This does not answer your question directly, but it might provide some insight. Tune an AM radio to a broadcast signal, and then introduce an unmodulated rf carrier injected onto the ac power line that powers the radio. Offset the rf carrier frequency from the broadcast frequency such that the resultant "beat" frequency falls within the audio band of the radio speaker. You will hear a tone at the offset or beat frequency. If you were to remove the broadcast signal, the tone disappears because there is nothing for the rf carrier to beat against. I used this technique to characterize the susceptibility of AM radios to rf signals on power lines, in order to investigate the rationale behind present FCC and CISPR controls on ITE conducted emissions. As far as I know, the CE parts of FCC and CISPR requirements are the only controls on conducted emissions to protect radios.
The modulation mechanism depends on the strength of the broadcast signal - the stronger it is, the higher the rf carrier power must be to cause a noticeable problem. What I found was that with a simulated broadcast signal providing a 10 dB S+N/N quality audio output (very poor quality audio, but this is how AM sensitivity is rated) the threshold of susceptibility (TOS) to common mode (cm) noise on a power bus was 48 dBuV, exactly the Class B FCC limit. For differential mode (dm) noise, the TOS was 20 dB higher, the 50/60 Hz power supply filter cap in the radio provided dm but no cm protection. If the simulated broadcast signal strength is increased, the susceptibility signal must be increased the same amount to get a beat frequency tone with the same S/N ratio as originally - the TOS scales linearly with broadcast signal strength. I am going to guess that the three devices you list as culprits did not have any requirement to meet a conducted emission limit. Just like an SCR-based light dimmer, they can chop the ac line with impunity. When I used to teach seminars, and do demos in hotels, I learned not to rely on being able to use AM radios in a hotel conference room - the large amounts of fluorescents and other dirty loads, totally uncontrolled by any standards - often made AM reception impossible. ---------- >From: don_borow...@selinc.com >To: emc-p...@majordomo.ieee.org >Subject: A Different Kind of EMC Problem >Date: Tue, Nov 12, 2002, 2:29 PM > > > I have three devices in my house that cause problems with AM radio band > reception, but not due to emissions, > > In each case, the device adds modulation to strong AM signals, synchronous > with the power line. > > The three devices are: 1. A stereo adaptor for a Beta VCR, 2. A dishwasher > with electronic controller, 3. An electric toothbrush charger. > > I can see a possible mechanism for the first two devices - the periodic > connecting and disconnecting at radio frequencies of two large metalic > structures via the power supply rectifier diodes. The power wiring is one > structure. For the VCR, the other structure is the cable TV wiring, and for > the dishwasher the pumbing is the other structure. The electric toothbrush > charger must do it all through the wiring. > > This is definitely modulation - the radio station having the worst > modulation goes off the air for about 1 second at dawn and dusk while they > change antenna pattern, and the interference goes away. It the problem were > emissions, the interference would be very audible with the radio station > off the air. > > I haven't had the time to try filtering the power line at the devices to > address the problem, but that is not what this posting is about. > > My question: Are there any regulatory standards that address modulation > interference like this? > > Don Borowski > Schweitzer Engineering Labs > Pullman, WA > > > > ------------------------------------------- > 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: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: davehe...@attbi.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" > ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
