Well I sure didn't think my response to Mr. Grasso's post would generate any controversy. I am humbled by how many times I am wrong. But, my original post was right.
First of all, using 600 Ohms as an impedance is an audio, not rf practice. 50 Ohms is the standard for rf test work, as 75 Ohms is for cable-distributed video-modulated rf. Mr. Grasso's original post was about rf test measurements in a 50 Ohm system. However, regardless of the impedance, the appeal to absolute measures (dB referenced to 1 mW or 1 mV) is invalid in the context of how an intermediate filter (if) band-pass is defined. The critical issues are the bandwidth where the response of the filter is down X dB, where X might be 3, 6, or 60. It is purely a ratio to the response on center, and has nothing whatsoever to do with any absolute measure. > From: [email protected] > Date: Tue, 2 Sep 2003 13:21:16 -0700 (PDT) > To: <[email protected]> > Cc: <[email protected]>, <[email protected]>, <[email protected]> > Subject: Re: Q. on Res Bandwith performace traceabiity > >> >> 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, [email protected] at [email protected] 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" <[email protected]> >>>>> Reply-To: "Charles Grasso" <[email protected]> >>>>> Date: Thu, 28 Aug 2003 07:06:13 -0700 >>>>> To: "Emc-Pstc" <[email protected]> >>>>> 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: >>>>> [email protected] >>>>> with the single line: >>>>> unsubscribe emc-pstc >>>>> >>>>> For help, send mail to the list administrators: >>>>> Ron Pickard: [email protected] >>>>> Dave Heald: [email protected] >>>>> >>>>> For policy questions, send mail to: >>>>> Richard Nute: [email protected] >>>>> Jim Bacher: [email protected] >>>>> >>>>> 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: >>>> [email protected] >>>> with the single line: >>>> unsubscribe emc-pstc >>>> >>>> For help, send mail to the list administrators: >>>> Ron Pickard: [email protected] >>>> Dave Heald: [email protected] >>>> >>>> For policy questions, send mail to: >>>> Richard Nute: [email protected] >>>> Jim Bacher: [email protected] >>>> >>>> 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: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Ron Pickard: [email protected] >> Dave Heald: [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >> 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] 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
