#1 Usefull Equations
Posted: Sat Jun 19, 2010 10:14 am
Ok, as requested a place to put things of use. The one that suggested the thread is how to calculate a voltage difference as a decibel value.
Vdecibel = 20log(v1/v2)
or to help the calculation and a sort of RP form
((v1/v2) log ) * 20
This is for voltage, for power change the 20 to a 10.
To go the other way, to get a voltage ratio from a dB value the inverse is
VRatio = 10 ^ ( VDecibel / 20 )
So to help with that (and how to do it using the windows calculator) to convert a -23dB (for example value to a ratio)
( -23 / 20 )
Press = to get the result of that, then check the Inv (for inverse) box and click the Log button. That gives the ratio, so the ratio of
1v to 0.0707v
Just to add to this, calculate the ration of -20dB
-20dB = 1 : 0.1
Then calculate the ratio of -3dB
-3dB = -0.707
And notice how adding the log value
-20 + -3 = -23
gives the same result as multiplying the voltage ratio
0.1 * 0.707 = 0.0707
You can always do that with decibels (and any logarithmic value).
So if you have a amplifying stage that has a gain of 10dB followed by a stage that has a gain of 5dB, the gain of the two combined is simply 15dB
Vdecibel = 20log(v1/v2)
or to help the calculation and a sort of RP form
((v1/v2) log ) * 20
This is for voltage, for power change the 20 to a 10.
To go the other way, to get a voltage ratio from a dB value the inverse is
VRatio = 10 ^ ( VDecibel / 20 )
So to help with that (and how to do it using the windows calculator) to convert a -23dB (for example value to a ratio)
( -23 / 20 )
Press = to get the result of that, then check the Inv (for inverse) box and click the Log button. That gives the ratio, so the ratio of
1v to 0.0707v
Just to add to this, calculate the ration of -20dB
-20dB = 1 : 0.1
Then calculate the ratio of -3dB
-3dB = -0.707
And notice how adding the log value
-20 + -3 = -23
gives the same result as multiplying the voltage ratio
0.1 * 0.707 = 0.0707
You can always do that with decibels (and any logarithmic value).
So if you have a amplifying stage that has a gain of 10dB followed by a stage that has a gain of 5dB, the gain of the two combined is simply 15dB