Path: ...!Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Sun, 26 May 2024 23:21:58 +0000 From: boB Newsgroups: sci.electronics.design Subject: Re: dBs Date: Sun, 26 May 2024 16:21:56 -0700 Message-ID: References: User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 77 X-Trace: sv3-PNs+U8f4Xac0PhHlyWazWWK1eoBxawffX5bi+N6WKVK42c7PL2xaR5xB8yWbJO/ZFJk4vgIJhn1ZiqH!T7bVF+JUKDqdn1SsxueHp+IXc0hHpemBUcokMw+CCsfd+uqkwdq/JzXmxfx7ug1T8Gy/Dmr7woF4!sCDuM69H X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 4026 On Sun, 26 May 2024 23:42:15 +0200, Jeroen Belleman wrote: >On 5/26/24 22:58, Cursitor Doom wrote: >> On Sun, 26 May 2024 21:48:21 +0200, Jeroen Belleman wrote: >> >>> On 5/26/24 19:58, Cursitor Doom wrote: >>>> On Sun, 26 May 2024 19:25:41 +0200, Jeroen Belleman wrote: >>>> >>>>> On 5/26/24 19:09, Cursitor Doom wrote: >>>>>> I'm feeling cognitively-declined today, probably as a consequence of >>>>>> my vast age and general ignorance of matters mathematical and >>>>>> everything else in fact, with the sole exception of "fatuous >>>>>> conspiracy theories." >>>>>> Can some kind soul assist? >>>>>> If my RF power meter is reading -13dbm when there's a 20dB attenuator >>>>>> in line, what is the true power level, please? >>>>>> I've got an exhaustive App Note from Rhode & Schwartz which claims to >>>>>> cover everything about decibels, but, er, doesn't. >>>>>> >>>>>> CD. >>>>> >>>>> That would be -13 + 20 = +7dBm, provided that impedances are matched >>>>> everywhere. >>>> >>>> I was under the impression that one couldn't simply just add dBs to >>>> dBms? >>> >>> You can. That's what decibels were invented for. >>> >>> Let's spell it out then. You know 0 dBm is 1 mW. So -13 dBm is >>> 10^(-13/10) times 1 mW, or 50 uW. >>> >>> A 20dB attenuator divides power by a factor of 10^(20/10), that is, a >>> factor of 100. So before the attenuator, you had 5 mW. >>> >>> 5mW is 10*log(5) is +7 dBm. >>> >>> Jeroen Belleman >> >> Oh I know you're figures are correct, Jeroen. But to check them I had to >> use look-up tables off the net: >> >> -13dBm = 0.05mW >> 20dB = 100X >> 0.05X100 = 5mW >> 5mW = =7dBm >> >> Sometimes you can just straight add-up dBs and other times you can't and I >> can never remember when it's appropriate and when it's not. To be safe, I >> revert to the method I showed above. It's longer, but at least I know I >> can rely on the result. Whoever invent dBs "to make things simpler" needs >> to have their grave desecrated and their name effaced from history IMO. It was Bell Labs a long time ago I believe that came up with that great system. Don't shoot them ! Well, they're no longer around, unfortunately. Yep, Jeroen is right. boB >> > >It's not that bad! Adding dB values works fine in the context of >chains of gain and attenuation. It's easier to add decibels than >to multiply gain values. One gets used to it. I can usually do >it in my head. Anyone working in RF or in control systems gets >proficient at it very quickly. > >I agree that there are situations where using decibels can get >confusing. For example, in light detectors, optical power gets >converted into current, so a 20dB change in optical power would >result in only 10dB change in the electrical signal power. But >let's not go there just yet. > >Jeroen Belleman