Scope Wars

[rhb]

The DS1202Z-E does quite nicely on the step response in single channel mode, but in dual channel with the corner at 80% of Nyquist it's rather strange.  I've not yet analyzed the error in dual channel mode.  I blew almost 2 hours this morning battling the server problem Dave was having trying to post a few plots before the server went completely belly up.

In single channel mode, the Nyquist is at 500 MHz which gives a generous margin and a good step response.  A DS1054Z should be much better as a 50 MHz scope in 4 channel mode.

I may try to get a minimum phase impulse response generated for the Butterworth filter examples tomorrow.  A lot depends upon how much time I spend cleaning up my shop so I can build my new lab.

As I have a 100 ps pulse generator, I'm going to take advantage of the simplicity of using that to measure the DSO AFEs.

Properly they should be switching anti-alias filters when they change the sample rate.  We shall see.  As I stated at the beginning, I have some large hammers to hit things with.  And know how to use them.

Have Fun!
Reg

[Elasia]

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.

Depends on structure but yeah usually... on pretty much all budget / mid 4 channel scopes its 1 adc rail per 2 ports so getting full specd bandwidth is almost always using 1+3,1+4,2+3,2+4

[David Hess]

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.

That is not exactly a recommendation for DSOs which suffer from that problem.  In the past DSOs which shared digitizers between channels supported equivalent time sampling so the result was longer display generation time.

For a long time Tektronix had distinct "real time" DSOs which dedicated one digitizer to each channel instead of sharing them so the problem was recognized not long after DSOs became generally available.

...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.

On a 4-channel DSO using one of the other channels as a trigger input also compromises performance if digital triggering is used which is almost always the case now.  A modern 2-channel DSO with an extra trigger channel might not suffer from that.

[rhb]

I'd like an explanation of why digital triggering on a live channel is a problem other than bad design or running out of FPGA fabric.

I can see many ways someone could bungle it, but no reason a skillful implementation would have problems.

Reg

[David Hess]

I'd like an explanation of why digital triggering on a live channel is a problem other than bad design or running out of FPGA fabric.

I can see many ways someone could bungle it, but no reason a skillful implementation would have problems.

If the digitizers are shared between channels like we are discussing, then using a channel as a trigger source without displaying it has the same effect as displaying it lowering the sample rate of the other channels.

[nctnico]

I'd like an explanation of why digital triggering on a live channel is a problem other than bad design or running out of FPGA fabric.

I can see many ways someone could bungle it, but no reason a skillful implementation would have problems.

If the digitizers are shared between channels like we are discussing, then using a channel as a trigger source without displaying it has the same effect as displaying it lowering the sample rate of the other channels.

Interestingly my GW Instek scope doesn't have this limitation.

[Kleinstein]

If the scope does not get the lower sampling rate from using a trigger channel, it may not have digital trigger, but still old style analog trigger. This can be an advantage with equivalent time sampling.

One could do a proper implementation of the digital trigger for equivalent time sampling, but it takes extra effort to remove the aliasing part from the trigger channel. This would be more like having a good AA filter for the trigger and removing (set to higher BW) it for the signal path for equivalent time sampling. So it may need 2 ADCs if the trigger is from the signal.

I don't think one would find and need 7 th order AA filters in the usual scopes, at least not the classical butterworth type (there are usually a few parasitic limitations involved anyway). Usually the amplitude to higher frequency goes down and some low level aliasing is accepted. So the AA filter is not suppressing down to the full ADC resolution. I would not expect much visible difference if only good for 5 bits.

Switching the speed between 1 or 2 channels per ADC (or the other way around) would need switching the AA filter, but the steps further down when the sampling rate is reduced because of limited memory / low horizontal speed usually use digital averaging, so that no extra filter is needed. With some of the cheap scopes the sampling rate is no longer that limiting so no need to use 50% or even 80% of the Nyquist limit.  Using a relatively large fraction of the Nyquist limit is more like a thing of the mid class scopes at some 500 MHz BW. The limited ADC speed was a big thing in the early days (1990s).

[Fungus]

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.

That is not exactly a recommendation for DSOs which suffer from that problem.

It's something that doesn't happen very often. If the "solution" is to pay triple price for something else then it's simply not worth it, I can use the money more productively elsewhere.

Some people still sleep at night despite things like this. Get over it.

