What Is a Digital Storage Oscilloscope and How Does It Work

What Is a Digital Storage Oscilloscope and How Does It Work

ByKen

What Is a Digital Storage Oscilloscope and How Does It Work

As an Amazon Associate we earn from qualifying purchases. This guide does not link to any products. It explains what a digital storage oscilloscope is, how it turns a signal into a stored record, how triggering works, and which two specifications decide whether a given scope can capture your signal. The explanation is based only on the manufacturer and Keysight pages listed in the sources section at the end.

Quick answer

  • A digital storage oscilloscope samples the input with an ADC, stores the values, triggers on a chosen condition, and draws the waveform from the stored record.
  • The two limits to check are analog bandwidth and sample rate. For a single event, real-time bandwidth is roughly the sample rate divided by 2.5.
  • The trigger decides when each capture starts. Edge, pulse-width and video triggers are the common families.

A digital storage oscilloscope (DSO) is an instrument that turns an electrical signal into numbers, stores those numbers, and then draws the waveform on a screen. It shows voltage on the vertical axis and time on the horizontal axis, the same as an older analog scope, but it does the drawing from recorded data instead of driving the beam directly. This guide explains the four steps a DSO performs, the trigger, and the two specifications that decide whether a given scope can capture the signal you care about.

The four steps: sample, store, trigger, display

According to Keysight’s overview of how an oscilloscope works, a DSO follows four main steps. The input signal first goes to an analog-to-digital converter (ADC), which samples the signal at discrete intervals and converts each sample into a digital value. Keysight notes that sampling rates are often in the gigasamples-per-second (GSa/s) range, and that higher rates capture fast-changing signals more accurately.

The scope then stores those values in memory. Because the waveform is stored, you can review it or analyze it after the input signal has gone away. Keysight describes the bit depth, typically 8 to 16 bits or more, as the setting that controls vertical resolution.

Triggering comes next. Keysight describes the trigger as the element that decides when the capture begins and how it proceeds. Edge triggers are the basic type. Pulse, runt and logic triggers respond to more specific conditions. The display step follows: the scope processes the stored data and shows it on an LCD or LED screen. Because the data is digital, the waveform can be zoomed, panned and processed with math functions.

Why the trigger matters

Keysight lists the trigger as the step that determines when and how the scope captures data. Without a trigger, successive captures of a repeating signal would not line up. You set a trigger level and an edge, and the scope captures when the input crosses that level in that direction.

Tektronix’s TBS1000 datasheet lists three trigger families on that series: edge, video, and pulse width (also called glitch). The pulse-width trigger fires when a pulse is shorter than, longer than, equal to, or not equal to a set time from 33 ns to 10 s, which is useful for catching a short glitch in a circuit that otherwise looks clean. The same datasheet lists the trigger modes Auto, Normal and Single Sequence.

Sample rate and bandwidth: the two numbers to check

A DSO has two limits that interact. The first is analog bandwidth, which Tektronix describes as the highest frequency the input circuitry can accurately capture. The second is sample rate. The two together decide what the scope can show for a single event.

Tektronix’s application note on real-time and equivalent-time sampling explains the rule of thumb. In real-time (single-shot) mode, the scope uses one trigger event to collect all the samples for a waveform. Dividing the sample rate by about 2.5 gives the approximate real-time bandwidth limit. The note’s example: a 2 GS/s scope has a theoretical real-time bandwidth near 800 MHz. The note also gives a counter-example. A 100 MS/s sampler on a 400 MHz analog scope can capture only about 40 MHz in real time, so a high analog bandwidth alone does not guarantee a fast single-shot measurement.

The samples-per-cycle rule explains the 2.5 figure. In theory two samples per cycle can reproduce a sine wave, but in practice Tektronix’s sin(x)/x interpolation needs at least 2.5 samples per period, and fewer samples produce a distorted waveform. For a single transient event, the sample rate, not the analog bandwidth alone, decides what the real-time capture can show. The TBS1000 datasheet, which covers discontinued 60 MHz, 100 MHz and 150 MHz four-channel models, lists 1 GS/s on each channel with the rate held on all channels at once.

What a DSO captures that an analog scope does not

The stored record is the main difference. Keysight’s overview describes the memory step as the point where sampled values are kept, so the waveform can be reviewed after the input signal is gone. Record length, which is the number of points stored, limits how much history is kept at a given sample rate. The TBS1000 datasheet lists a 2.5k-point record length; at 1 GS/s that is a capture window of 2.5 microseconds, so the record length and sample rate together set how much time a single capture covers.

Stored data also allows measurements. The display step is where cursors, averages and math traces are applied to the saved record. Those functions are not on the signal path, so they do not change the data already captured.

How to pick a DSO for a common job

  • Checking a repeating signal such as a clock, a PWM output or a sensor line: analog bandwidth and channel count matter most, and a modest sample rate is often enough.
  • Catching a single glitch or power-up event: sample rate matters as much as bandwidth, so check that the real-time rate is high enough for the event width.
  • Measuring a signal you cannot see on the screen at first: a trigger with pulse-width or runt options helps isolate the event.

Use the sample-rate and bandwidth figures from the datasheet for the exact model you are considering. Two scopes with the same analog bandwidth can differ a lot in single-shot capture, and the datasheet is the place to confirm the number for each one.

What we could not verify

We did not find a Keysight Learn or Tektronix Learn page that gives a single standard definition of a DSO in the same wording. The definitions above come from the Keysight used-equipment guide and the Tektronix application note and datasheet. We did not verify a general bit-depth or memory-depth figure for every DSO on the market, so this guide gives only the typical ranges that Keysight states.

How we chose these sources

We opened each source as a readable page before using it. The Keysight page is from its used-equipment knowledge section, and the Tektronix application note and datasheet are from the manufacturer. We did not use retailer listings, forums, or third-party manual mirrors for any claim in this guide. Where a source gave only a summary, we left the detail out rather than fill it in from memory.

Frequently asked questions

What makes a DSO different from an analog scope?

A DSO keeps a sampled record of the signal. The sampled values are what the display draws, and they can be reviewed after the input is gone. The sources used here describe the sampling, memory and display steps that make this possible.

Which number matters more, bandwidth or sample rate?

It depends on the signal. For a repeating signal the analog bandwidth is the main limit. For a single transient, the sample rate sets how much of the event the real-time capture can show, as the Tektronix example of a 100 MS/s sampler on a 400 MHz scope illustrates.

Do I need the record length to be long?

Only if the event is long compared with the capture window. The TBS1000 datasheet example of 2.5k points at 1 GS/s gives a 2.5-microsecond window, so a slow event may need a longer record or a lower sample rate setting.

Related reading on this site

For probe choices, read the guide on automotive oscilloscopes. For a broader look at the category, read the guide to digital oscilloscopes.

Sources

Similar Posts