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Simple Tips on What to Consider When Buying an Oscilloscope

Blog Post created by BoonCampbell Employee on Nov 8, 2017

Comparing different manufacturers’ oscilloscopes and their various specifications and features can be time-consuming and confusing. Streamline this process and select the oscilloscope that best fits your application with these top 10 considerations.

 

Streamline this process and select the oscilloscope that best fits your application with these top 10 considerations.

 

Bandwidth

The primary oscilloscope specification is bandwidth, and it is critical that you select an oscilloscope that has sufficient bandwidth to accurately capture the highest frequency content of your signals. Most oscilloscopes with bandwidth below 8 GHz have a Gaussian, or single-pole low-pass filter frequency response. The oscilloscope’s bandwidth is the frequency at which the input signal is attenuated by 3 dB. Because of this, you cannot expect to make accurate measurements near your oscilloscope’s specified bandwidth frequency.

 

Rule of Thumb:

  • Analog applications: Choose a bandwidth at least 3x higher than the highest sine wave frequencies you will measure
  • Digital applications: Choose a bandwidth at least 5x the highest clock rate in your system.

 

This will allow you to capture the fifth harmonic with minimum signal attenuation. This fifth harmonic of the signal is critical in determining the overall shape of your digital signals. This 5-to-1 rule-of-thumb does not take into account lower clock-rate signals that may have relatively fast edge speeds. These clock signals may contain significant frequency components beyond the fifth harmonic and require even higher bandwidths.

 

Keysight’s S-Series oscilloscope has bandwidth options of 1, 2, 2.5, 4, 6, and 8 GHz  

 

 

Figure 1 – Keysight’s S-Series Oscilloscope from 500 MHz to 8 GHz

 

Sample Rate

A digital oscilloscope can spend a lot of time calculating between the trigger event, the signal displayed, and the next trigger. This can result in only a few captures of your signal each second. Your oscilloscope will fail to capture intermittent errors or faults in your signal without a high enough update date.  This happens because the oscilloscope is busy calculating the last acquisition captured instead of acquiring. The higher the update rate, the higher your chances are of capturing that rare event.

 

Memory Depth

Closely related to an oscilloscope’s maximum sample rate is its maximum available acquisition memory depth. You should select an oscilloscope that has sufficient acquisition memory to capture your most complex signals with high resolution. Even though an oscilloscope’s banner specifications may list a high maximum sample rate, this does not mean that the oscilloscope always samples at this high rate. Oscilloscopes sample at their fastest rates when the time base is set on one of the faster time ranges. But when the time base is set to slower ranges to capture longer time spans across the oscilloscope’s display, the scope automatically reduces the sample rate based on the available acquisition memory. Maintaining the oscilloscope’s fastest sample rate at the slower time base ranges requires that the scope have additional acquisition memory. Determining the amount of acquisition memory you require is based on the time span of your signal and your desired maximum sample rate:

 

Acquisition Memory = Time Span x Required Sample Rate

 

Even though an oscilloscope’s banner specifications may list a high maximum sample rate, this does not mean that the oscilloscope always samples at this high rate. 

 

To better understand the relationship between bandwidth, sampling rate, and memory depth, let’s look at a real-world example. Consider trying to capture one frame data that lasts 1 ms and has serial data transmitted at 12 Mbps. So let’s assume that we have to capture a 12 MHz square wave for 1 ms.

  • Bandwidth — to measure the 12 MHz signal, we need an absolute minimum of 12 MHz, however, this will give a very distorted signal. So a scope with at least 50 MHz bandwidth should be selected.
  • Sampling rate — to reconstruct the 12 MHz signal, we need around 5 points per waveform, so a minimum sampling rate of 60 MS/s is required.
  • Memory depth — to capture data at 60 MS/s for 1 ms requires a minimum memory depth of 60,000 samples.

