Pseudorandom Noise Loading for Noise Power Ratio and Law of Addition

Noise loading has been used for many years to test wideband communications systems. 1 In this technique, the system load is simulated by thermal noise of the same bandwidth and total power as the actual load. This is particularly effective for equipment designed to carry hundreds of telephone circuits, because the multiplexing together of many independent circuits naturally leads to a total signal that is gaussian. Small frequency bands (notches), usually a few kilohertz wide, are eliminated from the thermal noise load at several places in the band. This noise signal is applied to the device under test, and the noise that appears at the notch frequencies after the device is detected and measured. The noise in these narrow notches is a fairly accurate measure of the circuit noise that a customer would overhear. This noise may be broken down into several components: thermal, intermodulation, interference, and so on. In modern, long-haul, repeatered telephone systems, intermodulation noise is usually a big contributor that must be held at or below the thermal noise, which limits the power at which each repeater may be operated. Until now, intermodulation noise was usually measured by raising the output power until intermodulation became far larger than the thermal noise. This is possible since intermodulation noise increases with an increase of input power and thermal noise does not. The difficulty with this approach is that the noise is then being measured outside of the equipment-design range.