Author Topic: Rigol DS1054Z bandwidth  (Read 17394 times)

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Offline rf-loop

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Re: Rigol DS1054Z bandwidth
« Reply #50 on: December 24, 2016, 02:13:32 pm »
the sin(x)/x interpolation is the most faithful representation the oscilloscope (any oscilloscope) can give you
Most faithful representation is given by excessive sampling rate, either RTS or ETS. Once gain - real data.
Even better: understand the math behind sin(x)/x and you'd know/understand you don't get any extra information from an excessive sampling rate where fs/2 lies beyond the range of the anti-aliasing filter.

Yes but if manufacturer do not follow any rules and name some kind of "smooth" function as Sinc  after then you can not trust any thing.
This joke violates rules. And more fun. it do not care real sampled data for Sinc draw. They want produce just "nice art image".

One limited wink what is going there is here:

https://www.eevblog.com/forum/testgear/rigol-ds1074z-weird-signal-level-problem/msg563208/#msg563208

After junk Sinc function first published by Rigol with DS1000E ancient model they have not repaired and learn anything.

1. Sinc (Sin(x)/x) function result need: Curve draw via real sampled data points. Rigol flush samples off out like if they are garbage.

If do small home work it can easy see that Rigol Sinc is joke what can result "what ever".
Example if look Siglent and and lot of others, even old Tektronix DSO (example 2440 or 2230 2430) draw Sinc tightly through the sample points (of course).
It can confirm very easy example stopping Siglent acquisition and then turn display mode dots or vectors and then Sinc on.  It can immediately see  that all draw go always via true sample points. 

Rigol do not. And even more bad, in many cases user can not even check because Sinc (better say "smooth" or what ever but not Sinc) can not switch on and off. Also it draw fake dots between sample dots. 
« Last Edit: December 25, 2016, 08:14:37 am by rf-loop »
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Offline David Hess

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Re: Rigol DS1054Z bandwidth
« Reply #51 on: December 24, 2016, 03:19:48 pm »
No one want old crap cheap scopes analog trigger system. These exist only in museum.

Ya, who needs equivalent time sampling when they can have Gibb's phenomena distorting their transient response!  100 MHz DSOs with 2 GS/s ETS sample rates and 500 picoseconds of jitter were junk!  And 20 GS/s with 50 picoseconds is even worse!

As a practical manner, analog trigger jitter in a 100 MHz oscilloscope is easy to make insignificant.  Even in a 300 to 500 MHz oscilloscope, it should be insignificant.

When a digital trigger is used, then it appears to produce zero jitter on the triggered edge however this is misleading.  The sinc reconstruction error caused by aliasing on a fast edge (wobulation) produces its own jitter which gets added to the edge of the following cycle and the rest of the waveform.  To put it another way, aliasing makes the trigger point ambiguous.  You can see this on the old LeCroy marketing videos where even when looking at a single edge, only the trigger point itself is noise free which to me looks as weird as Gibb's phenomena.

If do small home work it can easy see that Rigol Sinc is joke what can result "what ever".
Example if look Siglent and and lot of others, even old Tektronix DSO (example 2440 or 2230) draw Sinc tightly through the sample points (of course).

The old Tektronix 2440 series is a great example of this because it can draw the sinc interpolated signal while also highlighting the real sample points and they always line up like they should.  The 2230 and 2232 however do not support sinc interpolation at all which makes sense given their relatively slow 8088 and 80C188 processors running at 20MHz/3=6.7MHz and 100MHz/7.5=13.3MHz respectively.  The 2440 uses an 8MHz 6809 (system), 4 MHz 6805 (controls), and separate custom waveform processor.

If you drive the 2440 with a fast edge and do not use equivalent time sampling, then the Gibb's phenomena also shows up when sinc interpolation is used which is to be expected given a rise time to support 300 MHz operation and only a 500 MS/s sample rate.
 

Offline rf-loop

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Re: Rigol DS1054Z bandwidth
« Reply #52 on: December 25, 2016, 09:42:50 am »


If do small home work it can easy see that Rigol Sinc is joke what can result "what ever".
Example if look Siglent and and lot of others, even old Tektronix DSO (example 2440 or 2230) draw Sinc tightly through the sample points (of course).

The old Tektronix 2440 series is a great example of this because it can draw the sinc interpolated signal while also highlighting the real sample points and they always line up like they should.  The 2230 and 2232 however do not support sinc interpolation at all which makes sense given their relatively slow 8088 and 80C188 processors running at 20MHz/3=6.7MHz and 100MHz/7.5=13.3MHz respectively.  The 2440 uses an 8MHz 6809 (system), 4 MHz 6805 (controls), and separate custom waveform processor.

If you drive the 2440 with a fast edge and do not use equivalent time sampling, then the Gibb's phenomena also shows up when sinc interpolation is used which is to be expected given a rise time to support 300 MHz operation and only a 500 MS/s sample rate.

Sorry my old memory. It was Tek 2430 what I had years ago as also 2440. 2430 have also Sinc as 2440.  I have never used 2230.


Generally:

With fast edges (or what ever waveform)  as example rectangle/square wave. Corners "wobbling" is just aliasing because some harmonics in signal goes over Nyquist wall. (or better say, too near it). 
Still today I can see peoples who admire and want tune up frequency response like more is better. Bullshit. Less is better. Scopes are for inspect unknown signals, not known sinewave inputs.

I have not seen any entry level or bit over this level new digital scope what have acceptable analog side filter system. Still they design front ends like in cheap analog scopes where slow decaying gaussian BW shape is fully ok.
It is not ok in digital scopes in real time "one shot" sampling mode. (ETS is different - but only for repetitive signals). It looks like digital peoples are designing oscilloscopes and no one really care about analog front end before ADC.    Same can see in some scopes what have 50ohm  input. Not even close real 50ohm impedance, exept with DC. Perhaps designer do not know what is difference between 50ohm resistance and 50 ohm impedance in case that we handle any other than DC -  ELF - VLF. 

Some scope may have 100MHz label and then real time samplerate 250MSa/s. But analog side may have example -3dB over 150MHz and perhaps -6dB over 200MHz and even 300 MHz still leaks to ADC inputs. This is wrong. And looks like no one care. Designers comment is only "so what"... and here in forum many peoples try tweak and hack for more analog BW. 

Best modification is not rise analog bandwidth but reject it and/or modify bandwidth shape more steep like "brickwall" some amount under Nyquist wall.   When samplerate drops, corner f need also drop.
Construction where every input have its own ADC is more easy. Samperate do not drop if more or less channels are in use.

Who is first company (in entry level and bit higher level scopes) who really understand how important is analog front end and its BW filter - and also do it. But, it need also explain to entry level noobs why this is important.
If scope have (well made) ETS mode for repetitive signals, in this case BW shape can be "wide open".
Some times it is fun how they add some digital filter systems to scopes but not any half word about much more important analog side filters. They can not be replaced with the filtered digitized signal.

With well designed analog front end and its BW shape and then perfect made Sin(x)/x interpolation  there do not exist any form of signal aliasing. As long as analog BW and its shape is ok for used samplerate and signal reconstruction using Sinc.

Who is first manufacturer who make well designed analog front end for real time sample mode oscilloscope. Of course other method is rise ADC sampling rate. 1GSa/s for 100MHz scope with slow decay gaussian BW shape is ok if anything over 300 - 400MHz  dot leak from input connector to ADC.   


« Last Edit: December 25, 2016, 09:49:33 am by rf-loop »
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Offline David Hess

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Re: Rigol DS1054Z bandwidth
« Reply #53 on: December 25, 2016, 01:03:28 pm »
With fast edges (or what ever waveform)  as example rectangle/square wave. Corners "wobbling" is just aliasing because some harmonics in signal goes over Nyquist wall. (or better say, too near it).

With perfect sampling, the sinc filtering of a fast edge should only produce the Gibb's phenomena which is phase coherent with the input signal and looks like preshoot and overshoot.  I think the actual "wobulation" sometimes seen is caused by intermodulation distortion in the digitizer which results in additional frequency spurs which are also aliased.  In both cases, increasing the sample rate or bandwidth limiting the input signal helps.

Quote
I have not seen any entry level or bit over this level new digital scope what have acceptable analog side filter system. Still they design front ends like in cheap analog scopes where slow decaying gaussian BW shape is fully ok.

It is not ok in digital scopes in real time "one shot" sampling mode. (ETS is different - but only for repetitive signals). It looks like digital peoples are designing oscilloscopes and no one really care about analog front end before ADC.

Some of the very early Tektronix DSOs included four pole 24dB/octave Gaussian filters which should have helped with this but I do not know if their unfiltered input bandwidth also had a 4 pole Gaussian response.  On an analog oscilloscope it hardly matters except for noise measurements.

Quote
Same can see in some scopes what have 50ohm  input. Not even close real 50ohm impedance, exept with DC. Perhaps designer do not know what is difference between 50ohm resistance and 50 ohm impedance in case that we handle any other than DC -  ELF - VLF.

Most designs implement a switchable 50 ohm feedthrough termination in front of the high impedance buffer which adds significant input capacitance compromising the 50 ohm input impedance.  At 200 MHz and below this is not a problem but above that it can be; my 300 MHz 2440 handles it well enough.  Some oscilloscopes route the input signal around the high impedance buffer when in 50 ohm mode like the 350 MHz Tektronix 485 and dedicated high frequency oscilloscope usually lack a high input impedance buffer.

