Scope Wars

[bdunham7]

So can *you* figure out what the design error in the AFE of the Rigol and the Instek is?

I'm not sure the error is in the AFE, although the fact that it isn't in the AFE might be the error. 

[rhb]

It's pretty simple.  The anti-alias filter is *before* the amplifier, rather than after it.

I guess you could call it a "feature" as it lets you see a 1.25 GHz signal on a 200 MHz scope.

Have Fun!
Reg

[bdunham7]

It's pretty simple.  The anti-alias filter is *before* the amplifier, rather than after it.

I would have thought it would be 'part of' the amplifier.  But don't these scopes depend heavily on the ADC chip for all sorts of amplification and gain?  How much actual AFE is there?

I don't have a Rigol anymore, but on my Siglent which can be made to alias in the same way, albeit with significant (but not sensible) attentuation, but the behavior changes radically going from 500MSa/S to 1GSa/S by turning the paired channel off and on.  At 1GSa/S, it behaves more or less properly, with nothing folding back as far up as 900MHz.  My level sine wave capabilities stop at 990MHz. 

Do any of your scopes change their response, say looking at a 400MHz signal which will be aliased back to 100MHz at 500MSa/S and displayed more or less properly at 1GSa/S, less attenuation?

[Bud]

In entry level scopes anti-aliasing filter is part of the variable gain bandwidth limiting amplifier IC.  The ADC buffer also may have a rudimentary low pass filter on its output right before the ADC. Examples can be seen here

https://www.eevblog.com/forum/projects/project-yaigol-fixing-rigol-scope-design-problems/msg890966/#msg890966

https://www.eevblog.com/forum/blog/eevblog-978-keysight-1000x-hacking/msg1233118/#msg1233118

It can be seen there is no filtering before the VGA.

And there is no "amplification" in the ADC beside the stupid x2 magnification which simply stretches the signal vertically by displaying twice lower bits of resolution along the Y axis. All gain settings occurred in the front end before the ADC so the signal is scaled to the full ADC range before it hits the ADC. 

[rhb]

The DS1202Z-E does what one would expect.  The Instek also does, however, it sometimes gets into a weird state and displays completely bogus traces until you fiddle with the knobs and then it does what one would expect.

I ordered a pair of 935 MHz LoRa transceivers from Crowd supply today.  I'll be very interested to see what level of signal resolution I can get on those.

Having cut my teeth on an analog scope, I'm used to being able to go well past the -3 dB corner by turning up the gain.  But 1.25 GHz on a nominally 200 MHz DSO sort of blows my mind.  The Instek doesn't provide as clean a trace as the Rigol at 1.25 GHz.  Not yet sure why yet, though I suspect timebase jitter is the cause.

It would be really nice if someone built a low end DSO with the filter after the amplifier and circuitry to bypass the filter.  I'm sure it would confuse the hell out of a novice, but it would be very useful and an insignificant increase in cost.  Though one could use band pass filters to provide clean ETS operation with accurate displays of frequency to alleviate the confusion factor.

The major issue I have is that the user has almost no control over the parameters, especially for the FFT.

Once you've paid for the ADC, FPGA and display I look at a DSO and think, "It should be able to do everything."  Obviously DACs, buffers and such needed, but the major HW is already there.

I wish the person who asserted that the amplifiers had BW filters would post an example part number.

Have Fun!
Reg

PS  The filter is *before* the amplifier even if it's in the same chip.  Putting the filter before the amplifier improves the noise spec, but without a filter *after* the amplifier lowers the system performance.  You really need two.

[tom66]

And there is no "amplification" in the ADC beside the stupid x2 magnification which simply stretches the signal vertically by displaying twice lower bits of resolution along the Y axis. All gain settings occurred in the front end before the ADC so the signal is scaled to the full ADC range before it hits the ADC.

The HMCAD1511 ADC used in the Rigol DS1000Z series has 14-bit ADC chains internally.  It can provide any magnification up to 6-bits without reduction in sample rate or bit depth (as the output is 8-bit truncated), if I understand the datasheet correctly.

