Entry Level DSO FFT uses for low frequency noise analysis and investigation - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: August 11, 2025, 05:00:47 pm
content: This all started over here.https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg6008181/#msg6008181Best

author: mawyatt
author_rank: Super Contributor
timestamp: August 11, 2025, 05:08:35 pm
content: It seems the SDS800X HD can be quite useful at lower frequencies with the built-in FFT, ERES and FIR Filters. Allowing the FFT to "operate" on the math functions which include ERES and FIR Filters (LP, HP, Band Reject, and Band Pass) allows one an extremely large "playing field" for all sorts of uses including possible low frequency noise analysis.Have any others investigated this?Best

author: 2N3055
author_rank: Super Contributor
timestamp: August 11, 2025, 05:22:35 pm
content: Mike,As I posted here:https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg6008137/#msg6008137little SDS800xHD can be used for quite "fancy" stuff.As for FFT Decade plot starting from lower frequencies like 0.01Hz, I would like to see this too.So, I already asked the question and Performa01 also supports us this is a good idea..Now we wait.

author: mawyatt
author_rank: Super Contributor
timestamp: August 11, 2025, 05:35:37 pm
content: Don't forget to ask them about the lower limit for the Bode Function tooYou folks that have Siglent's "ear" are a great asset, thanks for all the help and support you provideBest

author: 2N3055
author_rank: Super Contributor
timestamp: August 11, 2025, 05:42:08 pm
content: Quote from: mawyatt on August 11, 2025, 05:35:37 pmGlad if we can help. Of course, we all benefit from improvements..

author: mawyatt
author_rank: Super Contributor
timestamp: August 11, 2025, 06:42:58 pm
content: Here's an example of what was discussed in the linked thread above. This is created with an input signal of 20uv (-94dBv from a 60dB attenuator and AWG set to 20mv (-34dBv)) at 3Hz. ERES 3 bit math channel created and used as input to FFT. Sweep set to 10s/div and Sample Rate 1MSa/s with 100Mpts memory. This produces an ERES low pass corner frequency of 7kHz due to 3 Bit ERES -3dB low pass filter effect of 0.007 * Sample Rate ( 1MSa/s * 0.007 = 7kHz).The FFT was setup with decimated sample rate of 25KSa/s with 2Mpts from 1Hz (Lowest limit) to 100Hz Log X Axis. Y axis is in dBv and shows the 3Hz signal at ~-94dBv as well as the Mains noise signal @ 60Hz with an FFT average of 50.With a good low noise, low frequency 60dB preamp this looks attractive for a low frequency noise analysis/investigation setup, especiallyIFSiglent allows lower corner frequencies with Log X Axis.Best

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2025, 08:54:56 pm
content: The noise curve looks funny to me. Can you run the same test to measure the oscilloscope's own input noise?

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 12:46:48 am
content: See link above. We first ran an Open BNC with nothing attached. Since this DSO has no 50Ω internal termination, the termination was standard 1MΩ.Best

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2025, 05:56:15 am
content: The 20 MHz bandwidth noise of 37 microvolts RMS is possible, but the lack of flicker noise does not look right when there is no increase in midband noise. It is difficult to tell what is going on without normalization to a resolution bandwidth of 1 Hz.Could the high frequency noise be aliased into the displayed FFT frequency range covering up the low frequency noise? This would make the displayed noise flat.

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 12:46:38 pm
content: Here's the averaged (34) FFT of nothing attached to the DSO BNC, thus open circuit channel 1 (C1) 1MΩ Input Z. This FFT (F2) is created with 2Mpts from 1Hz to 100Hz with a decimated sample rate of 25kSa/s with an average of 34. The input for the FFT is math channel (F1) created 3 Bit ERES which operates at 1MSa/s from the 10s/div Time Base. The 3 Bit ERES produces a FIR Low Pass filter of 0.007 * Sample Rate or 7kHz.So the input is being sampled at 1MSa/s, then FIR filtered by ERES to 7kHz, then decimate resampled at 25kSa/s for the FFT for a 1Hz to 100Hz range, don't think noise folding (aliased) is taking place here.Edit: Here's another (PNG_200) using the built-in FIR Low Pass Filter (F3) set to 10kHz and the FFT operating from the LPF (F3). The Adaptive Sample Rate was enabled for the LPF.Agree the lack of a bulging noise profile at the lower (<10Hz) frequency is apparent. Not being an expert in oscilloscope front end design, we won't comment on why this is so!Best

