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Instruments combine RF, digital test

Posted: 17 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:RF digital test? digital bus structure? cross-cultural approach to test?

Traditionally, digital and RF testing have been two different cultures. A substantial gap in the frequency domains of interest led to different test approaches and instrumentation requirements. Now, changes in both RF communications and digital bus structures are creating the need for cross-cultural approaches to testing, and instrument developers are responding.

In the RF world, growth in software-defined radio and DSP technology has brought digital test needs from the realm of baseband operation into the IF stage and parts of the carrier frequency range.

Furthermore, the adoption of frequency-hopping, bursted signals and adaptive modulation is moving RF testing beyond the range of traditional instruments that anticipate relatively steady-state conditions. Because signals change rapidly, the time domain can no longer be ignored in the RF world.

The need for time-based analysis in RF testing is particularly acute for radio-based communication systems such as Wi-Fi and wireless USB. Tight regulatory restrictions on channel frequency, bandwidth and output power spectrum for the radios in these systems require the use of frequency-domain testing. But adaptive frequency-hopping and modulations are best observed in the time domain. As a result, test equipment for such systems must blend both approaches.

Fortunately, advances in the digital world are prompting a technology push that is bringing the time domain into RF frequency ranges. The adoption of high-speed serial buses, for instance, is creating a need for high-speed digital test capabilities. Bus structures such as PCI Express are currently using frequencies of 2.5GHz, but are quickly moving to second- and third-generation interfaces at 5GHz and 10GHz. These frequencies impose an even greater speed requirement on the test equipment. A sampling oscilloscope, for instance, must offer a sampling clock rate of 5GHz to simply capture a 2.5GHz PCIe signal. To observe signal integrity, the instrument must also be able to capture third and fifth harmonics, stretching sample rate requirements to 12GHz and above.

Intersecting worlds
The digital and RF worlds begin to intersect at such frequencies, resulting in digital tools and techniques creating a presence in RF testing. This is happening in part because some tools that use time-domain techniques also offer traditional frequency-domain capabilities via FFT and other DSP techniques. Such cross-cultural tools include digital oscilloscopes that can work as spectrum analyzers, time-domain reflectometers that can double as network analyzers, and high-speed arbitrary waveform generators that can operate at RF frequencies.

Pushed by advanced modulation and DSP technologies, digital design approaches have been encroaching on traditional RF applications, moving from baseband (a) into the IF (b) and beyond.

Tektronix Inc. has been a prominent participant in this cross-cultural area with several instrument announcements during the past several quarters, including a spectrum analyzer, an oscilloscope and an arbitrary waveform generator. All are capable of handling signals at frequencies of at least 5GHz and represent a blend of digital and RF test techniques. The RSA6100A family of spectrum analyzers, for instance, uses digital sampling and FFTs to offer RF spectral analysis to 14GHz that captures short-duration signals that traditional swept-frequency approaches can miss. The AWG7000 arbitrary waveform generator can create digital RF signals of up to 5GHz for broadband applications, such as radar system testing.

The RSA6100A spectrum analyzers uses digital sampling and FFTs to offer RF spectral analysis to 14GHz.

The rise of high-speed serial buses is also creating opportunities for the design of new digital RF test gear. The PXI Systems Alliance has adopted the PCIe bus structure, which allows the streaming of digital data at many gigahertz. National Instruments Corp., for one, is working with Phase Matrix Inc. and BAE Systems to develop PXI Express synthetic instruments that can handle RF signals of up to 26.5GHz.

There is crossover in the other direction as well. Increasingly, high-speed digital signaling is facing concerns that traditionally belonged in the RF space. One such concern is the transmission-line nature of serial bus signal paths. Small errors in impedance matching can manifest as serious error rates, but the cause can be difficult to pin down in the time domain. Network analysis techniques from the RF world can help.

The rise of time-domain techniques to solve RF design issues has been subtle but is continuing unabated and gaining momentum. Most recently, Tektronix has developed software for its arbitrary waveform generators that directly target RF applications such as WiMedia.

Of course, the highest frequencies in RF will remain the domain of traditional equipment for now. Instruments such as the recently announced Rohde & Schwarz ZVA40 network analyzer, which works up to 40GHz, are beyond what can be accomplished using digital techniques. But that distinction is slowly eroding.

- Richard Quinnell
EE Times

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