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    Difference between conducted and radiated emission ?

    Can anyone tell me the difference between conducted and radiated emission, especially for digital circuits ?? For instance, inside a large size digital core what is source of the conducted and radiated emission ?

    regards,


    taofeng

    •   Alt31st March 2008, 12:09

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    Re: Difference between conducted and radiated emission ?

    Please refer to IEC standards for exact definitions and allowed levels.
    The word "conducted" implies that the signals are conducted along conductors, wires or printed lines; also dielectrics can and do conduct elmag.signals.
    The word" radiated" implies that signals can be radiated from various conductive or dielectric structures as an elmag.wave.
    Different test methods are then used to distinguish how interfering signals can get to a test point.



    •   Alt10th January 2011, 18:13

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    Re: Difference between conducted and radiated emission ?

    Hello,

    At relative low frequency, most spurious emissions are generated by common mode current in the connected cables. Measuring the true radiation from long cables at low frequency is very difficult and depends heavily on the routing of the cables. Therefore at low frequency they carry out common mode measurements with current clamps. They can also measure common mode voltage. In both cases the cables are terminated with predefined common mode impedance. These are the "conducted emission" measurements. Frequency range is generally up to 30 MHz.

    At high frequency, radiation from the case or PCB (in case of plastic case) becomes important (for example harmonics from clock oscillators). As the frequency is higher, they can measure the radiation directly as antennas have suitable size (log periodicals combined with bi-conicals or (large) horns ). These are the "radiated emission" measurements. Frequency range generally begins at 30 MHz up to above 1 GHz (depending on standard and highest clock frequency in the device under test). .


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    •   Alt10th January 2011, 19:04

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    Re: Difference between conducted and radiated emission ?

    All this is true. Due to various uncertainties, IEC introduced standards with well described procedures and equipment to obtain reliable and repeatable results.

    Concerning radiated emissions, starting at 30 MHz is not correct. I took part in an interesting emission test of a small uP driven telephone echange. Its clock harmonics and subharmonics were radiated far away from ~1 MHz up to >50 MHz as a many-line spectrum.
    With lots of digital hardware used often without any technical consideration, we now have an exceeding and growing interference everywhere. To get money, fools promoting "digital over power lines" radiate a heavy mess around, filling mainly short-wave bands with their well-paying traffic.
    And there is no legal defense- maybe we can equally well interfere with their traffic...



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    Re: Difference between conducted and radiated emission ?

    starting at 30 MHz is not correct
    Most designers will ignore emssions without limits in EMC standards. It's another question if you are able to measure them. "Radiated" emissions below 30 MHz are however pure nearfield phenomena, respectively magnetic and electric fields, that have to be accessed with field probes rather than antennas. Some limit values for fields below 30 MHz exist in specific EMC standards, but they are not of general importance.



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    Re: Difference between conducted and radiated emission ?

    The main difference between Conducted and Radiated emissions is in the frequency range. Conducted emissions are measured off the power cord of your system between 150 kHz and 30 MHz. Radiated emissions are measured from 30 MHz to 1GHz and higher, depending on the highest frequency being generated in your system.



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