I've yet to find a VGA amp, besides one Texas part, that has proper tuned analog filters on the input.  The Texas part only provided a 20MHz LP filter for the B/W limit function of most scopes.  I would expect that to be something that could be done with the right DSP.

[tomato]

Tektronix makes "Mixed Domain Oscilloscopes" that have real spectrum analyzers built into them, but the spectrum analyzer uses it's own hardware.  Why?  Because the hardware in a standard digital oscilloscope is not well suited for doing spectral analysis.  Putting the FFT from a normal digital oscilloscope under a microscope strikes me as a waste of time -- it will always be compromised because the hardware is designed for an oscilloscope, not a spectrum analyzer.

[bdunham7]

The DS1202Z-E does what one would expect.  The Instek also does, however, it sometimes gets into a weird state and displays completely bogus traces until you fiddle with the knobs and then
I wish the person who asserted that the amplifiers had BW filters would post an example part number.

Follow his links.  Photos, part numbers, diagrams, everything is there in quite a lot of detail.

I don't know how the filters are implemented in the chip, but they clearly aren't working the way I would 'expect'.  The data sheet for the LHM6518 shows selectable bandwidth filters of 20, 100, 200, 350, 650 and 750MHz.  That explains a lot in terms of available low-end DSO types (except 70 is missing...)

So assuming your DS1202Z-E is set to 200, what should the attenuation at 1.25GHz be?  And what do you actually see?

[David Hess]

The real advantages of modern DSOs are primarily in (1) the advanced triggering and protocol decoding stuff (2) single shot acquisitions (because the storage crts sucked when they were new, and they didn't get any better with age). I would wager a skilled operator with a good analog 'scope could do everything else.

Modern DSOs have many advantages over old ones but old DSOs work fine for single shot acquisitions in most applications as the photo I posted earlier shows.

Not much difference? The newer scope gives you 200x improvement in bandwidth, 400x improvement in sampling rate, and a 4,000x improvement in memory:

single shot bandwidth: 1 MHz vs. 200 MHz 
sampling rate: 10 MSa/s vs. 4 GSa/s
memory: 501 pts. vs. 2Mpts

It is not quite so stark and the HP54501A was never intended for real time sampling which is reflected in its name, digitizing oscilloscope; they had other DSOs at the time with high single shot sample rates.  They were not thinking in this way then but what the HP54501A does have is 10 Gsample/second equivalent time sampling which should be obvious because otherwise how could a 100 MHz bandwidth combined with 10 Msample/second sample rate make any sense?

It also lacks peak detection because its intended applications would never need it.  It is not the type of DSO I would expect to use for general purpose work.

In entry level scopes anti-aliasing filter is part of the variable gain bandwidth limiting amplifier IC.  The ADC buffer also may have a rudimentary low pass filter on its output right before the ADC. Examples can be seen here

That sure is not how the Rigol DS1000Z series was designed.  They implement discrete switched bandwidth limiting immediately after the discrete preamplifier, and it is not for anti-aliasing.

Like I said earlier, if the filter was for anti-aliasing, then shouldn't it track the decimated sample rate?  Doing this would have all kinds of negative consequences for how the DSO behaves.

[rhb]

So assuming your DS1202Z-E is set to 200, what should the attenuation at 1.25GHz be?  And what do you actually see?

The output of the 8648C was 0 dBm into a 50 ohm thru so you should be able to work out the attenuation from the photo.  It's rather severe.  I'm just amazed I could see anything.

Have Fun!
Reg

I never saw a link or part number until tom66 posted one.

[rhb]

Tektronix makes "Mixed Domain Oscilloscopes" that have real spectrum analyzers built into them, but the spectrum analyzer uses it's own hardware.  Why?  Because the hardware in a standard digital oscilloscope is not well suited for doing spectral analysis.  Putting the FFT from a normal digital oscilloscope under a microscope strikes me as a waste of time -- it will always be compromised because the hardware is designed for an oscilloscope, not a spectrum analyzer.

You clearly do not understand DSP.  The Tektronix MDO lines have separate HW for SA because they don't want to spend the money on the time domain acquisition HW required to achieve the same level of performance.  They also cost 10-100 times more money than the DSOs I'm testing in this thread.

