EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Mepakos on August 02, 2019, 08:49:52 am
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How do i calculate dBm with these Values.
R= 50Ω and 40dB
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dBm is a measure of power, dB referenced to 1 mW. Your starting information doesn't include a way to calculate power so you can't get to dBm.
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dBm means "decibel milliwatt", this is more convenient to represent power.
If you have power in Watts - Pwatt, you can convert it to power in dBm:
dBm = 10 * log10( Pwatt * 1000 ) = 30 + 10 * log10( Pwatt )
If you want to get power in Watts from power in dBm:
Pwatt = 10^( (dBm-30)/10 )
dB is just a ratio in logarithmic scale. dBm means power ratio to 1 mW, represented in logarithmic scale.
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If you have 40 Vrms (Volts RMS) on a load with R= 50Ω, then there is a power:
Pwatt = Vrms^2 / R = 40^2 / 50 = 1600 / 50 = 32 Watt
dBm = 30 + 10 * log10( 32 Watt ) = 45 dBm
Don't confuse RMS voltage with peak voltage, because for peak voltage power will be different. If you have 40 Vpk on a load with R= 50Ω, then there is a power:
Pwatt = Vpk^2 / (2*R) = 40^2 / (2*50) = 1600 / 100 = 16 Watt
dBm = 30 + 10 * log10( 16 Watt ) = 42 dBm
This calculation for peak voltage is valid for a sine waveform only. This is why RMS voltage is more proper way for AC measurements.
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How do i calculate dBm with these Values.
R= 50Ω and 40dB
You cant calculate that from your data. dB is a measure of ratio, not an absolute value. I have two sticks in my hands, one stick is twice as long as the other. Can you tell how long my sticks are in inches ?
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How do i calculate dBm with these Values.
R= 50Ω and 40dB
As others have pointed out, that "40dB" is at best ambiguous. If you mean "40dBW" that equals 10kW. To convert dBW to dBm, just add 30: 40dBW = 70dBm (adding 30dB is the same thing as multiplying by 1000.)