I agree with most of Mr. Woodgate's commentary, but I wonder if the presence of even an rf emission (as contrasted to 50 Hz) should cause any problem to a land-line phone with a wired handset. It wouldn't seem likely that the power available from a couple AA batteries would be sufficient for that. Now if it were a cordless phone and there were an rf emission in band to the cordless link, that would be much more credible.
> From: John Woodgate <[email protected]> > Reply-To: John Woodgate <[email protected]> > Date: Sat, 26 Jul 2003 10:38:36 +0100 > To: [email protected] > Subject: Re: self blinking LEDs as EMI sources WAS: LED lamps > > > I read in !emc-pstc that Wan Juang Foo <[email protected]> wrote (in > <of7340f59a.f0a87124-on48256d6f.000df921-48256d6f.0013d...@np.edu.sg>) > about 'self blinking LEDs as EMI sources WAS: LED lamps' on Sat, 26 Jul > 2003: > >> These circuits >> were found to (well at any rate, seems to) emit interference that cause >> a telephone to pick up (50Hz) hum! > > Up to that point, your treatment of the phenomenon was quite scientific. > >> It looks like the mains hum was pick >> up and modulated by the 'device' and reradiated or broadcasted... > > I think that is improbable. For such a small circuit to pick up > significant mains hum, the magnetic field would have to be very strong, > but in that case, why did not the circuits in the telephone, which are > much larger in area, pick up the hum directly? > > Then again, such a small circuit as you describe for the LEDs would not > re-radiate significantly at 50 Hz. And the presence of a flashing LED > would seem to play no part in any re-radiation. > > You don't say that the hum was modulated on/off or in level in time with > the flashing of the LED, and if there were no such modulation, that > would seem to confirm that the LED circuit was not causing the hum. > > The situation might be different if the LED circuit included a precision > timer, based on a high-frequency clock. This might radiate over short > distances, and the radiation *might* be modulated by quite a small > voltage induced by a weak 50 Hz magnetic field. In that case, you should > be able to find the high-frequency emission with a spectrum analyser and > sniffer loop, or a scanning receiver, whether or not the 50 Hz field is > present. But the induced hum would be modulated at the flash rate. > -- > Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk > Interested in professional sound reinforcement and distribution? Then go to > http://www.isce.org.uk > PLEASE do NOT copy news posts to me by E-MAIL! > > ------------------------------------------- > 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
