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Spectrum analysis revisited to seek out masked signals

Tektronix has upgraded its RSA6000 series of spectrum analysers with a feature that it calls Swept DPX, as well as with new triggering features. In the same way that the feature sets of digital oscilloscopes have evolved to help you capture events in the time-domain that are very infrequent, these spectrum analysers help—among other new capabilities— isolate events that might occupy the same spectrum space as other signals while being infrequent in time, or lower in level than the dominant signal.

The instrument spans 9 kHz to 14 GHz; it has an analysis bandwidth of 110 MHz, and in the DPX architecture, when you view a spectrum wider than 110 MHz, the instrument down-converts successive 110-MHz segments of the complete sweep to its baseband, generates a spectrum of each segment, then concatenates them on the display to show the full scan. One of the features of the new instrument is that, with greatly increased processing power—through the use of new-generation FPGAs—devoted to the core FFT functions, it can now capture many more spectra per second: a maximum of 292,000 per second. Tek has used this feature to increase the dwell-time—the period for which the instrument looks at any given 110-MHz-wide “slice” of the band of interest. It aggregates these spectra, so that when you look at the screen you see a spectrum that is a composite of many measurements over time. Therefore, if there is an infrequent event, you have a much greater chance of capturing it. The analyser represents the frequency of occurrence of a signal crossing any given pixel on the composite display by colour-grading the pixels. If you spot an infrequent event, you can set the instrument to trigger by choosing a window on the trace and by specifying a low value for the frequency of occurrence. You still, of course, require a signal that has at least some periodicity—the spectrum analyser is not an appropriate tool for single-shot events— but you can capture transients as short as 10.3 µsec. There is a statistical aspect to this parameter, and a Tek spokesman asserts that there is a good probability of capturing events as short as 1 µsec. The point-and-go aspect of this triggering tool leads Tek to call it “Trigger-on-this”. The instrument can also confi gure its basic capture capability into a 110-MHz digital oscilloscope with a full range of pulse-trigger facilities. The new series of instruments costs from €69,110 to around €130,000 in a maximum confi guration. If you have an earlier RSA 6000, you can carry out the upgrade yourself by doing a main-board swap.

Tektronix, www.tektronix.com.