An oscilloscope is a device that displays an electrical signal in the form of a graph. The graph in most cases shows how the signal changes over time. Y-axis represents voltage and X-axis represents time. These simple graphs can be used to extract highly useful information about the signals, such as amplitude, frequency, time period, rise time, duty cycle, jitter and much more.
Oscilloscopes can be classified as analogue and digital. Analogue oscilloscopes trace signals but digital oscilloscopes sample signals and construct waveforms on the display. Digital oscilloscopes use an analogue-to-digital converter (ADC) to convert the measured voltage into digital information. They acquire the waveform as a series of samples. These samples are continuously stored in the memory until the oscilloscope has enough samples to describe a waveform. The digital oscilloscope then reproduces the waveform on the screen by using the stored samples.
Digital oscilloscopes are further classified into digital storage oscilloscopes (DSOs), digital phosphor oscilloscopes (DPOs), mixed-signal oscilloscopes (MSOs) and digital sampling oscilloscopes. Here we will discuss only DSOs. DSOs allow you to capture and view transients. Since the signal information exists in digital form as a series of stored digital codes, it can be processed easily by the oscilloscope itself or by an external computer to extract various results from the measurements. The waveform can also be displayed even after the signal has disappeared.
Why do you need an oscilloscope?
A good oscilloscope is an indispensable tool for anyone who is into design, testing or repair of electronic equipment. The oscilloscope helps you visualise the functioning of electronic circuits better. If you are into design, the detailed measurements can help you optimise your design for improved performance. In testing, a good insight into the circuit can be gained very quickly using an oscilloscope. Repairing becomes easy and quick with a tool to measure accurately at various testpoints. These days, engineers need the best tools available to solve their measurement challenges quickly and accurately, and a digital storage oscilloscope is the best tool to do that.
Why is it important to select the right oscilloscope?
Accurate signal capture is of utmost importance for test and measurement equipment as all your decisions rely on the measured values. You can have correct measurement only when your oscilloscope is compatible with the range of signals you are trying to measure. That is why it is important to have a pretty good idea about the signals that you are going to measure before you buy an oscilloscope. Once you know your signals, you need to understand specifications in the datasheet to select an oscilloscope that will accurately measure all your signals. Also consider its upgradeability to meet your future needs.
While looking at technical specifications, check whether the values are guaranteed or typical. You cannot completely rely on the typical value of any specification as it will give you only an idea of the oscilloscope’s performance. To have the measurement that complies with various quality standards, you need guaranteed specifications.
Technical specifications mentioned in a datasheet are usually difficult to understand. For easy understanding, we can divide specifications into three groups: measurement accuracy, measurement convenience and construction.
Specifications governing measurement accuracy
First, ascertain the signals that you are going to measure. To make sure that the oscilloscope measures all your signals accurately, the below-mentioned specifications of the oscilloscope need to comply with your signals’ range.
Bandwidth. Bandwidth indicates an oscilloscope’s fundamental ability to measure a signal. It determines the maximum frequency signal that the oscilloscope can accurately measure. Accuracy decreases with increase in the signal’s frequency. Bandwidth mentioned in the datasheet (say, 100 MHz) is actually the frequency at which a sinusoidal input is attenuated to 70.7 per cent of its true amplitude. Beyond this frequency the oscilloscope cannot support reasonable accuracy.
The oscilloscope must have suitable bandwidth to accurately measure your signals. To decide how much bandwidth you need, find out the range of frequencies you would need to measure, then multiply the maximum frequency by ‘five.’ For example, to measure a signal with 10MHz frequency accurately, you need an oscilloscope with 50MHz bandwidth. Otherwise, high-frequency changes will not be resolved and the amplitude will be distorted.
In case of digital signals, it is important to capture the fundamental, third and fifth harmonics to display accurate results (as shown in the figure). Therefore the bandwidth of the scope, together with the probe, should be at least five times the frequency of the measured signal.
Rise time. Analogue engineers find bandwidth more interesting, but for a digital engineer rise time is critical. Carefully consider rise time specifications if you will be working with digital signals. Choose an oscilloscope with sufficient rise time to accurately capture the details of fast transitions. An oscilloscope with faster rise time will more accurately capture critical details of a fast transitioning signal.
As a thumb rule, similar to bandwidth, the rise time of the oscilloscope should be less than one-fifth of the fastest rise time of the signal to accurately measure it. For example, for a 5ns rise-time signal, an oscilloscope with less than 1ns rise time is required.