Whether an oscilloscope is new, used or refurbished, its performance properties directly determine what measurements it can provide and how accurate they will be. This article will discuss six basic and important characteristics, so you know what aspects to look out for.
An oscilloscope’s bandwidth, denoted in the unit Hertz, limits the frequency range it can operate in with reasonable accuracy. Signals have to fall within its range for the scope to render an accurate image. A signal exceeding the instrument’s bandwidth will lose too much detail to allow for useful measurements as the amplitude, edges and waveform details may all be distorted. Scaling the signal input through an attenuator effectively extends its bandwidth.
It may seem obvious, but the number and types of independent inputs available on the instrument really are essential to the kind of measurements it can perform. Oscilloscope can have as few as two or as many as twenty channels, so determine first how many you will need to find the most cost-effective solution for your project. Also consider whether only analog channels will be enough, or whether you will need both analog and digital channels to process your data comprehensively. For the latter case, mixed signal oscilloscopes (MSOs) are the ideal solution.
The resolution of the waveform displayed by your oscilloscope is determined by its sample rate, expressed in samples per second. Basically, it means how many samples, snapshots or images, the scope can acquire within one time increment. As a general guide line, the sample rate should exceed 250% of the scope’s bandwidth, ideally even 300% or more, to ensure a smooth and effective sampling process.
When comparing oscilloscopes’ specification sheets, please note that the sample rate listed is usually its maximum capacity, which might not be available when using all channels, but only when limited to one or two inputs. Using more channels at the same time might impact sample rate and slow it down significantly. The image below shows the immense difference sample has on the resulting waveform that can be displayed on the scope.
Oscilloscopes do utilize interpolation between data points, so a larger number of samples increases accuracy and clarity significantly, providing a much more reliable base for troubleshooting, debugging and decision-making.
As the oscilloscope acquires samples, processes them and updates the waveform on the display, there is a small gap before it can tackle the next data point. The update rate specifies how fast the scope can process batches of data and therefore, how long or short this gap is. A lower update rate means more dead time, during which the signal goes unobserved and glitches, errors and other infrequent events might be missed. With a faster update rate, the incoming signal can be monitored with minimal interruption, which makes missing such events much less likely.
As a a practical example, imagine your signal has an error that happens once per fifty thousand cycles. The oscilloscope you’re working with sports hundred thousand waveforms per second as an update rate. At these rates, the display will on average catch the error twice per second. Compare this with a much slower instrument that only has eight hundred waveforms per second as the update rate. All of a sudden you would have to watch the signal for one minute on average to see the glitch even once – the advantages of higher update rates are obvious.
As with sample rates, specification sheets will usually list the maximum update rate of an oscilloscope. These might only be attainable in specific modes of acquisition, which in turn might limit the scopes performance in other areas. Make sure to get a clear idea of the instruments actual performance while displaying at maximum update rates to decided whether its sufficient for your intended measurements.
In a digital scope, after analog input is fed into the ADC and digitized, the resulting data is stored in the built-in memory. How many data points the memory can hold at once is referred to as memory depth. A larger memory depth basically means you can capture longer stretches of the signal without having to overwrite previously gathered data. Memory depth therefore has an impact on the oscilloscope’s sample rate as well. The sampling rate does not remain constant, but slows down as memory fills up. The deeper the memory available, the longer the scope can maintain full speed while sampling. In a nutshell, should your projects require looking at longer signal stretches at once and you need a high resolution of the displayed waveform, a larger memory depth will help you quite a bit.
More memory depth does not only have advantages, though. Deep memory can also slow down the update rate as it demands more of the CPU, effectively resulting in more dead time between captures, the disadvantages of which are explained above. Weigh the scope’s responsiveness against the length of time you will want to observe in order to find the right balance for your project.
Lastly, the connectivity features on Oscilloscopes should not be underestimated as they can save – or cost! – you much time when working with it. It’s worth thinking through your processes first to decide in what instances and what kind of connectivity you need for the scope to fit seamlessly into your everyday work. Do you need additional options for storing data long term, such as an external hard drive? Will you need to share data with colleagues? Do they work in the same space as you do or are you in different physical locations? Do you want to connect external monitors or other items to your oscilloscope for more convenience? Will you need to embed it into existing systems? If you already have an oscilloscope model in mind – does it have the basic connectivity features you will require, such as USB or LAN connectors? Transferring data efficiently is just as important as accurate measurements for your projects to benefit from them long term.
These six aspects were just a quick overview of some basic performance characteristics that are important in every oscilloscope. The list of different models on the market, some with very specialized functions, is long and getting longer. Often, a used Keysight oscilloscope will offer you more bandwidth and faster sample rates at lower price points than a comparable new model, so be sure to check out the Keysight Used Equipment Platform for great savings.
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