Author Topic: EEVblog #845 - Oscilloscope FFT Comparison  (Read 44064 times)

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Offline EEVblog

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EEVblog #845 - Oscilloscope FFT Comparison
« on: January 26, 2016, 10:48:18 pm »
Dave compares the FFT modes on 7 different oscilloscopes:
Rohde & Schwarz HMO1202 Series
Tektronix MDO3000
Keysight 3000X Touch
Lecroy WaveJet 354 Touch
GW Instek GDS-1104B
Rigol DS1054Z
Rigol DS2000
Which is the best?
Which one sucks the most?

 
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Online chris_leyson

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #1 on: January 26, 2016, 11:06:34 pm »
Dave, my 30+ year old TEK 7L12 will do better than any of those scopes, 70dB to 80dB dynamic range and 1.8GHz, albiet very very very slowly. Try an Analog Discovery or Red Pitaya, 14-bit A-D, I don't why manufacturers bother putting FFT software in scopes with 8-bit or 10-bit digitizers.

OK, to be fair FFTs are a damn sight faster than a swept 7L12 and you get much better frequency resolution, but I'm thinking 16-bit A-D plumbed into 1st IF (105 MHz) or 2nd IF (10MHz).  ;)
 

Online wraper

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #2 on: January 27, 2016, 12:10:15 am »
When Dave said that FFT sucks in the cheap scopes, for some reason didn't mention that instek's 1000B series are even a bit cheaper than Rigol 1000Z (for 50 MHz model if comparing with 1054Z). Also from actual FFT performance it's certainly looks to work better than R&S and Agilent (except fiddly UI) and is second after Tek MDO.
 

Online chris_leyson

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #3 on: January 27, 2016, 12:12:05 am »
Forgot to mention that FFTs can't handle transients very well, and aliasing, that's always fun if you get your sampling rate wrong, but it can be a useful feature sometimes. They're good if you can fill your 50 to 60dB dynamic range with signal, fair enough. I've got the extra math module plugged into the back of my HP54610B so it will do integration, differentiation and FFTs, only used the FFT function to try and figure out where some noise was coming from, a few frequency domain clues are always useful.

If you could convolve or correlate two channels that would be a useful function, I don't know if this feature is available on Keysight or Tek scopes.

 

Offline karoru

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #4 on: January 27, 2016, 12:16:29 am »
With any algorithm it's "garbage in, garbage out" principle. FFT from 8-bit ADC in the scope will be bad, no matter how clever you try to analyse the data (unless they invent some oracle chip that will extract the additional information using heaven microwave link). We can compare mainly how software guys managed to put algorithms into software so it at least don't make you cry when after turning the knob it takes 10 sec to return the scope to responsive state.

Otherwise you can just dump the data from your scope and put it on your random Matlab/Octave/Scilab/whatever and do the data processing by yourself.
« Last Edit: January 27, 2016, 12:23:41 am by karoru »
 

Offline Froese

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #5 on: January 27, 2016, 01:24:17 am »
Sorry if this is a dumb question - I'm not a DSP-guy.

Why do these scopes need this huge number of FFT points?   Basically, if you have 1024 horizontal pixels on the screen, shouldn't a 1024-point FFT (+/- some slack) be good enough?  Couldn't they mix the selected window down to  DC (like a direct conversion receiver), filter, maybe resample  and then a 1024-FFT over DC to span-width?  Software mixing/filtering/resampling/1k-FFT should be much cheaper than a direct 128k-point FFT, shouldn't it?
« Last Edit: January 27, 2016, 12:35:39 pm by Froese »
 

Offline EEVblog

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #6 on: January 27, 2016, 01:39:57 am »
When Dave said that FFT sucks in the cheap scopes, for some reason didn't mention that instek's 1000B series are even a bit cheaper than Rigol 1000Z (for 50 MHz model if comparing with 1054Z). Also from actual FFT performance it's certainly looks to work better than R&S and Agilent (except fiddly UI) and is second after Tek MDO.

Yes I completely forgot the price of the GW-Instek, and stand corrected.
 

Offline Muxr

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #7 on: January 27, 2016, 02:06:41 am »
I remember having to use the FFT on my Rigol DS2k and how frustrated I was. It really is as useless as your video showed.
 

Offline EEVblog

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #8 on: January 27, 2016, 02:12:38 am »
I remember having to use the FFT on my Rigol DS2k and how frustrated I was. It really is as useless as your video showed.

You can see a signal is there, but that's about it.
 

Online wraper

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #9 on: January 27, 2016, 02:20:02 am »
I remember having to use the FFT on my Rigol DS2k and how frustrated I was. It really is as useless as your video showed.
I once switched on FFT on my hacked DS2072, thought wtf is this and did not even bother to try using it again. IIRC even on my old Instek GDS-1152A it worked better. Donated to my friend about a year ago, very similar with Rigol DS1000E series but IMO better quality. ADCs were of the same model but originally higher speed grade 100 MHz vs 40 MHz in Rigol. They were overclocked too but only by 25% not 150% as in Rigol, and there were four of them at 100 -> 125 MHz vs five of 40 -> 100 MHz in Rigol.
EDIT: and BTW in the 1152A CPU/FPGA/ADCs were on the daughter board too (GDS-1000B teardown) . So it might be common how instek design the things.
« Last Edit: January 27, 2016, 02:31:35 am by wraper »
 

Offline Muxr

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #10 on: January 27, 2016, 02:53:16 am »
I remember having to use the FFT on my Rigol DS2k and how frustrated I was. It really is as useless as your video showed.
I once switched on FFT on my hacked DS2072, thought wtf is this and did not even bother to try using it again. IIRC even on my old Instek GDS-1152A it worked better. Donated to my friend about a year ago, very similar with Rigol DS1000E series but IMO better quality. ADCs were of the same model but originally higher speed grade 100 MHz vs 40 MHz in Rigol. They were overclocked too but only by 25% not 150% as in Rigol, and there were four of them at 100 -> 125 MHz vs five of 40 -> 100 MHz in Rigol.
EDIT: and BTW in the 1152A CPU/FPGA/ADCs were on the daughter board too (GDS-1000B teardown) . So it might be common how instek design the things.
Yeah DS2072A is what I got (it's my main scope until I take delivery of my new to me RTM1054 in a few days). I am not getting rid of DS2072A. It really isn't a bad scope for time domain stuff it has served me well, but that FFT is just a major letdown.

edit: All these Chinese scopes seem to have major deficiencies, some are better than others in certain areas, but overall, and that could be just based on popularity, Rigol seems to do a ok enough job on most of it. I do think they should address the FFT though, because the audience they cater to, most likely doesn't own a dedicated Spectrum Analyser. Spectrum Analysers are generally pretty expensive, for this demographic, and having a usable FFT function should probably play a major role in deciding which first scope to get.
« Last Edit: January 27, 2016, 03:08:04 am by Muxr »
 

Offline orion242

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #11 on: January 27, 2016, 03:10:16 am »
14:43....its like "dick n' balls"

LMAO!!

Its that off the cuff commentary that makes your videos great.  Keep up the good work!
 

Online wraper

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #12 on: January 27, 2016, 03:19:46 am »
14:43....its like "dick n' balls"

LMAO!!

Its that off the cuff commentary that makes your videos great.  Keep up the good work!
Dave just confirmed after a moment my first thought what it looked like, when this thing appeared on the screen (I think that I even said it loudly on my language). Then I started to laugh loudly. It really looks like this.
 

Offline nixfu

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #13 on: January 27, 2016, 03:36:32 am »
Most of us hobby players are often interested in using FFT becuase we cant afford a proper SA for the bench.  And sometimes FFT is at least a chance to get some sort of data in the frequency domain that is better than nothing.

If there was just a little more competiton in the SA market and more decent options than the only one now (Rigol 815), then maybe more benches could have a real SA sitting on them and FFT on scopes would hardly be worth using.

Someday someone(Owan, Siglent, Hameg, GW)  surely will come out with something to compete in price and functiom against the Rigol 815 eventually.


« Last Edit: January 27, 2016, 03:40:37 am by nixfu »
 

Offline Someone

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #14 on: January 27, 2016, 04:24:16 am »
Sorry if this is a dumb question - I'm not a DSP-guy.

