# Virtual Oscilloscope - Online Scope Simulator, Triggering, Lissajous

> Online dual-channel oscilloscope simulator: volts/div, time/div, coupling, trigger level and slope, X-Y Lissajous mode, built-in function generator and cursors.

- URL: https://qilingtools.com/en/physics/oscilloscope
- Category: Physics Tools (https://qilingtools.com/en/physics)
- Tags: [Physics](https://qilingtools.com/en/tags/physics), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

A dual-channel oscilloscope that runs in your browser, with a green phosphor screen, a 10 × 8 grid and the same controls as a real scope: volts/div, time/div, vertical position, DC/AC/GND coupling, trigger source, level, slope and Auto/Normal/Single modes. Each channel is fed by a built-in function generator with sine, square, triangle, sawtooth and pulse waves, plus noise, DC offset and AM/FM modulation. Move the trigger level outside the signal and you can watch the trace lose lock exactly as on a real instrument, or switch to X-Y mode and measure phase with Lissajous figures.

## How to use

1. Pick a waveform, frequency and amplitude on the function generator, or click a preset
2. Turn the volts/div and time/div knobs until the trace fills most of the screen
3. Set the trigger level and slope for a stable trace, and try moving the level outside the signal
4. Read Vpp, frequency, duty cycle and phase difference with the cursors or automatic measurements
5. Open Reading practice, read an unknown signal from the grid and check your answers

## Features

- CRT-style green phosphor screen with persistence, 10 × 8 grid with minor ticks, two colored channels
- Realistic triggering: Auto free-runs when the level is outside the signal, Normal holds the last sweep, Single captures once
- DC, AC and GND coupling with adjustable vertical position and trigger position
- Built-in function generator: five waveforms, duty cycle, DC offset, noise and AM/FM modulation
- Time and voltage cursors plus automatic Vpp, Vrms, frequency, period, duty and phase; Lissajous intercept method in X-Y mode
- Randomized reading practice with 5% tolerance and a running score

## FAQ

### Why does the trace drift or smear across the screen?

The scope isn't triggering. Every sweep should start at the moment the signal crosses the trigger level with the chosen slope, so each sweep starts at the same phase and the picture stands still. If the level is above the signal's maximum or below its minimum there is never a trigger: Auto mode sweeps anyway, each sweep starts at a different phase and the traces pile up and roll; Normal mode just keeps the last triggered sweep. Bring the level back inside the signal (or press Set to 50%) and it locks. Noisy signals make the trigger point jitter, which is why real trigger circuits use hysteresis, as this one does.

### How do I read peak-to-peak voltage and frequency from the grid?

Peak-to-peak voltage = divisions from peak to trough × volts/div; 3.2 divisions at 0.5 V/div is 1.6 V. Period = divisions for one full cycle × time/div, and frequency = 1/period; 2.5 divisions at 0.2 ms/div gives T = 0.5 ms and f = 2 kHz. Use the minor ticks on the center axes (five per division, 0.2 div each), and first set volts/div and time/div so the trace is as tall as possible with one to three cycles on screen. That keeps the reading error small.

### What is the difference between DC, AC and GND coupling?

DC coupling passes the whole signal including its DC part, so an offset signal moves up or down the screen. AC coupling puts a blocking capacitor in series and shows only the AC part, which is handy for looking at small ripple on a large DC voltage. GND disconnects the input and grounds it, so the trace shows where 0 V sits and you can set the vertical position. On a real scope the AC coupling capacitor also attenuates very low frequencies (a few hertz) and makes low-frequency square waves tilt; here AC coupling is treated as ideal.

### How do Lissajous figures measure phase and frequency ratio?

In X-Y mode CH1 drives the horizontal deflection and CH2 the vertical. At equal frequencies the figure is an ellipse. If A is the maximum vertical deflection and B is half the height where the ellipse crosses the Y axis, then sin φ = B/A. An ellipse tilted up to the right means φ between 0 and 90°, tilted up to the left means 90–180°. φ = 0 collapses to a line, and 90° with equal amplitudes gives a circle. With different frequencies, f_y : f_x = tangent points on a horizontal edge : tangent points on a vertical edge, so 1:2 gives a figure eight.

### Why does the measured Vrms differ from Vpeak/√2?

Vpeak/√2 is only true for a pure sine. A square wave has an RMS equal to its peak, a triangle wave peak/√3, and an offset signal also includes its DC part. The scope computes Vrms by definition, √(mean of v²), including DC, so an offset signal reads higher than its AC RMS; the table also shows the AC RMS with the mean removed. When the screen does not hold a whole number of cycles the result is slightly off, just as on a real digital scope.

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