I have been doing SA on non-stationary data for 35 years.  That is pure DSP territory.  So if an SA is doing it, it is DSP.  All "real time" SAs are using DSP.  So far as I know there are *no* SAs capable of analyzing non-stationary data as it's not germane to electronics except in very exceptional cases.

Have Fun!
Reg

[Bud]

And there is no "amplification" in the ADC beside the stupid x2 magnification which simply stretches the signal vertically by displaying twice lower bits of resolution along the Y axis. All gain settings occurred in the front end before the ADC so the signal is scaled to the full ADC range before it hits the ADC.

The HMCAD1511 ADC used in the Rigol DS1000Z series has 14-bit ADC chains internally.  It can provide any magnification up to 6-bits without reduction in sample rate or bit depth (as the output is 8-bit truncated), if I understand the datasheet correctly.

I've yet to find a VGA amp, besides one Texas part, that has proper tuned analog filters on the input.  The Texas part only provided a 20MHz LP filter for the B/W limit function of most scopes.  I would expect that to be something that could be done with the right DSP.

LMH6518

" The following LMH6518 functions are controlled using the SPI-1 compatible bus:
• Filters (20, 100, 200, 350, 650, 750 MHz or full bandwidth) "

[rhb]

Thanks.  The block diagram confirms my interpretation of the experiment.  I look forward to studying the datasheet.

Have Fun!
Reg

[tomato]

Tektronix makes "Mixed Domain Oscilloscopes" that have real spectrum analyzers built into them, but the spectrum analyzer uses it's own hardware.  Why?  Because the hardware in a standard digital oscilloscope is not well suited for doing spectral analysis.  Putting the FFT from a normal digital oscilloscope under a microscope strikes me as a waste of time -- it will always be compromised because the hardware is designed for an oscilloscope, not a spectrum analyzer.

The Tektronix MDO lines have separate HW for SA because they don't want to spend the money on the time domain acquisition HW required to achieve the same level of performance.

No, they use separate hardware for the spectrum analyzer because oscilloscope hardware is not optimized for spectral analysis. If you want the best performance, you use hardware designed to do the intended application.  FFT capability on an oscilloscope is convenient, but the best oscilloscope cannot match the performance of the most basic spectrum analyzers and dynamic signal analyzers.  That's why nit-picking the FFT performance of an oscilloscope is kind of silly exercise. 

I have been doing SA on non-stationary data for 35 years.

Yes, we know.  You mention it in almost every thread.

[rhb]

As I stated earlier, you don't understand DSP.

Have fun!
Reg

[tomato]

As I stated earlier, you don't understand DSP.

Oh, please.

[bdunham7]

I am saying that applying a filter before decimation to prevent aliasing due to the lower sample rate after decimation is worse than not filtering at all because it corrupts the histogram of the input signal.  That is what an anti-aliasing filter would have to do.

So as I read along and try to keep up, sometimes I just don't get it.  Where is the 'histogram' you mention?  Is it an inherent property of the signal or are you assuming some transform has been done and it stored somewhere?  I thought I knew what a histogram was, but I'm not sure what you mean here by 'corrupts the histogram'.  As far as applying a digital filter before decimation, are you saying this is bad in general, bad for constructing a time-domain display (i.e. the oscilloscope trace) or for some other purpose, such as FFT?  This is done all the time in audio, it's just called oversampling.  When you record audio intended for 44kSa/S, you need to apply a brick wall filter, like 96db/octave to the input, which is commonly done by recording at 192kSa/S and then applying the brick wall digitally, then downsampling.  I can't see what harm this would do in the context of an oscilloscope display as long as it was done consistently and there was no expectation of a response beyond the bandwidth of the filter.

Like I said earlier, if the filter was for anti-aliasing, then shouldn't it track the decimated sample rate?  Doing this would have all kinds of negative consequences for how the DSO behaves.

