Scope Wars

[Fungus]

but fate seems already (almost) sealed for me: reg he ruled out for me Owon, rigol1102 it seems old of hardware, rigol 1202 they say out there that it's based on z old platform, instek is prohibited price... on the square he remains alone siglent 1202 

I think Charlotte is the one teaching many here... 

Only if you completely ignore the prices. It's obvious that you should get something better if you spend more money.

[Fungus]

In many respects, the DS1102E looks better to me than DS1202Z-E. 

It has a lot less features but it displays wiggly lines just fine. You can get the DS1052E even cheaper and hack the bandwidth to 100MHz.

[David Hess]

Step response *is* the last thing that a novice will think about.  It's also the single best test of the performance of the AFE and the DSP design.  With 30+ years of DSP experience in the oil industry, I can look at the step response and draw the AFE filter profile on a cocktail napkin.   TANSTAFL  You cannot have a DSO BW which is more than 50-70% of Nyquist and have the traditional analog Tek <3% overshoot.

Step response is the first thing I think about because oscilloscopes are primarily time domain instruments.

Something else I would test for now is variation of step response and bandwidth with signal level as this is a problem with the Rigol DS1000Z series which suffers from overload problems; this is why some users report 200+ MHz bandwidth while others report bandwidth commensurate with the specifications.

[Fungus]

Something else I would test for now is variation of step response and bandwidth with signal level as this is a problem with the Rigol DS1000Z series which suffers from overload problems

All 'scopes suffer that if the signal goes wildly offscreen, the DS1054Z is no worse.

Or maybe you're referring to abuse of the 'fine' vertical control - switch the vertical response to a range which is too low and crank the fine control to try and bring it back up again. I don't really see that as a problem despite it being used endlessly as a stick to beat Rigols with.

[CharlotteSwiss]

Instek can be had real cheap right now:
https://www.tequipment.net/Instek/GDS-1054B/Digital-Oscilloscopes/?search=true

for Americans yes, but for those who live in Europe, shipping and taxes are a drain 

[rhb]

A brief tutorial on the relationship between filter passband in the frequency domain and time domain response.

We consider 3 cases, full amplitude to Nyquist, a corner frequency at 80% of Nyquist and a corner frequency of 50% of Nyquist.  The respective cases are shown here:

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We next look at the time domain impulse response of this "ideal" low pass filter.  It's rather less than ideal if you want to discriminate multiple impulses.

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A 500 MSa/s DSO has a Nyquist at 250 MHz.  So if we set the corner frequency to 80% of Nyquist this is what we get:

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If we are conservative and choose a corner frequency of 50% of Nyquist we get this:

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The undershoot at the tail end of the impulse can be reduced by changing the shape of the ramp.  I chose to stick to the most basic demonstration to start with.  When I get to evaluating particular scopes I'll use the step response to show the actual filter passband.

the Octave and gnuplot files are attached.

If you want to get a detailed understanding of this I recommend:

The Fourier Transform and Its Application
Ronald Bracewell

I have the 2nd ed and it is my most frequently used math book because it has a pictorial dictionary of transform pairs in the back.

Have Fun!
Reg

Dave's server is confused and is not letting me post PNG files.  I had posted them when I saw I'd forgotten to change the X axis label.  It's been throwing a hissy ever since.

[SilverSolder]

Does multiple sampling of the same (repetitive) signal get around some of the limitations of the Nyquist sample rate?  E.g. if the impulse was made to repeat...

[rhb]

These are the plots for my previous post

[rhb]

Here are the Octave and gnuplot files

The point of this 3 piece :-(  post is TANSTAFL.  The time domain and frequency domain are bound together by the Fourier transform.

I can easily get a nice clean step with <1% overshoot.  But I can't claim a -3 dB corner at 80% or more of Nyquist.

Nyquist & Shannon showed what was required to accurately represent a *band limited* signal with *regular* sampling without aliasing.

Sampling scopes collect a single sample per waveform and dither the time of the sample to form a sweep.  The Tek 11801 does this at 20 femtosecond intervals.

It is possible to to use random sampling and avoid aliasing entirely, but that is a *very* different topic called "compressive sensing".  So far no one here has taken me up on a discussion of that publicly, though one person has in a PM revealed he understands the topic to some degree.

Have Fun!
Reg

[edigi]

Sampling scopes collect a single sample per waveform and dither the time of the sample to form a sweep.

You mean older scopes do this. Modern scopes that can be used also for non-repetitive signals (can be important even for hobbyist to check various buses and transients) just take samples at regular intervals dictated by the sampling rate.

