Quote:
Originally Posted by Sephiroth
The SH doesn't give a voltage reading. It gives a power reading based on a conversion from a reference point. Voltage is a field value; dBmv is a ratio and thus not a field value.
Can you tell me what the voltage is at 0 dBmv? It can't be zero!
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It gives a field value - dBmV
dBmV is the voltage ratio with respect to 1mV RMS across 75R i.e. 0dBmV is 1mV RMS (Interestingly the definition w.r.t. 1 volt, dBV, is independent of impedance)
No different to the concept of a power ratio. dBm (or dBmW) is the power ratio with respect to 1mW i.e. 0dBm is 1mW.
As the impedance is defined for dBmV then the power is also known - 0dBmV gives a power of −78.75 dBW (−48.75 dBm)
The confusion, as I've tried to explain, is that just quoting 'dB' without qualifying whether you're talking about amplitude or power is ambiguous.
Just saying '6dB is a ratio of 4' is true for power but not amplitude. Without context, both of these statements are true:
n increased by 6dB = n*2
n increased by 6dB = n*4
Saying the 'voltage has increased by 6dB' or 'the power has increased by 3dB' is non-ambiguous. In both case the parameter has gone up by two.