Scope Wars

[tv84]

While fun and quick and dirty way to check if you "upgraded" your scope from base 100 MHz to something else, pulse response is not very relatable to usual scope work we all do. Unless you're making pulsers and you need to check them.. In which case you get scope with adequate bandwidth and calibrated and well defined pulse response for that particular purpose.

My intent was not having an absolute characterization of an equipment BUT only doing a comparison between 2 equipments.

Why wouldn't the pulse response be useful in comparing 2 brickwall response scopes? Because of the greater probability that they would look the same? But, in the BW of interest, for me, that would mean that both had the same fidelity. Correct?

[rhb]

A fast square wave is only one instrument needed to do a full evaluation of a scope.   However, if you know what to look for it will tell you quite a lot.

I'm using the following:

Bodnar GPSDO
Keysight 33622A 120 MHz AWG <1 ps jitter
Bodnar <40 ps rise time square wave and 100 ps pulse generators
HP 8648C

I'll also be using Octave and waveform files for a lot of tests.

I recommend reading the performance check and calibration sections of the Tektronix 465 service manual to get an idea of what is involved. It's beautiful reading and in addition the theory of operation discusses how every circuit in the scope works.  So you'll not only learn how to evaluate the performance, you'll learn how a good analog scope works.

I made a photo yesterday of the 100 ps pulse as shown by my 485 to send to a friend so I've added it just because it's a nice image of a minimum phase impulse response.

Have Fun!
Reg

[rhb]

These are the plots for my previous post

(Attachment Link)

(Attachment Link)

(Attachment Link)

(Attachment Link)

There is an unalterable relationship between the shape of the passband and the time domain response.  And a step response will tell you which scope has a better passband.  I generated impulse responses because it was easier than generating a minimum phase step response and they show the same thing.

The "ideal" low pass filter has rather less than an ideal time domain response.  TANSTAFL

Reg

[2N3055]

While fun and quick and dirty way to check if you "upgraded" your scope from base 100 MHz to something else, pulse response is not very relatable to usual scope work we all do. Unless you're making pulsers and you need to check them.. In which case you get scope with adequate bandwidth and calibrated and well defined pulse response for that particular purpose.

My intent was not having an absolute characterization of an equipment BUT only doing a comparison between 2 equipments.

Why wouldn't the pulse response be useful in comparing 2 brickwall response scopes? Because of the greater probability that they would look the same? But, in the BW of interest, for me, that would mean that both had the same fidelity. Correct?

Absolutely you could relatively compare. As long as there is no simplistic expectation like some (not you) that rise time directly correlates with bandwidth using some fixed simple formula. And expecting that perfect pulse response will give "perfect" frequency response. It will give frequency rolloff that is perfect for looking at pulses and will have nice response for mathematical analysis.

If you want bandwidth flatness, brickwall is better..

https://www.euramet.org/Media/docs/Publications/calguides/EURAMET_cg-7__v_1.0_Calibration_of_Oscilloscopes.pdf
https://www.ece.ubc.ca/~robertor/Links_files/Files/TEK-Understanding-Scope-BW-tr-Fidelity.pdf
https://m.eet.com/media/1119902/c0882paged.pdf

[SilverSolder]

A fast square wave is only one instrument needed to do a full evaluation of a scope.   However, if you know what to look for it will tell you quite a lot.

I'm using the following:

Bodnar GPSDO
Keysight 33622A 120 MHz AWG <1 ps jitter
Bodnar <40 ps rise time square wave and 100 ps pulse generators
HP 8648C

I'll also be using Octave and waveform files for a lot of tests.

I recommend reading the performance check and calibration sections of the Tektronix 465 service manual to get an idea of what is involved. It's beautiful reading and in addition the theory of operation discusses how every circuit in the scope works.  So you'll not only learn how to evaluate the performance, you'll learn how a good analog scope works.

I made a photo yesterday of the 100 ps pulse as shown by my 485 to send to a friend so I've added it just because it's a nice image of a minimum phase impulse response.

Have Fun!
Reg

Is the timebase 100ps per division in the picture?

[SilverSolder]

[...]
The entire issue is under what conditions is discretely sampled data *accurately* representative of the analog reality?  Seismic was recorded on 2" analog tape with 21 tracks before digital became possible.  And *no one* was recording digital data for actual exploration work until it was as good as the analog data.
[...]

Very interesting stuff about what you do in the seismic space, I know very little about it but it sounds fascinating.

