Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 37

One figure of distortion common with modulated signals is the adjacent channel power (ACP) and adjacent channel level ratio (ACLR). Sometimes a third term, adjacent channel power ratio (ACPR), is used instead of ACLR. All are measures of out‐of‐channel spectral regrowth caused principally by the third‐order intermodulation distortion occurring because of a modulated signal. During testing, a modulated signal waveform is applied to the DUT. Figure 1.9 shows the output spectrum of a signal modulated with 16 quadrature amplitude modulation (16 QAM) over a 40 MHz BW, applied to an amplifier.

Figure 1.9 Spectral regrowth causing ACP in a 16 QAM signal.

It is a repetitive periodic waveform from an arbitrary waveform generator, which must be comprised of a multiple sinewave signals, typically thousands of tones, each of which can intermodulate with each other one. In a typical modulated signal, each tone can have a nearly random amplitude and phase, so it is quite complicated to measure each distortion product directly. In general, this figure of merit measures the intermodulation products, which appear in the adjacent channel to the channel under test, as a total integrated power using band power measurements.

In the figure, the lower and upper ACP region is identified, and the signal here is caused by the third‐order distortion in the amplifiers. Also identified is the outline of the distortion profile of the amplifier. ACP is used as a figure of merit as it is easy to discern the distortion level in the adjacent channel where there is no signal. However, the distortion occurs in‐channel as well as out‐of‐channel and is usually a bit higher in the center of the channel. The sloped response of the distortion profile is typical and can be understood by considering the density of signals that can create the intermodulation distortion. Note the outer edges of the adjacent channel where the distortion signal is lowest; only the outermost tones of the main signal can intermodulate to create a signal at these outer reaches of the adjacent channel. At the edge of the adjacent channel nearest the main signal, any two signals that are separated by one‐half the main‐signal bandwidth can intermodulate to create a signal here. The density of these signals is quite high, roughly half the power of the main signal. In the center of the main signal, where the distortion is not apparent because it is masked by the main‐signal power, it is outlined by the distortion profile curve in the figure; any two closely spaced signals can cause distortion power here. The density of such signals is over the whole bandwidth, so the distortion level here is roughly twice that at the close‐in edge of the ACP signal. Even though this distortion is masked by the main signal, it is still present and causes errors in the transmitted signal.

The total integrated power is the ACP. The ratio of the ACP to the total power in the main channel is the ACLR, shown by the Markers 1 and 2 in the figure (they are set to be a delta‐marker with respect to the reference Marker R, which shows the main tone absolute power). Often, test system noise can mask the ACP or ACLR to some extent and becomes the limitation of the measurement. Details of the ACP and ACLR measurements are found in Chapter 8.