Why do they these scopes need this huge number of FFT points?   Basically, if you have 1024 horizontal pixels on the screen, shouldn't a 1024-point FFT (+/- some slack) be good enough?  Couldn't they mix the selected window down to  DC (like a direct conversion receiver), filter, maybe resample  and then a 1024-FFT over DC to span-width?  Software mixing/filtering/resampling/1k-FFT should be much cheaper than a direct 128k-point FFT, shouldn't it?
One power of the FFT with a large number of points is when you zoom on its horizontal scale (frequency) and can see that your original signal contains not one but two or more closely located frequencies, similar to how you might have a memory depth of 1M but the screen shows the overview in 1k points until you zoom in. The other reason to use lots of points is that the noise in the final result is reduced which is why you can see 80-100dB of signal to noise ratio and turn on dB scaling for the vertical axis even though in time domain the 1000px linear in the vertical axis was enough to see the limits of the ADC.
 

Offline Muxr

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #15 on: January 27, 2016, 04:54:57 am »
Most of us hobby players are often interested in using FFT becuase we cant afford a proper SA for the bench.  And sometimes FFT is at least a chance to get some sort of data in the frequency domain that is better than nothing.

If there was just a little more competiton in the SA market and more decent options than the only one now (Rigol 815), then maybe more benches could have a real SA sitting on them and FFT on scopes would hardly be worth using.

Someday someone(Owan, Siglent, Hameg, GW)  surely will come out with something to compete in price and functiom against the Rigol 815 eventually.
Of all the options for an SA I am leaning towards the Signal Hound the most. It has higher bandwidth, and it covers audio frequencies. It's USB based but from what I understand the software is quite usable.
 

Offline scopeman

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #16 on: January 27, 2016, 06:41:12 am »
I know that this is about low cost scopes but below is a link to a good introduction to FFT on LeCroy scopes.

Deep memory on a software based FFT is a must. The Chinese scopes are no match for the FFT and math functions on even the older 9300 series LeCroy scopes with deep memory. LeCroy had deep memory scopes with FFT probably 10 years before anyone else. Even before HP got into the business with their scopes that were lucky to do 6 bits and did not have FFT, and before Tek had anything that would do FFT.

With the advancement in processing speed they are more responsive than ever.

Scopes that do the FFT in hardware (dedicated hardware memory with dedicated FFT processing in hardware as opposed to a mixed approach) have the advantage of speed (Rohde & Schwarz does this on their high line scopes) but limitations in effective resolution bandwidth due to the fixed number of bins.

The ability to adjust the depth of the sample memory to optimize the required frequency resolution and or the processing speed is quite handy. Once you get your hands on a deep memory scope with FFT and see the advantages of variable depth sample memory you will never want to go back to wimpy or fixed memory scopes again.

My old 9354AL can vary the sample memory from 500 samples to 2M samples and I can do an FFT to zoom memory and then do additional math on the FFT like FFT average. Math on Math on Math is one of the great features of these scopes and they were built in the 1990's. The follow on scope generations are built on the same proven architecture with faster processing, more acquisition memory and even more math power.

These scopes trace their lineage back to Walter LeCroys' involvement with measurements in high energy physics and that is why they were way ahead of the curve in all matters of mathematical processing, including FFT.


See:

http://teledynelecroy.com/doc/setting-up-an-fft

It is still true that there are many occasions where you do need a good RF spectrum analyzer but for a lot of uses FFT on a good scope is still quite handy.

Sam
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Offline daqq

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #17 on: January 27, 2016, 07:26:31 am »
Thanks for the comparison Dave!

It would be a great idea for a Fundamentals Friday to show how Fourier (and Fast Fourier) transformation actually works - the mathy bits.
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Offline Lukas

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #18 on: January 27, 2016, 09:57:48 am »
Yeah, I know, everyone hates USB oscilloscopes, but leveraging a modern PC's processing power for FFTs could really improve things. My 2015 ultrabook i7 can easily do a 10M-point FFT (using libfftw) every second. No need for FPGA or ASIC wizardry. Compare this to the MDO3000 where a 1M point FFT took like 10 seconds.

Take a closer at how various oscilloscopes respond to turning the knobs while they're number crunching: Some scopes like the instek one don't give priority to the user and just keep going on crunching, making the oscilloscope seem unresponsive. OTOH the Keysight one suspends the acquisition in the instant the user turns the knob and waits some time after the user has done the last interaction. I'm wondering why they need to do this, since they claim to have hardware FFT. So changing the FFT parameters should just be poking some registers...
 

Offline EEVblog

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #19 on: January 27, 2016, 11:06:02 am »
Yeah, I know, everyone hates USB oscilloscopes, but leveraging a modern PC's processing power for FFTs could really improve things.

This has always been a potential advantage of USB scopes. Particularly when they quite often have >8bit converters.
 

Offline EPTech

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #20 on: January 27, 2016, 11:25:46 am »
Oops,

I just bought a Rigol DS4204. I did not buy it specifically for it's FFT features but I hope they are better than what I saw on Dave's review.

Anyway, it will arrive next week and I will put it through the test and get back to you. I only have an analogue function generator running max 2 Mhz.
What is the best benchmark using that? Maybe a square wave to get some harmonics?

« Last Edit: January 29, 2016, 06:35:15 pm by EPTech »
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Offline Mosaic

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #21 on: January 27, 2016, 11:56:34 am »
Too bad about the Rigol....I have one, but also a DSA 815 TG...so i use that....I also have an 1102D for Q&D fft at low freqs.

I did some quick bandwidth testing on the DS2072A up'd to DS2302a...seems to be around 310Mhz , 3dB down....using a Tek SG504 leveled sinewave gen. via 50ohm RG58 cable. I need to up that cable to an RG400 or LMR400 for  more reliability. Getting a couple megaphase orange 4Ghz rated cables soon, so I can use those.
I can do a proper test soon as I got an HP8753D 3Ghz VNA with full cal kit. I can tap off a minicircuits coupler and sweep the Rigol  for leveled sinewave and compare it to the VNA for  a precision check.

Let me know if that data useful to anyone.
 

Offline Karel

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #22 on: January 27, 2016, 11:57:52 am »
Dave, on the DS1054z, did you set the "FFT mode" to "Trace" or "Memory"?

In "Trace mode" it's only 1200 pts. In "Memory mode" it's 16384 pts.

From the video it's not clear to me which mode you selected.
Could it be that the FFT mode selected was "Trace"?
 

Offline Stupid Beard

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #23 on: January 27, 2016, 12:14:47 pm »
Yeah, I know, everyone hates USB oscilloscopes, but leveraging a modern PC's processing power for FFTs could really improve things. My 2015 ultrabook i7 can easily do a 10M-point FFT (using libfftw) every second. No need for FPGA or ASIC wizardry. Compare this to the MDO3000 where a 1M point FFT took like 10 seconds.

You don't need a USB scope to do that. You can just get the waveform data out of any modern scope and do it. Someone has already done that for the rigol scopes; I think http://rheslip.blogspot.ca/2015/09/software-spectrum-analyzer-for-rigol.html is the right URL. I've never used it so I don't know how useful it actually is.
 

Offline TiN

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #24 on: January 27, 2016, 12:38:58 pm »
Quote
14:43....its like "dick n' balls"
LMAO!!

 :-DD :-+

TDS7000 (on example of mine CSA7404) is similar for FFT like MDO, superslow, but pretty accurate when it gets there.
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Offline ADT123

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #25 on: January 27, 2016, 12:41:44 pm »
Yeah, I know, everyone hates USB oscilloscopes, but leveraging a modern PC's processing power for FFTs could really improve things. My 2015 ultrabook i7 can easily do a 10M-point FFT (using libfftw) every second. No need for FPGA or ASIC wizardry. Compare this to the MDO3000 where a 1M point FFT took like 10 seconds.


Bear in mind Dave was doing an FFT over a frequency of DC just over 1MHz (all FFTs are from DC, the center / span displays in these scopes are zooms of the full data).  A 1 million point FFT over a 1MHz span will require 1 seconds to acquire the data and then an extra amount of time to do the FFT.   

The collection time depends on the frequency range you are measuring and the number of bins.

The time to process the FFT depends on how fast the hardware can crunch the numbers.

Suspect the above is why the Tek was so slow with 2 million points.