[SilverSolder]

The acquisition mode makes a difference here, right? 

I.e. the behaviour you describe assumes "Normal" acquisition mode?

[David Hess]

Something else I would test for now is variation of step response and bandwidth with signal level as this is a problem with the Rigol DS1000Z series which suffers from overload problems

All 'scopes suffer that if the signal goes wildly offscreen, the DS1054Z is no worse.

Not all oscilloscopes suffer signal distortion from overload; sampling oscilloscopes by the nature of how their sampling works are immune.  Some oscilloscopes implement series, shunt, or feedback clamping to reduce or eliminate overload recovery time.

The DS1000Z series suffers from overload problems even for signals which are completely within its dynamic range and this explains the great variations in reported bandwidth but months ago I pointed out other evidence of overload caused by insufficient full power bandwidth.

Or maybe you're referring to abuse of the 'fine' vertical control - switch the vertical response to a range which is too low and crank the fine control to try and bring it back up again. I don't really see that as a problem despite it being used endlessly as a stick to beat Rigols with.

Whatever the detailed cause, better oscilloscopes do not suffer from it.  The shape of a signal within the input range should never change with amplitude or position.  Good oscilloscope manufacturers figured this out in the 1950s.

Sampling scopes collect a single sample per waveform and dither the time of the sample to form a sweep.

You mean older scopes do this. Modern scopes that can be used also for non-repetitive signals (can be important even for hobbyist to check various buses and transients) just take samples at regular intervals dictated by the sampling rate.

He means *sampling* oscilloscopes which sample before amplification.  DSO stands for digital storage oscilloscope which whether modern or old, operates in a completely different way.

It is possible to to use random sampling and avoid aliasing entirely, but that is a *very* different topic called "compressive sensing".  So far no one here has taken me up on a discussion of that publicly, though one person has in a PM revealed he understands the topic to some degree.

I am aware of it but do not find it that interesting because HP was the only one to implement it in DSOs that I know of:

https://www.keysight.com/upload/cmc_upload/All/exp66.pdf

It does not avoid aliasing entirely but instead converts the tones produced by aliasing into noise.

[rhb]

I mean what I said.  A "sampling scope" is a very different instrument from a DSO.  Do a search on the term.

Here's an aliasing example.  I've generated a 10 Hz sine wave sampled at 100 samples/second and a 60 Hz sine wave sampled at the same sample rate.







The Nyquist is 50 Hz, so the aliasing makes it appear that the 60 Hz signal is at 40 Hz rather than 60 Hz.




Here's what the actual samples look like with straight line interpolation between sample points.




Edit:  This is completely borked.  These are not the figures I posted for this.

[SilverSolder]

On my scope, you can choose acquisition modes that "oversample" the signal (e.g. averaging, high resolution, peak detect etc.) and I was wondering how that affects Nyquist (if at all!).

[edigi]

The 60Hz signal is 10Hz over the 50Hz Nyquist limit and folded back to 40Hz. I think I've seen already couple of times like that before. Is there anything more to be seen here?

[David Hess]

On my scope, you can choose acquisition modes that "oversample" the signal (e.g. averaging, high resolution, peak detect etc.) and I was wondering how that affects Nyquist (if at all!).

Averaging happens after the aliasing produced by decimation so has no useful effect on aliasing.  High resolution mode and peak detection happen before decimation so prevent aliasing up to (half) the maximum sample rate of the digitizer; high resolution does this by limiting bandwidth with a real time FIR filter and peak detection does this by returning a limited histogram of the original signal.

I consider peak detection invaluable but it is less necessary on DSOs with long record lengths where the maximum sample rate can be sustained at slower time/div settings.  My ancient Tektronix 2230 with a 1k record length and peak detection can pick up a 100 nanosecond GPS pulse per second signal at any sweep speed which would require at least a 10 megasample record length (or segmented memory) otherwise.  Peak detection might be considered a very limited form of DPO (digital phosphor oscilloscope) operation where DPO mode returns a complete 3D histogram of the signal.

[Kleinstein]

Many DSO can still use equivalent time sampling. This is similar to the sampling scopes in many respects. It was especially popular with the early DSOs from the 1990s, but many new one still have it as an option.
Besides using it for a substitute for a higher sampling rate, it may also be used as averaging to get high amplitude resolution / less noise.

It can make a difference to the Nyquist limit, though depending on the way it is implemented (e.g. if the is a fixed ADC clock) it may be help in all cases.

