Which oscilloscope to buy? Things to consider when choosing a scope

 

Oscilloscopes are used as windows into signals for troubleshooting circuits or checking signal quality.

A digital oscilloscope, which is what we’re going to be concentrating on in this article, acquires and stores waveforms. The waveforms show a signal’s voltage and frequency, whether the signal is distorted, timing between signals, how much of a signal is noise, and much, much more.

But how to know which oscilloscope you need? Below are five things to consider when you’re scoping out a new scope (pun intended!)

What oscilloscope bandwidth do I need?

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

amplitude degradation

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.

What is rise time and why is it important?

While analogue engineers look at bandwidth, digital engineers are more interested in the rise time of signals like pulses and steps.

what is rise time

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

The faster the rise time, the more accurate are the critical details of fast transitions.

Fast rise time is also needed for accurate time measurements. Rise time is defined as, where k is between 0.35 (typically for scopes with bandwidth 1 GHz).

Similar to bandwidth, an oscilloscope’s rise time should be < 1/5 x fastest rise time of signal.

E.g. 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.

What sample rate should I look for?
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, all to suit your application and the type of signal you wish to capture.

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

What channel density do you need?

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

Determine what you need

mixed signal oscilloscope

Whether to select 2, 4, 6 or even 8 analog channels depends on your application. Two channels let you compare a component’s input to its output, for example. Four analogue 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), when 6 or 8 channels allows for either multiple bus analysis whilst simultaenously 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 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.

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

Select passive probes that have sufficient bandwidth. The probe’s bandwidth should match that ofthe 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

use the right probe for the job

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

Determine what you need

triggered vs untriggered display

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.

 

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

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 optimise 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. 

When choosing an oscilloscope use the rule of 5

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