Edit:

It's obviously quite severe on a LeCroy DDA-125 with a 1.5 GHz BW and a 1 GHz Nyquist in 4 channel mode.

Lecroys and Keysights need to turn off channels, too. How do you feel about that?

[nctnico]

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.

That is not exactly a recommendation for DSOs which suffer from that problem.

It's something that doesn't happen very often. If the "solution" is to pay triple price for something else then it's simply not worth it, I can use the money more productively elsewhere.

Not just that but many high-end DSOs (>1GHz  >4Gs/s) from Lecroy and HP/Agilent/Keysight offer to trade-off samplerate versus number of channels available. In some cases (Agilent 54845A for example) even the bandwidth is limited when using a lower samplerate.

A much more interesting question would be: for what kind of measurements does the step response matter and for which ones it doesn't. And if it matters, can the situation be improved by adding an external filter to get a more gradual roll-off?

[RoGeorge]

Another trick when having many channels to spare is to put them in parallel, to look at the same signal, and thus eliminate some noise from the analog stages.  This is good especially for single events, when averaging can not be used.

[rhb]

I cannot see *any* reason that aliasing is acceptable on anything other than a sampling scope where it is defacto unavoidable if you set the instrument up so it is aliased.  However, that proved useful as a means of testing the linearity of the time base in my 11801.  The manual suggested that factory calibration data had been lost when the NVRAM failed, but a lengthy investigation showed that there were no errors.  I deliberately aliased a ramp from my Keysight 33622A which has a spec of <1 ps jitter.

To the extent that it is present in commercial DSOs is to be determined.  It's obviously quite severe on a LeCroy DDA-125 with a 1.5 GHz BW and a 1 GHz Nyquist in 4 channel mode.

Have Fun!
Reg

[Fungus]

I cannot see *any* reason that aliasing is acceptable

It's not excusable on an expensive 'scope where there's no reason to skimp on the filters in the analog front end.

[SilverSolder]

I cannot see *any* reason that aliasing is acceptable

It's not excusable on an expensive 'scope where there's no reason to skimp on the filters in the analog front end.

Sometimes, in photography, photographers will forgo the optical anti-alias filter in front of the digitizing sensor in order to get higher level of detail, on the understanding that it can lead to aliasing in some circumstances (moiré patterns if there are fine regular repeating patterns in the image - but in practice most images do not have details that are repetitive "sine waves" so for most images moiré is not a problem, so you can enjoy the higher level of detail).

Is there an equivalent way to think about an electronic signal - i.e. where you may expect most of the high frequency content to be noisy rather than distinct sine waves, so you are actually better off overall with a less aggressive AA filter?

[Fungus]

Is there an equivalent way to think about an electronic signal - i.e. where you may expect most of the high frequency content to be noisy rather than distinct sine waves, so you are actually better off overall with a less aggressive AA filter?

Definitely.

eg. The owner of this thread is having to use special pulse generators and cables to generate frequencies high enough to reveal these "problems".

Most oscilloscope users will never see them because they'll be pre-filtered by the capacitance in ordinary 'scope probes and cables.

[edigi]

I think you hit the nail on the head many times with your post.

I don't think one would find and need 7 th order AA filters in the usual scopes, at least not the classical butterworth type (there are usually a few parasitic limitations involved anyway). Usually the amplitude to higher frequency goes down and some low level aliasing is accepted. So the AA filter is not suppressing down to the full ADC resolution. I would not expect much visible difference if only good for 5 bits.

The probes supplied with the entry level scopes contribute a lot to the BW limitation, that is in real life the trouble is more to get enough BW than that aliasing has noticeable effect to signal shape. Especially that harmonics have smaller and smaller amplitude both by nature and due to BW limitation. There is one place though where aliasing do show up quite remarkable and that's the FFT output. This is due to that some of the scopes have excellent dynamic range (approx. 100dB) after the FFT.
Normally it's more an issue to get the probing to such level that the output is not a degraded sine with fast signals. While it's very easy to provoke scopes via hanging a purpose made circuit to the BNC input that outputs a low fundamental but fast rise time signal probably nothing could be further away from the typical real life use than this.

There is though one user error that can get quite much overshoot/ringing: Impedance misalignments and reflections. These scopes typically have only high impedance input.