There typically isn't a decimated sample rate on these scopes unless you set the timebase so slow that memory is limited--in which case you have to decide which path to take--or in the case of Rigol, more likely just ignore the issue.  I don't have a DS1054Z to take apart, but on scopes that have the LMH6518, which has the BW filter in between amp stages, I think that it is intended to be the anti-aliasing filter and it just doesnt' work well enough.  Looking at the datasheet, it appears to have approximately a 6db/octave rolloff, which makes sense for a simple RC circuit.  In each case, the advertised BW of the scope matches the 2.5X minimum full-speed sample rate (or better) and again, the only time the sample rate slows is at low sweep rates where I guess they just give up in budget scope-land.  So theoretically, most of the time, the BW filter gives you maybe -10db at Nyquist--quite consistently.  It just isn't good enough.  There isn't any other anti-aliasing that I know of unless they implement a digital filter when they do decimate at slow sweeps.  What other negative consequences do you refer to?

Now my Siglent 1104X-E (100MHz, unhacked) is another story.  It strongly aliases a 400MHz signal at 500MSa/S (both paired channels running) but completely suppresses it at 1GSa/S (one channel) indicating that something is changing between sample rates.  If it were a front end issue, I'd expect to see it display the 400MHz at least as strongly as it aliases it.

[rf-loop]

Now my Siglent 1104X-E (100MHz, unhacked) is another story.  It strongly aliases a 400MHz signal at 500MSa/S (both paired channels running) but completely suppresses it at 1GSa/S (one channel) indicating that something is changing between sample rates.  If it were a front end issue,  I'd expect to see it display the 400MHz at least as strongly as it aliases it.

Are you sure... is it possible your mind have dropped to one very common and easy trap...

Take a mm or other paper,  draw 400MHz signal samples to paper using 2ns sample interval... draw as many times as need and also draw original signal shape there and then alias...  think. And think again. After you find it... 1st Nyquist-Shannon lesson done. may I help... These our 400MHz signal samples are there, so it display your 400MHs perfectly same level as its display alias, think why you see alias instead of original input. Alias is nothing but original samples from original signal. Turn Sinc off...

Also note things you see in this image. Think twise... and this is unmod SDS1104X-E.
Btw, if you continue your pen and paper exercise yoy meet also nearly exactly one datail you find there in this image...
And last note. Only SDS1x04X-E hardware model Siglent make for export is SDS1204X-E afaicg.

[David Hess]

You clearly do not understand DSP.  The Tektronix MDO lines have separate HW for SA because they don't want to spend the money on the time domain acquisition HW required to achieve the same level of performance.  They also cost 10-100 times more money than the DSOs I'm testing in this thread.

The time domain acquisition HW required to achieve the same level of performance also does not exist.

Oscilloscope signal chains have extra requirements which are incompatible with spectrum analyzer signal chains including thermal balance and DC coupling which would compromise the performance of a *simpler* signal chain for RF spectrum analysis.  Both are different from the signal chains required for audio and other high resolution low frequency instruments which demand very high linearity produced with shunt feedback (oscilloscope vertical amplifiers rarely use shunt feedback) and low flicker noise if extended to low frequencies.  Oscilloscopes and spectrum analyzers typically have terrible flicker noise but that is what external low noise amplifiers are for.

I am saying that applying a filter before decimation to prevent aliasing due to the lower sample rate after decimation is worse than not filtering at all because it corrupts the histogram of the input signal.  That is what an anti-aliasing filter would have to do.

So as I read along and try to keep up, sometimes I just don't get it.  Where is the 'histogram' you mention?  Is it an inherent property of the signal or are you assuming some transform has been done and it stored somewhere?  I thought I knew what a histogram was, but I'm not sure what you mean here by 'corrupts the histogram'.

Whether it is explicitly identified or not, there is at least one histogram produced from the acquisition record and often two.  One is the display record which you see and the other if present is what measurements use.  In the case of DSOs which perform measurements on the display record, it serves for both.

What you see on an analog oscilloscope is also a histogram, graded in intensity by the gamma of the CRT which is oddly useful as it allows analog oscilloscopes to make accurate RMS noise measurements.  On old DSOs the display histogram may be only 1 bit deep like in the example I gave earlier while measurements are made on a separate deep histogram.

As far as applying a digital filter before decimation, are you saying this is bad in general, bad for constructing a time-domain display (i.e. the oscilloscope trace) or for some other purpose, such as FFT?

It is bad for time domain displays and measurements.  For displays, it will change the shape of the displayed waveform in non-linear ways which vary depending on the decimated sample rate.  The same applies to measurements which is even worse; which measurement at what time/div (and record length) is the most accurate, or even correct?

I consider DSO FFTs to be so broken that they are gimmicks for marketing so I am not sure anti-aliasing matters.  Where is the noise marker function?  How are they calibrated for spot noise?  Where are the phase results?

This is done all the time in audio, it's just called oversampling.  When you record audio intended for 44kSa/S, you need to apply a brick wall filter, like 96db/octave to the input, which is commonly done by recording at 192kSa/S and then applying the brick wall digitally, then downsampling.  I can't see what harm this would do in the context of an oscilloscope display as long as it was done consistently and there was no expectation of a response beyond the bandwidth of the filter.

That might be acceptable however it would mean sacrificing significant performance or economy for little gain.  It is difficult enough now to handle the data rates from existing oscilloscope digitizers which have data rates at least 3 order of magnitude higher and only the highest end DSOs are that capable.

And I am not sure anything would be gained.  Modern DSOs are function rich on the software side of things but they are not particularly more effective at displaying and making the same measurements which DSOs handled 30+ years ago, and I would argue that they are worse in many respect but admittedly they cost less.  (1) So what deficiency does oversampling and filtering correct?  If I could oversample, then why wouldn't I just make that the new higher maximum sample rate?

Like I said earlier, if the filter was for anti-aliasing, then shouldn't it track the decimated sample rate?  Doing this would have all kinds of negative consequences for how the DSO behaves.

There typically isn't a decimated sample rate on these scopes unless you set the timebase so slow that memory is limited--in which case you have to decide which path to take--or in the case of Rigol, more likely just ignore the issue.

I agree that long record lengths cure many ills but they are not a universal solution and they limit performance.  Somewhere that data has to be processed to be useful and it slows things down.

I don't have a DS1054Z to take apart, but on scopes that have the LMH6518, which has the BW filter in between amp stages, I think that it is intended to be the anti-aliasing filter and it just doesnt' work well enough.  Looking at the datasheet, it appears to have approximately a 6db/octave rolloff, which makes sense for a simple RC circuit.  In each case, the advertised BW of the scope matches the 2.5X minimum full-speed sample rate (or better) and again, the only time the sample rate slows is at low sweep rates where I guess they just give up in budget scope-land.  So theoretically, most of the time, the BW filter gives you maybe -10db at Nyquist--quite consistently.  It just isn't good enough.  There isn't any other anti-aliasing that I know of unless they implement a digital filter when they do decimate at slow sweeps.  What other negative consequences do you refer to?

That filter was never intended to be an anti-aliasing filter.  It allows one part to be sold for multiple applications.  It also allows for easy market segmentation.  On the practical side, it limits excess noise and provides for a predictable transition band.

DSOs like the DS1000Z series implement the bandwidth filters early in the signal chain for 50 and 70 MHz which when combined make 20 MHz.  That never seems right to me but I have done the math based on the values twice.

(1) As has been pointed out occasionally, this is not really fair.  Those old DSOs are comparable to modern DSOs costing thousands of dollars and not modern low cost DSOs which provide a very high capability for their price.

[rhb]

If you have a 400 MHz signal and a 250 MHz Nyquist, the alias is at 100 MHz.  If you have a 500 MHz Nyquist there is no alias.

The alias  is at 2*Nyquist - frequency.

Anything that can be done in the analog domain can be done in the digital domain.  Depending upon the required performance one or the other may be cheaper.  At the extremes it gets ambiguous as to what can be done with either.  I don't know of any analog scopes faster than the 1 GHz  Tek 7104.  There are plenty of DSOs which are faster.

State an SA performance specification that you believe cannot be met by time domain acquisition: center frequency, span, dynamic range, RBW and sweep time.

Have Fun!
Reg

[bdunham7]

Are you sure... is it possible your mind have dropped to one very common and easy trap...

I'm still trying to understand your message, but as to if I'm sure...look at the pictures in my post.

https://www.eevblog.com/forum/testgear/scope-wars/msg3119156/#msg3119156

The 4th and last pictures are of the same input signal at all the same settings except the sample rate changed because I turned CH2 on.

