| Products > Test Equipment |
| Siglent SDG1032X sine distortion at 1 kHz ? |
| << < (8/11) > >> |
| electr_peter:
--- Quote from: rf-loop on December 27, 2022, 12:18:58 pm ---I do not believe these all harmonics and/or these harmonics levels are true out from SDG1032X/62X output! --- End quote --- You are right to be suspicious. Harmonics are an effect (mostly) of digitization in ADC which creates additional spurs due to "sharpness" of digitization levels and low internal noise in ADC. Additional dither noise before ADC helps to smooth out these spurs and achieve better SFDR. This is an issue with a scope, not with AWG. Sources (to name a few): ADC Input Noise: The Good, The Bad, and The Ugly. Is No Noise Good Noise? Dithering in Analog-to-digital Conversion AN-804 Improving A/D Converter Performance Using Dither |
| Martin72:
FFT, two different vertical settings: https://www.eevblog.com/forum/testgear/comparison-between-siglent-sdg1000x-and-2000x/msg4604809/#msg4604809 |
| electr_peter:
Can you try this measuring setup: * SDG1032X 1 kHz signal with added controllable random noise level which has RMS of around 1/3-1/2 of LSB (LSB w.r.t. to SDS scope) * SDS2000X input with FFT With zero noise FFT looks like a comb (which is BS). With added noise noise floor should rise, but most higher harmonics should disappear. Noise level should be controllable to see the effect. If dither hypothesis is correct, dither noise should help to improve FFT view. Problem without dither is that averaging does not fully work in time domain, which results in "sharp" corners on a waveform, which are shown as a comb on FFT. If signal is 0.8LSB, ADC will always show 1LSB and average will be 1LSB. With dither, ADC sometimes will show 0LSB and average as correct 0.8LSB. |
| rf-loop:
--- Quote from: electr_peter on December 28, 2022, 07:33:42 am ---Can you try this measuring setup: * SDG1032X 1 kHz signal with added controllable random noise level which has RMS of around 1/3-1/2 of LSB (LSB w.r.t. to SDS scope) * SDS2000X input with FFT With zero noise FFT looks like a comb (which is BS). With added noise noise floor should rise, but most higher harmonics should disappear. Noise level should be controllable to see the effect. If dither hypothesis is correct, dither noise should help to improve FFT view. Problem without dither is that averaging does not fully work in time domain, which results in "sharp" corners on a waveform, which are shown as a comb on FFT. If signal is 0.8LSB, ADC will always show 1LSB and average will be 1LSB. With dither, ADC sometimes will show 0LSB and average as correct 0.8LSB. --- End quote --- There is not zero noise in ADC input. There is "lot" of wide band random noise from different parts in front end. As can see in this image below what include noise before ADC and ADC itself. Input CH1 no signal (just open BNC, AC 50ohm) 100mV/div. Zoomed in 2mV/div for display noise level. (every horizontal line is one ADC step) Display have 30s persistence. Then there is FFT. (FFT split window mode and then time domain zoomed in (time domain zoom displayed overlayed FFT display area) zoomed in horizontally and vertically zoomed in enough for count ADC steps (in this display area either the time domain scale or the FFT scale is visible, not both) Upper FFT cursor is 2dBm level. It is same as oscilloscope vertical displayed full scale (if go more details ADC full scale is tiny bit more but nonsense here) FFT (using these used settings) noise floor top is around -99dB down from full scale. (2dBm to -97dBm) When look Ch1 noise peak level it is roughly 4.8mVpp what looks like 24 steps. (if it is just 0.2mV/step then full scale is 819mV (display vertical 800mV+ bit overlap )) |
| electr_peter:
Noise is definitely non zero. Does 20MHz limit has an effect on this noise? This leaves the question what is going on with the FFT comb. DNL error or something funky inside SDS? :-// |
| Navigation |
| Message Index |
| Next page |
| Previous page |