Muriel et al, Scott is essentially correct.
The only thing to add is that the emission limits and immunity tests have very different goals and can't be compared easily. The emission limits are designed to help protect broadcast and communication radio receivers. These devices are sensitive in the sub-microvolt region, down to about -15 dBuV (about 20 kHz BW) such as some of the VHF/UHF radio gear I handled many years ago. The emission test levels are so low as to make it insignificant for even the poorest shielded digital circuit to be affected, but still allows some noticable interference to radio receivers to occur. I don't think we'd want to consider the costs to guarantee an interference-free spectrum. So the emission limits provide no real immunity protection for most modern electronics but help only the extremely sensitive analog devices and RF equipment. In contrast, the immunity tests help assure that common radio transmitters don't upset equipment, and a 3 V common mode RF at 129 dBuV is much more stressful than an incidental conducted RF emission at 66 dBuV. It would be interesting if our cellphones had a diagnostic mode to show us the percentage of all available channels that are unusable due to noise, forcing the phone to hunt for a new channel. (Hello Motorola, Nokia...) This channel hunting feature is the only reliable way a cell phone can function around other electronics. And, don't forget broadband noise emissions which can disturb numerous communication channels and still comply with the emission limits. Regards, Eric Lifsey Compliance Manager National Instruments Please respond to "Scott Lacey" <sco...@world.std.com> To: "Muriel Bittencourt de Liz" <mur...@grucad.ufsc.br> cc: emc-p...@ieee.org (bcc: Eric Lifsey/AUS/NIC) Subject: RE: doubt on conducted emissions
Muriel, See answers below. Scott Lacey -----Original Message----- From: owner-emc-p...@ieee.org [mailto:owner-emc-p...@ieee.org]On Behalf Of Muriel Bittencourt de Liz Sent: Thursday, August 03, 2000 5:46 PM To: Lista de EMC da IEEE Subject: doubt on conducted emissions Hello Group, I have some doubts concerning conducted emissions: 1. I'll make a hypothetical case: Let's say I have 2 electronic equipment (they can be switched mode power supplies). Equipment A requires 100W. Equipment B requires 3W. Let's say that my readings of conducted emissions, collected in a receiver, are: [EMI of equipment A at f=200kHz]=90 dBuV [EMI of equipment B at f=200kHz]=90 dBuV Making some calculations, I evaluate the interference voltage relative to 90dBuV ==> EMI in volts= 31.6mV Here begin my questions: # This 31.6mV is a voltage that propagates on the mains wires. I understand that the purpose of the EMC regulations (in the frequency range of 150kHz-30MHz) is to prevent that this voltage interfere with a radio receiver equipment or other electronic equipment. How long this voltage propagates in the mains wires (until which distance it is significant)?? A: Anything on the same branch circuit might be affected. The wire may also act as an antenna and radiate the interference. It may also couple to other circuits that run near it. # Is there any difference between the 90dBuV that equipment A (100W) generates to the 90dBuV that equipment B (3W) generates (Qualitative and quantitative)?? A: No. Both are 90 dB relative to 1 uV. The interference, in this case 200 kHz, is mostly a result of circuit layout inefficiencies. The voltage rise time (dv/dt) of the switching element is what generates the interference. Regardless of power ratings, most switch-mode supplies operate at a nominal primary voltage of either 150Vdc (110-120Vac in), or 300Vdc (230-240Vac in or 110-120Vac doubler). # This 31.6mV has a perturbing effect to the equipment A?? And to equipment B?? A: This interference may have a perturbing effect to any equipment that is sensitive at that frequency or one of its harmonics. # Concluding: This voltage is perturbing only for radio receivers?? Is this the goal of the regulations imposed by the agencies (CISPR, FCC, etc.)??? A: This voltage may also affect sensitive analog instruments (causes shifts of reading), may cause false clocking of digital circuits, and so forth. The agencies attempt to address this issue in two ways. First, they set limits for conducted and radiated emissions. Second, they (EC, etc.) require RF immunity testing so that a piece of equipment will not be adversely affected. Thanks in advance for the answers Regards Muriel Bittencourt de Liz Group for Conception and Analysis of Electromagnetic Devices GRUCAD/EEL/UFSC Florianópolis, SC Brazil
