Technical Insights > Benchtop

Tips: How to Select an Oscilloscope Before you Buy - Part I

2020-08-28  |  9 min read 

You know you want a new oscilloscope and you’re finally ready to purchase one that needs to last the next 10 years. How do you choose the best one to meet your needs — that’s also within budget? Here are some things to consider before buying:

Bandwidth

Select a scope that has enough bandwidth to accurately capture the highest-frequency content of your signals

How much bandwidth do you need? For purely analog signal measurements, you should choose an oscilloscope that has a bandwidth specification at least three times higher than the highest sine wave frequencies that you might need to measure. For digital applications, in general, Keysight Technologies recommends you choose a bandwidth at least five times the highest clock rate in your systems. (This oscilloscope-bandwidth-relative-to-clock rate recommendation doesn’t consider lower clock-rate signals that have relatively fast edge speeds.) You can upgrade the bandwidths in Keysight Technologies’ InfiniiVision and Infiniium Series oscilloscopes after the initial purchase. For extra buying flexibility, you can upgrade the bandwidths in Keysight Technologies’ InfiniiVision and Infiniium Series oscilloscopes after the initial purchase. To learn more about how to determine required bandwidth based on signal edge speeds, refer to the Keysight application note Evaluating Oscilloscope Bandwidths for Your Applications.

Sample Rate

Select a scope that has a maximum specified sample rate that’s fast enough to deliver its specified real-time bandwidth

An oscilloscope’s real-time bandwidth is closely related to its maximum specified sample rate. (Here, “real-time” means the oscilloscope can capture and display signals that are comparable to its specified bandwidth in a single acquisition.) 
Keysight recommends that an oscilloscope’s maximum specified sample rate should be at least four to five times higher than the scope’s specified real-time bandwidth. The core component of all digital storage oscilloscopes is the high-speed analog-to-digital converter (ADC) system. Keysight invests heavily in ADC technology and has the highest sample rate and highest-fidelity monolithic ADCs in the oscilloscope industry. To learn more about oscilloscope real-time sampling, refer to the Keysight application note Evaluating Oscilloscope Sample Rates Versus Sampling Fidelity.

Number of Channels

Select a scope that has enough channels of acquisition to perform critical time-correlated measurements across multiple waveforms

The number of oscilloscope channels you require will depend on how many signals you need to observe and compare to each other. If you need more than eight analog channels of acquisition, your choices become limited. (Keysight does offer solutions such as the N8834A MultiScope application, which allows you to tie multiple oscilloscopes together.) As mixed-signal designs become more complex, you may require more channels of acquisition and display. A Mixed Signal Oscilloscope (MSO) combines oscilloscope capabilities with some of the capabilities of logic analyzers and serial bus protocol analyzers. With a Keysight MSO, you can simultaneously capture several oscilloscope and logic signals with a time-correlated display of waveforms. Keysight oscilloscopes combine two, four, or eight dedicated analog channels with eight or 16 dedicated MSO channels, depending on which oscilloscope family you choose from, so you’re not stuck with giving up an analog channel to substitute as an analyzer. If you would like to learn more about making measurements with an MSO, look at the Evaluating Oscilloscopes to Debug Mixed-Signal Designs application note.

Display Quality

Select a scope that can display subtle waveform details and signal anomalies 

Select an oscilloscope that provides multiple levels of trace intensity gradation so you can see subtle waveform details and signal anomalies. A waveform’s brightness (intensity) on a display tells you how often a signal shows up at that particular spot on the display. That is, you can find information in the intensity of the waveform on the display. Oscilloscopes are not two-dimensional tools; they help us detect subtle signal differences that can make the difference between discovering an issue early on or after ten prototypes have already been made.

 

multiple levels of trace intensity gradation reveal jitter distribution

 

Figure 1. A Keysight InfiniiVision X-Series oscilloscope that’s set up to monitor jitter on a digital signal. With the oscilloscope’s fast waveform update rate (up to 1 million waveforms/sec), along with 64 levels of trace intensity gradation, multiple levels of trace intensity gradation reveal jitter distribution.

 

Memory Depth

Select a scope that has enough acquisition memory to capture your most complex signals with high resolution

Select an oscilloscope that has enough acquisition memory to capture your most complex signals with high resolution. The maximum available acquisition memory depth is closely related to an oscilloscope’s maximum sample rate. Oscilloscopes sample at their fastest rates when the timebase is set on a fast time range. But you may have to set the timebase to slower ranges to capture longer periods across the oscilloscope’s display. In this case, a scope automatically reduces the sample rate based on how much acquisition memory it has to work with. Although you may intuitively think deeper is always better, using deep memory often means making trade-offs. First, oscilloscopes with deep memory typically have a higher price. Second, acquiring long waveforms using deep memory requires additional waveform processing time. This typically means reduced waveform update rates, sometimes significantly so. With Keysight’s exclusive MegaZoom technology, you don’t have to be an oscilloscope expert. MegaZoom automatically selects deeper memory when needed to maintain fast sample rates. MegaZoom deep memory is not a special mode: it operates with the same familiar controls used for regular scope measurements.

Segmented Memory

Select a scope that can help you dig into the details by extending memory capability
 

Some scopes have a special operation mode called “segmented memory acquisition.” Segmented memory can essentially extend the scope’s total acquisition time. The scope does this by dividing its available acquisition memory into smaller memory segments, as illustrated in Figure 8. The oscilloscope then selectively digitizes just the important portions of the waveform under test at a high sample rate and time-tags each segment so you know the precise time between each occurrence of trigger events. This process enables your oscilloscope to capture many successive single-shot waveforms with a very fast rearm time — without missing important signal information. This mode of operation is especially useful when capturing a burst of signals. Examples of burst-type signals include pulsed radar, laser bursts, and packetized serial bus signals. To learn more about oscilloscope segmented memory acquisition, refer to the Keysight application note Using Oscilloscope Segmented Memory for Serial Bus Applications. Even so, Keysight scopes can be upgraded in acquisition memory and more, even years after the initial purchase, which means that you can buy what you need now and add additional capability later, often without shipping your scope in for service. 

Figure 2. Visual of how segmented memory works

Oscilloscope segmented memory and waveforms


If  you are interested in learning more about all the tips mentioned above and more, check out Keysight’s whitepaper: How to Select Your Next Oscilloscope: 12 Tips on What to Consider Before You Buy. Happy hunting!