DSOs break speed barriers in dramatically different ways
By Dan Strassberg -- EDN, January 10, 2002
Two test-and-measurement competitors, Tektronix and Agilent, are kicking off the new year with major digital-oscilloscope introductions. Tektronix's less-than-$60,000 TDS 6604 offers 6-GHz bandwidth on four channels, a two-channel-mode sampling rate of 20G samples/sec/channel, and a two-channel-mode memory depth of 250k samples/channel. The scope owes its bandwidth, which is unprecedented in a real-time-sampling instrument, to the use of silicon-germanium IC technology.
Meanwhile, thanks to long-awaited additions, Agilent's Infiniium family finally combines wide bandwidth with deep memory (to 8M samples/channel or 16M samples/channel when you interleave pairs of channels). The 54830B series includes four-channel units with bandwidths of 600 MHz and 1 GHz and two-channel units with bandwidth of 600-MHz. The maximum sampling rate of all family members is 4G samples/sec with channels interleaved and 2G samples/sec without interleaving (Picture 1).
Scopes from Tektronix and LeCroy (www.lecroy.com) have for years combined deep memory with wide bandwidth, but Agilent's prices, which start at $12,995, are attractive. Moreover, since the family's introduction several years ago, Infiniium scopes have offered a Windows-based user interface that many consider to be the industry's most user-friendly. In addition, the new units provide unprecedented speed of response when you use them in the way that scope users typically employ DSOs.
With a capture rate of more than 7000 waveforms/sec, the 54830B-series scopes acquire fewer waveforms per second than do Tektronix's DPOs (digital-phosphor oscilloscopes), many of which can capture more than 100,000 waveforms/sec. However, to achieve high-speed capture with the Tektronix scopes, you must operate them in the special DPO mode that creates a 3-D pixel map in which the "height" dimension, which the display usually translates to color grading, indicates the frequency of occurrence of each pixel on the screen. The pixel map discards what often turns out to be vitally important information. On the other hand, even when you trigger the Infiniium units at top speed, you still can view every sample of a complex waveform in its true time sequence. To preserve equivalent detail on Tektronix's DPOs, you must operate the scopes as standard DSOs. In that mode, their highest trigger rate is a fraction of that of the Agilent units.
The 54830B scopes achieve their fast screen updates through a major upgrade of the company's MegaZoom technology, which was previously available only in the lower bandwidth 54600 series. MegaZoom provides fast response by moving many of the bit manipulations away from the scope's main processor and into a very-high-speed ASIC.
Tektronix calls the TDS 6604 a DSO—not a DPO (Picture 2). Despite its unprecedented bandwidth, the scope operates as a conventional DSO rather than in the DPO mode. For many engineers, the TDS 6604 will make sequential-equivalent-time-sampling scopes a thing of the past. Although they can capture 50-GHz signals, sequential-sampling scopes can be frustrating to use. Because they capture just one waveform point per signal iteration—and, hence, one point per trigger—their response can be painfully slow. This slow response better suits sequential-sampling scopes to device characterization than to troubleshooting.
The TDS 6604's 6-GHz bandwidth not only is higher than that of any other real-time scope, but also is simultaneously available on all four channels. Moreover, when you operate the TDS 6604 in the two-channel mode, the scope can take 20G samples/sec on both active channels. The four-channel mode's 10G-sample/sec acquisition rate samples 6-GHz signals a bit sparsely, however. If sparse sampling becomes a problem and the signals are repetitive, you can increase the effective sampling rate by operating the scope in the random-repetitive-sampling mode, which does not exhibit sequential sampling's slow response. By comparison, Tektronix's CSA 7404 delivers 4-GHz bandwidth—two-thirds that of the TDS 6604—but if you want it to take 20G samples/sec, you must operate the scope in the single-channel mode; the CSA 7404's maximum sampling rate on two channels is 10G samples/sec.
Besides the scopes themselves, both Agilent and Tektronix simultaneously announced a large number of accessories, such as wideband probes, which scope users must have to make measurements on the wide-bandwidth fast-rise-time signals the new scopes are intended to capture.
Agilent Technologies, 1-800-452-484, www.agilent.com.
Tektronix Inc, 1-800-426-2200, www.tektronix.com.
