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G8MNY  > TECH     27.01.05 09:33l 70 Lines 3484 Bytes #999 (0) @ WW
BID : 30355_GB7CIP
Read: GUEST OE7FMI
Subj: dBW & dB Levels made easy.
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0GOS<DB0EEO<DB0RES<DK0WUE<7M3TJZ<ZL2TZE<
      GB7YFS<GB7CIP
Sent: 050126/2351Z @:GB7CIP.#32.GBR.EU #:30355 [Caterham] $:30355_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To  : TECH@WW

                                                    (Corrected Oct 04)
Deci Bells (1/10 Bell) which are on a logarithmic scale are used when large
ranges are used or many gain/loss terms have to be considerd. As these are logs
the dB can just be added rather than using gain factors or loss fractions that
have to be multiplied together.

dBW
The Formulae for POWER is 10log POut/1WATT, which gives this easy to remember
1dB step table based only on +/- 3, 6 & 10dB. Where Log = log base 10, not E or
Natural Logs.

Note, dBW has "0" referenced to 1 Watt.
  And dBm has "0" referenced to 1 mW, so 30dBm = 0dBW
  So 1uW = -30dBm or -60dBW

 +3dB is doubling the power,  -3dB is only a half.
 +6dB is Quadrupling power,   -6dB is only a quarter.
+10dB is 10 times the power, -10dB is only a tenth.

mW     dBW       Watts  dBW      Watts  dBW       Watts   dBW      Watts   dBW
10     -20       0.1    -10       1      0        10      10       100      20
12.5   -19       0.125   -9       1.25   1        12.5    11       125      21
15.65  -18       0.1565  -8       1.565  2        15.65   12       156.5    22
20     -17       0.2     -7       2      3        20      13       200      23
25     -16       0.25    -6       2.5    4        25      14       250      24
31.25  -15       0.3125  -5       3.125  5        31.25   15       312.5    25
40     -14       0.4     -4       4      6        40      16       400      26
50     -13       0.5     -3       5      7        50      17       500      27
62.5   -13       0.625   -2       6.25   8        62.5    18       625      28
80     -11       0.8     -1       8      9        80      19       800      29

dB
For Voltage and Currents the formulae is 20log out/in, which gives this easy to
remember 2dB step table based only on +/- 6, 12 & 20dB. Where Log = log base 10
not E or Natural Logs.

 +6dB is doubling the level,  -6dB is only a half.
+12dB is Quadrupling level,  -12dB is only a quarter.
+20dB is 10 times the level, -20dB is only a tenth.

Note +1dB is about +11% (111%), & -1dB is about -9% (91%) of the level.

Ratio    dB      Ratio   dB       Ratio   dB       Ratio    dB       Ratio   dB
0.01    -40      0.1    -20        1      +0        10     +20       100    +40
0.0125  -38      0.125  -18        1.25   +2        12.5   +22       125    +42
0.01565 -36      0.1565 -16        1.565  +4        15.65  +24       156.5  +44
0.02    -34      0.2    -14        2      +6        20     +26       200    +46
0.025   -32      0.25   -12        2.5    +8        25     +28       250    +48
0.03125 -20      0.3125 -10        3.125 +10        31.25  +30       312.5  +50
0.04    -28      0.4     -8        4     +12        40     +32       400    +52
0.05    -26      0.5     -6        5     +14        50     +34       500    +54
0.0625  -24      0.625   -4        6.25  +16        62.5   +36       625    +56
0.08    -22      0.8     -2        8     +18        80     +38       800    +58

There are other dB standards in use not always measuring the same item!
eg..
    dB SINAD is a ratio of signal in noise & distortion.
    dBuV where the "0" = 1uV of signal. eg 12dB sinad @ -6dBuV in 75 or 50ê!
    dBvu as displayed on a Voltage Unit meter. 0dBvu=100% etc.
    dBc  where the "0" = full carrier power. eg spurious = -60dBc.
    Not to be confused with the sound dB"C" weighting standard.


Why Don't U send an interesting bul>

73 De John, G8MNY @ GB7CIP


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