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| Siglent SDG1032X sine distortion at 1 kHz ? |
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| gf:
--- Quote from: 2N3055 on December 26, 2022, 11:34:50 am --- --- Quote from: gf on December 26, 2022, 11:30:59 am --- --- Quote from: Martin72 on December 24, 2022, 02:09:43 pm ---Ah, dBm... |O --- End quote --- No need to bang the head. The unit (dBV, dBm,...) does not really matter. What matters is the relative amplitude of the harmonics w.r.t. the fundamental, which is usually denoted dBc. BTW, another thing which does influence the SRDF and harmonic distortion is the utilization of the DAC's and ADC's full scale range. SFDR of a DAC or ADC is usually specified in dBFS units, i.e. relative to a full-scale sine wave. I.e. the specified numbers apply to full-scale sine waves. For lower amplitudes the numbers are correspondingly worse. --- End quote --- Respectfully, it matters because distortion won't be same at different amplitudes. Relative dB will be the same of course... --- End quote --- Agreed. I mean, it does not matter, whether the FFT results are displayed in dBm or dBV units. The amplitude of the signal does of course matter, due to different distortion at different amplitudes. |
| rf-loop:
--- Quote from: markone on December 26, 2022, 11:59:45 am --- Just out of curiosity, what are (and where are specified) DANL and Phase Noise numbers for SDS2504X HD in FFT mode ? --- End quote --- Nowhere. Also it is not so straightforward in this case how to setup example for get DANL. It depends... example here in image you can see also one kind of Displayed Average Noise Level in just this used setup. /window used in this image is Flat Top. Input Signal is -40dBm ~1MHz Note: during this long averaging there have been input signal frequency drift so marker signal displayed level is dropped nearly 1dB . True signal is ~-40dBm |
| Hexley:
--- Quote from: David Hess on December 26, 2022, 05:58:55 am --- --- 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. --- End quote --- In case the OP is still looking for a homebrew solution, perhaps this would be of interest: There is a clever alternative to the Wien bridge -- the "Phase Shift Oscillator" -- that can offer somewhat lower distortion. Roger Rosens published a description in Wireless World in 1982 https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/80s/Wireless-World-1982-02-S-OCR-IDX-40.pdf of an implementation that uses a distortion cancellation technique to reduce the third harmonic component. A later version of the circuit may be found here http://www.redcircuits.com/Page82.htm. This one replaces the gain-stabilizing thermistor with an LED/photo resistor setup. I was able to get a measured 0.00031% THD at 2 kHz with this circuit. |
| Martin72:
--- Quote from: Martin72 on December 26, 2022, 10:36:25 am ---Thank you both ! --- End quote --- So some things have become clearer to me now, the first result is the re-enactment of the FFT measurement. Looks like I got higher distortions but still it´s good. My new SDG2122X looks worse with the same settings. ;) (Post it in the comparison thread) |
| JBeale:
--- Quote from: David Hess on December 26, 2022, 05:58:55 am ---An audio frequency ADC should be a lot better than -60dB THD, so that must be your Wein bridge oscillator. [...] 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. --- End quote --- Words of wisdom! I had paid no attention to capacitor type. I was guilty of using X7R for the RC tuning KYOCERA AVX SR215C104KAA (MLCC - Leaded 50V 0.1 uF X7R ) and simply swapping those out for PANASONIC ECW-FD2W104KQ (Polypropylene Film 450VDC 0.1uF) gives me a far better result: Audacity spectrum plot now shows around -90 dBc, and my cheap laptop audio input may start to become a factor here. This is as good a number as I had hoped to achieve. I had not realized how nonlinear the X7R could get in such an application, but thank you for pointing that out! |
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