On Sun, 27 Feb 2005, David Cain wrote:
You are correct, and well-said. It's just one of my pet peeves when someone says, "My wireless card has XXX dB of power output"... doesn't mean anything since dB is inherently relative. The only number in the original post that's suspect is the +3.5 "dB" from the cable company. It should have some sort of reference attached (maybe it's understood in the cable circle to be dBm or whatever).I'm expecting to be schooled here, but if I remember correctly, dB in isolation is the logarithmic expression of a ratio.
In the case of the 2-way splitter it would refer to the ratio of input level to level on a given outut from the splitter. No reference level needed.
However, the "measurement" of the signal from the cable company would have to be in relation to some reference level. That reference level would be assumed or calibrated by the test equipment, and may be printed on the unit someplace.
dBm, dBW, dB�V et al are needed when a specification or measurement is being compared against an established reference level (i.e. a "+4 dB" line level input to professional recording equipment is actually +4dBm - a specific power ratio).
But it's OK to say a splitter costs 3.5 dB - you're describing the ratio of input signal to output. You can describe it as power or voltage and the numbers should still add up, at least if the loss is resistive or distributive in nature.
Also, the usual definition of the ratio is in terms of the *power* between the two signals. That's a big point of confusion for EE's first introduced to dBs... power goes as the *square* of voltage. Thus, voltage gain formula is 20*log10(Vout/Vin), whereas for power it's 10*log10(Pout/Pin).
Just for the record:
dBW <- Relative to 1.0 Watts
dBm <- Relative to 1.0 milliwatts
dBuV <- Relative to 1.0 microvolt (can't recall if this one is correctly
defined in terms of power or not)Oh, and for antennas, it's assumed that it's "gain compared to an isotropic radiator," so saying XX dB gain for an antenna is acceptable.
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
