Horizontal: time and sample rate

Time per division

How much time each of the ten divisions across the screen represents. Smaller values spread a fast signal out; larger values show more cycles or a slow change. The keys step through the usual 1, 2, 5 sequence. Keyboard: Left arrow and Right arrow, or the mouse wheel over the screen.

Sample rate

How many times a second the board measures the input. The value shown is the rate the board reports it actually used.

• A higher rate shows fast detail, but each capture covers less time.
• A signal must be slower than half the sample rate to be shown faithfully. Faster signals appear as a slower, false wave (this is called aliasing).
• Each board has a fastest rate; see Connect a board.

Rate setting

Auto (the usual choice) sets the sample rate and how long each capture is for you, from the time per division:

• At everyday settings it keeps the board’s usual rate, so readings and the spectrum stay as you know them.
• At a long time per division it lowers the rate and lengthens the capture, so the trace fills the screen and the trigger can still be found.
• At a very short time per division it raises the rate, so the screen shows detail rather than a few straight lines.
• It keeps at least about four samples a cycle of the signal on screen, raising the rate for a fast signal (up to the board’s fastest), and never lowering it below that. A slower rate would fill the screen, but with a false, slower wave (see Alias check). The rest of the screen is hatched instead, and a note says so.

Pressing a sample rate key switches to Manual: the rate stays where you put it. If a capture is then shorter than the screen, the part with no samples is hatched, and a note under the measurements says how to fill it. Choose Auto again to hand the rate back.

At a very long time per division even the board’s longest capture cannot fill the screen; the hatching and the note say so, and a shorter time per division fills it again.