> > Incorrect. A dB is a dB. Correct! But who is talking about db? I'm sure you know that db is a dimensionless ratio. I believe we were discussing power.
Let’s make sure we all have the formula: db = 10 log (P1/P2) or = 20 log (V1/V2) + 10 log (Z2/Z1) A quick scan of the formula will show that we multiply the voltage term by twice as much as the power term. Also, if Z1=Z2, then we have 10 log of 1 or zero so the impedance (resistance in this case) term drops out. OK now here is the important part. To convert db from a ratio to units of power we define 0dbm = 1mwatt into 600 ohms. Since not everyone measures power we also define 0dbv = 1 mvolt into 600 ohms. Since a millivolt into 600 ohms is not a milliwatt into 600 ohms, using the formula P = (V squared)/R, we find that 0 dbm is (almost) equal to 6 dbv into 600 ohms. Finally back to the point at hand, if the scale on our meter is in dbm then the half power point is at -3 dbm (not -3 db). If the scale is in dbv then the half power point is – 6 dbv (not -6db). In this context -3 dbm is equivalent to -6 dbv. I apologize to all for using the equal sign instead of the equivalent. I think my statements are otherwise accurate. And to give Ken his due, a db is unquestionably equal to itself. QED Fred Townsend 3 dB down is half power but 70.7% voltage. 6 > dB down is 1/4 power, or one half voltage or current. > > > on 8/31/03 10:42 AM, f...@dctolight.net at f...@dctolight.net wrote: > >>> >> A few basics: If we are talking about power points (no software puns >> intended) then it is the 3 db points. If we are taking about voltage >> (or current) points then it is the 6 db points. As the professors >> would say, it is left to the student to prove that 3 dbm = 6 dbv >> through the formula P = EE/R. >> QED? >> >>> My understanding is that it is the 6 dB points which are cited as the >>> bandwidth. I'm not up on CISPR 16 but to entirely specify the >>> bandwidth the 60 dB down points are also specified. The slope you >>> get from the 6 dB to the 60 dB points is called the shape factor. >>>> From: "Charles Grasso" <cgrassospri...@earthlink.net> >>>> Reply-To: "Charles Grasso" <cgrassospri...@earthlink.net> >>>> Date: Thu, 28 Aug 2003 07:06:13 -0700 >>>> To: "Emc-Pstc" <emc-p...@ieee.org> >>>> Subject: Q. on Res Bandwith performace traceabiity >>>> >>>> >>>> Greetings: >>>> >>>> Does anyone know if the Resolution Bandwidth >>>> filter performance is tracable to a given standard >>>> and which standard that might be? >>>> >>>> I was wondering if the rool-off after the 3dB points >>>> is specificed as a standard for ALL analysers. >>>> >>>> I am assuming that the anaswer is Yes as chaos >>>> would reign!! >>>> >>>> ------------------------------------------- >>>> This message is from the IEEE EMC Society Product Safety >>>> Technical Committee emc-pstc discussion list. >>>> >>>> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >>>> >>>> To cancel your subscription, send mail to: >>>> majord...@ieee.org >>>> with the single line: >>>> unsubscribe emc-pstc >>>> >>>> For help, send mail to the list administrators: >>>> Ron Pickard: emc-p...@hypercom.com >>>> Dave Heald: emc_p...@symbol.com >>>> >>>> For policy questions, send mail to: >>>> Richard Nute: ri...@ieee.org >>>> Jim Bacher: j.bac...@ieee.org >>>> >>>> Archive is being moved, we will announce when it is back on-line. >>>> All emc-pstc postings are archived and searchable on the web at: >>>> http://www.ieeecommunities.org/emc-pstc >>>> >>> >>> >>> ------------------------------------------- >>> This message is from the IEEE EMC Society Product Safety >>> Technical Committee emc-pstc discussion list. >>> >>> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >>> >>> To cancel your subscription, send mail to: >>> majord...@ieee.org >>> with the single line: >>> unsubscribe emc-pstc >>> >>> For help, send mail to the list administrators: >>> Ron Pickard: emc-p...@hypercom.com >>> Dave Heald: emc_p...@symbol.com >>> >>> For policy questions, send mail to: >>> Richard Nute: ri...@ieee.org >>> Jim Bacher: j.bac...@ieee.org >>> >>> Archive is being moved, we will announce when it is back on-line. All >>> emc-pstc postings are archived and searchable on the web at: >>> http://www.ieeecommunities.org/emc-pstc >> >> >> > > -- > > Ken Javor > EMC Compliance > Huntsville, Alabama > 256/650-5261 > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: emc_p...@symbol.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
