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Tektronix MDO4104-3 Mixed Domain Oscilloscope

A digital scope with a spectrum analyser!

By Nicholas Vinen

Tektronix must think that they’ve come up with a new type of test instrument since they’ve invented a name for it: “mixed domain oscilloscope”. We reckon they’re probably right. Digital storage oscilloscopes (DSOs) and spectrum analysers have both been around for yonks but while they’re individually useful, this unit can do some things that they can’t do by themselves.

So what do they mean by “mixed domain”? It may help to think back to high-school mathematics. If you weren’t too busy making paper planes or programming games into your graphing calculator, you may remember that when a function is plotted on a graph, the x-axis is called the “domain” and the y-axis the “range”.

For the classic oscilloscope display, the x-axis is time and the y-axis is voltage (or current). Hence these scopes operate within the “time domain”. Similarly, a spectrum analyser plots frequency on the x-axis and power on the y-axis. So we can say that a spectrum analyser operates with a “frequency domain”.

So a mixed domain oscilloscope can display data in either or both forms. We should point out that you can view the same signal either way, eg, as a plot of voltage-versus-time or power-versus-frequency. Each view is useful for different purposes; a spectrum is invaluable for analysing a radio frequency (RF) signal but is not so useful for debugging a serial bus!

It would have been tempting for Tektronix to just shoe-horn two instruments into one box and call it something new. That is definitely not what they have done though. Clearly, a lot of effort has gone into integrating the two and the result is a device which allows you to capture and analyse data in ways that were not possible before.

The power of mixed domains

A digital spectrum analyser typically samples the signal at a high rate for some period (say 1ms), then converts the captured data to the frequency domain using a mathematical transform (eg, a fast Fourier transform or FFT). The display then shows the signal frequencies present during the capture period. For better frequency resolution (“resolution bandwidth”), a longer sampling period is necessary, to acquire more data for analysis.

Click for larger image
Fig.1: both analog and digital channels are enabled here, as well as the spectrum display. You can see the serial commands between the controller IC and the voltage controlled oscillator (VCO). This also demonstrates the incredible capture bandwidth available as we can observe the output shifting from 900MHz to 2.4GHz without having to re-sample the data (orange trace shows frequency).

If data is captured over a longer period than merely necessary for the analysis, it is possible to “slide the window” (ie, the portion of data being analysed) within this period. This results in a series of spectrum plots, showing how the frequencies present in the signal shift over time. This can then be correlated with the time domain data captured by the oscilloscope portion of the instrument, so that the operation of the RF control circuitry can be observed simultaneously with the RF output.

By this point, you should be starting to get an idea of what this device is capable of.

In practice, the data for time and frequency domain analyses are stored separately. For the regular scope functions (ie, time domain), a generous 20Mpoints of storage is available. The spectrum analyser can capture an astounding one gigapoint (ie, one billion points). That corresponds to 2.5 milliseconds of signal when the spectrum analysis window has maximum span (>2GHz) and longer for smaller spans, to a maximum of 79ms (span of <125MHz).

As well as allowing for a large “sliding window”, this also gives you a lot of capture bandwidth. This is the difference between the lowest and highest frequencies which can be displayed simultaneously. So you can, for example, monitor the RF output of a circuit at 900MHz and 2.4GHz simultaneously (see Fig.1) or even 2.4GHz and 5.6GHz (with the 6GHz model). Since many digital wireless devices can operate on multiple frequencies, this can be handy.

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