Update - just tested this on a PicoScope 5000 PC scope
1 million point FFT measuring up to 1MHz takes 1.1 second per screen update
128K FFT measuring up to 1MHz takes 0.3 seconds per screen update
1 million point FFT measuring up to 200MHz takes 0.2 seconds per screen update
128K FFT measuring up to 200MHz takes 0.15 seconds per screen update

If I get a chance I will put in a similar signal to the one Dave used - suspect the PicoScope would have been the winner in terms of low noise / speed of update etc
« Last Edit: January 27, 2016, 12:57:36 pm by ADT123 »
Disclaimer:  I have worked for Pico Technology for over 25 years and designed some of their early oscilloscopes. 

Happy to answer Pico related questions when I have time but I am on eevblog in a personal capacity as electronics is a hobby.
 

Offline ADT123

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #26 on: January 27, 2016, 01:14:00 pm »
OK, similar FM modulated signal from a R&S sig gen.  PicoScope 5444B with a million point FFT.  One advantage this scope has is that you can change the resolution in hardware, I also enabled averaging (after all Dave did when available).  Combined this with the noise reduction you get from 1 million point FFTs and the noise floor is literally off the chart.



« Last Edit: January 27, 2016, 01:20:18 pm by ADT123 »
Disclaimer:  I have worked for Pico Technology for over 25 years and designed some of their early oscilloscopes. 

Happy to answer Pico related questions when I have time but I am on eevblog in a personal capacity as electronics is a hobby.
 

Offline dr.diesel

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #27 on: January 27, 2016, 02:29:09 pm »
The DS/MSO4000s are just as horrible as their little brothers.   :--

Offline nixfu

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #28 on: January 27, 2016, 02:50:40 pm »
A fellow forum member has written software you can use to run live FFT against up to the entire sample memory of the Rigols using the much more powerful PC CPU.

https://www.youtube.com/watch?v=twRke3suKpE&feature=youtu.be

Project site:
http://rheslip.blogspot.com/

« Last Edit: January 27, 2016, 02:58:50 pm by nixfu »
 
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Offline Kalvin

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #29 on: January 27, 2016, 05:50:02 pm »
Even a poor FFT of a cheap oscilloscope is better than nothing if you do not have a proper analyzer at hand to do the spectral analysis.
 

Offline Neganur

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #30 on: January 27, 2016, 06:00:35 pm »
Dave, my 30+ year old TEK 7L12 will do better than any of those scopes, 70dB to 80dB dynamic range and 1.8GHz, albiet very very very slowly. Try an Analog Discovery or Red Pitaya, 14-bit A-D, I don't why manufacturers bother putting FFT software in scopes with 8-bit or 10-bit digitizers.

OK, to be fair FFTs are a damn sight faster than a swept 7L12 and you get much better frequency resolution, but I'm thinking 16-bit A-D plumbed into 1st IF (105 MHz) or 2nd IF (10MHz).  ;)

Comparing apples to oranges. And an ADC behind the IF of the 7L is still a full blown SA compared to the FFT of those oscilloscopes.

(Edit: also, the Pitaya is only 125MSa/s)
« Last Edit: January 27, 2016, 06:04:27 pm by Neganur »
 

Offline Karel

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #31 on: January 27, 2016, 06:37:46 pm »
I did a quick check with DSRemote and EDFbrowser.
The signal source is the 1KHz calibration signal from the scope itself.
(I don't have a signal generator at hand at home.)

I downloaded 24 Mpts. Then I downsampled the data from 1GHz to 1MHz with the builtin reducer tool of EDFbrowser.
(It makes no sense to use 1GHz samplefrequency when you are interested in a 1KHz signal only.)



 

Offline Lightages

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #32 on: January 27, 2016, 06:56:48 pm »
I was hoping to see a comparison between the DS1052E and the DS1054Z in this video as well.
 

Offline mxmxmx

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #33 on: January 27, 2016, 06:57:21 pm »
By the way: Given that the FFT window in question was named in honor of Julius von Hann, LeCroy is actually doing the right thing in naming the window "von Hann". "Hanning window" is a misnomer, albeit a quite popular one, so LeCroy deserves a little bit of praise for using the correct name.
 

Offline karoru

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #34 on: January 27, 2016, 08:26:28 pm »
By the way: Given that the FFT window in question was named in honor of Julius von Hann, LeCroy is actually doing the right thing in naming the window "von Hann". "Hanning window" is a misnomer, albeit a quite popular one, so LeCroy deserves a little bit of praise for using the correct name.

I've been living my entire life in error - I've always thought it's "Hamming window"...
 

Offline Neganur

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #35 on: January 27, 2016, 08:42:23 pm »
That's the thing, there is Hamming. There isn't Hanning tough, it's supposed to be Hann. German names confused the hell out of the English speaking people (or so my Signals & Systems prof claimed :P)
 

Online janoc

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #36 on: January 27, 2016, 08:53:04 pm »
A fellow forum member has written software you can use to run live FFT against up to the entire sample memory of the Rigols using the much more powerful PC CPU.

https://www.youtube.com/watch?v=twRke3suKpE&feature=youtu.be

Project site:
http://rheslip.blogspot.com/


Or, an even simpler solution if you don't need realtime analysis - capture the waveform on an USB stick and then analyze it on a PC in something like Matlab or using Python. It certainly works much better.

https://github.com/mabl/pyRigolWFM

« Last Edit: January 27, 2016, 08:55:52 pm by janoc »
 

Offline max666

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #37 on: January 27, 2016, 09:40:54 pm »
By the way: Given that the FFT window in question was named in honor of Julius von Hann, LeCroy is actually doing the right thing in naming the window "von Hann". "Hanning window" is a misnomer, albeit a quite popular one, so LeCroy deserves a little bit of praise for using the correct name.

That nobiliary particle "von" can suck my 'dick n' balls'. I will not play along with that nonsense, it's Hann window period.
 

Offline TinkerFan

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #38 on: January 27, 2016, 09:42:36 pm »
I'm not an owner of one of those scopes (above my budget), but I like to dream  :)

Is it possible to use the scopes in FFT-Mode and in a delayed timebase mode simultanously? That would be such a great feature...
But I'm just curious...
"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering." - Freeman Dyson
 

Offline orion242

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #39 on: January 28, 2016, 03:32:15 am »
I think we need an audio guy to make some of Dave's famous phrases into ring tones.

This video definitely has one that I could assign to a few contacts....lol.
 

Offline Maxlor

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #40 on: January 28, 2016, 03:54:53 pm »
Actually, the Rigol isn't quite as horrible as it appears in Dave's video, once you update it.



This is what dave shows in the video. Frequency resolution is extremely low and the noise floor is high, because the Rigol appears to work with the screen buffer only, that's only a couple hundred data points.

In newer firmware versions (04.01 didn't have it, 04.03.SP2 has it), they added a "Mode" option which lets you switch between "Trace" (old behaviour) and "Memory*" which I believe works with the sample memory. Switching to mode, the FFT looks quite a bit better (better frequency resolution and better SNR), although still nowhere near as good as some of the other scopes:



Adjusting the memory depth has no effect though, I'm guessing to always use just a few thousand points. What matters in the sampling speed: the highest resolution you get (Hz/Div) is 1/1000th of the sampling speed.
 

Offline voltsandjolts

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #41 on: January 28, 2016, 09:04:05 pm »
Haha, my Agilent 6000 series FFT is no better than the Rigol. Well, maybe a little less noisy but... jeez.
 

Offline artag

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #42 on: January 29, 2016, 02:08:27 am »
Haha, my Agilent 6000 series FFT is no better than the Rigol. Well, maybe a little less noisy but... jeez.

Try turning the precision mode on (in the utility menu). It slows it down a lot, but the FFT is much improved.
 

Offline uncle_bob

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #43 on: January 29, 2016, 02:37:13 am »
Hi

If you take a look at one of the other videos on the Rigol scopes, Dave goes into the master oscillator that drives the sampler on the scope. What he finds (pre fix) is really ugly. In the case on the video they fix it to improve a jitter problem with the scope.

To some extent an FFT is measuring the clock signal as much as the test signal. A "dirty" clock will indeed limit your FFT floor and muck up the resolution of your bins. To get a good FFT, you need a good clock into the ADC. Yes, you also need the other stuff discussed at length in other posts here. The net result will still only be as good as the "weakest link" in the signal processing chain.

