Strictly defined, ACPR measures the ratio of the power in a main channel versus the power in the adjacent channel. Many wireless standards use some form of an adjacent channel power ratio (ACPR) measurement to characterize the interference from one channel into the adjacent channel. Although the drawback of a narrow IF bandwidth is longer measurement times, using a narrow IF bandwidth can significantly improve the instrument’s performance in some applications, such as measuring the adjacent channel power (ACP) for a modulated signal. By contrast, narrow IF bandwidth configurations require more spectrum acquisitions for a given span and lead to longer measurement times. Wider IF bandwidths translate to fewer individual spectrum acquisitions and faster measurement speeds. The choice of the IF filter bandwidth has a significant effect on overall measurement speed. Finally, the signal analyzer “stitches” each acquisition together in the frequency domain to present the entire span. If the signal analyzer’s instantaneous bandwidth is 50 MHz, the instrument will perform 10 GHz/50 MHz = 200 individual spectrum acquisitions. In this case, the instrument divides the 10 GHz span into smaller “chunks” based on its instantaneous bandwidth. Suppose one configures the signal analyzer to perform a spectrum measurement with a span of 10 GHz. In other applications, such as a fast spectrum sweep, the instantaneous bandwidth of the signal analyzer directly affects measurement speed. This file type includes high resolution graphics and schematics when applicable. For example, modulation quality measurements such as error vector magnitude (EVM) require the signal analyzer to capture the entire modulation bandwidth to demodulate the signal. In some applications, wide IF bandwidth is highly desirable. The variability of this IF filter stage gives engineers the tradeoff of wide instantaneous bandwidth versus better dynamic range. In most modern signal analyzers, a third IF filtering stage is often implemented with a bank of filters, each with different bandwidths and centered at the same center frequency. Instantaneous Bandwidth And The Spectrum Sweep Today’s spectrum analyzers use up to three mixing stages to downconvert a signal from RF to baseband. The signal analyzer executes the “spectrum” portion of the spectrum analyzer digitally simply by applying a complex fast Fourier transform (FFT) to the baseband waveform. The modern “spectrum analyzer” is really just a vector signal analyzer (VSA) that uses up to three mixing stages to downconvert a signal from RF to baseband (Fig. Understanding Signal Analyzer Architectures.Understanding Your Signal Analyzer's Dynamic Range.
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