I think it is obvious what is happening.  The scope is using post-acquisition digital low pass filtering in addition to the puny 6db/octave pre-acquisition analog BW filter.  The 200MHz and 300MHz-folded-back-to 200MHz are attenuated some, while the 400MHz and up are completely blocked by this filter.  However, at the lower sampling rate, the 400MHz is folded back to 100MHz, where it can't be attenuated because the digital filter can't tell the difference.  It is also possible that this digital filter doesn't work at the lower sampling rate, or works differently. The 400 MHz is still attenuated by the analog filter, but only by 10-12db or so.

Anyone have a different idea?

[nfmax]

If you have a 400 MHz signal and a 250 MHz Nyquist, the alias is at 100 MHz.  If you have a 500 MHz Nyquist there is no alias.

The alias  is at 2*Nyquist - frequency.

With a 500 MHz signal the alias is at zero frequency. In seismic applications you don't care about DC, but for oscilloscopes DC performance is absolutely vital.

[rf-loop]

Are you sure... is it possible your mind have dropped to one very common and easy trap...

I'm still trying to understand your message, but as to if I'm sure...look at the pictures in my post.

https://www.eevblog.com/forum/testgear/scope-wars/msg3119156/#msg3119156

The 4th and last pictures are of the same input signal at all the same settings except the sample rate changed because I turned CH2 on.

I think it is obvious what is happening.  The scope is using post-acquisition digital low pass filtering in addition to the puny 6db/octave pre-acquisition analog BW filter.  The 200MHz and 300MHz-folded-back-to 200MHz are attenuated some, while the 400MHz and up are completely blocked by this filter.  However, at the lower sampling rate, the 400MHz is folded back to 100MHz, where it can't be attenuated because the digital filter can't tell the difference.  It is also possible that this digital filter doesn't work at the lower sampling rate, or works differently. The 400 MHz is still attenuated by the analog filter, but only by 10-12db or so.

Anyone have a different idea?

If you looked my old test image it is told there, just as your tests with 200, 300 and 400MHz. It is all there. You can see attenuation there - roughly for different frequencies, also over fNyq (note x and y scales)  with 1Gsa/s and 500Msa/s (with sidenote: every scope and test setup is some amount different). Of course real high grade test need do using leveling just in scope terminal. But no one is doing now rocket science... after this scope have published it have been quite clear there is post acquisition DSP for make cheap 100MHz version. Turn it to 200MHz model what it really is and it change things.

As I have previously told this aliasing is not so big problem in normal use with probes and usual DUT things. BUT use need know and understand it. User need know limits and how to ge more reliable results and how to avoid aliasing. Even hammer need some knowledge how to use it for better results and for avoid bad effects.
It is not so easy to get high amount over 500MHz to scope using probes with usual DUTs.  2 channel can keep 1Gsa/s.  If channels selected so that 500Msa/s is max, it need teach to users that it produce aliases if there is over 250MHz enough strong frequency components going to ADC isput.  It is made for 100 or 200MHz BW and just do not use it for 400MHz. So simple. If you usuallu use it in situations where your signals are far over what your scope can handle you have wrong tool.
 
Of course if it is designed more carefully and there is good LPF before ADC it is better.  Example bit over 200MHz and 400MHz brick wall filters or others but...then some peoples start barking about risetime... and so on. Oscilloscope is compromice and all you see on screen is sum of errors and lies. 

My opinion have been and is and stay: Only right place for anti alias filter for reduce ADC true sampling speed aliasing is before ADC. And this is analog side. It is only place! Period. Why. Because digital side can not differentiate alias from true, both looks exactly same. If we make down conversion using perfect ADC no one can see this product is down conversion. Sample do not have any other information but its value and known sample interval. Totally different thing is after ADC digital side where can handle decimated samplerates aliasing if we do not directly throw away samples what have information. So it is true that stupid simple decimation is stupid but simple and cheap.. This is totally other story.

[gf]

State an SA performance specification that you believe cannot be met by time domain acquisition: center frequency, span, dynamic range, RBW and sweep time.

