FROM EDN EUROPE: Capture spectral data on pulse waveforms
By Graham Prophet -- EDN, February 5, 2004
If you are making measurements on a one-time, intermittent, or "bursty," signal, the usefulness of a conventional spectrum analyser is somewhat limited. It operates by scanning a narrowband receiver window across a range of frequencies and plotting received amplitude against a frequency baseline to display the spectral content of the signal. If your bursty signal is absent when the analyser's "window" sweeps across it, you don't see anything at all; at best, the results are difficult to interpret. Such signals are increasingly commonplace, according to Tektronix, citing waveforms such as those of RFID tags. The signal they send is not particularly demanding or complex, but it is present only once, when you interrogate the tag, and you need to characterise it.
Tek claims to have solved this problem with its real-time spectrum analyser series (Picture). With a total span reaching 8 GHz, it lets you look at any 30-MHz (or 15-MHz, depending on model) slice of that spectrum in real time and see any signals that are present in that passband. The instrument is a receiver front end with a variable bandwidth (reaching that 30-MHz figure), and an FFT (fast-Fourier transform) captures, samples, and processes all the signal energy in the selected passband to show its spectral content. You can display information in the frequency domain (a conventional spectrum-analyser view), the modulation domain, or in time or data domains. You can also have multiple views on screen at one time, with correlation between the traces.
One of the options is a spectrogram—a 3-D display with frequency span along one axis and time along the other. Each sample interval therefore appears as a line, and a colour scale represents signal intensity. As the display builds up, you can therefore see the changing distribution of spectral energy in the signal over time. The spectrogram display is similar to displays that have been available in the audio domain for voiceprints and similar analyses. In applications such as surveillance and signal intelligence, you can watch a complete signal band and capture every short-lived signal that emerges from it.
The instrument's architecture, as you might expect, has a receiver/mixer front end, downconverting to an IF filter, followed by an ADC and a great deal of DSP processing. It uses a two-stage frequency analysis—an initial quick analysis is for triggering and, when triggered, a detailed algorithm that processes captured data. This setup allows you to use a "frequency mask trigger"; you can set the instrument to trigger on a signal that is between certain frequency limits and meets specified levels. In this way, you can choose to trigger on some feature that characterises the start of, say, an intermittent pulse waveform. You can specify as much as 256 Mbytes of signal-capture memory, and there is a trade-off between frequency span (which affects the amount of data acquires in each sample event), sample rate, record length, and spectrum frame time.
The ADC at the instrument's core runs at a constant 51 MHz, using a 14-bit conversion. With a front end that has a wide bandwidth open all the time, a challenge of the receiver design lies in the dynamic range that it can achieve. Tek quotes a figure of 72 dB, noting that you have to go to a "high-end" conventional spectrum analyser to achieve better. That figure is good enough to carry out adjacent channel-level measurements on digital cellular signals. Depending on the specification, pricing begins at around €23,000.
Tektronix, +44 1344 392000, www.tektronix.com.
