Cabling Affects Electromagnetic Interference in Complex Ways
At EMI Software our focus is electromagnetic interference on cables.
Why? Because cables are a central element of most EMI test failures and cables can create some of the most difficult to solve interference problems.
Three things make cables problematic:
1. Cables are electrically long
2. Cables often carry unwanted energy
3. Cable conductors can resonate
Electrically Long Cables. Most electrical cables are much longer than the wavelengths of the signals they carry. Being electrically long they are quite efficient at transferring electromagnetic energy to and from the surrounding environment.
Electrical cables are pretty good antennas — unfortunately.
Take, for example, a 10 MHz digital data signal. If it has a typical rise/fall time of 3 nanoseconds, its spectrum contains significant energy up to more than 200 MHz (see figures below.) A 2-meter long cable is one and a third wavelengths long at 200 MHz, electrically long indeed. Conditions are right for EMI problems.
Noise Contamination. High frequency energy on cables originates at circuitry connected to the cables and or is coupled from the surrounding environment.
Common mode noise from circuitry is particularly problematic. Even a small amount of high frequency voltage on cable-connected interface circuits is capable of causing the cables to radiate fields greater than allowable radiated emissions limits.
Similarly, fields produced by external sources can couple quite efficiently to cable conductors, either during radiated susceptibility testing or in the final installation. If field levels are of sufficient magnitude they can degrade circuit performance, cause upset, or in extreme cases damage the interface circuits.
Cable Resonance. A cable that is perfectly terminated in its characteristic impedance at both ends and has no impedance discontinuities won’t resonate. Otherwise, it will. The challenge is, except for coaxial cables that are very carefully terminated at both ends, nearly all cables will resonate at certain frequencies. Even good quality data cables that are terminated in their characteristic impedance will resonate to some degree.
The magnitude of the resonances and the frequency of the resonances are a function of many variables, including cable length, circuit impedance, and distance from surrounding conductors and chassis.
Since it is impractical to run all cables as coaxial lines, cable resonances are an inescapable reality in electronic systems.
Small noise on long cables causes big EMI problems.
Take for example the circuit below. It carries the same 10 MHz waveform and spectrum shown above. Nominally the driver puts out a spectrum that decreases predictably with increasing frequency. But when the data cable connects the driver and receiver, everything changes.
Because the cable input impedance is not constant over its length and over frequency, the voltage on the cable varies with frequency and with position on the cable.
The two graphs below illustrate what happens. The graph on the left shows the 10 MHz signal spectrum at the driver end of the cable. The graph on the right shows its spectrum at the receiver end of the cable.
The voltage at the receiver is double (6 dB) the driver voltage at 10 MHz due to impedance mismatch. At 50 MHz the voltage at the receiver is 14 times (22.9 dB) greater than at the driver. In the time domain this equates to 2.5V ringing at 50 MHz! That’s enough to cause serious signal integrity and EMI problems. At many frequencies, cable voltage at the receiver is much greater than at the driver.
Cable radiation at these frequencies will likely be much higher than if the cable were properly terminated.
At frequencies above about 2 GHz the voltage at the receiver is lower than at the driver, indicating that the signal edges have rolled off by the time they reach the receiver. This too may have signal integrity repercussions if signal timing is critical.
This is just one example of why EMI problems involving cables can be difficult to understand and control.
That is why at EMI Software our focus is electromagnetic interference on cables.