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2025, 02:09:37 pm
content: From your graph I calculated a noise spectral density of about 1.6 microvolts/SqrtHz, which is at least a factor of 20 too low, and probably 200 too low, so I have no idea what is going on, but I sure would not rely on your results until I verified them with a calibrated external noise source.Averaging before the FFT might explain it.

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 02:55:25 pm
content: The 3Hz input signal of 2uv and 20uv FFTs confirms the amplitudes are correct (why we did so). The FFT average is completedpostcomputations and FFT average shown, otherwise ifpreFFT computations the 3Hz and mains 60Hz signals would not be displayed as they would "average" out before performing FFT since a reliable uniform trigger can't be achieved at this low level.BTW we see a similar result with the higher end SDS3000X HD, even using the hardware enabled 3 bit ERES (SDS800X HD uses software ERES), or the software ERES, or the FIR LPF. Here you can see the 20uv signal (-94dBv) and Mains (60Hz) signals with the SDS3000X HD using hardware 3 Bit ERES.Also, note the resolution bandwidths.Best

author: 2N3055
author_rank: Super Contributor
timestamp: August 12, 2025, 03:09:01 pm
content: Quote from: David Hess on August 12, 2025, 02:09:37 pmActually, ERES before FFT is not needed, for FFT alone.With FFT averaging, after FFT, signal floor looks the same. Without any ERES or Filters.P.S: Mike was faster..

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 03:28:22 pm
content: Quote from: 2N3055 on August 12, 2025, 03:09:01 pmAgree ERES is not required, but we included it for concerns of any possible noise folding or aliased signals as the pre FFT low pass filtering effect. Here we've included FFTs without ERES or Filtering of any kind from the SDS800X HD and SDS3000X HD and showing a 20uv (-94dBv) 3Hz signal, also note the Mains 60Hz "hum". Didn't wait for the full average of 64 tho, so results are somewhat more "noisy".Hopefully the SDS3000X HD updated firmware will correct the X axis FFT display showing some of the Time Domain axis labels not removed when using Exclusive FFT DisplayBest

author: gf
author_rank: Super Contributor
timestamp: August 12, 2025, 03:30:00 pm
content: Quote from: 2N3055 on August 12, 2025, 03:09:01 pmIt acts as anti-aliasing filter for the downsampling from 1 MSa/s to 25 kSa/s, reducing noise aliasing.[ Note that FFT sample rate is 40x lower than acquisition sample rate. ]Still not a good anti-aliasing filter, but better than nothing.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2025, 03:33:26 pm
content: Quote from: mawyatt on August 12, 2025, 02:55:25 pmI converted dBV to uV, and then normalized from the stated 0.04 Hz RBW to get 1.6 microvolts/SqrtHz. I did not bother correcting for the window function of the FFT because it is unknown and insignificant in this case.Maybe I did it wrong, but that result is way off from what it should be unless the Siglent high impedance buffer uses alien technology. Whatever else is happening, that is not the correct level of noise.Update: Nevermind. I swapped microvolts and nanovolts. My result should have been 1600 nanovolts/SqrtHz which is believable, but the 1/f noise curve appears to still be missing. Maybe a display range of 1 to 1 MHz would reveal it as the corner frequency may be quite high. 1600 nV/SqrtHz should be well into the 1/f noise curve.Update Update: Hmm, I could calculate the 1/f corner frequency knowing where 1600 nanovolts/SqrtHz is and a white noise estimate from the full bandwidth RMS noise. This would tell us where to look for the corner frequency, but is hardly required.

