It don't see how this makes the calculation of RF power any easier, to the contrary it confuses the issue.
cheerio Berndt On Thursday 28 September 2006 17:50, John Gilmore wrote: > > transmit power converted to dBm (1 dBm == 1 mW) minus the attenuator > > loss = output power in dBm. > > > > E.g. > > 100 mW -> 20dBm > > 20dBm - 15 db att = 5 dBm > > 5 dBm -> 3.2 mW > > Actually, I think 0 dBm = 1 mW. > > dB's are a royal pain in the butt. They eluded me for years because > they required a lot of rote memorization and made no sense. For those > of us not pickled in radio-speak from an early age, but who know basic > algebra, there's a simple way to deal. Ignore deciBels. Use Bels. > > Bels are easy and obvious. They're a straight logarithmic scale in Base > 10. 100 mW is 2 Bm. 10 mW is 1 Bm. 1 mW is 0 Bm. 0.1 mW is -1 Bm. > > DeciBels are just tenths of a bel. So if you shift the decimal point > one place, you're suddenly calculating in an easy to use notation. > > Here's the above calculation in Bels: > > 100 mW -> 2 Bm > > 2 Bm - 1.5 B att = 0.5 Bm > > 0.5 Bm -> 10 to the 0.5 power -> the square root of 10 -> about 3.2 mW > > See, now you not only know the answer, but you know WHY "5dBm" is 3.2 mW. > > Why the EE universe settled on doing everything in tenths of a > logarithmic unit is way beyond me. It's as if every carpenter figured > every length in deciInches or decimeters, even if inches, kilometers > or meters would be the more straightforward unit. How often do you > calculate in decivolts, deciwatts, or decimeters per second per > second? > > The rumor is that decibels were invented because somebody at Bell Labs > couldn't cope with decimal points or negative numbers, in the days when > equipment wasn't capable of dealing with large orders of magnitude > (e.g. the painful-to-someone 0.3 Bel became the friendly-to-someone 3 > deciBel). Of course, now that people regularly see 5 to 10 orders of > magnitude (5 to 10 Bels) (50 to 100 deciBels) (factors of 10000 to 10 > billion) in ratios, such as in radar, digital signal processing, or > fiber optics, the "deci" has just become a hindrance. > > You can do your part to clear up this idiocy by using Bels in most > places where the lemmings use deciBels. You may actually get them to > think (briefly). > > John > > PS: Don't even get me started about why dBm's aren't referenced to > watts rather than milliwatts! Since a "milli" is 1/1000th and that's > just 3 orders of magnitude, referencing to ordinary watts would merely > involve subtracting 3 or 30 from the number, e.g. 40 dBm = 4 Bm = 1 BW > = 10 dBW. It reminds me of how we're still calculating speeds in > 5280-foot units per 3600-second units rather than in some sane system > using basic decimal units. Actually using BW notation in your > thinking and writing may overload lemming brains, though. > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