The modern entry level DSOs tend to be 1-2 GSPS and some 100 MHz BW, so not that much limited by the sampling rate. It however gets worse if all channels are used and the sampling rate usually cut in half.

Besides the BW limit, there may also be a slew rate limit in some cases and thus amplitude dependent BW.

[SilverSolder]

Good article on equivalent time vs real time sampling here:  https://www.edn.com/random-repetitive-sampling-take-2/

[Fungus]

Or maybe you're referring to abuse of the 'fine' vertical control - switch the vertical response to a range which is too low and crank the fine control to try and bring it back up again. I don't really see that as a problem despite it being used endlessly as a stick to beat Rigols with.

Whatever the detailed cause, better oscilloscopes do not suffer from it.  The shape of a signal within the input range should never change with amplitude or position.  Good oscilloscope manufacturers figured this out in the 1950s.

"Better" oscilloscopes cost more then $350 and they'll still have design compromises somewhere.

The DS1054Z is beneath your sensibilities? Fine, don't use one...  but understand that many people still find it a very useful tool.

[rhb]

"Better" oscilloscopes cost more then $350 and they'll still have design compromises somewhere.

My observation with the Keysight MSOX3104T and Rohde & Schwarz RTM3104 is the biggest compromise is software quality in addition to the same cheating on specs as Rigol, etc.

Reg

[Fungus]

... in addition to the same cheating on specs as Rigol, etc.

The "cheating" is the industry standard.

(and some of the very few that don't do it are Rigols, eg. MSO5000 series)

[rhb]

Here's a rigorous explanation of the issue with the way the industry specifies DSOs:



Note that whereas you only need a 7 pole filter to adequately suppress aliasing using an Fc of 50% of Nyquist, if you raise that to 80% you need a 22 pole filter which is pretty much impossible to build on a production line.  So what they do is they let the signal alias and they apply a digital filter to kill the region in which the aliasing appears.

BTW I misstated the suppression at Nyquist earlier.  It's -6*(#bits-1).

I'll add the minimum phase time domain impulse response later.  The server problem this morning beat me up pretty badly trying to post the previous plots.  I spent over an hour trying to upload the figures.

Have Fun!
Reg

[rhb]

This is the step response of my Tek 485, easily the best portable analog scope ever built.  This one was refurbished and given a rough cal.  The vertical attenuator section is *not* well adjusted.  The procedure takes a lot of time and had the person I bought it from done that to my standards he'd have been lucky to make minimum wage.  So no criticism, I just want people to understand that a well tuned 485 is a good bit better.  I'll eventually get around to adjusting this one, but it takes most of a day because everything interacts with everything else.  So you iterate through all the settings.

The scope is set at 1 ns/div



The dotted lines are 5 divisions apart, so the solid graticules are the 10% and 90% points and it easily meets the 0.35/BW rise time with minimal overshoot.  In this case none, but at other attenuator settings there is some, hence my assessment that it needs to be given a full cal treatment.

This is a "good" scope from the days when Tektronix still set the standard for scopes and set a very high bar.  Not the best scope Tek made, but the best portable scope they ever made.

Have Fun!
Reg

[David Hess]

Many DSO can still use equivalent time sampling. This is similar to the sampling scopes in many respects. It was especially popular with the early DSOs from the 1990s, but many new one still have it as an option.
Besides using it for a substitute for a higher sampling rate, it may also be used as averaging to get high amplitude resolution / less noise.

Some DSOs which use digital triggering support equivalent time sampling but any aliasing if present corrupts the digital trigger so I am not sure they should be counted.  If aliasing is present, then the sin(x)/x reconstruction has multiple solutions so the trigger position is indeterminate.

Besides the BW limit, there may also be a slew rate limit in some cases and thus amplitude dependent BW.

I am sure diagnostics and measurement is great fun when the results depend on the volt/div setting.  Where have I seen that before?  Oh, yes, it was Rigol.

[Fungus]

Note that whereas you only need a 7 pole filter to adequately suppress aliasing using an Fc of 50% of Nyquist, if you raise that to 80% you need a 22 pole filter which is pretty much impossible to build on a production line.  So what they do is they let the signal alias and they apply a digital filter to kill the region in which the aliasing appears.

When I need to take a good look at a signal I simply turn off the channels I don't need and that raises the sample rate.

...maybe this is a big advantage of the 4-channel, 100MHz 'scopes over the 2-channel, 200Mhz 'scopes. They both have the same sample rate but one has a much bigger margin between bandwidth and Nyquist.