Regarding accuracy/precision, I guess nobody likes going backwards - which was often a problem with early digital stuff, like the first CD players which were not really as good as the best analog vinyl tech at the time.

Is there still any question today, that digital methods can be as good or better than anything analog (given money is no object) for seismic purposes?

[rhb]

Seismic went all digital as soon as they could build ADCs.  You cannot do the exotic processing which is the norm today in an analog system.  Imagine 3D ultrasound except that it's 10-15 TB of data with 3x velocity contrasts.  That takes a 10-20,000 core cluster 7-10 days per iteration.  After each iteration the velocity model has to be updated.

My PhD supervisor's claim to fame was doing deconvolution to remove water column reverberation using a magnetic drum with movable heads and adjustable gain on analog data around 1959.  It had been done digitally by hand by Enders Robsinson a classmate of my supervisor's around 1952 by hand digitizing the paper sections and then computing the Wiener inverse aka prediction error aka predictive deconvolution filter and applying it with a desk calculator.

The publication of "The Extrapolation, interpolation and Smoothing of Stationary Time Series" by Norbert Wiener in 1949 really got the oil industries attention and a consortium, the Geophysical analysis Group, was organized at MIT working under Wiener.   Two of the 7 students, Enders Robinson and Sven Treitel, wrote the papers that established digital signal processing.  These were republished for many years by Seismograph Service Company as "The Robinson and Treitel Reader" for may years.  They eventually merged it into a single monograph, Geophysical Signal Analysis",  in 1980 just before I started in the oil industry.  I learned DSP from that and a book by Kanasewich. "Time Sequence Analysis in Geophysics".

My MS was in igneous aka "hard rock" petrology.  I'd never even seen a seismic section before I arrived at Amoco in 1982.  They had promised lavish training, but I got only a single 2 week course after 18 months on the job.  I taught myself DSP from those two books evenings and weekends. In addition to those two books I had two books on reflection seismology.  So I cycled through the stack of 4 books, each time learning a little more that had been incomprehensible the previous pass.

Sadly, the EE community doesn't seem to recognize the importance of seismic exploration as a driving force second only to the military in the development of ADCs.

Here's a bit of background:

https://wiki.seg.org/wiki/Ralph_Harris
 
http://gsinet.us/Grapevine.html

TI was a subsidiary of GSI formed to make recording equipment.

Have fun!
Reg

[SilverSolder]

Thanks for the background @Reg, a whole fascinating area of applied science that isn't as well known as it ought to be.

[Bud]

rhb you stil can't refrain from throwing names, book titles and from praising your own ingenuity, can you 

[Elasia]

rhb you stil can't refrain from throwing names, book titles and from praising your own ingenuity, can you 

Eh.. better reading than some of the other junk posts that crop up.. i thought it was a decent history post anyway

rhb.. i was wondering if you were oil or not when you said seismic awhile ago.. guess that answers that lol

[rhb]

Well I've been fooling with the DS1202Z-E. 

I setup my Keysight 33622A for a 10 MHz, 20 mVpp square wave.

First, the spectrum as reported by an HP 8560A



Next the best I have been able to get from the DS1202Z-E.  I really can't figure out what they are doing wrong.  I'm in discussion with the Rigol support guy who supplied me with a step response.

Edit:  This was a consequence of my not realizing there was another menu of settings.  This display is limited to the screen data and uses a boxcar window.  I'll post a better result later.



For comparison the FFT from the Instek MSO-2204EA:



And the SA app from the Instek MDO-2000E



The SA app has a much better UI than the FFT function, but as you can see there are significant artifacts.  Unfortunately, as this is an unauthorized hack it is unlikely that Good Will will address the issue.

Have Fun!
Reg

[rhb]

At present I cannot save more than 12,000 samples to a CSV file.  If I try to save a longer file it shows a progress bar for a while and then says "Saving failed"





Reg

[David Hess]

On an analog oscilloscope, there will be no doubt that the signal wiggles right before each edge.

That is not completely true.  Frequency and phase compensation of the delay line used in an analog oscilloscope to show the leading edge produces pre-shoot which will become visible if the input edge and oscilloscope is fast enough.  The 300 to 400 MHz Tektronix 2465 series suffers from this, but the effect is very minor.  Oddly enough, the Tektronix 300 MHz 2440 DSO suffers from it also for some weird reason.

On a digital oscilloscope, we don't know.  Might be a perfectly square wave with digital artifacts (because of the oscilloscope's DSP), or it might be because the signal really wiggles before each edge.

