Basic Oscilloscope Operation

A student just learning to use oscilloscopes connects one directly to the output of a signal generator, with these results:

As you can see, the function generator is configured to output a square wave, but the oscilloscope does not register a square wave. Perplexed, the student takes the function generator to a different oscilloscope. At the second oscilloscope, the student sees a proper square wave on the screen:

It is then that the student realizes the first oscilloscope has its “coupling” control set to AC, while the second oscilloscope was set to DC. Now the student is really confused! The signal is obviously AC, as it oscillates above and below the centerline of the screen, but yet the “DC” setting appears to give the most accurate results: a true-to-form square wave.

How would you explain what is happening to this student, and also describe the appropriate uses of the ÄC” and “DC” coupling settings so he or she knows better how to use it in the future?

There are times when you need to use an oscilloscope to measure a differential voltage that also has a significant common-mode voltage: an application where you cannot connect the oscilloscope’s ground lead to either point of contact. One application is measuring the voltage pulses on an RS-485 digital communications network, where neither conductor in the two-wire cable is at ground potential, and where connecting either wire to ground (via the oscilloscope’s ground clip) may cause problems:

One solution to this problem is to use both probes of a dual-trace oscilloscope, and set it up for differential measurement. In this mode, only one waveform will be shown on the screen, even though two probes are being used. No ground clips need be connected to the circuit under test, and the waveform shown will be indicative of the voltage between the two probe tips.

Describe how a typical oscilloscope may be set up to perform differential voltage measurement. Be sure to include descriptions of all knob and button settings (with reference to the oscilloscope shown in this question):

A very common accessory for oscilloscopes is a ×10 probe, which effectively acts as a 10:1 voltage divider for any measured signals. Thus, an oscilloscope showing a waveform with a peak-to-peak amplitude of 4 divisions, with a vertical sensitivity setting of 1 volt per division, using a ×10 probe, would actually be measuring a signal of 40 volts peak-peak:

Obviously, one use for a ×10 probe is measuring voltages beyond the normal range of an oscilloscope. However, there is another application that is less obvious, and it regards the input impedance of the oscilloscope. A ×10 probe gives the oscilloscope 10 times more input impedance (as seen from the probe tip to ground). Typically this means an input impedance of 10 MΩ (with the ×10 probe) rather than 1 MΩ (with a normal 1:1 probe). Identify an application where this feature could be useful.