] 0 dbd=2.15 dBi.

[Example 5] The GSM900 antenna gain can be in the DBD (DBI), and the GSM1800 antenna gain can be in the DBD (. dbi).

3. DB

A DB is a value that represents a relative value, and when considering the power of a is compared to a large or small number of DB, the formula is calculated as follows: 10LG (a power/b power)

[Example 6] a power greater than B power one times, then 10LG (a power/b power) =10lg2=3db. In other words, the power of A is 3 DB larger than that of B.

[Example 7] The 7/8-inch GSM900 feeder has a transmission loss of approximately 3.9 DB for 100 meters.

[Example 8] If the power of a is dbm, the power of B is at + dbm, it can be said that a greater than B 6 DB.

[Example 9] If a antenna is a dbd, b antenna for the DBD, you can say a smaller than B 2 DB.

4. DBc

DBc is also sometimes seen, which is also a unit representing the relative value of the power, exactly as the DB is calculated. In general, DBc is relative to the carrier (Carrier) power, in many cases, to measure the relative value of the carrier power, such as to measure interference (same-frequency interference, intermodulation interference, intermodulation interference, out-of-band interference, etc.) as well as the relative magnitude of the coupling, spurious, etc. In principle, dB substitution can be used where dBc is used.

When the DB is converted to voltage (V, MV, UV), the voltage is more intuitive, it is necessary to consider the load resistance size, school to now do not understand the DB, is because there is no load resistor concept, mainly those teachers teach good, scripted, the book did not write, rightfully can not speak. There are 75ω and 50ω resistors commonly used in RF and communication, and I see that the data is also calculated with 600Ω as the load. The following calculation takes 75ω as an example.

Here are a few examples to help understand:

Recalling the decibel ratio of power (or voltage) to a reference power (or voltage): LG (P/P0) = LG (U/U0), it is clear that the level of the reference power (i.e. p=p0) is zero. For the same power, choose different reference power P0 (or voltage U0) The value of the level is different, after the addition of different units. If the 1W is the reference power, the power is P, the corresponding level is 10LG (P/1W), the unit is recorded as DBW (Decibel watts).

For example, when the power is 1W, the level is 0 DBW, the power is 100W, the level is DBW, the power is DBW, the corresponding level is: 10LG (100mw/1w) = 10LG (100/1000) = -10DBW;

The voltage of a point in a known system can also be dBW to indicate the level of the point.

For example, the voltage at an input is -4 MV, then its input power: P = u^2/z = 0.1^2/75 = 1.3x10^ () W; the corresponding level is 10LG (1.3x10^ ( -4)/1) = -38.75dbw

If the 1mW is the reference power, then the power is P at the corresponding level of 10LG (P/1MW), the unit is recorded as DBMW (decibel MW). For example, when the power is 1W, the level is 1 dbm, the level is 0 dbm, the power is 1 UW, the level is -30 dbm, the voltage is 1 MV, the corresponding power: P = u^2/z = 0.001^2/75 = 1.3x10^ (-8 ) W =1.3x10^ ( -5) MW, corresponding to a level of 10LG (1.3x10^ ( -5) MW/1MW) = -48.75dbm.

If you use 1 MV as the reference voltage, the voltage is U at the corresponding level of 20LG (U/1MV), the unit is recorded as DBmV (db Millivolt). For example, when the voltage is 1V, the corresponding level is DBmV, the voltage is 1 UV, the corresponding level is -60 DBmV, the power is 1 MW, the voltage U = Sqr (p*z) = Sqr (75*10^ ( -3)) V = 274 MV; the corresponding level is: 20 LG (274MV/1MV) = 48.75dbmv.

If 1 UV is the reference voltage, the voltage is U at the corresponding level of 20LG (U/1UV), the unit is recorded as DBUV (DB micro-volt). For example, when the voltage is 1 mv, the level is DBUV, when the voltage is 1 MW, the voltage: U = 274 MV = 2.74x10^ ( -5) UV; the corresponding level is 20LG (2.74x10^ (- 5)/1) = 108.75 DBUV level of four units DBW, DBm, DBmV, DBμV there is a certain conversion relationship, the table shows the left of the original unit is converted to the upper part of the new unit need to increase the value.

	DBW (New)	DBm (New)	DBmV (New)	DBμV (New)
DBW (formerly)	0	30	78.75	138.75
DBm (original)	-30	0	48.75	108.75
DBmV (formerly)	-78.75	-48.75	0	60
DBμV (formerly)	-138.75	-108.75	-60	0

Calculation method for "turn" db