Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 38

Widely found in the satellite communications industry, noise power ratio (NPR) is a measure of distortion, and not of noise at all. In the early days of satellite development, the industry needed a measure of distortion for satellite components but could not use the more common IMD or ACP. Most satellite systems have strongly channelized amplifiers, where the communication signals fill an entire channel and are filtered at the output so adjacent channel distortion would be filtered away, and could not be used as a figure of merit for the in‐channel distortion. Furthermore, the communications protocols for satellites could change over the life of the satellite, and often many different communication methods could be used in the same channel. NPR was developed to emulate a densely loaded communications channel but still provide a means to determine distortion.

In the early days, NPR signals were generated by using a noise diode followed by a filtered amplifier. This would produce a noise signal at high power, of the specified channel. This was followed by a narrow band‐stop filter, which blocked the noise signal in the middle of the channel. When this signal was applied to the system component, distortion of the amplifier could be seen in the notch of the NPR signal. Figure 1.10 shows an example NPR signal, after passing through an amplifier. This is not one created by a noise diode, but rather using an arbitrary waveform generator, which is programmed to produce an additive‐white‐gaussian‐noise (AWGN) signal with a notch at its center. In fact, the use of noise diodes to produce NPR signals has been essentially replaced throughout the industry with arbitrary waveform‐generated signals. In this example, the AWGN signal is created in a low‐frequency baseband generator and then upconverted inside the signal source to the desired center frequency. Some unflatness is apparent in the passband of the signal due to frequency response of the signal source.

Figure 1.10 An NPR signal showing the total power and ratios of band power.

Apparent in the figure is also the ACLR level, which is nearly the same at the edge of the main signal as the NPR signal in the middle. It is clear from this figure that ACP and NPR are closely related. Imagine, though, if the DUT is followed by a sharp channelizing filter; the ACLR would be removed by the filter and could not be used to determine the distortion but the NPR signal allows one to see the in‐channel distortion. NPR measurements are covered extensively in Chapter 8.