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 03:57:34 pm
content: Quote from: David Hess on August 12, 2025, 03:33:26 pmWe see a 444uVPP and 60uV SD (RMS) across 10s/div (100 second sweep span) for the SDS800 and a 421uVPP and 39uV SD (RMS) with the SDS3000, both with input BW set to 20MHz. BTW Window function is Flat Top.Best

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2025, 03:59:24 pm
content: Quote from: mawyatt on August 12, 2025, 03:57:34 pmWhich are believable. It was the FFT noise results that I was questioning, and I am still not sure they are correct.See my update above.

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 04:06:13 pm
content: Quote from: David Hess on August 12, 2025, 03:33:26 pm1600nV/Rt Hz is 1.6uV/Rt Hz!!Based upon the SD we get 13nv/Rt Hzbroadbandacross 20MHz (set by channel input LPF) for SDS800 and 9nv/Rt Hz for SDS3000. This assumes a brick wall 20MHz input filter which it's not, if it's a 1st order then Effective Noise Bandwidth becomes ~31MHz and density becomes 11nV/Rt Hz and 7nV/Rt Hz respectively.Best

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2025, 04:13:58 pm
content: Quote from: mawyatt on August 12, 2025, 04:06:13 pmYes, that is what I meant by confusing the two. I calculated the number and units correctly, but wrote it out in microvolts while making the comparison in my mind to the more familiar nanovolts.QuoteThat is about what it should be in a good design. I was thinking 14nV/SqrtHz but what is a nanovolt here or there?The SD measurement of 60 microvolts RMS or 39 microvolts RMS over 20 MHz indicates a huge amount of flicker noise, with an associated high corner frequency, compared to vintage oscilloscopes. An FFT over the proper range of frequencies should very easily show it.

author: 2N3055
author_rank: Super Contributor
timestamp: August 12, 2025, 04:33:50 pm
content: Quote from: David Hess on August 12, 2025, 03:33:26 pmRBW is stated in addition to Bin Width and is already corrected for window used.To make sure I understand: You are referring that sub 1 Hz noise is too low? NSD of 1.6µV/√Hz , but for area below 1Hz?It is not like it is 1.6nV/√Hz so we can scratch our heads..... But that is wrong calc..Front end design is split path, and low frequencies are servo-ed by precision opamp down to DC..If I put a 50 Ω terminator on input, it gets a bit better...

author: 2N3055
author_rank: Super Contributor
timestamp: August 12, 2025, 04:39:59 pm
content: Quote from: David Hess on August 12, 2025, 04:13:58 pm34µV RMS over 20 MHz BW drills down to 7.6nV/√Hz...Around 6nV/√Hz over 30ish MHz...that is not bad ....

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 04:50:41 pm
content: All our measurements were with Open Input DC coupled (worst case), using AC coupling improves due to partial removal of flicker noise, 50Ω terminated AC even better, and Shorted even better.In fact the SDS800 shows 521nv/Rt Hz SD with GND Input enabled, yes that's nV!! The display is just a flat line at 500uV/div!!! Which is questionable as the SDS3000 shows 29uV SD with GND enabled, so wouldn't put much into the SDS800 GND performance.With 50Ω Termination we get ~5.3nV/RT Hz on SDS3000 and using and external 50Ω on SDS800 we get ~5.7nv/Rt Hz.Best

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 05:06:34 pm
content: Quote from: mawyatt on August 12, 2025, 04:50:41 pmThis begs the test with an external BNC short which we did. Now we see a believable result of 31.6uV SD which makes sense and the display shows a typical result.Think we know why the SDS800 Internal Select GND is showing the result it does. This is because the GND is not actuallyat the inputbut behind some attenuators and/or gain blocks downstream towards the ADC.Anyway, this cleared this up for our understanding.Best

author: 2N3055
author_rank: Super Contributor
timestamp: August 12, 2025, 05:10:08 pm
content: Quote from: mawyatt on August 12, 2025, 04:50:41 pmHonestly, to me it looks like SDS800xHD nulls the data after ADC when you set it to ground. I can hear relay click, but I think data is just nulled.EDIT: Mike, today you are just too fast for me...520 nV is exactly half of the one ADC step at that V/div. Full scale is 4mV, divided by 4096..So it is digitally nulled.. MEAN is also 520nV, and Stdev P-P is zero. not a single flick...

