In a message dated 10/16/2003 10:14:54 AM Pacific Standard Time, jeff.loyer@xxxxxxxxx writes: Just to be clear - are you saying that if I connected 2 properly designed chips (driver and receiver) together with many properly designed single-ended transmission lines, they would likely fail FCC standards? Yes, but only with qualifications. Picture a single-ended, 50-Ohm microstrip, not multiple closely spaced traces. If excited with a 1 ns risetime signal at 33 MHz clock frequency from a 50-Ohm generator, and terminated with a 50-Ohm chip resistor, expect failure. For example, the EMCAD1 software application (circa 1992) is based on radiated emissions equations and predicts ~39 dBuV at 33 MHz at 30 meters antenna distance. The first few harmonics are also predicted to be out of limits. Note that this software uses worst-case predictions. The same software also indicates an increase in the single-trace emission level by SQRT(N), where N = the number of traces; however, the assumption is that all traces are carrying the same excitation (which is unrealistic). Now, if you place grounded planar areas around the trace, or have a densely routed board, the additional copper provides a substantial reduction in the measured emissions from that single trace because many field lines that would otherwise leave the board terminate on the added copper (even though they may be additional 50-Ohm traces). And, in a normal board design, many of the signals on these surface traces will generate fields that cancel each other. Therefore, the bare (i.e., unshielded, and unenclosed) PCB MAY or MAY NOT fail an FCC or CISPR radiated emissions test. The IBM paper I mentioned recorded up to 20 dB variation in radiated emissions from a single microstrip on a bare board as the trace was moved in-board from the PCB edge. This measured reduction also illustrates the field-capturing effect of surrounding copper, reference planes, or other conductors. The PCB I created contained significant open surface space around the traces (including the guarded trace structures) which tended to increase the radiated emissions relative to a real board design, but this was on purpose to illustrate the effect. As an added note, I found that the radiated emissions from a single trace increased more than linearly with the impedance of the line; i.e., a 100-Ohm trace radiated >6 dB more than a 50-Ohm trace. This result makes sense as a better match to the 120*pi = 377 Ohms of free space is achieved with the 100-Ohm line. Mike Michael L. Conn Owner/Principal Consultant Mikon Consulting *** Serving Your Needs with Technical Excellence *** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu