Measuring DUT low frequency noise 0.01-100 Hz Below noise floor of the test equi - Page 1
Forum section: electronics / projects

author: MasterT
author_rank: Frequent Contributor
timestamp: January 16, 2024, 11:25:24 pm
content: Direct approach to measure noise is too complicated and very expensive. Accuracy below 1 nVis not precise, if trusted at all.Propose: implement "lock-in" trick to measure noise Below noise floor.1. Gonna to use ads131m04 - 4 channel simultaneously sampling adc, the more channels the bettersince each test run takes more than 100 seconds.2. Input switch toogling between DUT output (voltage reference) and ground. Switching only ones percycle, 100 sec data from DUT, another 100 sec data for test pre-amp + adc.3. Calculate fft to get energy distribution. Sum & accumulate separately data for grounded input and DUT4. Repeating for 100 or so periods. 20.000 seconds seems too long time, but measured noise level expectedto be down to 0.1 nV (!!!) that can't be reach in any other way.5. Last step calculate difference in two data array, that belongs to DUT exclusively.As sampling goes in "interleaved" manner, drift due to temperature variation or any other reason shouldbe nulled out.

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2024, 11:48:32 pm
content: And the question is?

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: January 17, 2024, 12:23:27 am
content: I have an Agilent 89441A dual channel FFT analyzer that can docross correlation. I tried it and the results were so good that Iassumed operator error.Amplifiers with too much noise current create a noise voltageover the DUT that does not average away since that voltage iscommon to both channels. One of the reasons of the JFET amplifierin the other thread.regards, Gerhard

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 12:46:12 am
content: To Gerhard_dk4xp: Reffering to your design with 16x ADA489x , my approach would allow to use TL081 and get better results.4 channels to run comparative test with 4 references same time.And Agilent is not required, so far my nucleo H-743 is more powerfull than actually required.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: January 17, 2024, 01:03:35 am
content: Come on, TL08XXX is one of the worst noise generators available.1/f corner at 800 Hz, and even the flat part is an offense.And for me, the Agilent is required. It goes from mHz to 2.65 GHzwith mHz resolution.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 01:48:34 am
content: If you understand "lock-in" concept, than you should realize that it's just takes more time if using TL081. Certanly I exaggerated, idea is not to buy higly overpriced OPA, something like opa145 probably 'd be fine. I'm experimenting with TLV6742 at the moment, the best noise/ price ratio OPA so far.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 17, 2024, 01:49:26 am
content: day dreaming hereas Gerhard_dk4xp said get yourself the right equipment, the way you go youll never get a low floor noise,grab yourself an expensive generator who can do -110 db and well talki'm working with expensive acquisition stuff, i'm sampling 8 channels at 125Mbs minimum for very faint spikes, doing fft and more, and believe me going down pass -70 -80 db and more, to get looooooooooow noise floor is difficult, adc noises, yes they make some, supply issues, impedances etc ... the ADC cost around 50-60$ usd per channel, and the box cost 11,000 usd and can go up to 2 gig of frequencies, and do have peak and hold circuitry, and specially created software to do soEDIT oh and i'm down to nanovolts btwi have ssg3032x and ssa3032x stuff just to do thatand you suggest a mere TL08xxx op amp ooofffffffffare you here to ask for a design ?

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 02:01:53 am
content: Quote from: coromonadalix on January 17, 2024, 01:49:26 amNo. What I'm saying is iwritten in the topic name.Open to discussion, if this already was done - I'm no inventing a bycicle? It should be in radioastronomy or in scienceMy google search play "imbecile" for unknown reasonWill publish results as I get, work in progress.

author: moffy
author_rank: Super Contributor
timestamp: January 17, 2024, 02:54:36 am
content: Quote from: MasterT on January 17, 2024, 01:48:34 amI know what a "lock-in" amplifier is, modulating a low frequency signal, amplifying then demodulating. I also know that if you parallel the same type of low noise amplifier and their noise is non correlated that you improve the noise performance as the square root of the number of devices but I am not 100% certain as to what you mean by "lock-in"?

