---
title: Oscilloscope
lang: en
type: tuto
tags:
  - eee
  - tuto
prerequis:
  - instruments-de-mesure-en
  - chronogramme-en
aa: []
phases:
  - preuve-de-concept
  - integration-et-tests
draft: false
source_fr: embarque/mesure/oscilloscope.md
source_sha256: e8e6425183f2e65c5b59bade2547cc2f89a890fec48c0bf8173a51e75fade0f8
---

**The oscilloscope** displays **voltage as a function of time**: where the [[multimetre-en|multimeter]] gives *a value*, the oscilloscope shows *a waveform* — the signal's evolution, instant by instant. It is the indispensable instrument as soon as a signal **changes fast**: visualising a [[pwm-en|PWM]], measuring a frequency and a duty cycle, spotting a button's bounce, checking that a serial frame or a [[bus-de-communication-en|bus]] signal has the right shape. It only delivers with three settings properly understood — timebase, vertical scale and **triggering** (*trigger*). This page is a tool tutorial of the [[instruments-de-mesure-en|measuring instruments]] hub. For reading a waveform in itself, see [[chronogramme-en|timing diagram]].

![Front panel of an oscilloscope in functional blocks: on the left the gridded screen with a square signal and, below it, the CH1 and CH2 probe inputs; on the right three stacked blocks — vertical (Volts/div), horizontal (Time/div) and trigger (level and edge).|640](/ressources/img/oscilloscope/face-avant.svg)

## What is it for?

The oscilloscope answers questions the multimeter cannot handle:

- **seeing the shape** of a signal — square, sinusoidal, sawtooth, noisy — and not just its average value;
- **measuring in time** — the period and hence the **frequency**, the **duty cycle** of a PWM, a rise time, the duration of a pulse;
- **spotting a dynamic fault** — a bouncing contact, a parasitic oscillation, a signal collapsing under load, a *glitch* too brief for a multimeter;
- **checking a communication** — the shape of a [[bus-de-communication-en|UART, I²C or SPI]] frame (a [[analyseur-logique-en|logic analyzer]] *decodes* it, the oscilloscope shows its *electrical quality*).

For a simple DC value — a supply voltage, a resistance — the [[multimetre-en|multimeter]] stays faster. The oscilloscope is reserved for signals that **move**.

## Connecting the probe

The probe is the only link between the circuit and the screen. Connecting it properly is half the job:

1. **the probe cable screws onto a channel** (CH1, CH2…) on the front panel;
2. at the other end, two contacts: the **tip**, which goes on the point to observe, and the **ground alligator clip**, which goes on the **circuit ground** — and nowhere else;
3. the why: through its power cord, the oscilloscope's ground is **tied to the mains earth**. The clip therefore *imposes* earth on whatever point it touches — on the circuit ground, it establishes the common reference. On any other point, it creates a **short circuit through the earth** (see *Pitfalls*).

![Connecting an oscilloscope probe: the cable leaves channel CH1, the tip goes on the board's signal pin, the alligator clip on the GND ground; the power cord ties the oscilloscope's ground to the mains earth, which is why the clip is only ever clipped onto the circuit ground.](/ressources/img/oscilloscope/branchement-sonde.svg)

## Getting started

1. **Connect the probe** as above — tip on the signal, clip on the circuit ground.
2. **Set the vertical scale (Volts/div).** The vertical axis is in volts: choose the scale so the signal fills a good part of the screen without overflowing.
3. **Set the timebase (Time/div).** The horizontal axis is in seconds: adjust it to see a few periods of the signal — neither a mush of squeezed oscillations, nor a single stretched period.
4. **Set the trigger.** This is the key setting: it tells the oscilloscope **when** to start tracing (for example "on the rising edge, when the signal crosses 1.6 V"). Without a correct trigger, the image scrolls and stays unreadable.
5. **Account for the probe (×1 / ×10).** A ×10 probe divides the signal by 10 — it allows measuring higher voltages without saturating, and **loads the circuit less** (cf. *Measuring is not neutral*, at the hub): the oscilloscope must be set to ×10 to display the true value.
6. **Read** amplitudes and durations on the grid, or through the meter's automatic measurements.

> [!note]
> **The trigger turns a scrolling signal into a stable image.** A periodic signal retraced without synchronisation "slides" across the screen. The trigger sets a common starting point for every sweep (an edge, a level): the traces stack up and the signal looks frozen, hence readable. An unstable signal on screen is almost always a trigger problem, not a signal problem.

## Example — Watching a PWM signal

The goal is to check the signal produced by an `analogWrite()` on an Arduino board: its shape, its frequency and its duty cycle.

![Oscilloscope screen set to 1 V/div and 0.5 ms/div: a 0-5 V square wave, the period marked between two rising edges — about 4 divisions, that is 2.04 ms, hence 490 Hz — and the high part dimensioned at 50% duty cycle.|600](/ressources/img/oscilloscope/ecran-pwm.svg)

1. **Connect** the probe tip on the PWM pin, the ground clip on the board's GND.
2. **Set** the vertical scale around 1 to 2 V/div (0–5 V signal) and the timebase to see a few periods.
3. **Trigger** on the rising edge, level ≈ 2.5 V: the square signal freezes.
4. **Read the frequency**: measure the period T on the horizontal axis. The frequency is 1/T (≈ 490 Hz on most Arduino PWM pins by default).
5. **Read the duty cycle**: the share of time spent high over one period. An `analogWrite(broche, 128)` command (half of 255) should give ≈ 50%, a command of 64 ≈ 25%.

You have confirmed not only *that* the signal comes out, but *at what frequency* and *with what duty cycle* — information the multimeter, which would only have shown an average voltage, could not give.

## Pitfalls

**Forgetting the common ground — short circuit through the earth.** The ground of a bench oscilloscope is tied to the mains earth. Clipping the ground clip onto a point that is *not* the circuit ground ties that point to earth: short circuit, possible damage. Always connect the ground clip to the circuit ground, and be doubly careful on a circuit connected to the mains.

**Undeclared ×10 probe.** If the probe is in the ×10 position but the oscilloscope set to ×1, all the amplitudes read are wrong, by a factor of 10. Check the probe / setting agreement before measuring.

**Wrong trigger.** A trigger level outside the signal's range, or the wrong edge, leaves the image scrolling. Set the level within the signal's real amplitude.

**Unsuitable timebase.** Too fast, you only see a piece of the period. Too slow, the signal squeezes into a line. Adjust it for a few periods on screen.

**Mixing up the amplitudes.** Peak, peak-to-peak (Vpp) and RMS describe different quantities: read the right one for what you are after (see [[chronogramme-en|timing diagram]]).

**Underestimating the bandwidth.** An oscilloscope and its probe have a maximum bandwidth: a signal too fast comes out rounded or attenuated. For the slow signals of a student project this is rarely limiting, but keep it in mind on very steep edges.

## See also

- [[instruments-de-mesure-en|Measuring instruments]] — the hub: method and choice of instrument
- [[multimetre-en|Multimeter]] — when a single value is enough
- [[generateur-de-signaux-en|Function generator (GBF)]] — the natural pair: inject on one side, watch on the other
- [[chronogramme-en|Timing diagram]] — reading and interpreting a waveform (conceptual prerequisite)
- [[simulation-electronique-en|Electronic simulation]] — the simulated curve, the expectation to compare the real trace with
- [[pwm-en|PWM]] — the square signal most often watched on an oscilloscope
- [[lire-une-datasheet-en|Reading a datasheet]] — the characteristics (levels, *timings*) to check