Learning Library · Signals and oscilloscopes · 1
Frequency, period, and amplitude
Three numbers describe almost any repeating signal: how often it repeats, how long each repeat takes, and how big it is. Here is where they come from, and how to read them off an oscilloscope screen.
Top: a sine wave is the height of a point on a rotating circle, shown slowed down. Bottom: the same signal on a simulated oscilloscope, 10 horizontal by 8 vertical divisions.
Frequency and period are two views of one thing
Frequency is how many complete cycles happen each second, measured in hertz (Hz). Period is how long one cycle takes, in seconds. A 1 kHz signal repeats 1,000 times a second, so each cycle lasts 1 ms. Raise the frequency and the period shrinks by the same factor.
The rotating circle shows why a sine wave has the shape it does. One full turn of the vector is one cycle. The frequency is how many turns it makes per second, and the radius is the amplitude.
Reading frequency off the screen
An oscilloscope's horizontal axis is time. The timebase setting tells you how much time each division represents. Count the divisions across one cycle and multiply:
- Set 1 kHz and 200 µs/div. Ten divisions show 2 ms, so you see exactly two cycles, each 5 divisions wide.
- Raise the frequency to 10 kHz without touching the timebase. Twenty cycles crowd the screen. Turn the timebase down to 20 µs/div to spread them out again.
- Slow the timebase to 2 ms/div at 100 kHz. Two thousand cycles land on screen and blur into a solid band. That is a cue to speed the timebase up.
Why the trigger matters
A scope redraws the screen many times a second. Without a trigger, each sweep starts at a random point in the cycle and the traces pile up into a blur. A rising-edge trigger tells the scope to start each sweep at the same point, here as the signal rises through 0 V at the centre of the screen. Every sweep lands in the same place and the waveform stands still. Turn the trigger off to see the difference.
Four ways to state amplitude
- Peak (Vpk): from 0 V to the highest point.
- Peak-to-peak (Vpp): from the lowest point to the highest, twice the peak for a symmetric wave. This is the easiest to read on a scope: count vertical divisions and multiply by volts per division.
- RMS (root mean square): the DC voltage that would deliver the same heating power into a resistor. For a sine it is Vpk ÷ √2, about 0.707 × Vpk. For a square wave it equals Vpk. For a triangle it is Vpk ÷ √3.
- Average: zero for any wave symmetric about 0 V, which is why it is rarely used for AC on its own.
Many lower-cost meters measure the average and scale it as if the signal were a sine. On a square or triangle wave that reading is wrong. A true-RMS meter computes RMS directly and reads correctly whatever the waveform shape.
See your own signals. Oscilloscopes with automatic frequency, period, Vpp, and RMS measurements.
Shop UNI-T oscilloscopesUNI-T Learning Library. Design. Debug. Deploy.™