Santa Claus brought me a very nice gift this year. He used his excellent connections with eBay and got me a Rigol DS1102E oscilloscope for Christmas! In my letter to him, I told him of the great reviews I’ve found on it; in particular several videos by Dave Jones of EEVblog and also the personal experience of Bill N2CQR as described in his SolderSmoke podcasts. Bill also has a video showing the DS1102 in action displaying a frequency sweep of a bandpass IF filter: “Sweeping a Filter with a FeelTech Sig Gen and a Rigol ‘scope.”
In a few SolderSmoke podcasts, Bill also talked about using the FFT (“fast-fourier transform”) feature on the Rigol’s math menu as a “good-enough” spectrum analyzer. That’s what sold me on the DS1102E.
After a quick trip to the local Fry’s Electronics to pick up a 10x scope probe (Santa forgot that), I gave my Rigol a little test using the built-in 1KHz square-wave generator that’s used to adjust the compensation capacitor in the probe. After adjusting the probe for nice sharp corners on the square wave with no over or under shoot, I turned on the FFT function.
I was delighted to find just what I expected in the spectral content of the square wave: pronounced peaks on each of the odd harmonics (1st, 3rd, 5th, etc.) and only a gradual decline in the amplitude of each. Here’s the screenshot made with another nice feature of the DS1102E: screen capture as a bmp file and savable to a USB drive.
You can see Cursor A on the fundamental 1KHz wave, with Cursor B on the 7th harmonic at 7KHz. The vertical scale of the FFT portion of the screen is 20dB per division. Notice that by the 25th harmonic (!!) at the far right of the display that it’s down by only about 20dB (for more on the harmonic content of square waves see en.wikipedia.org/wiki/Square_wave and www.allaboutcircuits.com/textbook/alternating-current/chpt-7/square-wave-signals/).
Here’s an especially-cool simulation that shows a series of sine waves at odd harmonics adding up to a square wave: Sine Wave to Square Wave using Fourier Series. This one is even more cool: Fourier Series Animation (Square Wave).
Tomorrow, I hope to pump some RF into the Rigol and see how it likes it. I’m very eager to know how well the FFT function works on the HF-bands frequencies.