-8.3 dBmV. What is this in dB

[nickip_22]

i need help converting this. I have a book, but there is not much about this in there.

 

[E-design]

Not enough information as explained below- db is a ratio of levels and dBm is a referenced measurement.

https://www.mathhelpforum.com/math-...pics/6506-urgent-unit-conversion-dbmv-db.html

 

[nickip_22]

My modems downstream power is -8.3 dBmV. What is this in dB? What does the negative number signify

This is the whole questions. Thanks.

 

[FvM]

It's 8.3 dB below 1 mV respectively 0.385 mV, isn't it obvious?

The confusing point is to ask "What's this in dB". Unlike dBV, dBmV, dBµV or dBm, dB is only a relative unit, so it's meaningless
without specifying a reference level. Curiously, the same confusion shows in the above quoted link, that explains nothing, in
my opinion.

 

[albbg]

dBmV and dBm are the measurement of voltage and power expressed in dB and are referred respectively to 1 mV and 1 mW.
If you have negative dBm this means the power is less than 1 mW; if you have negative dBmV this means the voltage is less than 1 mV.
The conversion between linear and "dB" measurements are given by:

dBmV = 20*log10(V/1mV)
dBm = 10*log10(P/1mW)

so to convert -8.3 dBmV to mV you have to write:

10^(-8.3/20) = 0.385 mV = 3.85e-4 V

Now to convert it into a power you must know the impedance to which this voltage is applied and use P=V^2/R. If you know, for instance, R=50 ohm you will have:

P = (3.85e-4)^2/50 = 3e-9 W = 3e-6 mW

Hence:

P(dBm) = 10*log10(3e-6mW/1mW) = -55 dBm

if your impedance was, instead, 1 kohm:

P = (3.85e-4)^2/1000 = 1.5e-10 W = 1.5e-7 mW

then:

P(dBm) = 10*log10(1.5e-7mW/1mW) = -68 dBm

Regards