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Figure 1. Cable Subjected to Fields from an IC
Internal cables in systems can pickup fields radiated from devices and the board itself. Resulting currents induced in the cables can conduct noise into other parts of the system or cause emission problems if those cables are not contained within a shielded enclosure. The situation pictured in Figure 1 is not all that uncommon, a system cable is allowed to come close to an integrated circuit (or possibly a switching power supply). The amount of noise current induced in the cable can be significant and more than one might imagine.
Figure 2 shows the amount of current induced on a 4 m long UTP Category 5 cable (similar to the one shown in Figure 1) by a conductor carrying the signal from an HC240 gate. Upper and lower traces in Figure 2 represent the outputs of two matched Fischer F-33-1 current probes positioned on each side of the location were the signal was coupled into the wire, as shown in Figure 3. Signal current was a few tens of milliamperes with a risetime of about 2 ns. The signal conductor was parallel and adjacent to the UTP cable for about 2 cm at a signal frequency of about 33 MHz, which excited a resonance in the cable. A 2 cm coupling length is not very long, especially when considering the low frequency involved. The experimental setup simulates the induction that bonding wires in a large chip package are capable of.
Figure 2. Common Mode Current Induced on a 4 Meter UTP Cable
Figure 3. Current Probe and Oscillator Test Setup
Note that the waveforms in Figure 2 are in phase. This confirms inductive coupling (as opposed to capacitive coupling). Inductive coupling manifests itself as a series voltage in the receiving conductor. The series induced voltage, pushes current along in the wire, through both current probes in the same direction and hence the result in Figure 2. If the coupling were primarily capacitive, the phases would have been opposite, more on that next Technical Tidbit.
The amplitude of the current in Figure 2 is about 4 mA peak. If we assume an rms value of 1 mA for the current component at 33 MHz, then the current magnitude would likely radiate about 100 times the level allowed by international emissions requirements at this frequency, a 40 dB failure!. Common mode currents on the order of 10 microamps are capable of becoming an emissions problem.
I have also observed cases where magnetic fields from switching supplies caused induction into cables that ultimately caused failures of FCC Part 68 telephone interface requirements in the range of hundreds of kHz.
Executive Summary: Control system cable placement in your next design!
Note: Currents in the range of tens of microamps are not readily observable
on a scope, however a spectrum analyzer can measure these kind of amplitudes
easily. The phase between two current probes can also be displayed on a
spectrum analyzer to confirm the inductive or capacitive mode. A technique
for doing this is described in my 1998 IEEE EMC Symposium paper "Current
Probes, More Useful Than You Think."
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