Bob
 

Offline blueskull

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #44 on: January 29, 2016, 05:06:25 am »
I don't really think FFT makes sense these days. Cheap true SA for <$1k, or even $400 are everywhere, using crappy SDR chips you can even get one for <$100.

A properly designed lab grade USB SA, such as a Signal Hound, is just $900, which I got a new one for only $600, probably cheaper than a single license option on a mid range scope.

After all, you get only 8 bits from your ADC if you are using a scope, and that resembles only 50dB of SNR. A modern SA can get you much higher SNR.

The only reason not to use a true SA is probably because you do not have 50 Ohm output IMHO. Whenever I can get a 50 Ohm output with proper signal level, I will not use FFT.
 

Offline Muxr

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #45 on: January 29, 2016, 05:34:11 am »
I don't really think FFT makes sense these days. Cheap true SA for <$1k, or even $400 are everywhere, using crappy SDR chips you can even get one for <$100.

A properly designed lab grade USB SA, such as a Signal Hound, is just $900, which I got a new one for only $600, probably cheaper than a single license option on a mid range scope.

After all, you get only 8 bits from your ADC if you are using a scope, and that resembles only 50dB of SNR. A modern SA can get you much higher SNR.

The only reason not to use a true SA is probably because you do not have 50 Ohm output IMHO. Whenever I can get a 50 Ohm output with proper signal level, I will not use FFT.
Folks who break a bank to get a $400-$800 scope aren't going to be able to afford an SA, the scope already is a stretch and it's a far more important instrument in general electronics outside of RF/Radio work. Having a usable FFT even if limited in a pinch is nice. R&S and Gw-Instek demonstrate that it's possible to implement a half decent FFT in a scope.

This is why I think FFT in entry level scopes can be a deal breaker between different scopes, I will probably be tempted to recommend R&S and Gw-Instek to friends.

Even for me personally, the cost isn't necessarily the issue, I would be tempted to get a Signal Hound but it's not supported by my OS. And I don't need an SA enough to justify making bench space for a standalone unit which usually doesn't even cover audio frequencies I sometimes dabble in.
« Last Edit: January 29, 2016, 05:37:14 am by Muxr »
 

Offline AlessandroAU

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #46 on: January 29, 2016, 02:23:41 pm »
I just want to point out something that may not be apparent. Everyone loves to complain about the 8 bit limitation of scope ADC's and how it limits you to 50dB of dynamic range.

This is plainly NOT TRUE. For any FFT you have an increase in the "base" SNR based on the number of samples you take, it's called processing gain. With the rigol's 24M memory you can easily push 120dB... if the internal clock wasn't so shit.

http://www.analog.com/media/en/training-seminars/tutorials/MT-001.pdf
 

Offline uncle_bob

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #47 on: January 29, 2016, 06:08:08 pm »
I just want to point out something that may not be apparent. Everyone loves to complain about the 8 bit limitation of scope ADC's and how it limits you to 50dB of dynamic range.

This is plainly NOT TRUE. For any FFT you have an increase in the "base" SNR based on the number of samples you take, it's called processing gain. With the rigol's 24M memory you can easily push 120dB... if the internal clock wasn't so shit.

http://www.analog.com/media/en/training-seminars/tutorials/MT-001.pdf

Hi

The bit gain in the decimation process is highly dependent on a number of factors. It works well for a small signal buried in a bunch of random noise. GPS receivers do amazing things with one or two bits on the input. It may not work quite as well for a large signal into the scope that has relatively little noise. That's one of the reasons "IF based" post narrowband filter DSP approaches often needs to add a dither source to keep things sane. 

Bob
 

Online nfmax

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #48 on: January 29, 2016, 09:17:51 pm »
Sorry if this is a dumb question - I'm not a DSP-guy.

Why do these scopes need this huge number of FFT points?   Basically, if you have 1024 horizontal pixels on the screen, shouldn't a 1024-point FFT (+/- some slack) be good enough?  Couldn't they mix the selected window down to  DC (like a direct conversion receiver), filter, maybe resample  and then a 1024-FFT over DC to span-width?  Software mixing/filtering/resampling/1k-FFT should be much cheaper than a direct 128k-point FFT, shouldn't it?

Yes, you can do this, and in a rather cleverer technique called 'zoom FFT' processing. What Dave didn't mention, and indeed almost nobody does, is that the FFT mode in these scopes is not just an FFT, it also includes a power calculation, so that the trace on the screen is a power spectrum. They all do this because the output from an FFT is actually complex, and scopes have difficulty displaying complex waveforms. The power in a complex waveform is the sum of squares of the real and imaginary components, so we can easily show that. However, if you don't apply the power calculation you can do additional processing before display.

Suppose we make a 4096 point FFT. The output of this comprises 2048 frequency bins plus a DC bin (for a real input). You can think of each of these bins as the output of a narrow band filter, with an amplitude and phase response that depends on the window used, and a centre frequency which is a multiple of the basic frequency resolution of the 4096 point FFT, i.e. 1 over the time duration of the 4096 points. The real and imaginary components of the FFT bin output give you the signal that gets through this filter, down converted to DC, like the I and Q outputs of a 'zero IF' receiver. If we want a finer frequency resolution, we have to acquire more points, over a longer time, so that the bins are more closely spaced, and make a longer FFT.

However, if don't want to see all the frequency range, an alternative is to make a series of short FFTs of successive 4096 sample sections of the signal, and look at the sucessive complex outputs of the bins in just the frequency range we are interested in. This is another signal - complex valued this time - which varies over time and represents how the output signal from the filter corresponding to the FFT bin changed over time. For example, 8 successive 4096 point FFTs gives, for each frequency bin, a time sequence of 8 complex numbers. Here's the clever bit: for each bin we are interested in, we make another, 8-point FFT of the complex bin output. Because the data is complex, this gives us 8 frequency mini-bins in place of the original big bin. We apply the power calculation on the output of these FFTs and display the result. You can do this to as many of the original FFT bins as you want, but because of the extra overhead in the short FFTs it's only really worth doing if you are interested in just a small part of the frequency range. But it does let you show a low-resolution, full bandwidth power spectrum (based on the successive 4096 point FFTs) which is updated rapidly, at the same time as the higher resolution, zoomed section, updated less often. Hence the name of the technique.

We were using this approach in the '80s on a 68000 micro in a dynamic signal analyser for machinery vibration, but I believe it was first used in sonar processing some years earlier. Since memory is cheap and processors are fast, it seems to have fallen by the wayside, but heck, it still works!

Max
 

Offline G0HZU

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #49 on: January 29, 2016, 09:40:03 pm »
It was interesting to see the various scopes in FFT mode but I was left a bit disappointed because I don't think enough time was spent on each one to see how much you can exploit FFT mode and make some decent measurements that would be difficult on a conventional swept spectrum analyser.

For example, I still use an old Tek TDS2012 DSO quite regularly and I use the FFT mode on it quite regularly too. It only has a 2K FFT but you can still do a lot with a 2K FFT if you exploit the sample rate and alias limitations of the scope and turn them to your advantage. Obviously, you have to be wary of alias issues but a skilled operator can spot these and know to ignore them or manage/remove them.

Here's a few things I can do with the creaky old 100MHz TDS2012 in FFT mode.

I can look at an unmodulated  FM signal from an old 2m Yaesu radio (FT290R 145MHz) and zoom in to a narrow span of a few kHz or less and look at the close in phase noise of the radio's carrier with a decent refresh rate.

I can also watch the spectrum in 'close to real time' when speaking into the microphone and watching the scope display the (<20kHz) BW of the approx 145MHz FM test signal.

I can also watch the FT290R transmission as it slowly drifts in frequency by a few Hz per second as it warms up from a cold start. Obviously, the scope has to be warmed up and stable beforehand.

I can look at a ~145MHz cw test signal being on/off keyed at a 1Hz rate (once a second) and zoom right in and see the modulation sidebands at 1Hz, 3Hz, 5Hz spacings from the carrier etc. Obviously, the scope will be slow at this but try doing this on a typical/conventional spectrum analyser. Not many will be able to do it.

It's probably at its most useful when looking at classic SSB, AM or FM modulation in FFT mode. It can display a span of a few kHz really fast/fluidly. This old scope doesn't have much in the way of spurious free dynamic range but it's going to be better than the typical IMD distortion seen on an AM or SSB signal from a ham or CB radio. You can also do Bessel nulling on NBFM to measure FM deviation with it and the display refresh will be quite fluid.

It would be interesting to see if the Rigol or the other scopes can be configured to do similar. My scope is very basic and old so I suspect the other scopes will be able to do similar tricks (but with even better results)
« Last Edit: January 29, 2016, 10:32:37 pm by G0HZU »
 

Offline Wolfgang61

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #50 on: January 30, 2016, 10:29:38 pm »
Hi Dave, maybe somebody said it already. I think on the Rohde & Schwarz Scope you don't need to turn off the FFT to change the timebase. I'm using an HMO2024, a similar model. Just press the Time/Div rotary button and it should switch between the different windows, where you can change the specific settings.
 

Offline rs20

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #51 on: January 30, 2016, 10:42:20 pm »
With any algorithm it's "garbage in, garbage out" principle. FFT from 8-bit ADC in the scope will be bad, no matter how clever you try to analyse the data (unless they invent some oracle chip that will extract the additional information using heaven microwave link).

Is that really true? If you have a one million point FFT on 8-bit data, any given point in your output spectrum will have noise reduce by a factor of sqrt(1M). Put another way, if you add twice as much 8-bit data, you are effectively averaging over twice as many cycles.

I just want to point out something that may not be apparent. Everyone loves to complain about the 8 bit limitation of scope ADC's and how it limits you to 50dB of dynamic range.

This is plainly NOT TRUE. For any FFT you have an increase in the "base" SNR based on the number of samples you take, it's called processing gain. With the rigol's 24M memory you can easily push 120dB... if the internal clock wasn't so shit.

http://www.analog.com/media/en/training-seminars/tutorials/MT-001.pdf

Precisely!

Why do these scopes need this huge number of FFT points?   Basically, if you have 1024 horizontal pixels on the screen, shouldn't a 1024-point FFT (+/- some slack) be good enough?  Couldn't they mix the selected window down to  DC (like a direct conversion receiver), filter, maybe resample  and then a 1024-FFT over DC to span-width?  Software mixing/filtering/resampling/1k-FFT should be much cheaper than a direct 128k-point FFT, shouldn't it?

The computational cost of FFT is only O(N log N); it's not some sort of horrendous polynomial monster or anything like that. So to presume that "just doing direction conversion" on millions of points is easier that doing a 16k FFT is, I suspect, simply wrong.
 

Offline KE5FX

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #52 on: January 30, 2016, 10:57:23 pm »
Is that really true? If you have a one million point FFT on 8-bit data, any given point in your output spectrum will have noise reduce by a factor of sqrt(1M). Put another way, if you add twice as much 8-bit data, you are effectively averaging over twice as many cycles.

Of course it's not true, but that doesn't keep people from mindlessly repeating it every time this subject comes up.  A personal pet peeve.

Wait'll they learn how a sigma-delta converter works.   ;D
 

Offline uncle_bob

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #53 on: January 31, 2016, 06:27:49 pm »


Wait'll they learn how a sigma-delta converter works.   ;D

Hi

Then throw something cross correlation into the mix .... That's one example of how you can (unless you are careful) take things to the point that the numbers you get actually are *better* than the reality of the signals involved.

Bob
 

Online nctnico

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #54 on: January 31, 2016, 10:41:39 pm »
Most of us hobby players are often interested in using FFT becuase we cant afford a proper SA for the bench.  And sometimes FFT is at least a chance to get some sort of data in the frequency domain that is better than nothing.
FFT is most certainly not a poor man's spectrum analyser! A spectrum analyser typically sweeps the signal so it is less effective in capturing glitches. With an oscilloscope you can capture a worse case trace and use FFT to get it's frequency contents. Also spectrum analysers typically start from 9kHz to several MHz. An oscilloscope can be useful to do analysis in lower frequency bands as well.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline uncle_bob

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #55 on: February 01, 2016, 12:38:45 am »
FFT is most certainly not a poor man's spectrum analyser! A spectrum analyser typically sweeps the signal so it is less effective in capturing glitches. With an oscilloscope you can capture a worse case trace and use FFT to get it's frequency contents. Also spectrum analysers typically start from 9kHz to several MHz. An oscilloscope can be useful to do analysis in lower frequency bands as well.

Hi

So in that context. If I pay nothing for FFT, does it matter that it sucks?

As usual, I'm babbling ...

I've been in a *lot* of design reviews / spec reviews where the concept of "marketing check box" comes up. Translated into common English that's a spec with no details and no apparent strong need behind it. The assumption is that the customer needs to have it present, but has no real need for it. As an engineer, I find this whole idea crazy (to say the least). The next step is to put the feature in with as low cost as you possibly can. If there is a problem, it will be fixed later. That part is driven by the common assumption that any problem can be fixed with a firmware update.

So here we are on the *other* end of that conversation. We are the customers. We bought the scope or MSO. It had FFT listed as a spec. Was that up there with bandwidth on a check list for any of us? I doubt it. It most certainly was not for me. Was it the first (or second or tenth) thing I checked on a new scope? Nope. For me at least, yes, it's a marketing check box. It's not something I expect to work well. If I need a spectrum, I grab one of .... errr ... several .... very large, very heavy, very used, HP analyzers.

So, did the marketing guys get it right? In my case they did. Does that make the whole idea of a "not a spec spec" any less crazy? I'm not sure it does. It does indeed validate the business sense of doing it that way.

Bob
 

Offline mikerj

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #56 on: February 01, 2016, 11:11:01 am »
FFT is most certainly not a poor man's spectrum analyser! A spectrum analyser typically sweeps the signal so it is less effective in capturing glitches.

Update rate is an advantage, but in many other areas it is a poor man's spectrum analyser.  For starters there is no ability to capture only signals of interest and filter out unwanted signals which may be at a much higher level, which a spectrum analyser does.  A spectrum analyser gives you greater dynamic range (the noise floor on the cheaper scopes is much higher than a proper SA).

With an oscilloscope you can capture a worse case trace and use FFT to get it's frequency contents. Also spectrum analysers typically start from 9kHz to several MHz. An oscilloscope can be useful to do analysis in lower frequency bands as well.

OTOH even low end spectrum analysers go up to 3GHz or more, well outside the range of any reasonably priced DSO.  At the high end (e.g. 50GHz or more) then you have little choice but to use a spectrum analyser.
 

Online Mechatrommer

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #57 on: February 01, 2016, 05:54:02 pm »
Actually, the Rigol isn't quite as horrible as it appears in Dave's video, once you update it.
In newer firmware versions (04.01 didn't have it, 04.03.SP2 has it), they added a "Mode" option which lets you switch between "Trace" (old behaviour) and "Memory*" which I believe works with the sample memory.
here's the review link, and the linked original youtube by Rigol's Application Engineer himself... i didnt know they have a very fluent english speaking engineer there...
http://www.arbenelux.com/en/improved-fft-on-rigol-msods1000-series/
https://www.youtube.com/watch?v=dD-3ni7tYM4&feature=youtu.be

Adjusting the memory depth has no effect though...
there is. i just dick around with manual memory depth (after changing Mode from Trace to Memory), there are difference. i think i like 12KPts more than larger memory. now Rigol gave the power  of manual memory length selection for FFT to us. attached pics is i tried dicking around to get what Dave's got in the OP review... i tried 2 memory depth, 12K and 120K... to get 5KHz/div, you need to dick around with timebase (see attached). excuse my poor FG jittering 1MHz unspecified distortion level. i set it at seomwhere 2.5Vpp 50ohm terminated coax (i forgot to reset the DSO to 1X probe, at 10X setting, it read as 25Vpp, just ignore it or divide by 10). although with this high depth memory i'm suspecting or seems to be FFT aliasing when we set to max freq/div (namely 250KHz/div) as shown in 3rd and 4th picture. 500KHz harmonics at 1MHz fundamental? weird... but when set to Dave's setting, it looks ok.. maybe he can re-evaluate with his FM generator and see if the DS1Z with 4.3.SP2 update (SP1 according to the Rigol's engineer) may catch up with the rest...

let me enlarge the 12KPts memory FFT that match up with Dave's FFT test range (1MHz center, 5KHz/div, 20dB/div) and its accompanying aliasing artifact @ 250KHz/div...



