Choosing an Oscilloscope

There are some important things to consider when you are looking for a new USB, handheld, or benchtop digital storage oscilloscope. First of all, keep in mind that the bandwidth specification of an oscilloscope is the frequency of the negative three decibels point of a sine-wave signal at a specific amplitude, such as one Vpp (peak-to-peak voltage). The measured amplitude decreases as the sine wave’s frequency increases.

The bandwidth of the oscilloscope is the frequency at which this amplitude is negative three decibels lower, meaning a one hundred megahertz oscilloscope will measure a one Vpp sine wave of one hundred megahertz at approximately point seven Vpp, which is an error of roughly thirty percent.

Use this rule of thumb for more precise measurements: BW/5 is equal to about three percent error and BW/3 equals about five percent error, which is to say select at least a three hundred megahertz oscilloscope if the highest frequency you plan to measure is one hundred megahertz. A five hundred megahertz oscilloscope would be a better albeit more expensive option.

You should be aware that most signals these days are square waves, which are created by combining the odd harmonics of the fundamental sine wave, meaning a ten megahertz square wave is the combination of a ten megahertz sine wave, a thirty megahertz sine wave, a fifty megahertz sine wave, and so on. Choose an oscilloscope with a bandwidth of at least the ninth harmonic. If you will be dealing with square waves, select an oscilloscope with a bandwidth that’s at least ten times the square wave’s frequency. If you’re working with one hundred megahertz square waves, you will want an oscilloscope with a bandwidth of one gigahertz or greater.

Because square waves have steep rise and fall times, you need to consider the rise and fall times when choosing an oscilloscope. Calculate the steepest rise/fall time as 0.35/BW for an oscilloscope with a bandwidth lower than two point five gigahertz. Thus, a one hundred megahertz oscilloscope is capable of measuring rise times of up to three point five nanoseconds. Use 0.30/BW for an oscilloscope in the two point five gigahertz to eight gigahertz range. If rise time is your starting point and you need to measure a rise time of one hundred picoseconds, you will want an oscilloscope with a bandwidth of four gigahertz or more.

The next consideration is sample speed. Almost all contemporary oscilloscopes are digital. However, all the previously mentioned processes concern the analog part of an oscilloscope, occurring before the signal reaches the analog-to-digital converter where the signal is digitized.

The previously mentioned bandwidth-to-rise-time calculation is helpful here: a five hundred megahertz oscilloscope has a rise time of seven hundred picoseconds and to reconstruct this you will need at least two sample points on the edge, which is to say at least a sample every three hundred fifty picoseconds, or two point eight gigasamples per second. Choose an oscilloscope with a faster sampling speed. A five gigasamples per second sample speed results in a time resolution of two hundred picoseconds.

Choose an oscilloscope with however many channels you will need. Most oscilloscopes have two or four channels and high-end oscilloscopes almost always have four channels. Thankfully, a four-channel oscilloscope doesn’t cost twice as much its two-channel counterpart, but the number of channels will have a significant impact on the price of the instrument.

Next you will calculate how much memory you’ll need, depending on how much of the signal you want to see in a single acquisition. At five gigasamples per second you sample each two hundred picoseconds, so an oscilloscope with a memory of ten thousand sample points can store two microseconds of the signal and a scope with a memory of one hundred million sample points can store an incredible twenty seconds. Memory is less important when dealing with repetitive signals or eye patterns.

Consider repetition rate. Digital oscilloscopes perform many time calculations. Most oscilloscopes consume several milliseconds during the time of triggering an event, capturing and displaying the signal, and capturing the next triggered event, resulting in comparatively few waveforms per second — usually one hundred to five hundred — a problem which many manufacturers compensate for using various technologies. Repetition rate is important because signal faults and errors may occur when the oscilloscope is calculating the most recent acquisition. A higher repetition rate increases the chance you’ll capture such rare faults and errors.

You need to know what type of errors to expect when choosing a digital storage oscilloscope. All digital oscilloscopes have built-in intelligent triggers that can occur on much more than just the signal’s rising or falling edge. You can see the rare glitch every other second if the scope’s repetition rate is high enough, which is why it’s good to have a glitch trigger.

The last thing to consider when choosing a benchtop or handheld oscilloscope is the resolution of the display. This isn’t an issue when using a USB oscilloscope, since you’ll be using a computer screen to view the signal.

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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