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Bench, handheld, or PC-based?
You can still find the old model of oscilloscope, with its dials, knobs, switches, and a cathode ray tube (CRT). In fact, professional electronics field technicians still prefer bench CRT scopes. But these days, you can choose from several types, and each type has its advantages and disadvantages. We take a moment to review them in the following sections.
The bench oscilloscope, like the one in Figure 10-7, provides a bright and clear image on its glowing green screen. Even on the latest models, you can get your hands on all the basic functions and controls with front-panel switches and dials. This setup allows you to quickly select the operating modes of the scope without meandering through a series of on-screen programming menus.
If youíre looking for a used oscilloscope, youíre most likely to find the bench variety. Theyíve been making them for decades, but the newer models include computer interfaces and other advanced features.
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218 Part IV: Getting Your Hands Dirty
Some enhanced features you should know about
Oscilloscopes have improved greatly over the years, with many added features and capabilities. Although you don't absolutely need any of the following features for routine testing, you may find them handy as you gain experience. Among the most useful features are
^ Delayed sweep: When analyzing a small portion of a long, complex signal, this feature helps because you can zoom into just a portion of the signal and examine it. This is ideal when you work with television signals.
^ Digital storage:This feature records signals in computerized memory for later recall. After you have it in the memory, you can expand the signal and analyze specific portions; again, helpful in television work. Digital storage also lets you compare signals, even if you take the measurements at different times.
As you may expect, these features can add to the cost of the scope. Balance the extra cost against the usefulness of the features.
For on-the-go work, nothing beats a portable handheld oscilloscope. Looking a bit like a Star Trek tri-corder, the handheld scope provides all the basic functions of a scope, but in a battery-operated, palm-sized tool. The screen is a liquid crystal display, and although smaller than the screen on the average bench scope, itís still functional and readable.
You may find handheld oscilloscopes very handy (no pun intended!) if youíre on the go, but they donít usually offer all the advanced features of the better bench oscilloscopes. If you need the latest and greatest, rely on a handheld model only when portability is a must.
The PC-based oscilloscope doesnít have a screen of its own. Instead, it uses your personal computer (either desktop or laptop) to store and display the electrical signals that you measure. Most PC-based scopes are self-contained in a small, external module. The scope connects to the desktop PC or laptop through a parallel, serial, or USB port. A few manufacturers have designed PC-based oscilloscopes that you can install inside your computer. These plug into one of the available expansion slots inside the chassis of your desktop PC.
Many PC-based oscilloscopes cost less than a similarly equipped bench scope. They also take advantage of the features built into your PC, such as disk storage and printing. Obviously, the PC-based scope has one big disadvantage: You have to have access to a PC, either permanently plopped on your electronics bench or temporarily brought in (as with a laptop) when you need it.
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 219
Understanding oscilloscope bandwidth and resolution
You should know about a couple of notable specifications for oscilloscopes. One of the most important specs is bandwidth. Bandwidth is the highest frequency signal that you can reliably test with your oscilloscope, measured in megahertz (MHz). PC-based scope probes tend to have the lowest bandwidth, usually about 5-10 MHz. This bandwidth works just fine for many tasks, including working with hobby circuits and even servicing VCRs and audio equipment.
The average bandwidth of a low-cost bench scope falls in the 20-35 MHz range. This range does the job for all but the most demanding applications. Specialized troubleshooting and repair, such as work on computers and ultrahigh-frequency radio gear, may require bandwidths exceeding 100 MHz. But remember that the price of an oscilloscope goes up considerably as the bandwidth gets higher.
Another important specification is resolution. The resolution of the scope has to do with its accuracy. The X (horizontal) axis on an oscilloscope displays time, and the Y (vertical) axis displays voltage. The horizontal amplifier indicates the X-axis resolution. Most scopes generally have a resolution of 0.5 microseconds (millionths of a second) or faster. You can adjust the sweep time so that you can test signal events that occur over a longer time period, usually as long as a half a second to a second. Note that the screen can display signal events faster than 0.5 microseconds, but such a small signal may appear as a fleeting glitch or voltage spike.