Switching the speed between 1 or 2 channels per ADC (or the other way around) would need switching the AA filter, but the steps further down when the sampling rate is reduced because of limited memory / low horizontal speed usually use digital averaging, so that no extra filter is needed. With some of the cheap scopes the sampling rate is no longer that limiting so no need to use 50% or even 80% of the Nyquist limit.  Using a relatively large fraction of the Nyquist limit is more like a thing of the mid class scopes at some 500 MHz BW. The limited ADC speed was a big thing in the early days (1990s).

Although I've brought up the idea in one of these threads, I pretty heavily suspect that few scopes (especially in entry level) implement filter switching when ADC has to be divided among multiple channels and as a consequence sampling rate is reduced per channel.


There are many hard to answer but interesting questions that you've brought up. Most VGAs have separate output for triggering where a lower rate ADC could be used. I don't remember any comparison or even advantage/disadvantage summary of this or the main channel FPGA based digital triggering.
For the reduced sample rate due to acquisition memory limitation two (multi) stage properly filtered decimation could indeed provide excellent result but does it fit to FPGAs of entry level scopes?

[edigi]

Sometimes, in photography, photographers will forgo the optical anti-alias filter in front of the digitizing sensor in order to get higher level of detail, on the understanding that it can lead to aliasing in some circumstances (moiré patterns if there are fine regular repeating patterns in the image - but in practice most images do not have details that are repetitive "sine waves" so for most images moiré is not a problem, so you can enjoy the higher level of detail).

Most lens are very far from that resolution (and hard to focus on that level) so that the sensor could cause moiré, so the "analogue front end" filters enough that no AA filter is needed. Excellent example and finally a non-car analogy.

[rhb]

Is there an equivalent way to think about an electronic signal - i.e. where you may expect most of the high frequency content to be noisy rather than distinct sine waves, so you are actually better off overall with a less aggressive AA filter?

Definitely.

eg. The owner of this thread is having to use special pulse generators and cables to generate frequencies high enough to reveal these "problems".

Most oscilloscope users will never see them because they'll be pre-filtered by the capacitance in ordinary 'scope probes and cables.

Most users aren't sufficiently sophisticated in their probing techniques and would not recognize the issues for what they are if they saw them.

Fast scopes such as the 7104 and 11801 are 50 ohm *only*.

I'm somewhat bemused that no one seems to have taken any note of the figures I posted yesterday.  And are offering up conjectural arguments instead of demonstrating their assertions with Octave or Matlab.

I'd very much welcome a demonstration of the difference between a 2 pole & 7 pole, causal anti-alias filters with Fc at 50% & 80% of Nyquist when fed a square wave.

I *know* the answer from spending 30+ years in seismic research and programming where waveform fidelity is paramount.  So consider it a challenge.  If you assert something doesn't matter, demonstrate it mathematically.  There are plenty of programs to simulate a scope probe response and Octave can handle most of the math.  Though I did discover that the Butterworth filter stuff in the signal package for Octave is duff.  Not quite sure what it actually does as it doesn't resemble the definition of a Butterworth filter in "An Introduction to Digital Signal Processing" by John H. Karl which I highly recommend.  I've got over 6 ft of books on DSP and it is by far the one I reference most often.

Have Fun!
Reg

[nctnico]

Is there an equivalent way to think about an electronic signal - i.e. where you may expect most of the high frequency content to be noisy rather than distinct sine waves, so you are actually better off overall with a less aggressive AA filter?

Definitely.

eg. The owner of this thread is having to use special pulse generators and cables to generate frequencies high enough to reveal these "problems".

Most oscilloscope users will never see them because they'll be pre-filtered by the capacitance in ordinary 'scope probes and cables.

Not just that. A step (from an infinite time -1 to an infinite time +1) contains a very wide frequency spectrum. Somewhat repetitive signals OTOH consist of harmonics of their fundamental frequency. In order to have problems with aliasing and/or sharp roll-off from the anti-aliasing filter the harmonics have to be in the frequency band where the anti-aliasing filter has an effect on the signal which is large enough to make it visible.

[SilverSolder]

The first several generations of quality digital cameras all had relatively strong anti-alias (AA) filters in front of their sensors to remove any chance of aliasing.  It turned out, in the long run, that as sensors got fine grained enough (high enough spatial sample rate), photographers preferred the extra sharpness of not having an AA filter at all... so the problems caused by aliasing (which are real, and can and do happen) were not considered "worth the price" of reduced performance in other situations (and, in any case, most problems can be mitigated later in the workflow). 

