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Maybe it's a silly question but I couldn't find any useful info about this. Sorry for that.

I've been asked to design a 72-to-24VDC/6A converter for railway vehicles. I designed one and it works perfect. But our customer wants that the converter should pass railway EMC (i.e. both conducted and radiated emissions defined under EN50121-3-2) tests.

We have a small measurement laboratory equipped with Rohde&Schwarz measurement devices. We perform some pre-measurements on the products so that we can see their performances and make necessary modifications before sending them to accredited companies for actual testing and certificating process. For now, we can only measure conducted emissions in our small lab. Anyway, I performed some measurements in our small lab and made some modifications on the circuit to have it finalized.

After finalizing the test sample, I showed the pre-measurement results to my executives and one of them told me that if I can have the product passed the conducted emission pre-test according to radiated emission limits then it can pass radiated emission test as well (NOTE: RE limits are lower than CE's and as I stated above, we can only measure conducted emissions). When I ask him to explain why, he only says that our other products could passed RE test when they pass CE as well. But this is not an answer for me.

Sorry for the looong story, and finally my question is: How can it be? As you can guess, CE can be thought as the emission conducted via supply (or other signalling, control etc.) cables to other devices and/or battery and it's measured directly from the cable via a spectrum analyzer but RE is defined as the emission radiated by the device itself via its case (or not, doesn't matter) and it's measured via a 10m-distant-antenna. So, is it possible to define a substantial relationship between conducted and radiated emission?

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    \$\begingroup\$ I would have to say no because conducted emissions can be controlled by filtering on all of signals leaving the device but these would have no effect on radiated emissions. Also, of course, both tests must be run to verify conformity with all emission standards. If only one was sufficient, you would think that wouldn't be necessary. \$\endgroup\$
    – Barry
    Dec 2 '16 at 22:30
  • \$\begingroup\$ Do you have a small spectrum analyzer and a wideband antenna? You could use a park or whatnot as an impromptu OATS... \$\endgroup\$ Dec 2 '16 at 23:24
  • \$\begingroup\$ @Barry You're right. I thought so. \$\endgroup\$ Dec 3 '16 at 5:56
  • \$\begingroup\$ @ThreePhaseEel Unfortunately we have a spectrum analyzer but no antenna. So we cannot measure radiated emissions. We can only measure CE with our spectrum analyzer. \$\endgroup\$ Dec 3 '16 at 5:57
  • \$\begingroup\$ @RohatKılıç -- hrm, maybe a TV antenna would do the trick for 30MHz-1GHz?\ \$\endgroup\$ Dec 3 '16 at 15:57
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The reason for this is that the most common cause of radiative emissions is due to the power or signal lines acting as antennas. This is because most circuits are small compared to the wavelength of the frequencies that you care about for emissions. For instance, c/(100 MHz) is 3 meters. A trace on a 15 cm PCB will be an inefficient antenna at 100 MHz. However, if you let any noise at 100 MHz leak onto your 5' power cord, it can act as a very effective antenna.

Obviously this reasoning doesn't necessarily apply if you are worried about radiative emissions at 1 GHz or if your PCB is a meter long. It is also possible to use impedance matching networks made from inductors and capacitors that allow a short antenna to radiate effectively, but this tends to not happen by accident.

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  • \$\begingroup\$ Thanks for the answer. RE is measured between 30M-1GHz and CE is measured between 0.15-30MHz. The product has an aluminum case and it's grounded during the tests (according to test procedures defined in Standard). So the waves radiated by PCB tracks or components' terminals should be trapped by grounded case. Thus, the only radiation source can be supply or load cable(s). Am I correct? .... \$\endgroup\$ Dec 3 '16 at 6:12
  • \$\begingroup\$ .... And, CE limits are set around 93-100dBμV and RE limits are set around 40-50dBμV. I'm not sure, but if a signal conducted by supply line has an amplitude of 50dBμV then it shouldn't be measured as 50dBμV via a 10-m-distant antenna during RE tests. But maybe you're right anyways. \$\endgroup\$ Dec 3 '16 at 6:14

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