A) I don't agree with Ken that: "Emissions from a laptop are naturally (without suppression) on the order of 10 uV/m to 100s of uV/m."
Maybe IBM PC clone laptops use similar enough architecture and chipsets and design techniques to be this consistent (I don't know) but I have seen the following emissions in portable computing products that would come under the same FCC/EMCD emissions standards as a laptop... A.1) A portable computing device with a large additional RAM array for use by people who had lost the power of speech. They would type the word they wanted to say and it would speak it for them using loudspeakers. This was a product that was sold in the UK without a CE mark at around the time the EMC directive came into force (1996). I only measured its with a 1cm diameter unshielded shorted-turn probe. I am used to seeing levels of around 60 to 100 dBmicrovolts from this probe when it is closer than one inch to typical laptop PCBs, with less than 20 dBmicrovolts at a distance of more than one foot. But with this product I saw over 50 dBmicrovolts at frequencies from 30 to 300MHz when my 1cm loop probe was 4 feet away! Maybe some less mathematically-challenged member of the group would like to work out what the electric field strength could have been measured at 10 metres. I've never seen anything quite as bad as that before or since, but it shows what poor EMC design can achieve in the hands of some designers of computing-related products. (Incidentally, they sold it with a clamp for fixing to wheelchairs - some models of which have already been mentioned in this thread as being known to run amok under certain conditions of RF field.) A.2) A portable computing device used in an automatic change machine on board transport was tested to be fully compliant with EN 55022 (approx = CISPR 22). I helped the manufacturer investigate complaints of interference and discovered that sub-fitted variant, which had not been tested for EMC compliance, left an HCMOS inverter IC with an unterminated inverter - which promptly decided to self-oscillate at 200MHz. (Many manufacturers of products with a number of build variants only test the fully-loaded one for EMCD compliance and assume the others are at least as good.) The very interesting thing about this example is that the power-ground structure of the PCB made a beautifully tuned antenna and resonant circuit at 200MHz, so although the inverter was hard-switching and did not run hot, the only emissions were at the 200MHz fundamental - no harmonics were emitted at all. Another very interesting thing is that some of the complainants had measured the equivalent radiated RF power from these devices as 2W. Who would have expected an unterminated HCMOS gate to be able to emit 2W at 200MHz? Well, now you know that it is possible (if not probable) it becomes something you should think about when designing safety-related systems. A.3) Laptops which are fully compliant with CISPR 22 or the equivalent US regs will have higher field strengths when they are less than 10 metres away. How many of us can guarantee that there is always at least 10 metres between each item of electronic equipment? Another thread to this discussion has discussed the problems of low-energy lamps interfering with bedside radios, and this shows up the difficulties of confusing compliance with standards intended for legal market entry and protection of the radio spectrum with the actual EMC engineering to prevent interference in real applications. Another problem is that when we are in the near field of a product its 10 metre emissions measurements are meaningless. There are components of near field emissions which fall off with the cube of the distance and are not usually detected at all by 10 metre tests. For example I understand that many products have very quite strong frequency magnetic fields nearby, at audio frequencies and lower, caused by variations in the loading on their DC power supplies. A.4) The standard commercial and industrial tests for emissions do not test the full range of possible emissions, since they were only ever intended to protect the radio communication and broadcasting spectrum. Actual electronic devices, especially certain kinds of transducers and their amplifiers, can be very sensitive to frequencies outside said spectrum. Military EMC standards recognise this real-life problem and typically test for emissions down to 100 or even 20Hz. B) I don't agree with Ken that "1000 uV/m would represent at least a 20 dB outage at frequencies that could possibly interfere with sensor electronics." See my other posting today adding to Tom Cokenias's calculations to show that RF field strengths of 500uV/m can easily cause severe errors (tens of degrees C) in some thermocouple temperature measuring systems. Also refer to my recent posting about the significant RF immunity problems experienced with a blood sample incubator intended for mobile use. C) Ken says: "Does anyone really see this as a remotely possible mechanism? I don't." Well, I do. Regards, Keith Armstrong In a message dated 03/01/02 23:27:19 GMT Standard Time, ken.ja...@emccompliance.com writes: > Subj:Re: EMC-related safety issues > Date:03/01/02 23:27:19 GMT Standard Time > From: ken.ja...@emccompliance.com (Ken Javor) > Sender: owner-emc-p...@majordomo.ieee.org > Reply-to: <A > HREF="mailto:ken.ja...@emccompliance.com";>ken.ja...@emccompliance.com</A> > (Ken Javor) > To: m...@california.com (Robert Macy), ghery.pet...@intel.com (Pettit, > Ghery), james.col...@usa.alcatel.com ('James Collum'), > emc-p...@majordomo.ieee.org > > Emissions from a laptop are naturally (without suppression) on the order of > 10 uV/m to 100s of uV/m. 1000 uV/m would represent at least a 20 dB outage > at frequencies that could possibly interfere with sensor electronics. The > coupling is lossy: 1 mV/m will generate far less than 1 mV signal in the > electronics, and this at rf. Does anyone really see this as a remotely > possible mechanism? I don't. >