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 04:59:06 am
content: Quote from: moffy on January 17, 2024, 02:54:36 amI have a switch as described above - connecting pre-amp to D.U.T. and to ground. ->ModulatorSum & accumulate in two different arrays ->Synchronous Demodulatorrunning in software.Since modulator clocked from the same uCPU, there is even option to remove glitches comming from the switch - discard a few samples right after switching happened.Idea is not new, only application for noise measurements. Both data arrays have noise spectrum data, one is noisepre-amp+adc, anotherDUT+pre-amp+adc. Math is primitive, right?

author: moffy
author_rank: Super Contributor
timestamp: January 17, 2024, 05:06:17 am
content: Quote from: MasterT on January 17, 2024, 04:59:06 amSo effectively you are using the principle of zero drift amplifiers, chopper stabilised, to shift the noise measurement out of the 1/f noise region, is that correct?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2024, 11:03:44 am
content: Chopping the signal at the input can move the frequency band of interrest to higher frequencies. This works OK for the voltage noise, but it can add current noise. Another point is that with a simple switching between the input and GND one only measures the input half the time and thus reduces the effective signal amplitude to half. Ideally one would at least have 2 such channels in parallel to alternate between them.It depends on the signal source if the current noise will be a problem. With the normal AC coupling the impedance can be quite high at low frequencies.

author: rhb
author_rank: Super Contributor
timestamp: January 17, 2024, 02:51:32 pm
content: A recommendation from a career reflection seismologist, someone who made his living doing this sort of stuff for large oil companies including super majors.Record 4 DUTs switching between DUT and ground with separate records for each DUT or ground series. Make sure you have no aliasing. For 100 Hz you needat least500 Sa/s and an analog filter which is -6 dB for every ADC bit at Nyquist. If you can sample faster you can relax the analog filter and kill the higher frequencies after the FFT. Attempting to set a corner frequency for the anti-alias filter which is above 1/2 of Nyquist will lead to trouble you really don't want to experience.FFT to frequency (make sure you detrend the data so the start and end of each series is zero and then double the length by adding zeros)Cross correlate all 4 traces in pairs by multiplicationAverage the cross correlations of all the recordsSubtract the averaged cross correlation from each traceAverage the differences for each DUT and ground traceQED if there aren't any mistakesThe cross correlation step removes external noise sources. Averaging reduces the Gaussian distributed noise in the estimate of the external noise by 1/sqrt(N) where N is 6 times the number of DUT or ground records.Getting the FFT correct is where most people fail by neglecting to remove the discontinuity between the first and last sample. That adds a spurious constant to every frequency.This is all very simple, but the nuances will eat you alive if you ignore them. The Seismic Unix package will do all the heavy lifting but may not have a detrend module. It's been many years since I last used it, but I did software support for 2 variants for most of my career. There is a github repository for the code, but Stockwell did not preserve the fantastic wiki that a post doc at MIT built with examples describing each module. However, it can be retrieved from the Internet Archive.Have Fun!RegBTW If you will post your raw data I'll do the processing to provide a complete example.

