Low freq noise from Siglent AWG or just scope ? - Page 1
Forum section: electronics / testgear

author: MathWizard
author_rank: Super Contributor
timestamp: October 27, 2021, 11:29:54 pm
content: I was making an amplifier circuit, and found it was oscillating at really low freq, down around 1Hz. My hacked sdg2042x was set to 800kHz. And then after playing around I noticed it is the AWG all on it's own doing it. I never noticed this before.Looking at the AWG alone with 1Vpp100kHz 1.24Vpp and 0.42Vrms ~0.05Hz (w/ scope on ~5sec/div)1MHz about the same voltage, about 0.5HzZooming in on the 1MHz signal, I see it's 1.25Vpp, 0.44VrmsSo without trying my analog scope, am I just seeing a frame rate effect, like when a spinning tire looks stopped ?? Or is it actually a real oscillation, maybe from ADC/DAC low freq noise??I swear I never noticed it like this before, when using the AWG and the same scope together.

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2021, 12:12:07 am
content: Here's how I saw this 1st, with a 2 stage CE amp from a simple radio, and it's doing it now, the green trace looks like it periodically gets bigger and smaller peak to peak, like it's breathing. Aand when I zoomed out I see the low Freq on screenHere it is at 200ms/divthen zooming back 1 more to 500ms/div, it draws it as a single wave

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: October 28, 2021, 03:36:39 am
content: Many years ago I was working on a modem.The output had a lot of 50Hz on it.I couldnt work out where it was coming from.Power supply was clean.Then it suddenly hit me, the sig gen was on top of the scope and interfering with it.Moved the sig gen away and the 50Hz disappeared.Another day I was working on an amplifier which had high frequency noise in it.Turned out to be my desk lamp CFL radiating HF.I designed a usb pc scope.The output had 10KHz on it.When I traced the pcb I had put the a2d input signal beneath a tc7660 negative voltage generator.The oscillator in it was radiating into the adjacent a2d input signal track.

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2021, 07:24:13 am
content: Quote from: MathWizard on October 27, 2021, 11:29:54 pmQuoteHave you ever noticed that producing some types of humor is not quite easy.(I can not believe you do not know basic fundamentals, so I first reaction is: this is some kind of humor)Also I can try humor. Problem is between chair and oscilloscope.Playing like kid, independent of true age, without full understanding is, of course, a fun pastime.But seriously. I'll give you a hint. The AD converter can also be used for frequency conversions. Sure, I could say more clearly, but then you wouldn’t start learning the basics of a digital oscilloscope.But then if this is real seriously made msg...Look your first image right top corner two numbers... 99.9999kHz and Sa 50kSa/sYour signal is seriously undersampled. fNyquist with 50kSa/s is 25kHz and you try look 100kHz signal...welcome to world of digital oscilloscopes. You are not first one.

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2021, 06:19:04 pm
content: What produces the effect where it seemed like the Vpp of the trace was getting bigger and smaller, at a rate like breathing, when zoomed at 2us/div or whatever it was, . At the time I had the AWG at 2mVpp, so I was figuring it was actual noise. It was a amplifier with postive feedback too, so it was oscillating on it own sometimes too, but at 100's of kHz.I'm not familiar with basic digital theory, I haven't made it that far yet. I generally let the scope choose whatever sampling rate it want's guess I better pay attention to it more.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 28, 2021, 06:46:22 pm
content: Most probably the very low frequency you see it's an aliasing between the sampling rate of the oscilloscope and the generator.Search inside the user manual of your oscilloscope for the word "aliasing", and if your oscilloscope has an anti-aliasing setting, try to enable that.

author: Performa01
author_rank: Super Contributor
timestamp: October 29, 2021, 09:40:35 am
content: The "anti aliazing setting" in any DSO is to set it up for max. memory usage.The fundamentals again: the sample rate has to be at least twice as high as the highest input frequency. In practice it has to be rather triple than twice.So in case of a pure sine wave (and only then!) a sample rate of at least 2.5x the signal frequency would suffice.In case of a 100 kHz sine, 250 kSa/s would be the bare minimum for a faithful reconstruction of the input signal.In case of an 800 kHz sine, 2 MSa/s would be the bare minimum for a faithful reconstruction of the input signal.How do you achieve higher sample rates? Two possibilities:1. select a faster (shorter) timebase2. increase the max. memory (in the Acquisition menu, this scope has 7/14 Mpts available)