I really hate that with DSOs opperating at an insufficient sample rate.  It is not a problem when equivalent time sampling is used because the sample rate is so much higher.

Scopes with Gaussian response have gradual rolloff from very low frequencies, so amplitude accuracy will be impacted. -3dB is 30% percent amplitude error. So for a Gaussian response scope, 1GHz scope will have good amplitude accuracy up to cca 300MHz or about.

Something to keep in mind is that a Gaussian response is *not* the same as a Bessel response.  Compare the shape of a fast edge on an analog oscilloscope at maximum bandwidth and with the bandwidth limiter; they are not remotely the same.  The Gaussian response looks like a slew rate limited edge while the Bessel response should show something like an exponential curve.  Most bandwidth limits have a single pole response but some use LC sections for a 2 pole response and I know of one example which was 4 poles which is unfortunate because they complicate noise calculations.

It will behave same as if you took 1 GHz brickwall response scope and put in 500-600 MHz low pass filter in front of it.

So it is *not* the same as that produced bv a typical Bessel low pass filter.

So overshoot is actually useful. It shows you you are trying to look at the signal that is too fast for your scope, instead of happily hiding anything it doesn't like... You fight it by getting faster scope, not one that hides it better...

Or the preshoot and/or overshoot was always there, which is why I have reference level pulse generators to test oscilloscopes.

[gf]

Is the timebase 100ps per division in the picture?

100ps/div would be indeed cool, but given that this is supposed to be the impulse response of a Tektronix 485 (-> 350 MHz),  I'm afraid your guess is not realistic and rather an order of magnitude too low.

[gf]

The first picture shows ringing which is not in the original signal.

You must admit, though, that the ringing is eventually introduced by the attempt to reconstruct the waveform between the sampled points with sinc interpolation, which was turned on although the prerequisite requirement for this reconstruction method is not fulfilled.

I guess, displaying this signal in points mode would not show any indications of ringing, and linear interpolation would likely show a more pleasing shape, too.

[ Of course nobody can deny that sampling loses (an infinte amount of) information. Any information about the waveform shape between the sampled points is lost. Consequently, an exact reconstruction of the original signal without any a priori knowledge about the signal is impossible. Knowing that the signal fed into the ADC is band-limited to < fs/2 is one kind of useful a priori knowledge, but other kinds of knowledge which could enable reconstruction are imaginable too. ]

[nctnico]

The first picture shows ringing which is not in the original signal.

You must admit, though, that the ringing is eventually introduced by the attempt to reconstruct the waveform between the sampled points with sinc interpolation, which was turned on although the prerequisite requirement for this reconstruction method is not fulfilled.

I guess, displaying this signal in points mode would not show any indications of ringing, and linear interpolation would likely show a more pleasing shape, too.

Not really. There is some ringing in the signal and in linear/dot mode you just get a large smeared area. Even with the extra ringing sin x/x interpolation gives the most accurate representation given the circumstances.

[SilverSolder]

Is the timebase 100ps per division in the picture?

100ps/div would be indeed cool, but given that this is supposed to be the impulse response of a Tektronix 485 (-> 350 MHz),  I'm afraid your guess is not realistic and rather an order of magnitude too low.

So the 100ps pulse has been "smeared out" to become 10x wider?

[Sighound36]

100ps time base on Rigol 1202 humm................ let me think

[SilverSolder]

It is probably a challenge for that particular model! 

[gf]

Not really. There is some ringing in the signal and in linear/dot mode you just get a large smeared area. Even with the extra ringing sin x/x interpolation gives the most accurate representation given the circumstances.

OK, so the square wave was already degraded before entering the ADC - e.g. by the frontend.

So the 100ps pulse has been "smeared out" to become 10x wider?

This is to be expected. Since 100ps is pretty narrow in relation to the bandwitdh, it can be considered an approximation of a dirac impule. So the smeared pulse you see is close to the scope's impulse response.

[RoGeorge]

Is the timebase 100ps per division in the picture?

100ps/div would be indeed cool, but given that this is supposed to be the impulse response of a Tektronix 485 (-> 350 MHz),  I'm afraid your guess is not realistic and rather an order of magnitude too low.

So the 100ps pulse has been "smeared out" to become 10x wider?

Yes, smearing out a sharp pulse is expected to happen in a low pass causal filter, and if we consider those 100ps to be practically zero time, what we see in the image is the impulse response of the system (filter, oscilloscope, whatever), h(t).



