In another lifetime I worked on a microphone design for a trunk-mounted radio with similar problems. The electret mic output was amplified. There was ample opportunity for demodulation including in the transistor junctions (anyone use transistors anymore?) which created an interested feedback loop. Strategically sized and placed decoupling caps took care of the problem.
Don Umbdenstock Manager Compliance Engineering Tyco Safety Products / Sensormatic 6600 Congress Avenue Boca Raton, FL 33487 USA 561.912.6440 [email protected] _____ From: [email protected] [mailto:[email protected]] Sent: Tuesday, August 30, 2005 11:59 PM To: [email protected] Subject: Re: Analog telephone - RF field susceptible A couple of pointers: Automatic Gain Control circuis (typuically in the phone chip) can be very susceptible - essentially they change gain with the 1 kHz rythm from the applied RF field. Decouple/filter them and as Robert pointed out - keep small loop areas on the PCB. Electret microphones, actually a transistor in the mic, can also be susceptible. The handset cable picks up the field, delivers RF to the mic, where it gets demodulated, and then the demodulated audio noise is fed back to the mic circuit on the phone. Neven -------------- Original message -------------- > Use twisted pairs and DO NOT UNBALLANCE the grounds. Make > everything as symmetrical as possible. > > Close ALL loop areas. > > - Robert - > > On Mon, 29 Aug 2005 06:45:06 +0200 > "Amund Westin" wrote: > > Many analog telephones are susceptible to radiated > > fields. It's easy to > > observe audible noise (the sine 1kHz modulation tone)when > > RF fields are > > introduced under a laboratory test (IEC61000-4-3). Do you > > have any good EMC > > design techniques which improve the resistance to such RF > > fields, when the > > telephone enlosure is plastic material and wire is > > unshielded? > > Thanks. > > > > Best reg! ards > > Amund Westin > > > > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > Instructions: http://listserv.ieee.org/listserv/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: > > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
