ANSI SCTE 119:2018 pdf free download – Measurement Procedure for Noise Power Ratio
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The ratio of the signal power density to the power density of the combined noise and intermodulation distortion in the channel. Essentially, NPR is the depth of notch. The signal power density is defined with the entire passband flled with energy. The power density of the noise and intermodulation distortion shall be measured by removing signal power from a range of frequencies with a notch filter while maintaining constant total signal power at the DUT input. In an NPR measurement, the injected noise is referred to as the “signal.” The range of input powers where the power in the notch is dominated by thermal noise, laser RIN, shot noise, and other noise contributors that do not change with signal level. The noise region is on the left side of a dynamic range plot. In the noise region, NPR increases approximately 1:1 with an increase in input power. The range of input powers where the power in the notch is dominated by intermodulation noise. In a dynamic range plot this is evidenced by a rounding of the top of the curve. The intermodulation region is between the noise and clipping regions. If the distortion performance of the DUT is extremely good, the NPR curve will transition from the noise region to the clipping region with a minimal or no intermodulation region present. The range of input powers where the power in the notch is dominated by high order intermodulation noise caused by clipping. Clipping occurs when RF or optical devices are driven into an operating region in which the input-to-output transfer function is quickly reduced. The clipping region is on the right side of a dynamic range plot. In the clipping region, NPR decreases rapidly with an increase in input power. See Figure 2 for an example of a DUT output in the clipping region.