author: gamalot
author_rank: Super Contributor
timestamp: August 12, 2025, 05:21:45 pm
content: I also tried it on my SDS800X HD, and the results were pretty much the same, except there was no 50Hz power frequency noise (but a small spike closer to 60Hz, which was weird).

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 05:41:35 pm
content: Quote from: 2N3055 on August 12, 2025, 05:10:08 pmGood spot and makes sense!! We see a 1.04uVPP which is twice SD and Mean!Thinking about this, since the SDS800 is a low cost DSO, the added cost of a relay for GND may have been the decision to avoid the relay and just do it digitally.Best

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 05:46:54 pm
content: Quote from: gamalot on August 12, 2025, 05:21:45 pmIf you look at some of our FFT plots the "close to 60Hz" also shows up.Best

author: gamalot
author_rank: Super Contributor
timestamp: August 12, 2025, 06:49:47 pm
content: After seeing that the spike was missing from the picture in post #11, I guessed it might be a side effect of ERES, but I was wrong. It was still there after turning off ERES.

author: mawyatt
author_rank: Super Contributor
timestamp: August 12, 2025, 08:26:37 pm
content: Here's another plot that shows the result of sweeping a -80dBv (100uv) and -100dBv (10uv) signal showing accurate low frequency results.

author: gamalot
author_rank: Super Contributor
timestamp: August 13, 2025, 06:12:07 am
content: It took me ages to figure out that the spike would only disappear if I set the window function to flat top, and no other settings would work.

author: gf
author_rank: Super Contributor
timestamp: August 13, 2025, 09:47:47 am
content: Quote from: David Hess on August 12, 2025, 03:33:26 pmThe 1/f corner seems to be > 100 kHz. See image SDS824X_HD_Noise_20Hz-25MHz_4Ch_F30M in the demonstration threadhttps://www.eevblog.com/forum/testgear/sds800x-hd-review-demonstration-thread/msg5293744/#msg5293744And of course, at 1MSa/s, the noise beyond 500 kHz aliases into the 0...500 kHz region, but I'm not sure if this makes a huge difference at low frequencies, where 1/f noise possibly still dominates. I find it interesting that the 1/f slope decreases in the region < 1 kHz.

author: David Hess
author_rank: Super Contributor
timestamp: August 13, 2025, 08:14:11 pm
content: Quote from: 2N3055 on August 12, 2025, 04:33:50 pmNo, I updated my post. 1.6 uV/SqrtHz, or 1600 nV/SqrtHz is believable from 1 to 10 Hz. I calculated uV/SqrtHz but compared it to nV/SqrtHz.The comparable measurement on an old JFET input oscilloscope could be maybe 80 nV/SqrtHz. The Siglent measurement seems high to me for a dual-path high impedance buffer, so I wonder if there is some systematic problem with the Siglent measurement. A CMOS input buffer by itself could easily generate those levels of flicker noise though, if not more. I think Texas Instruments has a CMOS high impedance buffer where the datasheet shows that kind of noise level.Quote from: 2N3055 on August 12, 2025, 04:39:59 pmIt is not bad, especially for a MOSFET input and a modern DSO. The benchmark I use for low noise are the old Tektronix 200 MHz vertical amplifiers which achieve 28 microvolts RMS over 100 MHz, or a noise bandwidth of 160 MHz. (1) So that Siglent is good, but not "low noise" except in comparison to modern instruments. The JFETs Tektronix was using also had 1/f corner frequencies below 1 KHz.(1) I made this measurement using a 200 MHz 7A26 in a 500 MHz 4-bay mainframe, driving a 100 MHz 7603 through the vertical output, and then making a tangent noise measurement on the 7603 to determine RMS noise. I verified the tangent noise measurement method with a lower frequency wideband noise source independently measured with an RMS voltmeter of sufficient bandwidth. Accuracy of the tangent noise measurement method was better than 5%, and probably within 2%.Quote from: 2N3055 on August 12, 2025, 05:10:08 pmA couple years ago on the Rigols we tracked that down to the DSO setting the gain of the variable gain preamplifier before the digitizer to absolute minimum, so the ground setting yielded very misleading noise measurements.