« Last Edit: February 01, 2016, 05:59:34 pm by Mechatrommer »
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Online Mechatrommer

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #58 on: February 01, 2016, 06:29:26 pm »
and to encounter this video from gwInstek... i've made some more capture at the same 1KHz square signal using DS1054Z 12KPts FFT. i believe the video below is superseded (aka obsolete) by the latest Rigol's FW. except the FFT respond time... FWIW... edit: ... "so what are you waiting for? ... update now!"

« Last Edit: February 01, 2016, 06:54:21 pm by Mechatrommer »
It's extremely difficult to start life.. one features of nature.. physical laws are mathematical theory of great beauty... You may wonder Why? our knowledge shows that nature is so constructed. We simply have to accept it. One could describe the situation by saying that... (Paul Dirac)
 

Online nctnico

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #59 on: February 01, 2016, 06:33:50 pm »
FFT is most certainly not a poor man's spectrum analyser! A spectrum analyser typically sweeps the signal so it is less effective in capturing glitches.

Update rate is an advantage, but in many other areas it is a poor man's spectrum analyser.  For starters there is no ability to capture only signals of interest and filter out unwanted signals which may be at a much higher level, which a spectrum analyser does.  A spectrum analyser gives you greater dynamic range (the noise floor on the cheaper scopes is much higher than a proper SA).

With an oscilloscope you can capture a worse case trace and use FFT to get it's frequency contents. Also spectrum analysers typically start from 9kHz to several MHz. An oscilloscope can be useful to do analysis in lower frequency bands as well.

OTOH even low end spectrum analysers go up to 3GHz or more, well outside the range of any reasonably priced DSO.  At the high end (e.g. 50GHz or more) then you have little choice but to use a spectrum analyser.
What I'm trying to say is that both a spectrum analyser and FFT in an oscilloscope are equally useful tools each with their own strong and weak points.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online Kleinstein

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #60 on: February 01, 2016, 07:49:19 pm »
In the lower frequency range, FFT is a rather good alternative to a classical SA. Some of the more modern SA actually also use FFT instead of the classical logarithmic converter. The somewhat limiting factor with a typical scope is the relatively low resolution of the converter and thus a limited dynamic range. With sufficient long averaging the dynamic range is not that bad - as all frequencies are samples simultaneously there is plenty of time compared to a slow scanning classical SA.

So in many cases the FFT is a cheap alternative / extension for a SA for lower frequency signals. Even if you have a SA, scanning gets very slow at low frequency / low filter bandwidth. If you don't have an SA, the FFT function can be a substitute in many cases - sometimes worse than the SA and sometimes better.

The test dave did, was also just one aspect of the FFT - using high frequency resolution, which is something classical SAs are not very good at. Another point to compare would be the noise level / dynamic range. Some of the scopes did quite well, especially when using averaging or large number of points. Also the linearity of the input / ADC might be important: a poor input could add harmonics or IM products that are not there. Here averaging / oversampling does not help.
 

Offline max666

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #61 on: February 01, 2016, 08:24:03 pm »
and to encounter this video from gwInstek... i've made some more capture at the same 1KHz square signal using DS1054Z 12KPts FFT. i believe the video below is superseded (aka obsolete) by the latest Rigol's FW. except the FFT respond time... FWIW... edit: ... "so what are you waiting for? ... update now!"

I have been thinking about updating ... the firmware that is ;)
But I've read there are bugs introduced by the 00.04.03 SP2 firmware, so I'm not sure if I should update from 00.04.02 SP4?

Edit: Oh and I too would like to see Dave show the FFT again with updated firmware  :D
« Last Edit: February 01, 2016, 08:26:01 pm by max666 »
 

Offline G0HZU

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #62 on: February 01, 2016, 11:27:38 pm »
Here's my old Tek TDS2012 scope displaying a bandpass sampled signal at about 70MHz using FFT mode. The input signal is being on/off keyed once a second and you can see the modulation sidebands at 1Hz, 3Hz and 5Hz (etc) offsets.

So this old scope can display the bandwidth detail of this very narrow signal in a 25Hz span at 70MHz. Not many conventional analysers can do this. Obviously, the signal has to be narrowband or the display will fill with alias terms but because this signal is quite narrow it can be bandpassed sampled in FFT mode at a very low sample rate.

It would be interesting to see how easily other scopes can be configured to do this. My Agilent E4406A signal analyser can display this signal very well but none of my other analysers can.
 

Online nctnico

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #63 on: February 02, 2016, 12:34:05 am »
Any scope can do that.. (if it has FFT). For fun show the time domain signal...
Gw Instek GDS2204E:
« Last Edit: February 02, 2016, 10:52:09 am by nctnico »
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Online Mechatrommer

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #64 on: February 02, 2016, 03:09:54 am »
But I've read there are bugs introduced by the 00.04.03 SP2 firmware
what bug? i havent brick any, except slower GUI respond than the older version, but if you rotate knob quickly, it will reflect on the scope, just slower GUI. the older version also got bugs, so it makes very little difference knowing the fixed some esp this FFT stuff...
It's extremely difficult to start life.. one features of nature.. physical laws are mathematical theory of great beauty... You may wonder Why? our knowledge shows that nature is so constructed. We simply have to accept it. One could describe the situation by saying that... (Paul Dirac)
 

Offline nixfu

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #65 on: February 02, 2016, 03:39:44 pm »
FFT is most certainly not a poor man's spectrum analyser! A spectrum analyser typically sweeps the signal so it is less effective in capturing glitches. With an oscilloscope you can capture a worse case trace and use FFT to get it's frequency  contents. Also spectrum analysers typically start from 9kHz to several MHz. An oscilloscope can be useful to do analysis in lower frequency bands as well.

A common technique for using FFT as a pseudo-SA is to use a broad spectrum  noise generator which basically outputs noise signals all across the spectrum at once, instead of sweeping. 

You can then use this to see the frequency response/cutoff points of filters, RLC circuits, etc. using the scope's FFT.     The same broad  noise signal technique is often used by people who have an SA, but don't have a tracking signal generator.
« Last Edit: February 02, 2016, 03:44:12 pm by nixfu »
 

Offline max666

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #66 on: February 02, 2016, 09:20:10 pm »
But I've read there are bugs introduced by the 00.04.03 SP2 firmware
what bug? i havent brick any, except slower GUI respond than the older version, but if you rotate knob quickly, it will reflect on the scope, just slower GUI. the older version also got bugs, so it makes very little difference knowing the fixed some esp this FFT stuff...

I was reading this:
upgrade New Rigol DS1054Z oscilloscope to firmware 00.04.03.SP2?

alsetalokin4017 didn't go into detail, he just said "The SP2 firmware did fix a couple of issues that I personally found annoying, but introduced a couple more annoying issues. For my purposes the first point outweighs the second point."

I guess if it's only the "Measurements Fail with Math On" bug, then I'm going to update.
 

Offline G0HZU

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #67 on: February 02, 2016, 11:17:28 pm »
Here's what the E4406A signal analyser shows for the 1Hz OOK signal at 70MHz. This is a plot I did some time ago.

In the second image I've used the Agilent 89600 SW to take data from the E4406A and then measure the 99% occupied BW of the 1Hz OOK signal and this should be about 38Hz in theory. This is also an old plot and it looks like it was done at 10.7MHz and not 70MHz.

I tried using my old HP54540C in FFT mode to look at this but it was a bit of a struggle to get a decent display. I've not had this scope long and I've only used it a few times but the FFT mode isn't as good as I expected. It can do up to a 32K FFT which is much better than the little Tek TDS2012 but I much prefer to use the Tek in FFT mode as it is much easier to drive.
« Last Edit: February 02, 2016, 11:20:26 pm by G0HZU »
 

Offline rs20

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #68 on: February 03, 2016, 05:07:51 am »
Apropos to nothing; I find it funny that oscilloscope vendors and users refer to it as "FFT", i.e., Fast Fourier Transform. FFT is just the fastest implementation of the DFT, the end user shouldn't care if a powerful FPGA running naive DFT was used instead.

Not a criticism per se, just something I find kinda funny -- the way that the optimized solution has become synonymous with the original problem.
 

