2 Noise Budget

In any communications system, the signal at the receiver does not exactly match the signal at the transmitter. The sources of the difference in a point-to-point system are shown in Figure 2-1. The PHY interfaces to the cable via a Discrete Front End (DFE), such as the Bus Interface Network (BIN)⁴ recommended for use with the Microchip PHYs. The cable interface is named the Media Dependent Interface (MDI) in the IEEE specification and the standard defines the characteristics at exactly this point. The specification defines the media as a balanced pair and specifies some characteristics of the media, including insertion loss. Insertion loss is a measurement of the impedance mismatch between the MDI and the cable. This system impedance mismatch will cause reflections and therefore signal distortion. The attenuation over the cable length between any two ports will cause additional signal loss. These all combine to distort the signal, even before the addition of noise. A system that is built on better quality components and cabling will result in better signal quality by minimizing reflections and attenuation. This remaining difference between unstressed signal and the tolerance through the DFE of the receiver is the amount of noise that the system can tolerate. Noise can be from electromagnetic interference as well as from power supply and other noise sources in the receiver.