First, let's discuss the use of common mode chokes as specialized EMI filters in a specific kind of circuit known as a differential transmission line or balanced line. In hard-wired systems, there are essentially two ways to transmit energy (whether data or power) from a source to a load: either over a single-ended line or a balanced (usually two-wire) line. In any case, there will inevitably be at least some degree of noise on the line. The topology of the line determines what kind of EMI filter is the best choice to deal with the problem. Noise shows up on a single-ended line in differential mode; this means the noise travels down the line to the load and back again through the grounded side of the transmission line. If noise is circulating around a signal loop in differential mode, whatever looked like ground to a signal does not look like ground to the noise. The solution is to apply either a series inductive or shunt capacitive filter (such as the BLM or NFM type) to either absorb the noise or to shunt it to ground through a filter deliberately designed to present a good ground at the noise frequency. The balanced transmission line has no ground and no electrically-convenient place to dispose of the offending noise signal as in the single-ended arrangement. Noise is usually coupled into a balanced pair of transmission lines through stray capacitance and continues to circulate throughout the system via this stray capacitance between the balanced line and whatever may look like a relatively low-impedance return path. Placing differential filter elements such as feed-through capacitors from each side of the balanced line to ground is generally neither practical nor functional, and individual ferrite beads in each line will not generate sufficient common mode impedance to relieve the common mode noise problem. Ideally, what is needed is a filter component which can reduce the common mode noise without being attached to ground. The type DLW, DLP, and DLM common mode chokes accomplish this through a clever arrangement of coil windings on a magnetic core material such as ferrite.