 

Triggering

Triggering allows you to synchronize the oscilloscope’s acquisition and display particular parts of your signal under test. Most digital oscilloscopes trigger on simple edge crossings, but you should select your oscilloscope based on the types of advanced triggering needed to help you isolate your most complex signals. Some oscilloscopes have the ability to trigger on pulses that meet a particular timing qualification. For example, trigger only when a pulse is less than 20 ns wide. This type of triggering (qualified pulse-width) can be very useful for triggering on unsuspected glitches. Pattern triggering is also very common and allows you to set up the oscilloscope to trigger on a logical/Boolean combination of highs (or 1s) and lows (or 0s) across two or more input channels. More advanced oscilloscopes even provide triggering that can synchronize on signals that have parametric violations. In other words, trigger only if the input signal violates a particular parametric condition such as reduced pulse height (runt trigger), edge speed violation (rise/fall time), or perhaps a clock to data timing violation (setup and hold time trigger).

 

Most digital oscilloscopes trigger on simple edge crossings, but you should select your oscilloscope based on the types of advanced triggering needed to help you isolate your most complex signals.

 

Display Quality

The quality of your oscilloscope’s display can make a big difference in your ability to effectively troubleshoot your designs. You should select an oscilloscope that provides multiple levels of trace intensity gradation in order to display subtle waveform details like noise distribution, jitter, and other signal anomalies. For the highest oscilloscope display quality in the industry, go to Keysight.com. An example is shown in Figure 2 below.

 

 

Figure 2 – High levels of display quality are required when viewing complex modulated signals such as video

 

Serial Bus Applications

To help you debug your designs faster, select an oscilloscope that can trigger on and decode serial buses. Serial buses such as I2C, SPI, RS232/UART, CAN, USB, etc., are pervasive in many of today’s digital and mixed-signal designs. Verifying proper bus communication along with analog signal quality measurements requires an oscilloscope. Many of today’s oscilloscopes have optional built-in serial bus protocol decode and triggering capabilities. If your designs include serial bus technology, then selecting an oscilloscope that can decode and trigger on these buses can be a significant time-saver to help you debug your systems faster.

 

If your designs include serial bus technology, then selecting an oscilloscope that can decode and trigger on these buses can be a significant time-saver to help you debug your systems faster. 

 

Connectivity and Documentation

Selecting an oscilloscope that meets your hardware connectivity, test automation, and electronic documentation requirements are key. Automated testing requires that the oscilloscope’s ports be programmable. So make sure any measurement that can be performed using the oscilloscope’s front panel and menu controls can also be programmed remotely via LAN or USB connectivity.

 

Keysight’s InfiniiVision X-Series and Infiniium Series oscilloscopes are all fully programmable via SCPI commands as well as National Instruments IVI drivers. Saved images (screen-shots) and data (waveforms) can also be easily imported into various word processors, spreadsheets, and applications such as MATLAB. Keysight’s N8900A InfiniiView offline analysis software lets you easily capture waveforms on your oscilloscope, save them to a file, and recall the waveforms into the application. With Keysight’s > 50 standard automated measurements with statistics and 16 independent math functions, you’ll be able to analyze a wide variety of tests.

 

Probing

Your oscilloscope measurements can only be as good as the data your probe delivers to the oscilloscope’s BNC inputs. Always keep in mind that when probing your circuit, your oscilloscope and probe become part of your device-under-test. This means it can change the behavior of your device-under-test signals due to capacitive or inductive loading. Select an appropriate probe for your measurement to minimize these loading effects. This will prevent disturbance of the input signal and deliver a true representation of your signal as it existed in your circuit before the probe was attached.

 

Select an appropriate probe for your measurement to minimize these loading effects.

 

Ease of Use

Your oscilloscope should be user-friendly and intuitive. This can be just as important as specified performance characteristics.  Oscilloscopes have evolved over the years with many additional features and capabilities but ease of use should not be compromised. Although most oscilloscope vendors will claim that their oscilloscopes are the easiest to use, usability is not a specified parameter that you can compare against in a product’s data sheet. Ease-of-use is subjective, and you must evaluate it for yourself. However, there are a few things you can look for when evaluating ease of use:

  • Built-in help menus, which reduce the need to reference manuals
  • Large color displays, which allow views of waveforms and measurement data at the same time
  • Voice control, which allows for hands-free control

Always request a demo when picking out your oscilloscope with a reputable company providing field engineers to help demo the scope or go to trade shows that provide hands-on demos.

 

Learn more about these and other oscilloscope selection topics:

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