About 3dB

Hello all, I am a new bie here.
May I ask a question about 3dB? I am using Timefeko for my simulation, but has no information about the f3dB. where can I get the info of that f3dB?
Thanks
dr_al

3dB=10*log10(Power out/Power in)
3dB=10*log10(1/2)

3dB=10*log10(0.5)
3dB=10*(-0.3010299957)
3dB=-3.010299957dB less than what is initially was sent. So also 3dB is called a)Half-Power or b)Bandwidth

in voltage
3dB=20*log10(Voltage out/Voltage in)

Thanks Djalli
AFAIK dB = 20*log10(Vout/Vin).
or since it use Power (that is square of the V) then dB = 10*log10(Pout/Pin)?

Regards

Alec

That is true. when is is Power Ratio it is 10*log( and when voltage ratio is 20*log( to get values in "dB"

also a great discussion for 3dB people in edaboard here did recently and it is good to read it.
https://www.edaboard.com/viewtopic.p...&highlight=3db

djalli


extra
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Now you willl see units "dB" and "dBm" very often.

-20dB is 10^(-20/10)=0.01Watts=10mWatts
I can say -20dB or I can say 10dBm. It is the same power.

dBm is power in MiliWatts.
30dBm = 10^(30/10)=1000mW=1 Watt

but 30dB = 10^(30/10)=1000 Watts

DeciBel is for the name of Alexander Bell and Deci=10 unit of tens
It simplifies addition and substraction.

3dB is usually used to define the power difference, half or double in linear field. You can easily work out the relation from equations above.

Selecting the 3 dB down frequency comes from the old days of hand calculations. It was easy to calculate it from the analysis because the reactance was equal to the resistance of the coupling networks (low frequency cutoff) and the parasitic capacitance and circuit resistance (high frequency cutoff) at that frequency. In the days of valves/tubes the equivalent circuits of the active devices had very easy to define values found in data tables for the active devices.

This was carried over in different methods to extend the bandwidth with peaking. The peaking was set to 3 dB above the flat line of gain and so the specifications of bandwidth were stated as +/- 3 dB.

hi dr-al,
If "f" of f3db is frequency, then f3db is the cut off frequency of a filter (low pass or high pass) or amplifier. In the cut off frequency the gain of a filter drop 3 dB (the gain is 0,7071 x flat gain).

LuizKapa.