Electronics > Beginners
How to setup equip to measure FFT
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adauphin:
Brief background, I picked up a 1054Z last year, have not had much time to practice with it, I just acquired a SDG 2042X and I would like to take some FFT measurements through a few components.

Would like to pretty much practice using a few RCA interconnect cables, speaker cables, just to get the hang of the equipment.

Just curious what amplitude (Vpp) is generally used for this, I'm assuming 1kHz as a start.

Curious to see the difference in waveforms through various components.

Also, would you leave the ground clips dis connected or connect to the outer shield?

I have a few T connectors, 50 ohm adapters, BNC banana connectors...I'm a quick learner, just want to get started in the right direction.  :-+

rstofer:
To get started with FFT, read the manual on how to use Math->FFT Operation->On and set the rest of the controls.  A source with rich harmonics is the compensation signal right on the scope itself.  All you need to do is connect the probe to the top terminal.  You don't need to connect the ground but if you wish to do so, connect it to the bottom terminal.

Your signal generator has the ability to add harmonics to a fundamental sine wave with varying voltage and phase offset.  RTFM on this.  I would probably set the AWG for 50 Ohm and run a coax from the generator to a tee at the scope.  I would plug a 50 Ohm terminator into the other side of the T.

The nice thing about using the AWG is that you know exactly how many harmonics are included and you know their amplitude.

I suppose you can add various filters on the way to the scope and see how it knocks down harmonics in the FFT but the more conventional way to study filters is with a Bode' Plot.  The new Siglent scopes are supposed to do a nice job of this, especially when coupled to a Siglent AWG via the network.  Other attempts have been made but they're not all the useful (like there is no scale).  The best way I have found for doing this at low frequency is with the Digilent Analog Discovery 2.  It will also do very nice FFTs with a lot more flexibility than the DS1054Z.
Mechatrommer:

--- Quote from: adauphin on February 03, 2019, 06:10:10 am ---Brief background, I picked up a 1054Z last year, have not had much time to practice with it, I just acquired a SDG 2042X and I would like to take some FFT measurements through a few components.

--- End quote ---
yeah, and before you make conclusion that 1054Z built in FFT is useless because 2042X has better 1Mpts FFT, you may try to comment on PC FFT SW tool for 1054Z that you can find in the net, there is one Windows SW that i know that can plot 10M FFT points from 1054Z, and there is another that can be run under linux, but no demonstrable capability to plot 10Mpts FFT. for 2042X there is no PC/Windows tool that i know that can plot greater than built in 1Mpts FFT, ie fully utilizing the scope's built in memory. about the probing technique, you may do whatever you like as a start (as long as you dont connect ground clip to the mains) and learn things in the process, if you worry too much on the technique, you may never get the chance to play with them anytime soon, just get any signal to the dso view and you can start at playing with FFT view/plot, ymmv.
Doctorandus_P:
Some time ago I saw a youtube guy (German?) with a Siglent scope (not spectum analyser) who was doing FFT's with just a piece of wire straight into the BNC, and picked up a lot of local radio stations with the FFT.

For FFT the input is usually set to 50Ohm, or an external 50Ohm termination resistor is used. The max input signal is defined by the power handling of that resistor.

You may also be interested in this nifty circuit:
http://rheslip.blogspot.com/2015/08/the-simple-scalar-network-analyzer.html
rstofer:
Dave's video on how not to destroy your scope:

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