Knowing the types of electrical noise and how to measure them are important first steps in enhancing your ability to troubleshoot.
Electrical noise currents on data communication cables are a real problem. They can cause corruption of the desired signals being sent across the cable by the equipment connected to its ends. In extreme cases, these noise currents may even become great enough to cause electrical damage, such as component burnout, to the circuit elements used at either end of the cable. Learning how to make some simple noise current measurements on such cables without disturbing the signal transmission itself is important if you want to troubleshoot such a problem. Let's talk about some of the basics of making these kinds of measurements.
Types of electrical noise
Most of the noise currents that get onto a victim data communication cable are of the common-mode (CM) type, as opposed to being the signal- or differential-mode (DM) type. What's the difference between the two? Let's discuss each in detail.
DM signal current. Looking at Fig. 1, we see that a current can be exchanged between two conductors and that this may be done on either grounded or ungrounded circuits of any type. Such a current is called a differential-mode current. In some cases, it may also be referred to by another name: normal-mode (NM), but differential mode is probably the preferred name nowadays.
CM noise current. In Fig. 2 (on page 24), we see that a current may be caused to flow simultaneously in the same two conductors shown in Fig. 1, but with negligible DM potential difference between the two conductors due to this current. This kind of current flow is called common-mode current since it is simultaneously common to both conductors. A CM current typically (but not always) uses the grounding system to complete the circuit loop for its circulation between the victim cable's ends.
Creating CM Noise Current
DM current on a data communications cable is typically created by the desired action of cable's electronic circuits, which are used at the cable's ends to send and receive the desired signal, whatever it is. CM current is caused by an aggressor source of noise current and by any combination of the following means.
* The ends of the victim data communications cable are referenced to "ground" by either a direct connection, terminating electronics circuit impedances, or a stray capacitance of some sort. In addition, the two ends are not at the same potential. So, a conducted CM current flows as shown in Fig. 3 (on page 24).
* The victim data communications cable looks like a loop-antenna and, as shown in Fig. 4 (on page 24), receives a wide range of radiated radio signals from the air, circulating them in common-mode between the cable's ends via the grounding system.
* The victim data communications cable acts as half of a capacitor's plate and, as shown in Fig. 5 (on page 26), is electrostatically coupled to any nearby conductor's electric fields. Air is one of the dielectrics between the two "plates" that make up the capacitor.
* The victim data communications cable acts as a single-turn, air-core transformer secondary winding, as shown in Fig. 6 (on page 26), and is magnetically coupled to any nearby conductor's magnetic fields.
Coupled vs conducted CM currents
To further keep our descriptive terms straight, let's note that conducted CM current occurs because the ends of a data communications cable are in some way galvanically (e.g., metallically) connected into the grounding system by either a direct connection or via circuit impedance in the cable's terminating electronics.
In contrast, all other CM current is introduced due to coupled means. These latter kinds of unwanted CM currents are due to far fields (e.g., radio waves through the air), or near fields (e.g., electric or magnetic field action).
Using a clamp-on CT
Now that the preliminaries are over, we can take a look at how to make the necessary CM current measurements by using commonly available test equipment.
The key to making a CM current measurement is to use a clamp-on current transformer (CT) as shown in Photo 1, as opposed to trying to use a series-connected shunt and looking at the voltage drop across it. The shunt is not normally used since it obviously requires cutting into the victim circuit's conductors in order to insert it. Also, its installation may change the circuit's electrical characteristics. A CT may be installed onto a single or multiple conductor cable without affecting the circuit's performance or cutting into the conductors.
The CT picks up the CM current because its flow generates a magnetic field around the conductors that is proportional to the current's magnitude.
DM currents, on the other hand, do not generate very much magnetic field, since the signal pair is physically configured so that the magnetic fields cancel each other out. This is called a zero-sequence magnetic field condition, a phenomenon familiar to electricians using clamp-on ammeters when they gather all of a circuit's conductors through the meter's aperture at the same time. Any current then displayed is called unbalanced current or ground-leakage current, depending upon the situation.
What do you feed the CT into?
OK, now what does the CT feed into for an indicator?
Analog or digital meter. Typically, if you're interested only in CM currents in the range from DC through about the first 25th or so harmonic of the power system's fundamental frequency (e.g., to about 1.5 kHz on a 60-Hz AC system, etc.), a simple average responding or true rms type of analog or digital meter is often used, as shown in Photo 2 (on page 24). These kinds of meters are quite useful in finding and tracking CM currents on data communications cables, where the problem can be related to the AC system's fundamental and first few tens of harmonics. Such currents are common in most facilities and appear most often on cables that are suffering from conducted CM noise currents or from near-field magnetic field coupling.
Oscilloscope. The above kinds of restricted-bandwidth detector/indicators are often not useful in finding interfering CM currents that range into the tens of kHz or well into the MHz region. Also, more sensitivity than provided by the typical meter may be needed to adequately "see" the CM current. As such, the CT may be interfaced to an oscilloscope, as shown in Photo 3, for a simultaneous display of a broad range of frequencies as well as lower level signals. This ability to see the CM current in the time-domain (e.g., x = time, y = amplitude) also allows you to see what the CM current looks like (i.e., its signature). This may really help in identifying its source and in tracking the same CM current through several points of measurement.
Wide-band, laboratory grade CT. Where a broad-bandwidth detector is needed, such as where measurements are needed well into the MHz range, a wide-band, laboratory grade CT is used. A typical CT of this type is shown in Photo 4, where the bandwidth is from around 1 kHz to something over 150 MHz (other ranges are available). If CM current below a few Hz needs to be checked, then a Hall-Effect based CT will probably need to be used. Sensors of this type will simultaneously respond to DC and AC but may not be useable above about 100 kHz. Also, they may be affected by external magnetic fields.
Warren H. Lewis is President of Lewis Consulting Services, Inc., San Juan Capistrano, Calif. and Honorary Chairman of EC&M's Harmonics and Power Quality Steering Committee.