author: 2N3055
author_rank: Super Contributor
timestamp: August 13, 2025, 09:12:06 pm
content: Quote from: David Hess on August 13, 2025, 08:14:11 pmWe discussed this before, where I agreed with you on this. No contest there. Numbers don't lie.When we say low noise, we mean in comparison with modern digital scope on the market now.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2025, 03:17:55 am
content: Quote from: 2N3055 on August 13, 2025, 09:12:06 pmThe Siglent measurement is the lowest I have seen in recent years for an entry level DSO, or at least the lowest that I believe. Most are much higher.

author: Performa01
author_rank: Super Contributor
timestamp: August 14, 2025, 06:04:43 am
content: In addition to the measurements shown in my Review & Demonstration Thread, I've done one more low frequency noise plot, see attachment.The challenge is to avoid aliasing and there we can assume that the noise above some 200 kHz is negligible when compared to the strong LF noise caused by the dual path input buffer, that attenuates low frequencies by almost 20 dB, thus increasing the noise figure by that amount. Then add the high source impedance of about 100 kΩ for the LF-Amplifier (JFET-OpAmp). All this explains the massive increase of noise towards low frequencies.However, this is not the classic 1/f noise. We have the HF-buffer, which is perfectly low noise above 10 MHz, at <2,5 nV/√Hz! Of course, this buffer is CMOS and will produce a pronounces 1/f noise, but still far less than what we see because of the dual path design. The LF-Buffer with its >30dB noise figure is mixed into the HF path. We get a transitional zone between some kHz and 150 kHz, where the contribution of the LF-path decreases linearly. Below ~3 kHz we see the normal 1/f noise of the FET-OpAmp.So this tries to explain why the slope of the LF-noise is not constant and remind everyone that the noise performance for higher frequencies (already starting at 200 kHz) is absolutely flawless.Knowing all this, we can easily tolerate aliasing from above 1 MHz, hence my setup for the noise spectrum shown here is:- 20 MHz physical 1st order bandwidth limit at the output of the PGA.- Keeping the memory depth at or below 10 Mpts, because Math and Measurements cannot handle more data in a low-end SDS800X HD.- Keeping the acquisition sample rate at 5 MSa/s, in order to have enough headroom for a digital AA-filter.- Digital high order FIR filter with 1 MHz corner frequency, acting as AA-filter for the FFT, which in turn runs at only 1.25 MSa/s.SDS824X_HD_Noise_10Hz-625kHz_LPF

author: iMo
author_rank: Super Contributor
timestamp: August 14, 2025, 07:36:58 am
content: With an at least +40..+60dB LNA in front of your scope (you will need a variable gain amplifier as well) all these effects above become negligible, imho..PS: in other words - any scope with "a good fft functionality - like it can apply a windowing function and it can average N spectra.." will show you the same results..

author: mawyatt
author_rank: Super Contributor
timestamp: August 14, 2025, 02:40:42 pm
content: We've copied the setup from Performa01 above showing the Input Noise FFT without any FIR Pre-filter (F2 Red Trace), with FIR Pre-Filter (AA) @ 1MHz (F3 Blue Trace), and with FIR Pre-Filter (AA) @ 100kHz (F4 Green Trace).Note the effects of the 1MHz Pre-Filter (F3 Blue Trace) around ~150KHz (and F4) hinting some slight Noise-Folding (Aliasing) or Inter-Modulation is present.Anyway, this is three 2MPts FFTs, with two FIR LPFs all being processed simultaneously on the SDS800X HD!!!Best