I arrived at the FFT function, but despite having read some guides, I can't decipher what I see on the display! The manual is obviously based on initial firmware, the menu is almost totally different!
In the attached image it is the usual square wave signal Vpp3v 1Khz I simply did auto set from the menu on page 3/3.
From what I understand FFt shows us the frequency domain on the horizontal axis, while in the vertical axis it is represented by decibels.
But what is not clear to me would be: my signal is 1Khz, why is the graph full from 0Hz up to 50Mhz?
Shouldn't my signal just be represented at 1Khz in the graph?
This FFT confused me ...
![Embarrassed :-[](https://www.eevblog.com/forum/Smileys/default/embarrassed.gif)
Specifically, what you are seeing is the spikes at 0V, 1kHz, 3kHz, 5kHz...49999kHz - all of the odd harmonics up to 50 MHz. You need to stop the upper frequency at around 9 kHz or 11 kHz (the odd 9th and 11th harmonics).
The FFT confuses me as well. It was a 3rd year subject in undergrad and the reason it comes up so late in the 4 year program is the depth of the math underlying the transform. The transform itself and the plot of amplitude versus frequency is pretty simple to understand. Doing the math to dig out the coefficients for the harmonics gets a little more involved.
Around 11 minutes into that video, the author decomposes a signal with an annoying high frequency hum and plots the various spikes. It is then easy to see which frequency needs to be filtered and he shows the result after filtering out that specific frequency. This is what the FFT is used for! What frequencies are present in the signal and which would I like to remove (or amplify).
For the square wave, I gave you the idea that it is composed of all the odd harmonics from DC to daylight. As the risetime increases, the number of harmonics decreases until the square wave looks like a sine wave and there is only the fundamental frequency left.
ETA: When the rise time increases numerically, the edge moves away from vertical. The rise time value is higher (like 100 ns is greater than 10 ns) but the number of required harmonics is less because the edge is less vertical. When I wrote "rise time increases", I meant numerically larger or visually slower.
The rise time of the compensation signal on my Rigol is 10 us. That's pretty slow when you think in terms of a simple 74LS00 quad NAND gate switching in 10 ns. That gate switches about 1000 times faster than the compensation signal and it isn't even very fast logic.
Rise time is usually measured as the time between the 10% and 90% levels of the rising edge. There is a fall time spec as well. Your scope will provide these measurements.
It's going to take more than 1 video to catch what is happening. Google for 'fft on scope' and see what you can turn up.