Since the loop output is Mdi/dt (where the di/dt is ofthe current flowing on the bonding wires and lead frame), the current waveformis proportional to the integral of Figure 2. The positive going pulse isthe rising edge and the negative pulse is the falling edge. For a squarewave, these two pulses will be separated. In the plot of Figure 2 though,they are adjacent. This means the current rises, in a little over a nanosecond,only to immediately fall with a slightly slower fall time. This is not likelya real signal but possibly the current pulse that occurs when a pull-up deviceoverlaps in time with a pull-down device resulting in cross conduction betweenpower and ground for a brief period. There are several other possibilities,such as conflict with another chip on a bus, that can result in such a currentwaveform.

Figure 3, by contrast, shows a continuous signal from the loop. The fundamentalfrequency is about 125 MHz, but the interesting part is the easily seen thirdharmonic of about 375 MHz. The shape of the wave is similar to that shownin the April 2002 Technical Tidbit on this site, "Printed Wiring Board Couplingto a Nearby Metal Plane." In that article, the oscillator output excited a board resonance at the third harmonic of the oscillator frequency. In the present case, a board feature that resonates at 375 MHz may be excited. An example might be a path crossing a ground plane break or a misterminated transmission line. Another possibilityarises from circuitry and time delays internal to the device.

The difference between Figures 2 and 3 is that while the pulse in Figure2 may be a signal integrity problem because of its amplitude, the wave inFigure 3 may be an EMC emissions problem because it is continuous with significantenergy. Even though the peak amplitude is only about 13 mV, about one tenththe peak amplitude in Figure 2, the inductive voltage drop between the chipdie and one or more leads exiting the package is likely to be about 50 mV. Thiscould represent a noise plateau over the board ground plane driving all theleads of the chip with respect to the ground plane.

Considering that it only takes a couple of millivolts fed into an appropriate radiating structure to cause an emissions problem, every lead from this chipmay be a potential EMC problem waiting to happen. Even an output that ispulled low to drive an LED continuously, has the capability of deliveringsignificant energy to a radiating structure such as the board itself or connectedcables.