At 12:26 PM -0800 11/1/98, Dave Emery wrote:
On Sun, Nov 01, 1998 at 09:53:37AM -0800, Tim May wrote:
Anyway, were I to try to shield a laptop I'd start with microwave leakage meters, a couple of t.v.s and radio (of different types and bands), and then I'd start recording signal levels of various sorts as different shielding layers and types were applied to the laptop(s). Simple lab stuff.
As someone who has actually spent hellish weeks working to suppress the RF emissions of some commericial network gear I designed - to make it pass FCC B and VDE certification - let me say this is a black art and no fun. There are sophisticated electromagnetics programs that can sometimes succeed in modeling the radiation from a computer system, but they are very expensive and inputting all the required information is painful or downright impossible (data just not available or in a usable format), so this nasty job usually gets done seat of the pants style using rules of thumb and educated guesses and hard won experiance and lots of trial and error. The job consists of attaching lossy ferrite beads, copper tape and other RF and common mode current supressing devices, shielding plastic packages with spray metallic coatings, adding ...
All good points, but there's a big difference between trying to meet FCC emissions requirements for a commercial product that has to meet cost, weight, and cosmetic requirements (e.g., a plastic case!), and the scenario of making a TEMPEST-like box for a laptop. Ferrite beads and copper tape are a lot different from a sealed box made of 10-gauge copper sheet.
Nobody does this using ordinary radios and TVs, the standard tool is a broadband spectrum analyzer or special EMC receiver with quasipeak filters and special calibrated wideband dipole antennas that have known gain and pattern characteristics. Isolating of radiating sites is often done with near field probes or sniffers attached to the spectrum analyzer that allow hot spots to be tracked to within a few cm. Often in order to get enough sensitivity one also needs special preamps, and a RF quiet site where signals from the DUT aren't drowned out by pagers and cellphones and emissions from nearby computers. A good bit of this work is done way out in the country under non-metallic fiberglass buildings that don't create reflections that confuse the measurements. And conducted as opposed to radiated noise is measured with special power line filters and cable filters...
Sure, but my point was that John Young should *at least* start with actual measurements, as opposed to putting most of the onus on a FOIA request to get TEMPEST docs declassified. If he can get spectrum analyzers and all that stuff, so much the better. --Tim May Y2K: A good chance to reformat America's hard drive and empty the trash. ---------:---------:---------:---------:---------:---------:---------:---- Timothy C. May | Crypto Anarchy: encryption, digital money, ComSec 3DES: 831-728-0152 | anonymous networks, digital pseudonyms, zero W.A.S.T.E.: Corralitos, CA | knowledge, reputations, information markets, Licensed Ontologist | black markets, collapse of governments.