Quote
Best modification is not rise analog bandwidth but reject it and/or modify bandwidth shape more steep like "brickwall" some amount under Nyquist wall.   When samplerate drops, corner f need also drop.

...

With well designed analog front end and its BW shape and then perfect made Sin(x)/x interpolation  there do not exist any form of signal aliasing. As long as analog BW and its shape is ok for used samplerate and signal reconstruction using Sinc.

Except for raising the real time sample rate or using equivalent time sampling, there is no good solution for this in a time domain instrument.  If the filter shape does not have linear phase, then the filter itself will effectively create the same problem which rules out filters which have higher performance in the frequency domain.  The best solution I have seen is in some early Tektronix DSOs which implemented 4 pole Gaussian bandwidth filters.
 

Offline rf-loop

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Re: Rigol DS1054Z bandwidth
« Reply #54 on: December 25, 2016, 03:33:55 pm »
With fast edges (or what ever waveform)  as example rectangle/square wave. Corners "wobbling" is just aliasing because some harmonics in signal goes over Nyquist wall. (or better say, too near it).

With perfect sampling, the sinc filtering of a fast edge should only produce the Gibb's phenomena which is phase coherent with the input signal and looks like preshoot and overshoot.  I think the actual "wobulation" sometimes seen is caused by intermodulation distortion in the digitizer which results in additional frequency spurs which are also aliased.  In both cases, increasing the sample rate or bandwidth limiting the input signal helps.

Quote
I have not seen any entry level or bit over this level new digital scope what have acceptable analog side filter system. Still they design front ends like in cheap analog scopes where slow decaying gaussian BW shape is fully ok.

It is not ok in digital scopes in real time "one shot" sampling mode. (ETS is different - but only for repetitive signals). It looks like digital peoples are designing oscilloscopes and no one really care about analog front end before ADC.

Some of the very early Tektronix DSOs included four pole 24dB/octave Gaussian filters which should have helped with this but I do not know if their unfiltered input bandwidth also had a 4 pole Gaussian response.  On an analog oscilloscope it hardly matters except for noise measurements.

Quote
Same can see in some scopes what have 50ohm  input. Not even close real 50ohm impedance, exept with DC. Perhaps designer do not know what is difference between 50ohm resistance and 50 ohm impedance in case that we handle any other than DC -  ELF - VLF.

Most designs implement a switchable 50 ohm feedthrough termination in front of the high impedance buffer which adds significant input capacitance compromising the 50 ohm input impedance.  At 200 MHz and below this is not a problem but above that it can be; my 300 MHz 2440 handles it well enough.  Some oscilloscopes route the input signal around the high impedance buffer when in 50 ohm mode like the 350 MHz Tektronix 485 and dedicated high frequency oscilloscope usually lack a high input impedance buffer.

Quote
Best modification is not rise analog bandwidth but reject it and/or modify bandwidth shape more steep like "brickwall" some amount under Nyquist wall.   When samplerate drops, corner f need also drop.

...

With well designed analog front end and its BW shape and then perfect made Sin(x)/x interpolation  there do not exist any form of signal aliasing. As long as analog BW and its shape is ok for used samplerate and signal reconstruction using Sinc.

Except for raising the real time sample rate or using equivalent time sampling, there is no good solution for this in a time domain instrument.  If the filter shape does not have linear phase, then the filter itself will effectively create the same problem which rules out filters which have higher performance in the frequency domain.  The best solution I have seen is in some early Tektronix DSOs which implemented 4 pole Gaussian bandwidth filters.

Yes, all this is, in practice, quite complex and need do some compromise.

Of course there is many solutions and all these
Rising real time samplerate related to BW.  (in low gfrequency scopes today quite easy)
ETS is still useful in some cases but only if quality is good
Better filters in analog front end.  Too often designers forget this in low end scopes!
Doing as perfect Sinc as possible.  Even most of low end scopes they try - but as seen, some not even try.

Here is examples and not for argumenting if Siglent is good or bad. Only just for example for some random readers who have total lack of knowledge about these things. (Examples made using Siglent SDS1102X+ but this is not important here)

First image:
Some rough image about BW shape.  (but not for evaluate this naamed scope real BW or BW shape, only for example for this discuss)
Note that this scope is named as 100MHz.
Image is made using slow sweep from around 1MHz to 360MHz linearly and sweep time 6s. One horizontal div 30MHz
There is vertical cursors what show around -3dB level related to low freq start.
Then there is marker A. It is around 100MHz position. (model name freq)
Then there is marker B. It is around 250MHz. If this scope is used both channels on, samplerate is 500MSa/s what means that Nyquist wall is 250MHz.
We can see that even with this frequency leevel have not dropped even 6dB. All what ADC get over this frequency when 2 channels is in use produce aliasing. And this need understand that also example square wave harmonics go very easy over this if there is enough fast risetime.