author: macaba
author_rank: Regular Contributor
timestamp: January 17, 2024, 05:32:28 pm
content: The existing literature has titles like "low frequency noise measurement with cross correlation", if you're interested.They tend to use expensive devices to do it, so I am very interested about the result of doing it with cheap parts.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2024, 06:22:25 pm
content: Even when correlation and averaging can reduce noise by a longer averaging time, it is still worth to start with low noise. For a target of 0.01 Hz some chopper action can be a good idea, but it needs care with current noise. The TLV6742 looks reasonable low noise for the power, but it is not really worth saving a few cents on the frist amplifier that sets the noise level. Normally I would consider this a case for still using low noise discrete JFET amplifiers. It would take 9 times the measurment time to compensate for 3 times the noise to start with.A multi channel ADC is a good start for doing correlation measurements, though ideally one wants good isolation. Not sure if one really needs 4 channels, 2 may be enough.The equipment for correlation was expensive as ADCs and the computing part were quite expensive in the old times and it is still a niche application, so no large numbers. For the correlation there is also a big difference in the computationaly effort in whether one wants correlation with zero delay, or correlation with delay / frequency resolution. The computation also gets a lot less demanding for more low frequencies, and not higher frequencies like the 100s of kHz or even more.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 07:29:17 pm
content: Quote from: moffy on January 17, 2024, 05:06:17 amNo. Has nothing to do with AZ /zero-drift , there is no freq. shift up/ down, switch operates at 10 mHz -> completely in the 1/f."LOCK-IN". Period.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 07:40:32 pm
content: Quote from: macaba on January 17, 2024, 05:32:28 pmNo, wrong term cross-correlation was brought here by someone else. It has nothig to do with CC. 4 channels processed completely separately, comparison results at last point by operator. Multichannels necessary to null out enviroment interference, and to accelerate throubleshooting or adjustment related to low freq. noise.Lock-in correct name, see picture. The fact that it shows optical signal doesn't change anything. Light source -> ground in this case. Optical sample -> DUT. FFT complicates lightly, but concept would works if simple low pass filter introduced. Difference in DC level output of the two channels would directly indicate noise level

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2024, 10:32:22 pm
content: Quote from: MasterT on January 17, 2024, 07:29:17 pmIt is still a kind of chopping / AZ switching, just too slow to be effective for the relvant frequency range. This way there is nothing really suppressing the noise - just 2 parallel channels averages and with most amplifiers a rather poor noise performance.The cross correlation method is the standard way to measure very low noise levels, even below the amplifier noise level.One can calculate correlation via FFT, as the correlation is related to folding and FFT can be used as on effective way to calculate folding. So it may be possible to get the cross correlation without knowing it by its normal name.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 17, 2024, 11:15:56 pm
content: Quote from: Kleinstein on January 17, 2024, 10:32:22 pmYou know when you are talking with AI. It's the same like talk to imbecile with encyclopedia , big data base of knowledge in his hand but no brain to differentiate what is related and what is not. Idiot just trash whatever it could pick up out of book on you, and watching if you could possible find something interesting.

author: moffy
author_rank: Super Contributor
timestamp: January 17, 2024, 11:33:36 pm
content: Quote from: rhb on January 17, 2024, 02:51:32 pmA fascinating process, using the randomness of two channels noise to extract the common input signal via correlation.P.S. Saving your explanation for reference purposes.

author: rhb
author_rank: Super Contributor
timestamp: January 18, 2024, 02:34:51 am
content: Quote from: Kleinstein on January 17, 2024, 06:22:25 pmDelay is just a linear phase shift with frequency. Proper name is "the shift theorem". It's a simple linear fit to the phase of the product of the 2 FFTs. Computational effort is the same, 2 FFTs and a straight line fit. DSP was developed by the oil industry because we needed data below 100 Hz. No one else did, so big oil paid for it all. When the ADCs got faster other fields started using it. Anyone doubting that should look up the history of Norbert Weiner's Geophysical Analysis Group and Enders Robinson and Sven Treitel's papers in the '50s and early '60s. TI started life as the instrument design and manufacturing division of Geophysical Services Incorporated. I know a lot of people who get misty eyed talking about GSI. Seismograph Service Corporation published a series of compilations of reprints of their papers as "The Robinson and Treitel Reader". The book is a legend in seismic exploration.Have Fun!RegThe more channels you have the more cross correlations you can do. You need at least 2.

