Oscilloscopes are electronic test instruments that measure the amplitude of signals as they change over time. Operating up to GHz (GigaHertz) frequencies, an Oscilloscope measures voltages in electronics that are changing rapidly from components like microprocessors, FPGAs, and simpler components like amplifiers. Mixed Signal Oscilloscopes measure digital signals as well as analog signals for easier debugging of complex electronics.
The most important feature of an Oscilloscope is its bandwidth. As signals go higher in frequency they are more difficult and therefore more expensive to measure. The bandwidth of an Oscilloscope is the max frequency at which about 70% of the signal’s strength can still be measured. This is often called the 3 dB point. Modern Oscilloscopes are said to have bandwidths from 50 MHz up to 1 GHz and beyond.
The second feature to consider is sampling rate. A digital Oscilloscope samples the voltage of the signal millions or billions of times per second to show the engineer as much detail as possible about what is occurring. A good rule of thumb is to sample at least 5 times for every cycle of the frequency you will be measuring. For instance, a 100 MHz signal has a period of 10 nanoseconds. To get good detail, sample that signal 5 times per cycle or every 2 nanoseconds. Oscilloscopes would call this 500 MSa/s (500 MegaSamples per second or 500 million samples per second). Modern Oscilloscopes are capable of sampling billions of times per second.
Number of Channels
Select an Oscilloscope with enough channels for your application. Most Oscilloscopes have either 2 or 4 standard channel. Mixed signal oscilloscopes have 8 or 16 digital channels as well. A digital channel is just one bit meaning it is measured as either HIGH or LOW. A standard channel is measured in voltage. An 8 bit Analog to digital converter is typical, but higher resolution functions or hardware is also available for specific applications.
Memory Depth is also an important consideration. How often the signal is sampled dictates how long information from that signal can be stored and shown on the display or analyzed later. This is expressed in memory depth. A modern Oscilloscope might have 12 million points of memory available. Therefore, if it is set to sample at 1 GSa/s the instrument can store 12 milliseconds of continuous data.
Waveforms Per Second
The other important fundamental characteristics of an Oscilloscope have to do with its display. How many waveforms can it show on the display per second? This is an indication of how quickly it can process all that data it collected. It is also important to usability and ease of use. Lastly, the size of the display is important as the bigger the screen the easier it often is to visualize the data on it.
An Oscilloscope should be able to take measurements and display them on the screen and allow the user to track signals with cursors. It should also have the ability to store images or data for lab reports to a memory stick or a PC over USB. Modern digital Oscilloscopes have additional features that include search and analysis, recording capabilities, serial data decoding, and advanced triggering. Some Oscilloscopes even include function generators.
With proper probes, Oscilloscopes can measure power and current as well. Some Oscilloscopes have advanced data analysis or triggering features enabling decoding of serial data buses, power analysis, and even advanced methods for waveform recording. Modern Oscilloscopes are also easily connected to a computer through USB or an Ethernet connection to enable further data analysis on the PC.