Microwave Engineering Europe - April 2009 - (Page 28)

28 TEST frequency spectrum has been removed (Figure 1). This stimulus is applied to the unit under test (UUT) and the response is measured with a spectrum or signal analyzer. Any nonlinearity in the UUT will cause spectral components to appear in its output signal within the notch (Figure 2). The calculated NPR value is the ratio of the average power across the notch spectrum versus the average power in an equal bandwidth of the stimulus passband. Comparing approaches and results Although the various analog and digital approaches produce equivalent results, analysis has shown that the digital method is superior in terms of repeatability and test time. These beneﬁts are important in both product development and manufacturing. The analog technique uses an analog white-noise source as the stimulus. A whitenoise signal is continuous and its spectral shape depends on the ﬁlters being used to control its bandwidth. Consequently, it is difﬁcult to obtain reproducible results between UUTs or across test systems. This can be remedied with longer averaging periods in the measuring receiver, but this will extend overall test time. In contrast, the digital technique uses an AWG to produce a stimulus that is more deterministic than white noise (Figure 3). Figure 2: Spectral components that decrease the depth of the notch in the output signal are used to calculate the NPR value. The required stimulus signal contains a series of equally spaced discrete tones that have random phase relationships. The most suitable AWG makes it possible to create an accurate representation of system data trafﬁc while also ensuring measurement repeatability. To achieve these goals, the AWG must provide control over key attributes of the stimulus signal: the number of spectral lines and their spacing; overall spectral shape; notch depth and width (1 to 10 percent of overall noise-signal bandwidth is recommended); and statistical characteristics such as the cumulative complementary density function (CCDF). The digital technique provides three important advantages. One is accuracy: Because NPR is a relative measurement, many uncertainties are removed from the test. The basic accuracy of the measurement depends on the dynamic accuracy of the spectrum or signal analyzer used to measure the UUT’s output spectrum. Characteristics of the NPR stimulus Eight key parameters deﬁne an effective NPR stimulus: • Average power • Occupied signal bandwidth • Notch bandwidth • Passband spectral shape • Noise statistics • Noise signal peak value • Probability density function (PDF) • Complementary cumulative density function (CCDF) Figure 3: The vector signal generator and AWG work together to produce a precisely controlled stimulus for the NPR measurement. Table 1: NPR repeatability versus test time. Microwave Engineering Europe ● April 2009 ● Digitally-generated synthetic noise has key attributes in both the time and frequency domains. In the time domain, the noise signal has a ﬁnite repetition period and is very repeatable in amplitude versus time. In the frequency domain, the noise signal has a ﬁnite spectrum. It also has band-limited spacing of the spectral lines that is equal to the inverse of the time-domain repetition period. Naturally, the proper choice of signal generator and spectrum analyzer depends on the bandwidth to be measured. Another consideration is software tools capable of deﬁning wideband multi-tone signals and then downloading the parameters into a compatible signal generator. www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - April 2009 News Contents Comment MMICs/RFICs: MMICs for Broadband Receiver Applications Antennas: Mutual Coupling Reduction in Compact Arrays for Wireless Sensors via a Pre-fractal Defected Ground Structure Repeatable Characterization of Distortion Caused by Nonlinearities in Wideband Communication Systems Products Calendar

Microwave Engineering Europe - April 2009

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