IC technology allows waveform access at previously inaccessible internal points
By Dan Strassberg, Contributing Technical Editor -- EDN, February 4, 2008
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Several of the latest high-end digital oscilloscopes incorporate facilities for reconstructing waveforms that probes can’t reach. However, Vitesse Semiconductor says that such de-embedding is no better than the device models it uses and that many of today’s models are inaccurate. The company instead expects the situation to only get worse because the DSP techniques that scopes use for signal reconstruction are based on linear-circuit models and new high-performance devices increasingly require nonlinear models to reconstruct inaccessible signals with sufficient accuracy. To solve the problem, Vitesse has announced the VScope technology, which builds into certain new ICs circuits that can measure the inaccessible signals. It also provides the hardware with a simple means of controlling the measurements and getting the results from the IC through its pins and—both to control cost and to maximize compatibility with older devices—avoids the need to add pins. VScope also ensures that the measurement circuits have negligible impact on chip area and, hence, on chip cost and makes it possible to disable the measurement circuits so that they consume power only when measuring. It also provides a software package with which users can select the measurements they need and display the results in easy-to-understand formats.
The company believes that the technology, which customers can implement for far less than the $100,000-plus cost of a high-end scope, can displace scopes in many—but not all—signal-integrity applications. Company representatives also describe as negligible the recurring cost of having the measurement circuits in every VScope IC.
At the heart of VScope is a scanner within the CDR (clock- and data-recovery) circuits. The scanner’s strobe point is adjustable in time in small increments over at least a full UI (unit interval), and its threshold level is adjustable in voltage in similarly small increments over somewhat more than the entire normal input-signal swing. Each scanner comprises two identical circuits, either of which you typically might assign to monitoring a reference point—usually at the center of an open eye diagram. You can control the placement of either circuit’s strobe time and threshold using a bus that Vitesse describes as similar to I2C (inter-integrated circuit).
You obtain a VScope measurement set by changing the strobe time and threshold voltage typically 216 times. The actual number of individual measurements is up to you, however. A satisfactory VScope hardware implementation allows increments of 1/64 of the time and voltage ranges, but smaller increments are possible, Nevertheless, Vitesse does not currently envision increments fine enough to replicate oscilloscope resolution.
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In reconstructing an eye diagram, you might categorize the state of each time/voltage point as always a logic one, always a logic zero, one and zero approximately equal fractions of the time (say, no more than 60% of individual measurements in either state), one slightly more often than zero (say, more than 60% but less than 95% of individual measurements), zero slightly more often than one, one considerably more often than zero (say, more than 95% but less than 100% of individual measurements), and zero considerably more often than one.
Vitesse points to the value of VScope in tuning filters in adaptive equalization, such as those for ultra-high-speed serial-bus signals in backplanes and cables. These filters, which are common in backplane transceivers, pre- and de-emphasize higher-frequency components of signals whose data rates now often extend beyond 10 Gbps. In the absence of equalization, these signals’ eye diagrams can be completely closed. In other words, in the time domain, the signals can be indistinguishable from white noise.
The first VScope IC, the VSC3406, a six-channel, 6.5-Gbps multirate backplane transceiver in a 10×10-mm package, costs $60 (1000/year).
