Author Topic: dBm calculating  (Read 2418 times)

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Offline MepakosTopic starter

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dBm calculating
« on: August 02, 2019, 08:49:52 am »
How do i calculate dBm with these Values.

R= 50Ω and 40dB

 

Offline richard.cs

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Re: dBm calculating
« Reply #1 on: August 02, 2019, 09:50:21 am »
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.
 

Offline radiolistener

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Re: dBm calculating
« Reply #2 on: August 02, 2019, 08:00:40 pm »
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.
« Last Edit: August 02, 2019, 08:14:12 pm by radiolistener »
 

Offline radiolistener

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Re: dBm calculating
« Reply #3 on: August 02, 2019, 08:11:36 pm »
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.
« Last Edit: August 02, 2019, 08:18:33 pm by radiolistener »
 

Offline Bud

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Re: dBm calculating
« Reply #4 on: August 02, 2019, 08:40:05 pm »
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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Offline fourfathom

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Re: dBm calculating
« Reply #5 on: August 02, 2019, 09:58:06 pm »
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.)
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