Scope Junction

Scope Junction is a now-defunct United Business Media (UBM) website devoted to OSCILLOSCOPES. Here is an article, titled "THE MATURE SCOPE NEOPHYTE", that I wrote that was published on the site. You can contact me at: dgary99@yahoo.com


THE MATURE SCOPE NEOPHYTE

A few weeks ago, Dave Pereles of Tektronix (this site’s sponsor) presented a workshop on how to purchase a scope. I felt a lot like Rip Van Winkle waking up from a very long sleep. The devices have certainly changed in the years since I last used one as a physics undergrad. Back then, scopes were based on cathode ray tubes (CRTs) and responded in real-time to the electrical impulses presented to them.
The most common type of scope available today is the digital storage oscilloscope. What most amazed my newbie sensibilities was the fact that, strictly speaking, we were not viewing a real-time signal deposited on the grid of a CRT and reproduced right then and there on the screen. These devices are actually more like digital signal processing units.
The input is sampled, and an A/D converter stores the analog value of the measurement as a digital value in memory. After many of these points are stored, the waveform is reproduced via mathematical algorithms on the oscilloscope’s display from the sampled points in memory. What we are viewing is a reconstruction of the electrical event from stored memory — reproduced so quickly that it seems immediate to our human senses, though it lacks the true immediacy of an analog scope.
This immediately brings to mind the most important point to consider when buying a scope: the sampling rate, or how often the A/D converter samples the input signal. The Nyquist Theorem requires sampling to be done at least twice for each cycle of the highest frequency that needs to be observed. If one needs to observe a 20MHz signal, the sampling rate must be 40MS/s at the very bare minimum. A rate any slower would produce a phenomenon called aliasing, where an incorrect value for the input will be calculated and displayed as truth to the viewer.
It is also important to remember that, in less expensive scopes, the sampling capability is often shared by as many as four digital inputs.
Another key specification to consider is bandwidth. The oft-stated 3db point is the frequency at which a sinusoidal signal is attenuated to 70.7 percent of its true value. If representation of amplitude to within 2 percent is important, it is important to purchase a scope whose bandwidth is five times the frequency of interest.
A similar rule of thumb is that, when studying digital signals, a risetime five times faster than the signal is recommended. If the signal’s risetime is 4ns, the scope’s must be 800ps (though Michael Dunn criticized these recommendations last week).
A mixed-signal scope does everything a DSO does, and it has inputs for measuring purely digital values. This device often comes with byte-sized arrays (no pun intended) of eight, 16, or more inputs. These inputs can be important factors in triggering. At what point will the scope start displaying inputs? It can trigger on a signal’s edge (edge triggering) or any boolean combination of the digital inputs. Some scopes can trigger on a specific character in an RS232 or USB 2.0 stream observed on an analog input. The more expensive scopes can also work with more esoteric protocols.
Another scope type is what Tek calls the digital phosphor oscilloscope (which Ransom Stephens discussed in a post last month). Though its architecture is quite similar to that of the lower-end DSOs, this device brings back memories of my beloved CRT scopes of yore. Less frequently observed signal trajectories are reproduced dimly on the screen, and more frequently followed ones are brighter.
Everything described so far is common, in degrees, to all major scope manufacturers, but there is one scope type that is still largely a Tektronix exclusive: the mixed domain oscilloscope. This device does everything a regular scope does, but it also has an RF input with spectrum analyzer functionality. The user can set a cursor to a given point in the time domain. In another window, the device will display the RF signal in the frequency domain, in a manner very much like a spectrum analyzer.
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