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #69 on: February 03, 2016, 06:04:11 am »
"Measurements Fail with Math On" bug, then I'm going to update.
i've tried to reproduce the "statistic measurement freeze failure during math on" bug, and "zoomed time activated, change persistent and brick the scope" bugs on SP2, they are not there anymore. whats a bit annoying is the slower respond time, but thats not a big deal for me as quick scroll still be reflected in the GUI albeit the slowleness... and i found trigger failure weirdnessness bug probing CAL signal https://www.eevblog.com/forum/testgear/rigol-ds1054z-unable-to-trigger-cal-signal-at-lt-0-6v but afaik its not happening on the other external signals, and i'm not sure if this is specific to SP2, this probably also happening in the older FW.. imho this high resolution FFT in SP2 is far outweigh any older (or probably newer) bugs, just stay away from them, i havent encounter anything serious so far, but there is another thread to report such bugs... ymmv.

« Last Edit: February 03, 2016, 06:11:53 am by Mechatrommer »
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Offline alsetalokin4017

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #70 on: February 03, 2016, 04:26:55 pm »
"Measurements Fail with Math On" bug, then I'm going to update.
i've tried to reproduce the "statistic measurement freeze failure during math on" bug, and "zoomed time activated, change persistent and brick the scope" bugs on SP2, they are not there anymore. whats a bit annoying is the slower respond time, but thats not a big deal for me as quick scroll still be reflected in the GUI albeit the slowleness... and i found trigger failure weirdnessness bug probing CAL signal https://www.eevblog.com/forum/testgear/rigol-ds1054z-unable-to-trigger-cal-signal-at-lt-0-6v but afaik its not happening on the other external signals, and i'm not sure if this is specific to SP2, this probably also happening in the older FW.. imho this high resolution FFT in SP2 is far outweigh any older (or probably newer) bugs, just stay away from them, i havent encounter anything serious so far, but there is another thread to report such bugs... ymmv.
When I reported the Measurements Fail bug to Rigol USA, they were immediately able to see it on their test scopes with the SP2 firmware, and agreed that it was a bug. The problem with reproducing this bug is that it happens after a totally random time interval, sometimes it will hit within seconds and sometimes it takes 30 minutes or more. It is definitely there though. I don't know why you can't see it on your scope. Lots of other people have also seen this bug with the new firmware. The workaround is to keep the Math trace set up, but turned OFF, when you are using measurements, and only turn the Math ON for a brief time when you need to see what it's doing. It still might bite you though, since sometimes it only takes a few seconds for the bug to hit.

As far as the Freeze bug goes: when I tested both my old scope running Boot Version 0.0.1.2 and the new scope running Boot Version 0.0.1.3, both running the latest SP2 firmware, the old scope still was able to freeze, the new firmware made no difference at all for this bug. I had them both for three days before I returned the "freezable"  one to Rigol, and I tested them side-by-side (actually one on top of the other) on my bench. The new firmware apparently does _not_ fix the freezing issue on scopes that have Boot Version 0.0.1.2, as far as I can tell.  The workaround for those who have this bug is to be very careful about using Persistence and Zoom at the same time.

The SP2 firmware also introduces the "Pluses" spelling error and sometimes the "pluses" counters and edge counters miscount the actual number of pulses on screen.
No workaround here--- just suck it up and ignore the spelling error, and count your pulses by eye to see if the counter is off-by-one.

And of course it makes the scope very slow to respond to some controls, like especially the channel vertical position controls. So use "safecracker mode" on your fingers and turn the knobs slowly.... very slowly.... and wait for the scope to catch up.

But your problems with trigger level are something else. As I posted at the time it was being discussed, my scope has no problem triggering on the Calibrate signal down to smaller voltage levels.

The FFT does seem to work better now though.
The easiest person to fool is yourself. -- Richard Feynman
 

Online Mechatrommer

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #71 on: February 04, 2016, 05:01:53 am »
So use "safecracker mode" on your fingers and turn the knobs slowly.... very slowly.... and wait for the scope to catch up.
you dont have to slow turn until you see something on the screen... if you turn quickly, the next screen update, the big offset you made on the knob will reflects, sometime it just goes off the grid. eventually the scope did catch up from behind, its just the GUI "update pooling" will take sometime before its visible on the screen. i just need to learn the right knob turn speed from experience, to get the increment i want on the next screen update... except if i want a very precise Voffset then i will go as you said "safecracker mode"?
It's extremely difficult to start life.. one features of nature.. physical laws are mathematical theory of great beauty... You may wonder Why? our knowledge shows that nature is so constructed. We simply have to accept it. One could describe the situation by saying that... (Paul Dirac)
 

Offline markone

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #72 on: February 05, 2016, 07:00:39 pm »
Sorry for the OT,

but take a look at this beauty, Dave's FFT reference signal (1Mhz, FM 5Khz, Dev 500Hz) under italian ELAD FDM-S2 16bit-122.88MSa/s sampler analysis, until 60Mhz and 6Mhz span blows out the windows everything showed here in terms of sensitivity, resolution, dynamic and update rate (almost real time) at mere 430 Euro plus taxes.

Not a DSO, but if you need high resolution FFT within its spec, coupled with a DC block and a fixed attenuator and / or active probe for High-Z it does a great job.


 

Offline AlessandroAU

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #73 on: February 06, 2016, 12:00:11 am »
He is the FFT from an AD7760 eval board.

2.5MSPS, 24bit
 

Offline rs20

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #74 on: February 06, 2016, 12:12:46 am »
He is the FFT from an AD7760 eval board.

2.5MSPS, 24bit

Now try it with the OP's 1 MHz carrier  :P

Also, did you do the FFT on your PC? How many points? How long did it take? Doing an FFT is trivial, after all, if you have GB of memory and no particular time limit...
 

Offline hpw

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #75 on: February 07, 2016, 12:20:54 pm »

If some is interested about the dynamic range on 12...40 bit

Hp

Offline rs20

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #76 on: February 07, 2016, 12:47:28 pm »

If some is interested about the dynamic range on 12...40 bit

Hp

That's just a simulation. Try putting in a vaguely reasonable AWGN representing an analog frontend and you'll get very different result. Processing gain from long FFTs is a much more practical way of getting a low noise floor than an absurd 40-bit DAC.
 

Offline hpw

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #77 on: February 08, 2016, 09:50:39 am »

>> That's just a simulation.

Yes it's a digital & dithered signal to test digital gear! Keep in mind that HW digital mixer often calculates using > 32 bits.

>> Try putting in a vaguely reasonable AWGN representing an analog frontend and you'll get very different result.

Well, I used here triangle dither. Gauss dither will not alter that much. it's also dither level dependent.
 
>> Processing gain from long FFTs is a much more practical way of getting a low noise floor than an absurd 40-bit DAC.

Why is this absurd? I do not play with marketing bits...  ::)

IMHO it was a demonstration of dynamic range f(bits) not more using internal IEEE 64/80 bits..

Keep in mind that PCM <> DSP conversion, even uses higher bit count.

Hp



Offline tautech

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #78 on: March 06, 2016, 12:53:52 pm »
An SDS1000X  FFT screenshot from member rf-loop, snatched from another thread:
https://www.eevblog.com/forum/testgear/siglent-sds1000x-series-oscilloscopes/msg832023/#msg832023

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #79 on: March 07, 2016, 04:42:27 am »
Wow, Dave used 10 Vpp which is crazy high if he terminated an input with 50 ohms. Seems like a bit of a rookie mistake and a bad example to use. 
VE7FM
 

Offline max666

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #80 on: March 07, 2016, 10:38:36 am »
Wow, Dave used 10 Vpp which is crazy high if he terminated an input with 50 ohms. Seems like a bit of a rookie mistake and a bad example to use.

Hmm ... is Dave terminating in at the Rigol DSA815 input? Because then I it would be too much, wouldn't it?
Quote from: Rigol UserGuide
CAUTION
To avoid damage to the instrument, for the signal input from the RF input terminal, the DC voltage component and the maximum continuous power of the AC (RF) signal component can not exceed 50 V and +20 dBm respectively.

+20 dBm = 0.1 W and 10 Vpp AC terminated at 50 Ohm would be 1 W, or am I missing something here?
 

