Scope Wars

[SilverSolder]

Let's see what the results say.   It is definitely interesting to see if there are significant differences between the different brands/models.

[med6753]

After reading most of this thread it seems to me there's whole bunch of this going on......



Some of you guys take this stuff way too serious.  Just think, you could be this poor sod. No aliasssing here. 

[2N3055]

@med6753
Ahahaha, LOL,  that commercial  is pure gold... Thank you for that...

[gf]

A DSO can allow aliasing and then slap a boxcar filter on the signal in the FPGA which eliminates the aliased part of the spectrum.  Point taken.

But one needs to take care not to end up with an overall frequency response like in the middle of picture 1.2 from the following message.
(which is supposed to be an unmodofied SDS1204X-E @500 MSPS)

While there is indeed a notch around fs/2, the attenuation recovers at ~2*fs/3 to only -10dB, and is still only -15dB at fs. I don't know, but I guess the apparent notch around fs/2 might be actually a digital low-pass filter, whose frequency response appears mirrored at fs/2 when considering the frequency band from fs/2...fs.

[rhb]

For a long time my only scope was a 5 MHz, recurrent sweep, Heathkit IO-18.  It has banana jacks for the input.  I've tried multiple times to find a new home for it, but none of them used it, so I took it back.  It is still looking for a new home.

It's not a great scope, but it is a *lot* better than no scope.

Reg

[rhb]

A DSO can allow aliasing and then slap a boxcar filter on the signal in the FPGA which eliminates the aliased part of the spectrum.  Point taken.

But one needs to take care not to end up with an overall frequency response like in the middle of picture 1.2 from the following message.
(which is supposed to be an unmodofied SDS1204X-E @500 MSPS)

While there is indeed a notch around fs/2, the attenuation recovers at ~2*fs/3 to only -10dB, and is still only -15dB at fs. I don't know, but I guess the apparent notch around fs/2 might be actually a digital low-pass filter, whose frequency response appears mirrored at fs/2 when considering the frequency band from fs/2...fs.

I'd like an impulse response from that as a CSV file.  I question whether the FFT is valid.  If it is, there are real problems.  But that's also the point of this thread.

Reg

[nctnico]

In analog scopes there was no real choice about the matter.  The BW had to extend far past the -3 dB point to be able to build it.

I woulnd't say far beyond. I used to own an old HP scope with 300MHz bandwidth and that rolled off pretty quick. The amplifier to drive the CRT had a bootstrap to get a bit more bandwidth but I don't think that worked miracles for the phase. I can't remember the model name but it was a mainframe which had a display section on top and room for 2 modules (vertical amplifier and time base).

[David Hess]

Could we please stick to something remotely relevant to a comparison of intro level DSOs?  I rather fear that no one except the very opinionated will ever read the actual test results.

The type of tests to be performed probably shouldn't be used as a definitive buyer's guide for people who buy $350 'scopes anyway, so... 

I agree but in the tests I would use to evaluate low end oscilloscopes, instruments like the Rigol DS1000Z series come up last because of functional, design, and documentation problems  What rhb proposed when he started this discussion is at least objective.

When I evaluate modern DSOs in person, one of the first things I use is a reference level pulse generator and precision RF stepped attenuator because if there are aberrations or differences at different volt/div settings, then I know the design is junk and no further testing is required.

[rhb]

In analog scopes there was no real choice about the matter.  The BW had to extend far past the -3 dB point to be able to build it.

I woulnd't say far beyond. I used to own an old HP scope with 300MHz bandwidth and that rolled off pretty quick. The amplifier to drive the CRT had a bootstrap to get a bit more bandwidth but I don't think that worked miracles for the phase. I can't remember the model name but it was a mainframe which had a display section on top and room for 2 modules (vertical amplifier and time base).

Depends on the overshoot.  The lower the cutoff slope the less the overshoot. Which is why that was a major criterion for analog scopes. 

Reg

[Fungus]

in the tests I would use to evaluate low end oscilloscopes, instruments like the Rigol DS1000Z series come up last

Instruments like the Rigol DS1000Z are among the cheapest available so that should surprise nobody.

The point of these tests should be to see if the more expensive instruments are really any better than the Rigol.

[rhb]

Here are the Rigol DS1102E and the Owon XDS2102A doing FFTs.    I'll need to investigate further, but it appears that the FFT on the DS1202Z-E is worse than the DS1102E. 

However, until I can get trace data off the DS102Z-E I don't really have a way to tell what is going on other than to feed a signal  from the 8648C which is 10-20% above Nyquist and examine the time domain traces.  There are a lot of ways to screw up an FFT spectral analysis and I've seen a lot of them over the years.,

Of note, the Owon cuts off the frequency range at 50% of Nyquist. If you switch to 2 channels or turn on 12 bit the sample rate drops to 500 MSa/s and the FFT changes to only show up to 125 MHz.

There is almost no user control of the FFT parameters on either of theses scopes as is the case with the DS1202Z-E also.  The MSO-2204EA allows some, but only the SA app provides reasonable control albeit with serious problems with spurious artifacts.

