Tektronix pushes real-time-DSO bandwidth to 7 GHz
By Dan Strassberg -- EDN, October 30, 2003
The maximum bandwidth of real-time-sampling DSOs stood at 6 GHz for more than a year with entrants from LeCroy (www.lecroy.com) and Agilent (www.agilent.com) joining a unit that Tektronix introduced. However, Tek’s 6-GHz unit differed significantly from those of its competitors. With a maximum memory depth of 125k samples/channel (250k samples/channel with one or two channels active), it lacked the deep memory of the competitive units (now as great as 96M samples/channel on one or two channels in certain LeCroy scopes). Those who wanted an ultrawideband, deep-memory DSO from Tek had to settle for a 4-GHz bandwidth unit with single-channel-mode memory as deep as 32M samples/channel.
Now, Tek has changed that picture; at an attractive base price of $59,900, its TDS7704B offers industry-leading 7-GHz bandwidth that combines with single-channel-mode memory as deep as 64M samples. Tek says that these capabilities make the TDS7704B the first real-time scope that is suited to capturing and analyzing 4.25-Gbps data streams. However, compared with the newest 6-GHz-bandwidth scopes from Agilent and LeCroy, which in real time can simultaneously capture four signals at the scopes’ –3-dB frequencies, the TDS7704B has a notable blemish: With more than two channels active, its maximum sampling rate is “only” 10G samples/sec—too slow to capture signals at the scope’s
–3-dB frequency. If you want the scope to simultaneously capture four 7-GHz signals, you have to operate it in the random-repetitive-sampling mode, in which it can’t capture single-shot phenomena.
Also, in specifying the TDS7704B, Tek somewhat clouded a de-facto industry standard—the way in which scope manufacturers specify rise time. Scope rise time used to refer to the time a displayed waveform took to go from 10 to 90% of an input step’s amplitude. Although Tek publishes the new scope’s 10 to 90% rise time (62 psec), prerelease materials emphasize the shorter 20 to 80% rise time (43 psec). From now on, when you compare 6- and 7-GHz-bandwidth scopes, you’ll have to be sure that you are comparing equivalent rise times.
On the other hand, the TDS7704B offers a feature competitors can’t provide: Tek’s patented DPX (digital-phosphor) technology, which allows the capture of as many as 400,000 relatively short waveforms/sec. When it first announced DPX approximately half a decade ago, Tek billed the technology as a way to rapidly uncover waveform anomalies that you could later reacquire and more completely investigate via conventional means. The company and its customers subsequently found several other uses for DPX, however, especially in characterizing jitter, an application that benefits from rapid acquisition of large numbers of waveforms. Tek now refers to the DPX pixel map as a waveform database and offers software tools to aid in analyzing its contents.
Another new feature of the TDS7704B is MultiView Zoom, with which you can zoom in on as many as four segments of a long waveform and zoom in on a waveform that you have derived by zooming in on a segment of a longer waveform. You can repeat the process three times to display the entire waveform and three successively shorter segments, each magnified in time with respect to its “parent.” Yet another new feature, a SiGe (silicon-germanium) trigger IC, which Tek calls the heart of the industry’s first all-SiGe trigger system, reduces the scope’s rms trigger jitter to as little as 1 psec.
Tektronix Inc, 1-800-426-2200, www.tektronix.com.