The Nikon D800E was the first camera model (to my knowledge) and allowed photographers to make the trade-off of having no AA filter versus the normal Nikon D800 that kept the traditional AA filter, and was otherwise exactly the same in every other respect.

If the same kind of thing happens when looking at an oscilloscope, we should be careful that we don't end up jumping to conclusions based on e.g.  taking a whole bunch of pictures where the unfiltered D800E performs worse than the filtered D800, and then concluding that the D800E sucks...   even though the E model takes sharper pictures in 99% of real world situations and you would only choose the filtered D800 if you were taking product shots of patterned fabrics or meshes, or just liked the slightly softer look better!

[David Hess]

If the digitizers are shared between channels like we are discussing, then using a channel as a trigger source without displaying it has the same effect as displaying it lowering the sample rate of the other channels.

Interestingly my GW Instek scope doesn't have this limitation.

It is not a universal problem but I think it is more common in 4-channel DSOs.  2-channel DSOs with a separate external trigger input may make other arrangements so use of the external trigger does not affect operation of the vertical channels.  For instance if the triggering is all done during decimation, then no acquisition memory is required for the external trigger channel.  But in some DSOs, the digital triggering has an additional trigger qualification step after acquisition so acquisition memory is still required.

If the scope does not get the lower sampling rate from using a trigger channel, it may not have digital trigger, but still old style analog trigger. This can be an advantage with equivalent time sampling.

How the external trigger operates might be known by checking which trigger modes are available.  If there is no difference between the vertical inputs and external trigger input, then the external trigger is digital.  But I remember some past DSOs where the external trigger only allowed level triggering indicating that it was analog.

[Fungus]

Fast scopes such as the 7104 and 11801 are 50 ohm *only*.

Sure, but their usage case is completely different than "manually poking around electronic circuits looking for signals".

[rhb]

Not just that. A step (from an infinite time -1 to an infinite time +1) contains a very wide frequency spectrum. Somewhat repetitive signals OTOH consist of harmonics of their fundamental frequency. In order to have problems with aliasing and/or sharp roll-off from the anti-aliasing filter the harmonics have to be in the frequency band where the anti-aliasing filter has an effect on the signal which is large enough to make it visible.

A fast (e.g 100 ps) pulse is an almost flat spectrum over a large BW.  This is why the seismic industry records the impulse responses  of the recording system input amplifiers and filters.  It verifies that all the channels are working properly which is a *big* deal when you're being paid $10-20 million to acquire the survey.  The client oil company has a person on board, the observer, whose sole job is to make sure that all the many pages of specifications about positioning, sea state and other factors are adhered to.  There are contractual allowances for how many guns and receivers may be inoperable.

If you want an accurate waveform, there are certain constraints about filter roll off and stop band at Nyquist which *must* be met.  Of course, if you don't care, then it hardly matters, does it?

I use my scopes for a lot more than just manually poking around a circuit.  For anything that needs significant BW you have to provide test points in the board design.  Otherwise you'll never get a valid signal sample.  U.FLs that you can jumper in with a blob of solder are very good for that.

If you provide a numerical example demonstrating your assertions above, I'll produce a counter example disproving them.  Otherwise I'm not going to bother until I need to demonstrate why certain DSOs behave the way they do.

Have Fun!
Reg

BTW "Infinite +/- 1" does not exist.

[David Hess]

Fast scopes such as the 7104 and 11801 are 50 ohm *only*.

Sure, but their usage case is completely different than "manually poking around electronic circuits looking for signals".

The 7104 with its MCP CRT is particularly useful for "manually poking around electronic circuits looking for signals" which works fine with low-z dividing probes, but the 7104 supports high impedance probes also up to 200 or 350 MHz and of course faster active probes were available.  Later Tektronix released slower but still fast oscilloscopes with MCP CRTs for exactly that application.

Consider what application the 7104 was specifically designed for and how the Soviets solved the exact same problem.  It had nothing to do with electronics.

[rhb]

Got a link for the Soviet solution?

[tv84]

BTW "Infinite +/- 1" does not exist.

Sure it exists. It's infinite.