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Siglent SDG1032X sine distortion at 1 kHz ?
JBeale:
Thanks everyone for the useful input. Looks like this model has reasonable performance and a decent spec, plus an array of other features that are probably useful in other ways, so I placed an order for one. Somehow I had the idea that -60 dBc was easy to obtain with a simple analog circuit, maybe with the right one but I've found that not just any circuit will necessarily do that.
Martin72:
--- Quote from: rf-loop on December 24, 2022, 02:01:01 pm ---I'm lazy and do not want calculate total from harmonics. ;)
--- End quote ---
Hopefully the siglent will do it for us someday. ;)
What your measure concerns:
Using flat top makes sense to me, as it got the best amplitude accuracy.
Also using averaging.
I see the values displayed in the screen upper right..
To be honest, in all the years I´ve used fft as it comes, adjusted as long as the output seems plausible to me, without thinking about it.
Something I want to change, so I´ve read the application notes from lecroy about setting up the fft.
Span = half of samplerate, OK got it.
"Delta"-f = 1/time per division *10 divisions, OK got it...
But there must be more when I look at your screenshot.
Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?
Is it because of the amount of fft points, we can choose up to 2M ?
And what about delta-f in general, the lower the better ?
radiolistener:
--- Quote from: rf-loop on December 24, 2022, 02:01:01 pm ---Here 1kHz sine out from SDG1000X and level 0dBm (load 50ohm).
--- End quote ---
Looks much better than Chinese generators which have about -40 dBc
David Hess:
--- Quote from: JBeale on December 24, 2022, 07:02:14 am ---I tried building a fixed 1 kHz Wien-bridge oscillator using a NE5534A opamp with the old-school filament lamp for AGC, and that works OK. The sinewave looks unclipped on my scope (which has only 8 bits of resolution I think) but my example barely reaches -60 dB THD, even after adding a few poles of RC low-pass on the output. Now I suppose the audio-frequency ADC I'm using to measure that with might also be far worse than its spec, or my garden-variety 0.1 uF ceramic caps are somehow nonlinear at that level (??) or something else, but so far I can't prove it.
--- End quote ---
An audio frequency ADC should be a lot better than -60dB THD, so that must be your Wein bridge oscillator.
Starting on page 29 of Linear Technology application note 43, Jim Williams discusses the distortion of various audio Wein bridge oscillator configurations. The most basic with a single good operational amplifier and a lamp for AGC achieves 0.007% or -83dB at 1 kHz with distortion limited by the lamp itself.
The NE5534A is pretty good so I suspect your capacitors are the problem. This is definitely a place for C0G or NP0 ceramic capacitors if you do not use film capacitors except for decoupling.
Performa01:
--- Quote from: Martin72 on December 25, 2022, 11:09:54 pm ---Span = half of samplerate, OK got it.
"Delta"-f = 1/time per division *10 divisions, OK got it...
But there must be more when I look at your screenshot.
Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?
Is it because of the amount of fft points, we can choose up to 2M ?
And what about delta-f in general, the lower the better ?
--- End quote ---
I have tried to explain the basics of the FFT setup several times before, e.g. here, Reply #23:
https://www.eevblog.com/forum/testgear/rohde-schwarz-rtb2002-vs-siglent-sds2104x-plus/msg3239832/#msg3239832
--- Quote ---• The analysis bandwidth (FFT-BW) is always half the FFT sample rate (FFT-SR).
• The frequency step (Δf or df) is the sample rate divided by the number of FFT points.
• The resolution bandwidth (RBW) is the frequency step multiplied by a factor specific for the
window function in use.
• The maximum number of FFT points depends on the record length, which in turn increases with slower timebase settings, but is ultimately limited by the maximum memory set in the Acquire menu and of course also the specified maximum possible FFT length. Apart from that, the max. number of FFT points can be further limited by the specific setting in the FFT menu.
SR (Sampe rate) = RBW * k, where k is the 3 dB bandwidth factor in bins, depending on the window function: Rectangle 0.99, Blackman 1.74, Hanning 1.62, Hamming 1.64, Flattop 3.73.
Blackman and especially Flattop are the most universal and useful window functions in practice, whereas Rectangle is rather specialized and should be avoided unless you absolutely know that you actually need it (e.g. for short transients).
Thus: df = RBW / 4 (rounded) in case of the flattop window.
To get the proper settings for any given FFT-BW and RBW pair, proceed as follows:
Determine the FFT samplerate: SR = FFT-BW * 2 [Sa/s];
Determine the number of FFT points: FFT-Pts >= SR / df [-];
Determine the timebase: TB >= FFT-Pts / SR / 10 [s/div];
--- End quote ---
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