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How to choose an oscilloscope

An oscilloscope is an instrument that graphically displays electrical signals and shows how those signals change over time. They are used by engineers to troubleshoot circuits and check signal quality. Most engineers use a digital oscilloscope, which is what we’re going to focus on here. Digital oscilloscopes acquire and store waveforms, which show a signal’s voltage, frequency, the portion of the signal that is noise, whether the signal is distorted, the timing between signals and more.

But when it comes to choosing an oscilloscope, how do you know which oscilloscope is right for your application? There are 10 primary factors to consider when you buy an oscilloscope. For a quick overview of the top factors, watch the short video below. Otherwise, keep reading for the full details on how to choose an oscilloscope for your application.

Oscilloscope bandwidth

System bandwidth determines an oscilloscope’s ability to measure a signal. Specifically it determines the maximum frequency that the instrument can accurately measure. Bandwidth is also a key determining factor in price.

0scilloscope bandwidth considerations

Determine what you need – use the ‘five times rule’

For example, a 100 MHz oscilloscope is usually guaranteed to have less than 30% attenuation at 100 MHz. To ensure better than 2% amplitude accuracy, inputs should be lower than 20 MHz. For digital signals, measuring rise and fall time is key. Bandwidth, along with sample rate, determines the smallest rise-time that an oscilloscope can measure. 

The probe and oscilloscope form a measurement system that has an overall bandwidth. Using a low-bandwidth probe will lower the overall bandwidth, so be sure to use probes that are matched to the scope.

Oscilloscope rise time

Rise time describes the useful frequency range of an oscilloscope, and this is a critical measurement in the digital world. Rise time is often considered when measuring digital signals like pulses and steps.

Determining rise time on an oscilloscope

Determine what you need – use the ‘five times rule’

In order to accurately capture the details of rapid transitions, an oscilloscope must have sufficient rise time. Fast rise time is also needed for accurate time measurements. To calculate the oscilloscope rise time required for your signal type, use this equation:

oscilloscope rise time equation

For instance, a 4-ns rise time needs a scope with faster than 800 ps rise time. Note: As with bandwidth, achieving this rule of thumb may not always be possible. Many logic families have faster rise times (edge speeds) than their clock rates suggest. A processor with a 20 MHz clock may well have signals with rise times similar to those of an 800 MHz processor. Rise times are critical for studying square waves and pulses.

Oscilloscope sample rate
Example of oscilloscope sample rate

The sample rate of an oscilloscope is similar to the frame rate of a movie camera. It determines how much waveform detail the scope can capture.

Determine what you need – use the ‘five times rule’

Sample rate (samples per second, S/s) is how often an oscilloscope samples the signal. Again, we recommend a ‘five times rule’. Use a sample rate of at least 5x your circuit’s highest frequency component.

Oscilloscopes come with a wide variety of sample rates, from 1 to 200 GS/s, to meet the needs of your application.

The faster you sample, the less information you’ll lose and the better the scope will represent the signal under test. However, fast sampling will also fill up your memory quickly, which limits the time you can capture.

Channel density of an oscilloscope

Digital oscilloscopes sample analog channels to store and display them. In general, the more channels the better, although adding channels adds to the price.

Determine what you need

Choose the right number of oscilloscope channels for your application

Your application will determine whether you need to choose an oscilloscope with two, four, six, or even eight analog channels. Two channels let you compare a component’s input to its output, for example. Four analog channels let you compare more signals and provides more flexibility to combine channels mathematically (multiplying to get power, or subtracting for differential signals, for example). Oscilloscopes with six or eight channels allows for multiple bus analysis while simultaneously viewing voltage or current type signals in a power related environment.

A Mixed Signal Oscilloscope adds digital timing channels, which indicate high or low states and can be displayed together as a bus waveform. Whatever you choose, all channels should have good range, linearity, gain accuracy, flatness, and resistance to static discharge.

Some instruments share the sampling system between channels to save money. But beware: the number of channels you turn on can reduce the sample rate.

Consider compatible oscilloscope probes

Good measurements begin at the probe tip. The scope and probe work together as a system, so be sure to consider probes when selecting an oscilloscope.

When taking measurements, probes actually become a part of the circuit, introducing resistive, capacitive, and inductive loading that alters the measurement. To minimize the effect, it’s best to use probes that are designed for use with your scope.

Select passive probes that have sufficient bandwidth. The probe’s bandwidth should match that of the oscilloscope.

A broad range of compatible probes will allow you to use your scope in more applications.

