How to Use an Oscilloscope

Contributors: jimblom

Oscilloscope Lexicon

Learning how to use an oscilloscope means being introduced to an entire lexicon of terms. On this page we'll introduce some of the important o-scope buzzwords you should be familiar with before turning one on.

Key Oscilloscope Specifications

Some scopes are better than others. These characteristics help define how well you might expect a scope to perform:

• Bandwidth -- Oscilloscopes are most commonly used to measure waveforms which have a defined frequency. No scope is perfect though: they all have limits as to how fast they can see a signal change. The bandwidth of a scope specifies the range of frequencies it can reliably measure.
• Digital vs. Analog -- As with most everything electronic, o-scopes can either be analog or digital. Analog scopes use an electron beam to directly map the input voltage to a display. Digital scopes incorporate microcontrollers, which sample the input signal with an analog-to-digital converter and map that reading to the display. Generally analog scopes are older, have a lower bandwidth, and less features, but they may have a faster response (and look much cooler).
• Channel Amount -- Many scopes can read more than one signal at a time, displaying them all on the screen simultaneously. Each signal read by a scope is fed into a separate channel. Two to four channel scopes are very common.
• Sampling Rate -- This characteristic is unique to digital scopes, it defines how many times per second a signal is read. For scopes that have more than one channel, this value may decrease if multiple channels are in use.
• Rise Time -- The specified rise time of a scope defines the fastest rising pulse it can measure. The rise time of a scope is very closely related to the bandwidth. It can be calculated as Rise Time = 0.35 / Bandwidth.
• Maximum Input Voltage -- Every piece of electronics has its limits when it comes to high voltage. Scopes should all be rated with a maximum input voltage. If your signal exceeds that voltage, there's a good chance the scope will be damaged.
• Resolution -- The resolution of a scope represents how precisely it can measure the input voltage. This value can change as the vertical scale is adjusted.
• Vertical Sensitivity -- This value represents the minimum and maximum values of your vertical, voltage scale. This value is listed in volts per div.
• Time Base -- Time base usually indicates the range of sensitivities on the horizontal, time axis. This value is listed in seconds per div.
• Input Impedance -- When signal frequencies get very high, even a small impedance (resistance, capacitance, or inductance) added to a circuit can affect the signal. Every oscilloscope will add a certain impedance to a circuit it's reading, called the input impedance. Input impedances are generally represented as a large resistive impedance (>1 MΩ) in parallel (||) with small capacitance (in the pF range). The impact of input impedance is more apparent when measuring very high frequency signals, and the probe you use may have to help compensate for it.

Using the GA1102CAL as an example, here are specifications you might expect from a mid-range scope:

Understanding these characteristics, you should be able to pick out an oscilloscope that'll best fit your needs. But you still have to know how to use it...onto the next page!