As with the MSO-2204EA and DS1202Z-E the input is a 20 mVpp 10 MHz square wave from the Keysight 33622A.  The low signal level is to avoid damaging the 8560A and the SDRs.

As I've also got an SDRplay RSP2 and  V2 & V3 RTL-SDR dongles I'll capture spectra with those to compare against the 8560A and the DSOs.

Have Fun!
Reg

Reg

[gf]

You mean, why the spectrum is not mirrored exactly at fs/2 (or even, why is it displayed beyond fs/2 at all)?

My Hantek seems to calculate the FFT from the interplated/upsampled data when I select a fast timebase where the displayed trace needs to be interpolated. Clearly the upsampled data (und thus their FFT) have a much higher Nyquist frequency (several GHz) than the raw ADC samples (fs/2 = 500MHz), and eventually I get a similarly strange display beyond 500 MHz as you. So what I see is obviously the spectrum of the trace as it appears on the screen, which is not necessarily the signal spectrum. Only with a much slower timebase setting (slowest timebase which still uses 1GSPS) I seem to get what I want/expect.

I'm not sure, of course, if the issue with the Rigol is the same. Does the apparent FFT spectrum change when interpolation is switched between sinc and linear?

Btw, hopefully you can save raw samples. My Hantek saves interpolated samples too (at a higher samling rate than ADC), if a too fast timebase was selected.

[David Hess]

in the tests I would use to evaluate low end oscilloscopes, instruments like the Rigol DS1000Z series come up last

Instruments like the Rigol DS1000Z are among the cheapest available so that should surprise nobody.

The point of these tests should be to see if the more expensive instruments are really any better than the Rigol.

Even ignoring design flaws, the Rigol DS1000Z series would come up last in my list only because Rigol is deliberately misleading with their documentation and design.  Being inexpensive has nothing to do with the manufacturer deliberately deceiving its customers.

[SilverSolder]

Just for fun, I fired up some ancient equipment to see how it compares with the newer scopes (i.e. interesting to compare used equipment in the entry level price range).

The scope is an Agilent 54622D, fed 400mV of 10MHz square wave from an HP8012A pulse generator synced to a HP3325A for the frequency.

First, the square wave on its own.  Is the overshoot due to the flanks of the pulses getting close to the scope's 3dB point (at 100MHz) ?


Close-up of the leading edge of the square.



Then, there is the FFT function.

How does this scope, with its modest 200M samples per second rate,  get as high as 4GSamples per second for its FFT function??

Vertical is 10dBV/division, offset by -54dBV

[rhb]

Even ignoring design flaws, the Rigol DS1000Z series would come up last in my list only because Rigol is deliberately misleading with their documentation and design.  Being inexpensive has nothing to do with the manufacturer deliberately deceiving its customers.

Would you please document those assertions?  I'd like to prove or disprove them and compare the performance metric to other OEM products.

Thanks,
Reg

[rhb]

How does this scope, with its modest 200M samples per second rate,  get as high as 4GSamples per second for its FFT function??

Vertical is 10dBV/division, offset by -54dBV

Probably the same way the DS1202Z-E gets 50 GSa/s, by reporting something which is incorrect because the UI programmer didn't understand how to do the calculations. 

Or not reading the API documentation as happened to me when I was the lead scientific programmer on a project.  one of the UI programmers complained that my code was crashing.  So I went and ran his example under the debugger.  He was passing the sample rate in the wrong units which caused my code to calculate the wrong offset in memory.  I went back and told him a few minutes later that he was passing an argument in the wrong units and went back to my work.  I don't think I said 5 sentences.  I just stated what the issue was and thought the matter settled.  No recrimination other than read the function header before calling a routine.

But he complained to the project manager and I got lectured about "not being a team player" or such.  She also made me go fishing all day as a "team building" exercise when the project started which I thought a huge waste of time.

It's going to get rather interesting when I start pulling sample data into Octave.

Have Fun!
Reg

[nctnico]

Or not reading the API documentation as happened to me when I was the lead scientific programmer on a project.  one of the UI programmers complained that my code was crashing.  So I went and ran his example under the debugger.  He was passing the sample rate in the wrong units which caused my code to calculate the wrong offset in memory.  I went back and told him a few minutes later that he was passing an argument in the wrong units and went back to my work.  I don't think I said 5 sentences.  I just stated what the issue was and thought the matter settled.  No recrimination other than read the function header before calling a routine.

Offtopic: you should write code which can deal with such conditions without crashing. Especially pointer errors shouldn't be allowed to happen because they can go unnoticed for a long time. If your code throws an error then people won't bother you; they figure out they did something wrong themselves.

[SilverSolder]

How does this scope, with its modest 200M samples per second rate,  get as high as 4GSamples per second for its FFT function??

Vertical is 10dBV/division, offset by -54dBV

Probably the same way the DS1202Z-E gets 50 GSa/s, by reporting something which is incorrect because the UI programmer didn't understand how to do the calculations. 

