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G8MNY > TECH 14.07.18 18:48l 91 Lines 4263 Bytes #999 (0) @ WW
BID : 22361_GB7CIP
Read: GUEST
Subj: dBW & dB Levels made easy.
Path: IZ3LSV<DB0ERF<OK0NAG<OK0NBR<IW0QNL<JH4XSY<JE7YGF<GB7CIP
Sent: 180531/0559Z @:GB7CIP.#32.GBR.EURO #:22361 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jun 16)
Deci Bells "dB" & not "db" (Dolby audio system logo) are a 1/10 Mr Bell's unit,
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 -12 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 & 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.
(Level is Current or Voltage NOT POWER)
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!
e.g.
dB SINAD is a ratio of signal in noise & distortion.
dBm used much for 600ê AF where 1mW = 0.774V RMS which ÷ 2V peak to peak.
dBV where the "0" = 1V RMS about 2.5dB bigger than dBm in 600ê
dBuV where the "0" = 1uV of signal. e.g. 12dB sinad @ -6dBuV in 75 or 50ê!
dBvu as displayed on a Voltage Unit meter. 0dBvu=100% level etc.
dBc where the "0" = full carrier power. e.g. spurious = -60dBc.
Not to be confused with the sound dB"C" weighting standard.
In 50 Ohms this is usefull..
Peak PEP Peak PEP
Volts Watts Volts Watts
2.8 0.08 28 8
3.3 0.1 33 10
4 0.16 40 16
5 0.25 50 25
5.6 0.32 56 32
7 0.5 71 50
8 0.64 80 64
10 1 100 100
14 2 141 200
16 2.5 160 250
20 4 200 400
See my Buls "QRO 1kW HF Metered Dummy Load" & "QRP Power Meter & Dummy Load"
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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