Clearly you will get slightly different answers if using different BWs. I'm not sure what the difference between a 3 and 6 dB BW filter is, in terms of the total noise power underneath the curve. It will depend on the shape factor of the IF filter.
But the difference between 3 dB, 6 dB and impulse BWs should not make a whole lot of difference, a couple dB at most. I know that's huge for you uncertainty types out there, but for those of us in the real world of EMI testing, a couple dB variation in the ultimate noise floor isn't a big deal unless it is pushing the noise floor up close to the limit. That shouldn't be the case, except when a combination of a possibly damaged receiver and unrealistic limit (Hello, Gert) combine to give you fits. The question of what BW is the "correct" one to use in this application is tailor-made for Werner Schaefer. He lives, eats and breathes this stuff. Werner, where are you? Ken Javor Phone: (256) 650-5261 > From: Brian O'Connell <[email protected]> > Date: Wed, 26 May 2010 11:20:51 -0700 > To: <[email protected]> > Subject: RE: [PSES] Radiated emission testing for automotive at low levels. > > Ken, > > From your explanation, I assume that B = 3dB noise bandwidth. > > Is the BW from the instrument spec, or is from the measurement data ? > > thanks, > Brian > > -----Original Message----- > From: [email protected] [mailto:[email protected]]On Behalf Of Ken Javor > Sent: Wednesday, May 26, 2010 11:09 AM > To: [email protected] > Subject: Re: [PSES] Radiated emission testing for automotive at low levels. > > The fundamental equation from statistical thermodynamics is: > > N = kTB, > > where, > > N is noise power in Watts, > k is Boltzmann¹s constant, which is the ideal gas law constant divided by > Avogadro¹s number, > T is temperature in Kelvin units, and > B is bandwidth in Hz. > > If you work that out at room temperature (298 K) and put it into dBm units, > you get 174 dBm in a 1 Hz BW. > > Ken Javor > > Phone: (256) 650-5261 > > From: <[email protected]> > Date: Wed, 26 May 2010 11:00:53 -0700 > To: <[email protected]> > Subject: Re: [PSES] Radiated emission testing for automotive at low levels. > > Ken, I'm curious about the origin of the -174dBm. > _____________________________________ ________________________________________ > _______ > > Ralph McDiarmid | Schneider Electric | Renewable Energies Business | > CANADA | Compliance Engineer > > From: Ken Javor <[email protected]> > To: [email protected] > Date: 05/26/2010 10:38 AM > Subject: Re: [PSES] Radiated emission testing for automotive at low levels. > > Noise floor (dBm) = -174 dBm + 10*log (BW) + F (dB) > > At room temperature, where F is noise figure. > > If F isn¹t given, but a noise floor is given with a stated BW, then using the > above equation, you can get the noise floor at any other BW using: > > Noise floor (dBm) = Noise floor at stated BW (dBm) + 10*log (BW desired/BW > stated) > > Keep in mind, as someone else noted, that receiver manufacturers often state > noise floor as ³displayed average noise level.² That uses video averaging, > which isn¹t allowable if making a peak detection measurement. You can add > about 12 dB for the difference between DANL vs. peak detection, other things > (BW) equal. > > Ken Javor > > Phone: (256) 650-5261David Heald <[email protected]> > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > <[email protected]> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to that URL. > > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
