Tektronix recently let me test their MDO4000 mixed-domain scope. I talked to them about it two months ago. I thought they forgot about it, but after the winter holidays a brand-new-looking MDO4014-3 arrived at my company with no letter or explanation. 
The MDO4000 series is mixed-domain because it has a spectrum analyzer input in addition to the normal time-domain inputs. You can trigger off any of the inputs or spectrum analyzer. From a triggering standpoint, the spectrum analyzer is another trace on the scope.
The lack of a zero-span turns out to be the most amazing thing about the spectrum analyzer. I attempted to enter a span of zero, and it gave me 1kHz. At first I thought thatu like the VIA Echo it simply lacked zero-span. It turns it doesn’t need it because you can view amplitude in the time domain and scroll along a window in the time-domain and see the entire spectrum during that window.
I put an antenna on it and monitored the Wi-Fi signals in my office. Using an older spectrum analyzer, I would look at it with max hold and resolution bandwidth of around 1 MHz. Eventually the sweep would cover all segments of the 20-MHz-wide Wi-Fi signal, and I’d get an idea what its spectrum looks like. To get time-domain info, I would set the analyzer to zero-span and set the resolution bandwidth (RBW) to maximum. If max RBW is 5 MHz but I know the signal I’m looking for is 20 MHz wide, I’d know my time-domain results will be 6dB down (i.e. 25%) of the actual signal strength. With the MDO4000, however, I can see the time-domain representation of signal strength in the top window. I can stop triggering and then scroll the window of time period whose spectrum I want to look at.
Look at this plot taken during a Wi-Fi packet. Note the tiny indicator below the time-domain plot showing the time period the spectrum analyzer is analyzing.
Now look below at that the spectrum when I’ve scrolled the time period to an area between Wi-Fi packets. (The little dot is under a segment of the time domain with low signal strength.)
You can get this info in the spectrogram mode too. This clearly works in my testing, but I felt like I got a much better feel for the spectrum when I scrolled manually in the time domain and watch the spectrum change.
I still found myself using peak hold. When I had it in free running mode, I could see that it didn’t capture the whole spectrum of brief packets. The peak hold is a different color, making it easy to distinguish peak from current values.
At one point I put the same signal, a sloppy square wave, into a time-domain port and the spectrum analyzer port. You can see the square wave and its frequency-domain harmonics.
I tired to make it alias on a 1MHz signal by setting the sweep rate way to low. Only a narrow range of settings, far from the correct sweep rate, resulted in aliasing. You can see it aliasing below.
Look at this noisy 1 MHz signal.
When I turn the band limit feature on, it looks the same as on my old analog scope.
When you’ve stopped triggering, you can zoom in and out on the waveform and scroll left and right using the large Wave Inspector knob. At first I thought this was a gimmick for people who don’t know to turn the sweep rate and x-position controls. It turns out to be an easier way to move around. The only complaint I had is that this control is larger than the sweep rate control.
It comes with PC software to control the unit and act as a display, but in my brief testing I found it no better than using the UI. This is in complete contrast to VIA Echo spectrum analyzer, which has a primitive UI on the unit and has PC software that displays much better detail.
The only negative thing about this scope is sometimes lags behind user actions by what felt like more than a second. It’s usually not noticeable, but sometimes it was confusing because it felt like turning a knob had no effect until a second later the scope responds to a knob adjustment and two keystrokes.
Conclusion
Every employer and client working for whom I’ve worked on RF projects has had an HP spectrum analyzer from the 80s. Using MDO4000 is a completely different experience. Moderns spectrum analyzers have the features like a different color for peak hold, but this is the only scope I’ve used that lets you trigger on a time domain signal and look at the spectrum.
The most basic version, 100MHz time-domain / 3GHz spectrum analyzer, sells for under $12,000. This is not that much more than a decent spectrum analyzer plus a decent scope. If you need a scope and a spectrum analyzer, it’s definitely worthwhile to get an MDO4000 series scope.
(Note: I tested the scope and wrote this article before I knew that Newark carried it. I have only talked to Tektronix about it as described at the beginning of the post.)