Offline saturation

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #81 on: December 12, 2016, 12:50:15 am »
Just for reference to this thread the relative prices:

Rohde & Schwarz HMO1202 Series;1232 $US2300
Tektronix MDO3000;  MDO3014. LIST PRICE. (US$4,110)
Keysight 3000X Touch, Keysight MSO 3054T ($13,000)
Lecroy WaveJet 354 Touch $US5000
GW Instek GDS-1104B GWinstek 1104 $700 ($830) 1102B $396, 1054B $366
Rigol 1054z ($US400)
Rigol DS2000 DS2202 ($US1790)

($) are list prices other price is discounted by one popular vendor here, before eevblog coupon code.  FWIW the other Instek is a 2 CH version and 50 MHZ version and have the same processing power, just reduced analog channels and bandwidth respectively.



When Dave said that FFT sucks in the cheap scopes, for some reason didn't mention that instek's 1000B series are even a bit cheaper than Rigol 1000Z (for 50 MHz model if comparing with 1054Z). Also from actual FFT performance it's certainly looks to work better than R&S and Agilent (except fiddly UI) and is second after Tek MDO.

Yes I completely forgot the price of the GW-Instek, and stand corrected.

Best Wishes,

 Saturation
 

Offline Arjan

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #82 on: July 11, 2017, 10:52:40 am »
Hi,

New member here. I'm considering getting a Rigol DS2074 after basically dimissing the Owon XDS series having seen the 14 bit teardown as I don't want a scope that's likely to break down in a few years due to bad soldering, bad caps or whatever. The Rigol stuff appears to have much better internal design and build quality but then I found this video on its mediocre FFT performance.

However, I'm wondering wich firmware was used in that video for the Rigols because I came accross this:

With the latest firmware (00.04.03 SP2) for the Rigol MSO/DS1000Z Series Oscilloscopes the  FFT functionality has been improved.

The FFT (Fast Fourier Transform) now can be calculated not only from the screen points but also on 16k points from the acquisition memory. This new enhancement enables the customer to perform FFT analyis in the frequency domain with increased frequency resolution.

Calculating FFT only from the screen points? What were they thinking? Anyway, cccording to the page info for that URL this appears to have been released in July 2015 and the 2000 series was released before that so I guess this enhancement was included in the 2000 series from the start but I don't like to guess or make assumptions.

So, is the FFT performance of the 2000 series still as mediocre with the latest firmware? Not sure if it's going to be a deal-breaker for me, I won't need FFT a lot and I suppose I could export the data and have it analyzed on my PC or something, right?

The 2074 seems rather attractive and it's currently on sale at Batronix for € 802.18 incl. VAT (and I don't have to pay VAT as a company) but are there any other scopes I should be looking at for that price or lower? I should say that the 2074 already offers much more than I would actually need, I could probably get by with a €300 scope but I just love the larger 800x480 display, graded display and better update rates and if I understand it can also function as a Wave Generator and replay back captured waveforms which for my application would be very convenient.

Thanks,
Arjan
 

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #83 on: July 11, 2017, 10:59:03 am »
Not long released SDS1202X-E:
1 Mpts FFT

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Offline Arjan

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #84 on: July 11, 2017, 12:56:16 pm »
Wow, thank you. That does look like an attractive alternative. I guess these are the main differences (the ones that matter to me):


Rigol DS2074           Siglent SDS1202X-E
2GSa/s                 1GSa/s[/font]
Waveform Gen           No Waveform Gen
Crap FFT               Decent FFT
€ 802                  € 427     at batronix.com (special 10% discount on the Rigol until 31 July 2017, regular price is €819)


I left out the bandwith because in essence these are both 200Mhz if you're ok hacking the Rigol. Not sure that I am but on the other hand both probably offer plenty bandwidth (even at 70Mhz) and Sa/s for my purposes so I guess I might have to accept the lack of WG and the lower Sa/s on the Siglent.

I have to say that in principle it's a lot easier for me to justify € 427 as I will be using this thing rather sporadically but I've wanted a scope for years and I don't like having to borrow one if I need it. On the other hand I want a quality product that's not going to break down on me. I was a little worried about the Lelon caps but after some Googling it seems they may not be that bad actually (provided they're genuine).
 

Offline Arjan

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #85 on: July 11, 2017, 02:54:53 pm »
Bit the bullet and ordered the Siglent SDS1202X-E, it's just an amazing price given the feature set and it does seem like the build quality is quite acceptable.
 

Online nctnico

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #86 on: July 12, 2017, 08:16:54 pm »
You could have looked further and bought the GW Instek 1054B. It has 4 channels, 1MPts FFT as well, (limited) lifetime warranty, mature firmware and a whole bunch of extra features (like input filtering) at the same price level.
« Last Edit: July 12, 2017, 08:25:39 pm by nctnico »
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Offline tautech

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #87 on: July 12, 2017, 08:26:48 pm »
You could have looked further and bought the GW Instek 1054B. It has 4 channels, 1MPts FFT as well, mature firmware and a whole bunch of extra features (like input filtering) at the same price level.
Maybe the 200 MHz BW and 500uV/div was more important to Arjan.
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Online nctnico

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #88 on: July 12, 2017, 09:14:36 pm »
You could have looked further and bought the GW Instek 1054B. It has 4 channels, 1MPts FFT as well, mature firmware and a whole bunch of extra features (like input filtering) at the same price level.
Maybe the 200 MHz BW and 500uV/div was more important to Arjan.
He wrote he would be content with 70MHz so I guess not. Also input filtering makes a lot of difference when looking at small signals. 500uV/div doesn't do any good if the signal is drowned in noise. It is not the big numbers that make all the difference but the small things which make life easier and the (recent) GW Instek scopes have those in abundance.
« Last Edit: July 12, 2017, 09:17:10 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online Fungus

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #89 on: July 13, 2017, 07:05:40 am »
Awww, is the GW Instek not getting enough love?

It seems to be a recurring theme with that 'scope. Nobody wants one. Maybe it's the horrible color and oversized screen bezel.

 

Offline borjam

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #90 on: August 03, 2017, 09:51:40 pm »
Awww, is the GW Instek not getting enough love?

It seems to be a recurring theme with that 'scope. Nobody wants one. Maybe it's the horrible color and oversized screen bezel.
Interesting. Seems that RS is selling the GW Instek rebranded as "RS Pro".

http://uk.rs-online.com/web/p/digital-oscilloscopes/1233543/

 

Offline tautech

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #91 on: August 03, 2017, 10:06:18 pm »
Awww, is the GW Instek not getting enough love?

It seems to be a recurring theme with that 'scope. Nobody wants one. Maybe it's the horrible color and oversized screen bezel.
Interesting. Seems that RS is selling the GW Instek rebranded as "RS Pro".

http://uk.rs-online.com/web/p/digital-oscilloscopes/1233543/
Yep. They even have rebranded Siglent bench DMM's.
http://nz.rs-online.com/web/p/digital-multimeters/1236466/
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Offline bctouw

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #92 on: October 09, 2018, 07:28:53 pm »
I have a recent Rigol 1054Z with all options enabled, exept the 100Mhz,
I have been testing the FFT indept, and i dissagree with you findings 

I have a perfectly usable FFT and even with th cursosr i can measure 60-70 dB of dynamic range.

If you want i can prove it !!,

Granted, it is fiddely to setup, but if yoy take a little bit of time you will get great results,   for a 400 dollar instrument.

Bas Touw, Holland
 

Online Fungus

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #93 on: October 10, 2018, 08:55:01 am »
I have a recent Rigol 1054Z with all options enabled, exept the 100Mhz,
I have been testing the FFT indept, and i dissagree with you findings 

I have a perfectly usable FFT and even with th cursosr i can measure 60-70 dB of dynamic range.

If you want i can prove it !!,

Rigol update the firmware since Dave's video, they improved the FFT.
 

Online nctnico

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #94 on: October 10, 2018, 09:36:12 am »
I have a recent Rigol 1054Z with all options enabled, exept the 100Mhz,
I have been testing the FFT indept, and i dissagree with you findings 

I have a perfectly usable FFT and even with th cursosr i can measure 60-70 dB of dynamic range.
You have to define useable. For example: try to look at a signal in a meaningfull way AND the FFT at the same time. With only a handful of kpts of FFT this is nearly impossible. The GW Instek scopes really set a new benchmark with their 1MPts FFT depths.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline bctouw

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Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #95 on: October 10, 2018, 06:18:17 pm »
That is true
 

Offline bctouw

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  • Posts: 4
  • Country: nl
Re: EEVblog #845 - Oscilloscope FFT Comparison
« Reply #96 on: October 10, 2018, 06:20:04 pm »
I can only say about the Rigol
 


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