Uniform field does insure good 'penetration' throughout the EUT. However,...
Gradient will specifically show up the following example of construction flaw: There are two traces on a PCB simulating a twisted pair of signal wires for noise rejection, but you didn't get to flip them as much as you like, plus you had to route one a little bit weirdly on one twist. To be certain all is well you calculated the area of the openess and made certain to match all loop areas within 1% of each other. After all you had to go around that cut out area, and didn't want to change sides too much. Now, the gradient presents a field in one loop of ?? and further away in the other loop of ??? [a different value!] But you were counting on the fields being uniform, and identical to gain cancellation. But instead, you have lost because now the pickup is subtracting two large numbers and will NEVER go to zero. That is one example of how gradient has worse effect than uniform field. Robert On Wed, 04 Jul 2007 16:28:55 -0500 Ken Javor <[email protected]> wrote: > I don't understand how a non-uniform illumination would > cause more > susceptibility than uniform illumination. While I agree > that non=-uniform > is more likely, I have always assumed you want uniformity > for repeatability > and to make it worst-case. > > > From: "Robert A. Macy" <[email protected]> > > Date: Wed, 04 Jul 2007 11:39:58 -0700 > > To: Ken Javor <[email protected]> > > Cc: [email protected] > > Subject: Re: Please reply to me (Magnetic Field testing > ) > > > > Sometimes, gradients generated by a dipole can > aggravate a > > weak design better than nice uniform fields. > Especially if > > there aren't enough "twists per inch" on the PCB > layout. > > > > And, probably more represent an interfering source. Few > > noise sources produce uniform fields. > > > > Robert > > > > On Wed, 04 Jul 2007 13:28:06 -0500 > > Ken Javor <[email protected]> wrote: > >> That is indeed the case. The coil diameter, the number > of > >> windings, the wire > >> gauge and the distance between the plane of the XMIT > loop > >> and the surface of > >> the test article are all specified, resulting in a > >> well-controlled test. > >> > >> An aspect that is well-controlled but perhaps not > >> optimized is the > >> orientation of the magnetic field relative to the > >> circuits within the test > >> article. The field orientation is determined by the > >> topology of the test > >> article enclosure, so the test will be performed the > same > >> at every test > >> facility, but the orientation of the magnetic field > lines > >> relative to the > >> plane of any victim circuit loop will be determined by > >> the layout within the > >> enclosure, and there is no guarantee that the test > field > >> is cutting the > >> victim circuit plane(s) at right angles. Therefore it > is > >> not assured that > >> meeting the test requirement demonstrates > compatibility > >> with a real world > >> source of even the same magnetic field intensity. > From > >> this point of view, > >> a Helmholtz coil of dimensions large enough to rotate > the > >> test article > >> within would seem a better choice, but these are hard > to > >> drive to desired > >> field intensities at frequencies that now extend to > 100 > >> kHz, and even at the > >> beginning extended to 30 kHz. It may be we need a > >> Helmholtz coil approach > >> for dc, the power frequency and maybe the first ten > >> harmonics, and if a > >> requirement extends above that, you use the small loop > - > >> the original > >> military driver for this requirement was protection of > a > >> VLF receiver, not > >> power frequency compatibility. > >> > >>> From: John Woodgate <[email protected]> > >>> Date: Wed, 4 Jul 2007 18:43:13 +0100 > >>> To: [email protected] > >>> Subject: Re: Please reply to me (Magnetic Field > testing > >> ) > >>> > >>> In message > >> <c2b141c4.58eb8%[email protected]>, dated > Wed, > >> 4 > >>> Jul 2007, Ken Javor <[email protected]> > >> writes: > >>> > >>>> I don't know what the commercial standards require, > >> but for instance > >>>> MIL-STD-461 has the field intensity (Tesla) limit > >> augmented by a > >>>> precise description of the coil diameter to be used > >> when generating the > >>>> field. This completes the requirement and makes it > >> repeatable across > >>>> test facilities. > >>> > >>> I think the distance of the coil from the surface of > >> the product is also > >>> specified, isn't it? That determines, in practical > >> terms, the field > >>> pattern of an inhomogeneous field, which is very > >> difficult to define > >>> otherwise. > >>> -- > >>> OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and > >> www.isce.org.uk > >>> There are benefits from being irrational - just ask > the > >> square root of 2. > >>> John Woodgate, J M Woodgate and Associates, Rayleigh, > >> Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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