Across the digital divide: Working with analog RF signals
Martin Rowe, Senior Technical Editor m.rowe@tmworld.com -- Test & Measurement World, 11/1/2006 2:00:00 AM
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Serial buses run through ICs, connectors, cables, and backplanes at speeds of 10 Gbps. Digital engineers must characterize these devices for impedance, crosstalk, reflections, and return loss. For many engineers, the transition into the analog RF world started by crossing a language barrier.
Mike Resso of Agilent Technologies made the transition, but needed help from microwave engineers. “In school, you decide if you're going to be a digital or analog engineer,” said Resso, “I studied digital and, until recently, didn't need to deal with analog. The microwave engineers helped me understand it, but only after we bridged a language gap.” What Resso called “crosstalk,” they called “isolation.” He had to learn to use a vector-network analyzer (VNA) because his time-domain reflectometer (TDR) lacked the bandwidth and dynamic range he needed.
Microwave engineers have to cross the chasm the other way. They often use two-port analyzers for single-ended devices. Now, they have to use four ports to characterize the differential pairs in a serial bus.
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VNAs now contain as many as 18 ports for characterizing differential signal paths. Courtesy of Anritsu. |
To characterize a differential-pair signal path, a VNA must excite one port side at a time and calculate the pair's impedance, loss, reflections, and crosstalk. “VNAs don't have differential drive,” said Tom Brinkoetter, product manager at Anritsu. “You need at least a four-port VNA to characterize one differential pair.” VNA manufacturers have responded by offering instruments with as many as 18 ports.
Unfortunately, you have to calibrate your VNA test setup every day. “Calibrating a 16-port setup can take an entire morning,” noted Brinkoetter.
VNA manufacturers have also simplified test setup and calibration. VNAs now have tools to streamline setups and calculations. Anritsu, for example, embeds Matlab into its VNAs so you can calculate S-parameters inside the instrument. Rohde & Schwarz uses wizards to help you with calibration and test setup, and the company's VNAs can import component models from popular design tools. “You can adjust the design model right in the instrument,” said R&S applications engineer Greg Bonaguide.
At microwave frequencies, you must compensate for the effects of your test setup when making VNA measurements. You can use the instrument to characterize your test setup, then import the design model and mathematically remove the effects of the test fixture. You can also adjust the design model in the instrument and plot the expected results.
Resso switched to a VNA from a TDR because of the VNA's wider bandwidth and lower noise floor. The bandwidth issue is changing because Tektronix has announced an oscilloscope with a TDR function that covers serial data streams up to 12.5 Gbps. “You don't need a –80-dB to –100-dB noise floor,” said product manager James Roth. “Digital designers need noise floors of –10 dB to –40 dB, and a TDR is within that range.”
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