Any engineer working with mixed-signal and/or RF circuitry would be able to relate to this scenario: Suppose you are working on a wireless system design and hoping to ferret out what effects the overall system might be having on your RF power amplifiers. How would you approach getting a handle on the issues?
For Ward Ramsdell, one of the principals of Prototype Engineering LLC, a design consultancy in Hillsboro, Ore., this has traditionally meant wrestling with both a scope and spectrum analyzer. “About 10 years ago, it was a mishmash of swept and real-time FFT analysis,” says Ramsdell. You’d run a trigger out of the scope into the spectrum analyzer, or use a signal generator to trigger both at the same time. “The issue was that the spectrum analyzers didn’t have pre-triggering. How could you take the data collected on both instruments and merge it?”
What options are there, especially if one wants to achieve even rudimentary time correlation of the measurements from the frequency and time domains? Take screenshots from both instruments and overlay them? That’s not much of an option. What Ramsdell would typically settle for, which is basically a hand correlation of the data from Excel spreadsheets, isn’t much better.
Ramsdell first caught wind of a radically different approach coming from Tektronix when he was still an applications engineer at Averna (he’s since left Averna to start his consultancy). “At the time I was working on an audio chip when Tek approached us about giving them feedback on a new class of instrument.” That instrument turned out to be the MDO4000 mixed-domain oscilloscope, which launched on August 30 to great fanfare. The MDO4000 combines a scope with a spectrum analyzer in the same instrument, allowing engineers to capture time-correlated analog, digital, and RF signals in an all-in-one system view (see the figure).
The old approach, according to Ramsdell, often meant more time spent figuring out the test methodology than actually making the necessary measurements. “You started by engineering a test solution rather than looking at the problem,” says Ramsdell. Now, with the merged-instrument approach of the MDO4000, users may find themselves inclined to take more and deeper measurements than they might have with a cobbled-together scope and spectrum analyzer.
Ramsdell cited the example of working on FCC qualification for a wireless product and finding a transient spur in its RF output that violates FCC standards. “In the past, I’d have to try to capture this spur on the spectrum analyzer and then answer a number of questions. How frequently does it happen? How do I correlate the periodicity? What is the spur’s offset from the carrier? Is there a clock in the system that corresponds with that?” All of this amounts to a differential diagnosis of the system in which you concoct a series of tests to prove or disprove a number of causes for the spectral anomaly.
With the MDO4000, it’s a relatively simple matter to look at the digital buses and analog regulators at the same time and examine the spur in relation to several aspects of system operation at once. You can quickly determine, for example, whether there is ringing on a regulator that’s causing the spur. “Rather than spending time coming up with the tests, I can actually look at the system,” says Ramsdell. “I’m still going through the differential diagnosis, but now I’m looking at them concurrently in a related fashion to easily get an overview of what’s happening at any point in time.” For Ramsdell, the instrument provides a much more ready window into system operation that frees him from thinking about how to correlate events in disparate domains to simply take the measurements.
When asked what might be lacking in the MDO4000 as presently constituted, Ramsdell points out that the MDO4000 is only the first in a long line of a new category of instruments. “Right now, the scope is actually gathering more data about the RF signal than it’s giving us access to,” says Ramsdell. “I look forward to more in-depth RF and modulation-domain analysis in future versions.” As an early beta user, Ramsdell has suggested to Tektronix that it might consider providing more advanced RF triggering and displaying more data in the modulation domain. “It’s safe to say that that’s coming, whether in a new instrument or in a future upgrade to this one.”
Prototype Engineering LLC