Basics of hand-held oscilloscopes

By David Herres

Hand-held oscilloscopes are available in a wide range of prices. For a robust instrument such as the Tektronix THS3024, expect to pay about one-third the cost of a mid-level bench model. Naturally, the screen size, display clarity and range of features including bandwidth are less, but there are some palpable advantages.

Today’s digital bench models, with flat-screen display rather than the ancient cathode-ray tube, are easy to move around, so portability is not the decisive issue. Some bench models work from batteries,  a plus for outdoor CATV, communication and data work, and the latest generation rechargeable batteries have a long life.

In view of these distinctions, it would seem that the hand-held oscilloscope, as opposed to a quality bench model like the Tektronix MDO3104, would be a good thing to have, but most of the advantages would not be decisive.

However, there is another set of considerations that weigh strongly in favor of the thrifty hand-held alternative. These considerations have to do with taking point-to-point or floating-ground readings. In a previous article we mentioned that such readings are hazardous when using a grounded bench-type instrument. The problem is in connecting the probe’s ground lead to a point in the circuitry of the equipment under examination. If that node is referenced to and floats above the electrical ground, there will be a low-impedance fault current path through the ground lead, through the oscilloscope and power cord, and eventually back to the grounding terminal in the building’s service entrance enclosure.

The end result will be smoke and sparks, with the ground lead hopefully burning up first, serving as a fuse to protect the oscilloscope’s inner workings as well as the equipment being examined.

Some electronics personnel make a practice of sawing off the ground prong of the oscilloscope power cord to isolate the instrument from ground and prevent the circumstances outlined above. The problem with this expedient is that the protective function of the oscilloscope’s equipment-grounding connection is defeated, giving rise to other safety (fire and shock) issues.

One way to confront the floating-ground measurement problem in a grounded bench-type oscilloscope is to use a differential probe set. Usually the high-voltage version is needed. The scope sees a signal that is the difference between the two inputs. The differential signal is isolated from ground, and there is no risk of sudden arc flash.

The other safe alternative is to use the hand-held oscilloscope. It is battery powered and thus totally isolated from any utility ground. Point-to-point measurements, floating ground measurements, and inadvertent polarity reversal are not problematic.

Furthermore, in the Tektronix THS3024 hand-held oscilloscope, there are no intra-channel common connections, so any combination of multi-channel measurements is safe provided overall voltage and power limitations are observed.