author: mikerj
author_rank: Super Contributor
timestamp: January 18, 2024, 01:17:47 pm
content: Quote from: MasterT on January 17, 2024, 11:15:56 pmIf you are really an AI then you are definitely advancing; you completely fooled me into thinking you were an obnoxious human with a few buzzwords but little actual knowledge.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 18, 2024, 02:17:09 pm
content: Quote from: mikerj on January 18, 2024, 01:17:47 pmI remeber, in 90-th one of those "mad cow desease victim" was pocking around with wooden stick, andmiracle happened - he accidentally hit a switch. Idiot was awarded a title "great inventor of the year".Elektor magazine start to publish enormous number of projects that use switches with plastic exenders,so PCB was intentionaly placed at the back of project case and extenders alow to switchas always from fron panel of the devise. Attaching PCB directly to front panel was illegal for long time,so engineer forced to use extenders.Later on same concept was apply to potentiometers.So get lost stupid.

author: rhb
author_rank: Super Contributor
timestamp: January 18, 2024, 02:59:58 pm
content: Quote from: moffy on January 17, 2024, 11:33:36 pmActually, you've got it backwards. In this case the signal is uncorrelated, but the noise is correlated. So one exploits the fact that a cross correlation between uncorrelated series is zero. This allows separating the correlated noise and removing it.The OP wants to measure low frequency voltage reference noise. All those simultaneous deviations of the reference graphs so popular among the volt-nuts. I have repeatedly asked for ASCII data so I could demonstrate the process, but for some reason volt-nuts don't like to share data in usable form. Same for time-nuts. As much verbiage as has been posted about thermal stability and other noise processes, etc, one would think they'd welcome the chance to obtain cleaner data, but you'd be very wrong. I was sent a plot of 4 frequency references by a time-nut. It had obvious strong correlated noise at femptoHz frequencies and would have been a snap to clean up. But when I asked for the data he had plotted he 'had to think about it". I never got any data and dropped off the time-nuts list.The only rationale I can imagine is that they are mortally afraid of mathematics. A successful proof would compel them to learn math. Lacking confidence in their abilities, they avoid the issue by refusing to allow me to have the data except as pictures which are not usable for mathematical analysis.Have Fun!Reg

author: moffy
author_rank: Super Contributor
timestamp: January 18, 2024, 10:44:40 pm
content: Quote from: rhb on January 18, 2024, 02:59:58 pmSorry I am slightly confused, I thought the use of two separate channels was because their noises would be uncorrelated but the common input signal would be correlated between the two channels thereby zeroing the uncorrelated input noises but extracting the common signal or have I got my terms wrong somehow? By signal I am referring to the voltage noise of the device under measurement. Sorry for my slowness but I must be missing a fundamental concept. How can two independent channels have correlated noise?I am very surprised that the Volts-Nuts would turn down an offer by you to have their data refined but they are admirably a very DIY and committed group.

author: rhb
author_rank: Super Contributor
timestamp: January 19, 2024, 02:31:33 pm
content: Temperature, supply voltage changes (mains voltages vary +/- 5%) are correlated noise. In this case the desired signal is the uncorrelated reference noise.I described the basic process of separating correlated and uncorrelated series. Which is the signal and which is a distinction made by the user. What I described is standard practice and one of the first things taught in DSP 101.I was and still am astonished by the consistent refusal to provide data. I was very accustomed to not having access to the raw data in the oil industry, but there are valid commercial reasons for that. I am forced to conclude that my mathematical background is threatening. Can't grasp why someone would feel threatened by my attempting to teach them math. If anyone has an explanation I'd love to hear it.@andreas posted 4 aging curves with the remark "try to find the exponent of these" as two of the curves were quite flat. I was ready to take him up on it and asked him to post the data in CSV format. Never happened. Andreas says it can't be done and makes sure he can't be proved wrong by not letting me try.So instead we must wait until I've collected several years of data myself. But I'll have a 100 channel 3497A & 3458A measuring about 80 Vrefs with the other channels monitoring the supply voltages and temperatures of the Vrefs.Have Fun!Reg

author: moffy
author_rank: Super Contributor
timestamp: January 20, 2024, 08:42:36 am
content: A nice simple explanation of using cross correlation for extracting a signal from noise, just for reference purposes:https://www.zhinst.com/others/en/blogs/how-reduce-noise-floor-and-improve-snr-employing-cross-correlation-techniquesalso:https://www.sciencedirect.com/science/article/pii/S0263224121008095

