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Spectrum analyzer spans time, frequency, modulation domains simultaneously

Posted: 15 Dec 2004 ?? ?Print Version ?Bookmark and Share

Keywords:tektronix? tek? rsa3408a? spectrum analyzer? rf system?

With its deep memory and noteworthy 2000-fold boost in resolution over its predecessors, Tektronix's new RSA3408A spectrum analyzer!priced reasonably at less than $50,000!debuts at a time when RF systems are racing up the curve in complexity and scope.

When Tek's director of RF product engineering Elaine May let me sit down in front of a new RSA3408A, I knew I was about to drive a state-of-the-art instrument. "This analyzer will let you view transients and signal instabilities that most engineers never know exist," she said. "Its high resolution gives a precision view so you can see things such as transients, spectral re-growth, and instabilities."

Watching its sparkling hi-res color display, I was reminded how all-pervasive DSP-based digital RF is. Signals are no longer basic-modulated carriers; gone are the days of looking at simple FSK, CW, FM, or voice-amplitude modulated signals and their sidebands. Today's digitally generated RF signals are pulsed, chirped, and compressed.

Some schemes even combine digital modulation and spread spectrum techniques. "We're seeing tremendous growth of DSP-based, modulation-agile communication systems," notes May. "That goes hand-in-glove with the pervasiveness of low cost RF in consumer electronics."

The RF environment has changed, too. Interference, co-located emitters, and crowding are commonplace predicaments faced by many RF designers!and end users.

Static snapshot limitations
In light of that, it's clear that conventional swept-spectrum current-generation realtime spectrum analyzers, and even VSAs (vector signal analyzers), aren't capable of effectively observing the kinds of signals in today's RF environments. Most spectrum analyzers are only able to compile a static snapshot of RF spectral activity.

Taking many acquisitions to plot a single frequency-domain image, or sweep, most spectrum analyzers can't provide a complete view of a complex modulated pulse train. They'll likely miss the kinds of short-lived transient events and anomalies that a realtime spectrum analyzer, such as Tek's new RSA3408A, can see.

Even VSAs are typically limited on RF performance, or they don't have a time-correlated multi-domain view, which is just what's needed to really understand what's transpiring with pulsed signals. As such, they can't readily let you comprehend relationships between modulation quality, pulse shape, and spectral footprints.

Until recently, it was usually good enough to look at RF signals in the frequency domain, using an instrument that displayed amplitude plotted across a frequency axis. Now, however, time is equally important. With a time axis, Tek's analyzer can help you pinpoint not only particular pulses within a pulse train, but also the RF activity occurring between those pulses.

Indeed, Tektronix claims its new instrument is the only spectrum analyzer available these days with the ability to capture and analyze individual pulses in a train of thousands of pulses. "You can zero in on one sample's worth like a microscope," said Elaine May.

With a single acquisition, the DSP-equipped RSA3408A!with its A/D converters that run twice as fast as Tek's RSA3300A Family forerunners!can simultaneously make pulse width, pulse-to-pulse phase, pulse ripple, carrier frequency, and emissions bandwidth measurements, to name just a few.

What's more, these measurements made on individual pulses can be viewed as a trend across all pulses in an acquisition. The data you get that way can provide a rather thorough depiction of signal behavior.

Color-coded third dimension
One of the most powerful capabilities of the RSA3408A is its so-called spectrogram display. The spectrogram is what adds the third dimension of time to the customary power-vs-frequency plot found in ordinary spectrum analyzers.

When you use a RSA3408A, the color-coded spectrogram gives you an almost intuitive view of how frequency, amplitude, bandwidth, and timing of a pulsed signal can change over continuous time.

Showing a sequence of measurements in the frequency domain, an RSA3408A delivers 36MHz realtime bandwidth, and up to 20ns frame-time resolution. Observing the spectrogram, you can see an uncanny amount of detail, even in a quickly changing RF spectra.

