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In message , Jeff Liebermann
writes On Sun, 3 Aug 2014 08:31:59 +0100, Ian Jackson wrote: In message , Jeff writes Jerry does not seem to take on board that this is an international group, not a group of ex-US cable technicians who slovenly misuse dBm to mean dBmV, when the rest of the world uses dBm correctly to mean dB relative to a milliwatt. I think only a small minority of US cable TV people would misuse dBm when they really mean dBmV. I worked in the industry all my life, 20 years of which were for an American company - and I never found this to be a common practice. A few do. The problem is not the cable distribution system levels, but rather the accessories attached to it. The RF levels on the cable will probably be in dBmV @ 75 ohm forever. OTA signals will probably remain as dBuV @ 75 or 300 ohms. Continental Europe usually uses dBuV for CATV, MATV and OTA. The UK has essentially always been dBmV for CATV, although we're quite happy to add or subtract the appropriate 60dB when required (or around 48dB for dBm). [I, for one, can't see any point in using large numbers when small numbers will do just as well.] However, the Wi-Fi signal levels from the cable modem/router/wireless box is in dBm. The MoCA bridge signal levels are also in dBm[1]. This is not really a problem because each technology usually involves different cable techs, engineers, and support groups. At one time, I worked on a project to distribute 2.4GHz Wi-Fi via the inside RG-6/u coax system. It rapidly increased in complexity and didn't work very well. It was quickly and mercifully killed. We initially has some confusion due to the different units, but the project manager simply demanded that we specify the full units of measure and equivalent units where appropriate, which largely eliminated the problem. That left the problem of people using 50 ohm test equipment on 75 ohm systems without a 50-75 ohm impedance matching pad. Impedance matching L pads: http://www.microwaves101.com/encyclopedias/380-l-pads 50/75 dB conversion formulas and cheat sheet: http://www.maximintegrated.com/en/app-notes/index.mvp/id/808 http://pdfserv.maximintegrated.com/en/an/AN808.pdf (106 KBytes) 50/75 ohm matching pads (and vice versa) can sometimes be more of a hindrance than a help (the 'horses-for-courses' principle needs to be sensibly applied). 1] Note that dBm and dBmW are independent of the impedance since xx milliwatts produce the same amount of smoke no matter what the impedance. However dBmV and dBuV require that the impedance be specified. In practice, dBm (or the rare dBmW) generally implies 50 ohms to the RF guy and 600 ohms to the audio/phone guy, whereas dBmV and dBuV generally implies 75 ohms. However, as usual, it usually depends on the context, and it's a case of "if in doubt, tell it as it is". One oddity I found (many years ago) was a piece of cable TV equipment had one RF parameter was specified in dBm, while all the others were in the usual dBmV. This was not a mistake, and the reason given was that the dBm signal was CW, while the rest were vision and sound IF and RF signals. I hope I may be forgiven for not understanding or agreeing with the reasoning for this decision, as not only was it unnecessary, but it could also have been considered a typo, and caused confusion. -- Ian |
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