-8.3 dBmV. What is this in dB

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

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

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.

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.

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