Check to see what’s available for the scope before you buy.

Use the right probe for the job

Oscilloscope probes

Passive probes: Probes with 10X attenuation present a controlled impedance and capacitance to your circuit, and are suitable for most ground-referenced measurements. They are included with most oscilloscopes – you’ll need one for each input channel.

High-voltage differential probes: Differential probes allow a ground-referenced oscilloscope to take safe, accurate floating and differential measurements. Every lab should have at least one.

Logic probes: Logic probes deliver digital signals to the front end of a Mixed Signal Oscilloscope. They include “flying leads” with accessories designed to connect to small test points on a circuit board.

Current Probes: Adding a current probe enables the scope to measure current, of course, but it also enables it to calculate and display instantaneous power.

 

Triggering capabilities of an oscilloscope

All oscilloscopes provide edge triggering, and most offer pulse width triggering. To acquire anomalies and make best use of the scope’s record length, look for a scope that offers advanced triggering on more challenging signals.

Determine what you need

Oscilloscope trigger options

 

The wider the range of trigger options available the more versatile the scope (and the faster you get to the root cause of a problem):

  • Digital/pulse triggers: pulse width, runt pulse, rise/fall time, setup-and-hold
  • Logic triggering
  • Serial data triggers: embedded system designs use both serial (I2C, SPI,CAN/ LIN…) and parallel buses.
  • Video triggering

Oscilloscope record Length

Record length is the number of points in a complete waveform record. A scope can store only a limited number of samples so, in general, the greater the record length, the better.

Determine what you need

Time captured = record length/sample rate. So, with a record length of 1 Mpoints and a sample rate of 250 MS/sec, the oscilloscope will capture 4 ms. Today’s scopes allow you to select the record length to optimize the level of detail needed for your application.

A good basic scope for example will store over 2,000 points, which is more than enough for a stable sine-wave signal (needing perhaps 500 points), whilst more advanced high-end scopes would have up to 1Gpoints, which is essential for working with high-speed serial data type applications. 

Oscilloscope waveform capture rate

Waveform capture rate, expressed as waveforms per second (wfms/s), refers to how quickly an oscilloscope acquires waveforms. The waveform capture rates of oscilloscopes vary greatly, so it’s important to find the right one for your application.

Determine what you need

Oscilloscopes with high waveform capture rates provide significantly more visual insight into signal behavior, and dramatically increase the probability that the oscilloscope will quickly capture transient anomalies such as jitter, runt pulses, glitches and transition errors.

Digital storage oscilloscopes (DSO) employ a serial processing architecture to capture from 10 to 5,000 wfms/s. Some DSOs provide a special mode that bursts multiple captures into long memory, temporarily delivering higher waveform capture rates followed by long processing dead times that reduce the probability of capturing rare, intermittent events.

Most digital phosphor oscilloscopes (DPO) employ a parallel processing architecture to deliver vastly greater waveform capture rates. Some DPOs can acquire millions of waveforms in just seconds, significantly increasing the probability of capturing intermittent and elusive events and allowing you to see the problems in your signal more quickly.

Oscilloscope expandability

As your needs change, you want an oscilloscope that can accommodate your needs with application modules and software updates.

Determine what you need

If you want to expand the capabilities of your oscilloscope over time, make sure your instrument has everything you need. For instance, some oscilloscopes allow you to:

  • Add memory to channels to analyze longer record lengths
  • Add application-specific measurement capabilities
  • Complement the power of the oscilloscope with a full range of probes and modules
  • Work with popular third-party analysis and productivity
  • Windows-compatible software
  • Add accessories, such as battery packs and rack mounts

 

Connectivity of an oscilloscope

After you’ve analyzed your oscilloscope measurements, you’ll need to document and share your findings. The connectivity of an oscilloscope delivers advanced analysis capabilities and simplifies the documentation and sharing of results.

Determine what you need

Depending on the oscilloscope, you may have access to standard interfaces (GPIB, RS-232, USB, and Ethernet), network communication modules, or advanced features that allow you to:

  • Create, edit and share documents on the oscilloscope, all while working with the instrument in your particular environment
  • Access network printing and file sharing resources
  • Access the Windows® desktop
  • Run third-party analysis and documentation software
  • Link to networks
  • Access the Internet
  • Send and receive e-mail

Need help choosing an oscilloscope? Download our oscilloscope selector guide or contact the experts at Tektronix to request a demo. If you already have a sense of which oscilloscope to buy, shop Tektronix oscilloscopes today.

Choose the right oscilloscope infographic