Modulation analysis
The RSA3408A box also includes built-in analog and general purpose digital modulation analysis. Using that, you can demodulate pulses and make time-correlated, multi-domain analyses of chirp or other modulation characteristics.

The device's automated pulse signal analysis capabilities can help you ferret out rogue pulses and jamming signals, as well as ease the task of understanding RF signal behavior, such as characterizing pulse distortion interference.

In addition, thanks to its DSP engine, Tek's analyzer provides overlap FFT (fast Fourier transform) processing, with user-settable resolution.

Trigger, capture, analyze
As you might expect, the RSA3408A analyzer also provides both frequency-domain and time-domain triggers. These event triggers ensure that you can capture a seamless record of RF signals into memory, and then perform time-correlated multi-domain analysis.

For example, a power trigger can enable spectrum capture whenever the total power of all signals in an analysis span crosses a threshold that you define. Tektronix claims that, together with the instrument's 36MHz bandwidth, ensures 100 percent probability of event capture.

In addition, a patented frequency-mask trigger can also capture a spectrum of interest. It works when a discrete change in signal frequency, amplitude, or bandwidth occurs, or when a signal appears or disappears. Like the unit's power triggering, the frequency mask trigger and the instrument's 36MHz window ensure 100 percent probability of event capture.

The system's triggering also works even if spectral events are detected at a much lower level than adjacent signals. In addition, the frequency-mask trigger system in the analyzer lets you create custom masks that can monitor multiple frequency bands within an analysis span.

A continuous trigger mode
Since some problematic signals might only appear, say, once every hour, or perhaps even as rarely as once a day, the RSA3408A also packs a nifty continuous trigger mode.

The continuous trigger mode constantly monitors a spectrum of interest, but only makes an acquisition when user-defined triggering criteria are met. Once triggered, the analyzer then captures and time-stamps the spectrum activity to memory, and then re-arms itself to trigger again if the event reappears.

Tek's continuous trigger mode also makes efficient use of the RSA3408A's capture memory by ensuring that it's only filled with relevant information. Moreover, it lets you leave the instrument unattended to acquire transient events over time.

With the ability to trigger on dynamic and transient signals based on specific events in time and frequency domains, the RSA3408A can identify and acquire single-shot events, or complex sequences of events, recording into memory for downstream and subsequent analysis.

The instrument does that with across a dynamic range of 66-dB ACLR (adjacent channel leakage ratio) and with a third order intermodulation spec of -78dBc. Tek's predecessor analyzers only had a dynamic range spec of -74dBc, so this is a substantial (>2X) improvement.

A trigger output also lets other instruments, such as oscilloscopes or logic analyzers, be triggered from events such as frequency-domain errors.

The ability to couple lab gear should go a long way to ensure a system-wide view of events, where you can diagnose the source of something such as a modulation error caused by a related time-domain event, such as a logic hit.

Modulation analysis
In addition to its versatile triggering, the RSA3408A includes built-in IEEE-802.11a/b/g WLAN analysis. Similarly, it's ready for RF identification (RFID) analysis, where it can analyze 2ASK (amplitude shift keyed) signals, and 2FSK (frequency shift keyed) modulation, with decoding for popular RFID formats.

Beyond that, the box also handles 128QAM and OQPSK quadrature modulation. Modulation displays can include constellation diagrams, I/Q level depictions, symbol tables, and eye diagrams, to name a few.

Multiple color displays let you observe spectra before interference occurs, for example, as well as during an interference event. You can see things such as a synchronized WLAN constellation, and then watch it go out of sync when an interfering RF signal comes on. For radar and pulsed signals you can see time vs. power views, as well as tables of pulse characteristics.

Finally, Tek has equipped this box with a 10/100 Base-T Ethernet LAN interface, two USB (Universal Serial Bus) v2.0 ports, and a VGA-compatible output jack. That should be more than enough connectivity to embed this instrument in your design and test environment.

- Alex Mendelsohn

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