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Oscilloscope Basics

Introduction

An oscilloscope (scope) is an electronic instrument that displays a voltage input on a screen as a function of time. It is used to display and measure repetitive waveforms or transient signals.

Picture of an oscilloscope


Common oscilloscope operating parameters

input impedance

> 1 megaohm
A high input impedance is used to prevent the scope from affecting the circuit being tested.
50 ohms
Used to match the 50 ohm output impedance of fast amplifiers and other devices.

input coupling

dc
Straight input connection, the screen trace shows both dc and ac components of the input signal.
ac
Places a capacitor in series with the signal input to capacitively couple only the ac signal into the scope. Useful for measuring small ac signals on a large dc offset.
ground
Useful for finding the zero volt level on the screen.

trigger source

signal input channel (e.g. CH1)
Triggers at some voltage on the input signal. Usually used for repetitive waveforms.
line
The ac power line, triggers the scope at 60 Hz.
external
An external trigger that is synchronizes the oscilloscope to the signal being displayed. Usually used for transient events.

trigger level and slope

level
The adjustable voltage level at which the scope triggers when the trigger input crosses this value.
slope
The slope of the trigger can be set for either the rising or falling edge.

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