Or not reading the API documentation as happened to me when I was the lead scientific programmer on a project.  one of the UI programmers complained that my code was crashing.  So I went and ran his example under the debugger.  He was passing the sample rate in the wrong units which caused my code to calculate the wrong offset in memory.  I went back and told him a few minutes later that he was passing an argument in the wrong units and went back to my work.  I don't think I said 5 sentences.  I just stated what the issue was and thought the matter settled.  No recrimination other than read the function header before calling a routine.

But he complained to the project manager and I got lectured about "not being a team player" or such.  She also made me go fishing all day as a "team building" exercise when the project started which I thought a huge waste of time.

It's going to get rather interesting when I start pulling sample data into Octave.

Have Fun!
Reg

Sometimes, engineers get very matter-of-factly in the interest of clarity, and that gets misinterpreted as aggression and/or rudeness by touchy feely people that prefer to present all matters of fact in ways that reduce the chances of anyone feeling offended by said facts.   I think a lot of the political divide can be explained by this mechanism.  And I don't think either side is right or wrong about this, there's no harm in being considerate, but on the other hand there is nothing wrong with being concise in a professional environment...

Anyway, back to the thread:   Is the answer to the conundrum that the real sample rate for the FFT function is the same as the scope's basic sample rate when it showed the waveform itself - in this case, 200Msamples/sec? 

[SilverSolder]

Here's another thing I don't understand about FFT on oscilloscopes.

How is the FFT function able to display signals above the Nyquist frequency (100MHz in this case, assuming 200Msamples/sec)?

[gf]

I still guess that it might be the same reason as on mine: The FFT is calulated from the samples in the display buffer which are interpolated and re-samped at a higher rate in order to fill the gaps between the original sampling points.

[rhb]

I don't know the details of this instrument, but HPAK introduced sample dithering which phase shifted the clock from sweep to sweep.  If you divide the actual sample rate by 20 and shift the phase of the clock by that amount on each sweep, then you get an effective sample rate on a repetitive waveform which is 20 times greater.  For spectrum analysis which has traditionally been done by sweeping the LO, that is a very viable approach and will produce the same answer.

I've started work on a spectrum analysis code for DSOs with the following API to  be released under a Gnu library license:

Input
-------
resolution BW from a fine grained set of choices
visual BW from a display limited range of choices
window choice for all or most of the windows on the Wikipedia list
number of sweeps to average
anti-alias filter complex frequency response

Output
---------
complex spectrum
display interpolation filter

There will probably be a few changes or enhancements, but the above is the minimum functionality.  So with a swept input it will provide vector network analysis to the limit of the DSO and signal source.

For those lacking a high level of DSP background, the display interpolation operator is  a function of the minimum phase anti-alias filter, which must also be corrected to zero phase, and the window operator.

It's routine stuff in seismic, so very familiar territory for me.  The sole limitation is free time. My hope is that this thread will lead OEMs to use it to replace the crap they currently have.

Have Fun!
Reg

[SilverSolder]

I still guess that it might be the same reason as on mine: The FFT is calulated from the samples in the display buffer which are interpolated and re-samped at a higher rate in order to fill the gaps between the original sampling points.

You may be right -  the FFT number of points is 2048, and the display is 1024...   so by doubling up the number of points and interpolating, the "new Nyquist" would now become 200MHz, which is what we can see in the screen grab.   

But...   how can there be information in the signal beyond the 100MHz bandwidth of the analog front end, even if you interpolate in the screen buffer you are not really adding signal information to what was there in the first place - right?

Perhaps they are actually doing wizardry along the lines that @Reg suggests above.

[rhb]

There are lies, damn lies and marketing brochures.

[Tomorokoshi]

Two tests using the FFT in an HP 54542A oscilloscope. 1 MHz square wave in each case, using an HP 3310A Function Generator and an HP 8011A Pulse Generator.

[gf]

But...   how can there be information in the signal beyond the 100MHz bandwidth of the analog front end, even if you interpolate in the screen buffer you are not really adding signal information to what was there in the first place - right?

Remember how upsampling by factor N is usually done. Insert N-1 zeros between the original samles, then apply a digital low pass filter. The zero insertion creates harmonics of the original ADC clock, and IM sidebands at each harmonic. The low-pass filter is supposed to eliminate anything beyond the original Nyqist then. Ideally it's a boxcar filter (in the frequency domain), having a sinc pulse response in the time domain (thus resultig in "sinc interpolation"). The ideal filter would indeed clean-up all generated frequencies beyond the original Nyqist. If there are still frequencies left, then the low-pass filter was not a perfect boxcar. Of course it can't in practice, since sinc has an infinite extent, so at least it needs to be truncated. And maybe some pulse-shaping is applied additionaly. While frequencies in the upsampled FFT spectrum beyond the original ADC's Nyqist does not provide additional information about the original signal, they do contain some information about the interpolation kernel which was used to generate the displayed trace from the raw samples. Not sure, though, whether it is sufficient to reconstruct the interpolation kernel, due to noise and zeros in the spectrum.