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: January 20, 2024, 11:41:31 am
content: <https://tf.nist.gov/general/pdf/1133.pdf><https://arxiv.org/pdf/1003.0113.pdf>Or the author's most interesting web site:<https://rubiola.org/>, for example item 11.regards, Gerhard

author: tomato
author_rank: Regular Contributor
timestamp: January 21, 2024, 05:06:15 pm
content: Quote from: rhb on January 19, 2024, 02:31:33 pmThe explanation is likely very simple; people don't offer you any help because they're put off by your attitude. The fact that you think it's because they're intimidated by your mathematical background is just one example of that attitude.

author: rhb
author_rank: Super Contributor
timestamp: January 21, 2024, 09:35:13 pm
content: Quote from: tomato on January 21, 2024, 05:06:15 pmMy attitude is a consequence, not native to me. When I came along asking for data it was refused. After many years, I still don't understand why trying to teach someone some very cool math is offensive. So I'm guessing.Ketchup!Edit: I just realized that from the number of posts and where they appear you are almost certainly someone's sock puppet. Question is, whose?

author: tomato
author_rank: Regular Contributor
timestamp: January 22, 2024, 12:33:51 am
content: Quote from: rhb on January 21, 2024, 09:35:13 pmRegardless, you still own your attitude.QuoteNo, I am not a sock puppet.

author: rhb
author_rank: Super Contributor
timestamp: January 22, 2024, 02:31:29 am
content: Actually, not only am I sure you're a sock puppet, but I am pretty sure I know whose hand is in the sock and can prove it. Little thing called "lexical analysis". Funny thing about humans. They all have unique signatures which can be identified based on word choice and sentence structure.I'd very much prefer to be left alone than have you banned.

author: tomato
author_rank: Regular Contributor
timestamp: January 22, 2024, 04:31:26 am
content: Quote from: rhb on January 22, 2024, 02:31:29 amI'm sure a moderator can verify that I'm not someone's sock puppet.You asked for an explanation of why people don't share their data with you, and I provided a possible reason. That's it, end of story.

author: Revky27
author_rank: Contributor
timestamp: February 09, 2024, 08:29:14 am
content: Quote from: rhb on January 18, 2024, 02:59:58 pmSorry but how can you say the 'noise is correlated'. What noise do you have that behaves like this?

author: rhb
author_rank: Super Contributor
timestamp: February 11, 2024, 02:52:08 pm
content: Temperature, barometric pressure, humidity, mains variation are all noise processes which are correlated among references in the same enclosing space. There are likely others, but you have to peel the onion one layer at a time.Those 4 are first order noise processes. at low frequencies.Have Fun!Reg

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 13, 2024, 09:24:47 am
content: Quote from: rhb on February 11, 2024, 02:52:08 pmI watch with great interest your desire to show good mathematics. Unfortunately I don't have much data.But perhaps you could show an example with numbers using some synthetic data generated artificially. Those. take the signal. Add noise to it. Add temperature and pressure effects. Evaluate the signal parameters using classical methods and the method you propose.It would be very interesting to see how it was possible to get the signal out of the noise when conventional methods cannot offer anything.Thanks in advance.