For an intuitive approach, you can think of the low pass filter like a system with a "speed limit" in energy transfer speed (the system is not allowed high frequency, or high speed transfers, only slow exchanges).  Now, if you suddenly drop a lump of energy (that 100ps pulse) in such a filter, the energy exchange "speed limit" will make that lump of energy to stay there for a while, until the circuit somehow manages to deal with it, therefore the smearing in time of the initially very narrow pulse.

Note the causal word in the name "causal filter".  A causal filter will never produce wiggles in front of a signal, while a DSP filter might produce such artifacts.  As an example, Fourier transform is not causal.  If you use Fourier to implement a filter, such a filter will produce some fake (artifacts) output before you even start to apply your signal.  That's not how physics works.  Other way to look at it is to say one can not do a Fourier transform live (in real time).

In the next image, you see before each edge some fake wiggling.



That is because of non-causal DSP.  Analog oscilloscopes never show that kind of artifacts, because they are made with physical analog components.  Physical world is always causal.

Back to the impulse response, h(t), of that analogue oscilloscope



that wiggling waveform completely characterizes the time response of a system.  Starting from it, one can deduce the time response (the waveform) for any shape of a given input signal (or else said, deduce what distorted waveshape we will see on that oscilloscope's display when we apply an arbitrary waveform input).

[Sighound36]

It is probably a challenge for that particular model! 

 

The Wavepro  4Gz model has 20ps HTB (analogue mode) and that's just a touch up the ladder from that Rigol   

The Rigol MSO8000 has 200ps max HTB in analogue mode

[SilverSolder]

[...]

Back to the impulse response, h(t), of that analogue oscilloscope



that wiggling waveform completely characterizes the time response of a system.  Starting from it, one can deduce the time response (the waveform) for any shape of a given input signal (or else said, deduce what distorted waveshape we will see on that oscilloscope's display when we apply an arbitrary waveform input).

Thank you for a very educational and concise post!

Presumably the delta function that is put into the causal system contains a certain defined amount of energy -  is there some "conservation of energy" going on so you can for example say that the area under the input pulse curve is always equal to the area under the smeared output, or something like that?

[rhb]

That pulse is 100 ps wide, so most of the energy is lost because the scope has only 350 MHz BW.  The time base is 1 ns/div which is as fast as the old girl can go.   However, in an ideal lossless case with a Dirac function which has unit area, the area of the  minimum phase BW limited impulse response is proportional to the area of the transfer function.

All the energy is being forced into sine waves which start at T0.

The requirement of  signal to be causal is that the real and imaginary parts in the frequency domain of a pure real function in the time domain be the Hilbert transform of each other.

I just found a fast one shot pulse generator I built 40 years ago out of a 7400 NAND gate.  The pulse width is governed by the propagation delay of the gates.  I couldn't measure it then, but I can now.

Have Fun!
Reg

[rhb]

Note the causal word in the name "causal filter".  A causal filter will never produce wiggles in front of a signal, while a DSP filter might produce such artifacts.  As an example, Fourier transform is not causal.  If you use Fourier to implement a filter, such a filter will produce some fake (artifacts) output before you even start to apply your signal.  That's not how physics works.  Other way to look at it is to say one can not do a Fourier transform live (in real time).

In the next image, you see before each edge some fake wiggling.



That is because of non-causal DSP.  Analog oscilloscopes never show that kind of artifacts, because they are made with physical analog components.  Physical world is always causal.

Back to the impulse response, h(t), of that analogue oscilloscope



that wiggling waveform completely characterizes the time response of a system.  Starting from it, one can deduce the time response (the waveform) for any shape of a given input signal (or else said, deduce what distorted waveshape we will see on that oscilloscope's display when we apply an arbitrary waveform input).

Actually you can apply a causal filter in the frequency domain, but you have to multiply by a complex series in which the imaginary and real parts are Hilbert transform pairs.  That's how I created the examples I posted for  boxcar, 80% and 50% of Nyquist passband impulse responses.  Commonly described in some circles as the Kramers-Kronig relation or condition.

The issue is that if the convolution is being done with an FIR, you have to have more multipliers for a causal filter than for an zero phase symmetric filter.  It's just crap DSP to allow using 1+(N-1)/2 multipliers instead of 1+N multipliers.

Thanks for the nice summary.  The noise level has gotten rather high.

Have Fun!
Reg