Many modern SAs obviously do time domain acquisition (particularly real-time ones). Frequently they are still hybrids, not sampling the RF directly, but sampling the IF band after analog down-mixing, and calulating the whole IF spectrum at once via FFT which is faster than a narrow-bandwidth sweep.

[2N3055]

I wanted to add few more observation, looking at discussion here.

Scope can have two reasons for aliasing:

- first, when running at fast timebases and fastest sampling speed it can alias because analog antialias filter in front end before A/D doesn't filter properly, and frequencies above Nyquist criteria reach A/D converter.

This happens when you connect 1 GHz signal to scope with 200 MHz rated bandwidth and push amplitude of generator up more than input attenuator is set for until something is show on a screen. If analog antialiasing filter is done right, when you connect out of band 100mV P-P signal on scope input set at 100mv full screen sensitivity (that would be V/div time vertical divisions) you shouldn't see any appreciable signal. If you do, analog filter is not steep enough. Mind you, you can always make something show, if you push 10V P-P into 1mV/div input. So that test has to always be with exactly the same amplitude as in band signal that shows correctly.

- second, when scope is running slow timebases, because of limited memory, it has to slow down effective sampling rate of A/D, otherwise it won't be able to capture such a long capture.
It can do that that by:
   1. Literally use slower clock for A/D converter. That is not always possible for various reasons.
   2. It keeps on sampling at maximum sample rate as always, but decimates by throwing away samples, and keeping only every 4th or 5th or 10th one. That is effectively the same as slowing down sampling clock for the same factor.
   3. It keeps on sampling at maximum sample rate as always, but downsamples by filtering. This will produce same number of samples as first two, by lowpass  filtering.

This, slow timebase induced one, is tricky one, and more often to happen than first occasion of aliasing.
Here , option at 1. is rarely used. Many reasons for it, including A/D converters designed for fixed clocks or limited frequency range, complications in datapump design etc etc.
Option 2. is mostly used. It's cheap and very easy to do. It is also compatible with Peak detect mode, that can run at the same time. This is what most scopes do.

Problem with option 1. and 2. is that as effective sample rate goes down, so does Nyquist criteria frequency.  But at the same time we keep input analog antialiasing filter the same, breaking Nyquist criteria. Hello aliasing. So instead of seeing solid block of signal (because periods are so close together they actually blend together horizontaly, keeping vertical amplitude size, showing signal envelope a that time scale)  it starts showing downconverted imaginary signal.

Option 3. doesn't have that problem. Since it is averaging with low pass filter it applies to sampled signal a digital antialiasing filter that filters out anything above the Nyquist at target sample rate. By virtue of that, it will pass through only signals that can be properly reconstructed, and nothing more. And being digital, you can make it pretty ideal if you want.

So option 3. is best one. It automatically creates signal that is free of aliasing errors, right? And that is what you wan't to do?
Well, not so fast.
I will demonstrate on Keysight 3000T. Make sure you get stable trigger and use holdoff if needed. Scope might show it uses FFT. I do that to force it to disable built in antialiasing algorithm.

Let's take that simple 50 MHz signal AM modulated with 100Hz as an example. That is carrier with period of 20ns (1/f), and modulation with period of 10ms (1/f)
Let's put scope at 10 ns/div. We want to see the carrier. Scope samples at 5 GS/s.

   If you have scope that does 1. or 2. or 3. you get nice 50 MHz sinewave on screen. 



Let's put scope at 10 ms/div. We want to see the modulation envelope. Scope samples at 20 MS/s.

  If you have scope that does 1. or 2. , you get this:



That is wrong right?

Now you might be tempted to say: let's use option 3.  It will fix this problem.
No, it would be right thing to do for spectrum analyser, not scope.  Why?

Here, this is what we want to see:




It is nice amplitude envelope at 100Hz and a solid block of 50 Mhz carrier inside.


But if we go with option  3. (downsampling by filtering), as suggested, we will filter out anything above 10 MHz..
On spectrum analyser, we would be looking at scale of 0-10 MHz and see nice clean spectra of that without any folding from upper bandwidth. Perfect.

What we will see on the screen of oscilloscope ?