author: rhb
author_rank: Super Contributor
timestamp: February 13, 2024, 01:46:28 pm
content: Please read:https://www.eevblog.com/forum/projects/so-you-want-to-do-digital-signal-processing/msg5284960/#msg5284960Try doing some basic experiments using Octave. I've been doing DSP for 42 years. I don't need to repeat my first 6 weeks of DSP. Which was on the job reading 4 books evenings and weekends and processing seismic data during the day.Generate 3 random series, cross correlate them, then add one series two the other two series and cross correlate the pair. The central value of each of the initial cross correlations will be zero. The 3rd xcor will not be zero. The longer your time series, the better you will be able to separate the added series from the other 2 series to which it has been added.FWIW This *is* the conventional method. Please read Bendat & Piersol. They are the masters of the conventional method which was pioneered by Norbert Wiener are MIT. Sparse L1 pursuits is a step farther.Have Fun!Reg

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 13, 2024, 03:58:42 pm
content: Quote from: rhb on February 13, 2024, 01:46:28 pmI have basic knowledge of DSP. I have a gap in using cross-correlation. The correlation between two sequences is one number. Let's say I got 0.5.How will this number help me remove correlated noise from one of the sequences?

author: rhb
author_rank: Super Contributor
timestamp: February 13, 2024, 05:34:29 pm
content: You average all the cross correlation to derive an estimate of the correlated noise which you then subtract from each time series.The longer the series, the better your estimate of the correlated noise. As the series of random uncorrelated series gets longer the cross correlation approaches zero. That improves the averaged cross correlations accuracy.Random Data by Bendat & Piersol is the canonical master treatment of the mathematics of DSP. I started with the 2nd ed in the late '80s and bought the 3rd & 4th. It is all pure applied mathematics with equations on almost every page. But if you really want to understand DSP, it is the "sine quo non" for mastering the topic.Temperature, pressure and humidity affect the heat flow from the Vrefs. As it is radiative, conductive and convective the mathematics get very messy. Carslaw & Jaeger have the basic details in their classic on "Conduction of Heat in Solids". An empirical solution would be a 6-8 week job, $$$$$. With real data spanning 10 years I'll do it for free. Making a numerical example is almost as much work. I have better uses of my time.Thanks for asking a sensible question instead of a demand that I do a bunch of work which is of no use to me.In regard to sparse L1 pursuits, section 12.4 of "A Wavelet Tour of Signal Processing" 3rd ed will give you the flavor. Understanding the proofs requires reading Mallat cover to cover and then Foucart & Rauhut. That took me 3 years to do including the original papers which were about 2/3 of what I read in those 3 years.My BA is in English lit. If *I* can master Foucart & Rauhut, anyone can. But it demands very strong motivation. The proof of the first of 3 theorems by Donoho took 14 of 15 pages!NB: The first order step would be to sum all the series for an initial estimate of the correlated noise and then refine that estimate using the cross spectra. So my description is what I'd tell the PhD across the hall at work, not a step by step description. We *all* forget stuff. I ran "an orphan home for lost problems" at 5 oil companies. Most of the time all I did was ask a few questions and the person responded, "I know how to do that!" and rushed back to their office.Have Fun!Reg

author: moffy
author_rank: Super Contributor
timestamp: February 13, 2024, 07:34:52 pm
content: So if I can chip in what I think, if I have 3 inputs all going to an ADC or multiple ADCs, if I connect a signal to two of them and zero to the input of the third, the third input would provide the common correlated noise due to environmental factors and can then be used against the two inputs with a signal to help remove just this common environmentally induced noise, or noise common to all channels. Once this noise is removed from the two channels with the signal cross correlation on the two signal channels can be used to uncover the original signal buried in the uncorrelated noise.

author: rhb
author_rank: Super Contributor
timestamp: February 14, 2024, 03:25:05 am
content: That's the cool part. You can recover the uncorrelated noise as in the Vref case, or you can recover the correlated noise.It's a signal & noise separation technique. Totally agnostic about what is signal and what is noise.In the seismic world this is done in multiple dimensions with complex spatial operators.Really recommend Bendat & Piersol.Have Fun!Reg