Sine wave aliasing is what it is  all know it.

Here is example about other form of aliasing. Corners wobbling.

Second  image:
1GSa/s and not visible aliasing. Also it can see when look rise time measured statistics (look sdev)

Third image:
sample rate is lowered to 500MSa/s. (two channels in use, CH2 trace is top of image)
Now we can see high amount of aliasing.  Also risetime statistics show it.
But quess what, if we take now couple of one shots we can find also waveforms what show "better risetime" . With 500MSa/s there can find single shots what have 1.9ns risetime and with 1GSa/s fastest risetime is 2.1ns. Fun, is it.  If I do some "trics" using example transfer line ringing I can get really weird results.  Here in these examples signal is known. Source is health Tektronix 284 pulse generator what produce around 70ps risetime without here meaningful overshoots and it have also quite flat top enough time.  But, as can see in first image, analog front end filter is not ok for 500MSa/s.  If analog front end have more steep filter so that 100MHz is not attenuated and -3dB is example 150MHz and then 200MHz is highly attenuated - corners wobbling (risetime variations) is away.
 
After then think Rigol 250MSa/s... and its wide open analog front end... and still some peoples want it even more wide BW.
When you see something you need next think is it DUT or scope what do it. Experienced people may easy regognize things quite easy (but in some cases it also need extra thinking and time more or less)

Of course analog scopes are naaturally totally free from these aliasing things what ever channels are in use and what ever horizontal speed. (this is why I still use analog scope in some repair/service situations wwith really unknown signals. I need make money, not spend my time for fun)




This image is NOT VALID for evaluate this used oscilloscope real BW or its shape. This is because sweep generator level accuracy (flatness) is not checked with sufficient reliability. But this image can use for this subject in this message.


Fast edge from TEK 284. Scope 1GSa/s and Sin(x)/x ON
No visible aliasing.



Fast edge from TEK 284. Scope 500MSa/s and Sin(x)/x ON
Strong visisible aliasing.  Because there is frequency components over Nyquist limit what are not enough reduced before ADC
(look BW shape image.)
« Last Edit: December 25, 2016, 08:40:38 pm by rf-loop »
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Offline David Hess

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Re: Rigol DS1054Z bandwidth
« Reply #55 on: December 25, 2016, 08:44:55 pm »
I know of at least one person who put a motorized control on their SG503 or SG504 leveled generator so they could do accurate oscilloscope and probe sweeps.

I wish someone with a Tektronix 284 pulse generator or equivalent like a PG506 which is acceptable up to 100 MHz would run these tests on a DS1054Z before and after the bandwidth hack.  The best test I have seen shows a problem.
 

Offline nctnico

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Re: Rigol DS1054Z bandwidth
« Reply #56 on: December 25, 2016, 09:40:34 pm »
IMHO there is not much use in trying to use an oscilloscope beyonds it's specifications especially a lower end one without dedicated 50 Ohm inputs. For starters there is the capacitance of the typical 1:10 probes which is a huge load for frequency components in the 100MHz region. And even if you use external 50 Ohm terminators the input capacitance of the scope will screw things up. At the end of the day you are just stacking errors on top of errors when trying to use an instrument beyond it's limits.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline David Hess

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Re: Rigol DS1054Z bandwidth
« Reply #57 on: December 26, 2016, 07:00:22 am »
IMHO there is not much use in trying to use an oscilloscope beyonds it's specifications especially a lower end one without dedicated 50 Ohm inputs. For starters there is the capacitance of the typical 1:10 probes which is a huge load for frequency components in the 100MHz region. And even if you use external 50 Ohm terminators the input capacitance of the scope will screw things up. At the end of the day you are just stacking errors on top of errors when trying to use an instrument beyond it's limits.

It is not quite as bad as that.  I think a larger problem is verifying performance without having a reference level pulse generator and leveled RF source.  And if you build, buy, or repair either, how do you calibrate them?

Probes are specified with a terminated 50 ohm source so they see a 25 ohm source impedance.  This is unrealistic of course for most applications but the 15 picofarad shunt capacitance of a x10 probe isn't too bad at 100 MHz if the source impedance is low.  It is even acceptable at 200 MHz.  A ground lead if used is at least as big a problem.

As the source impedance or frequency rises, other probe types need to be used like low-z passive probes or active probes.  Sometimes I just design in an emitter or source follower or transconductance output to drive a probe or 50 ohm transmission line to the test instrument. 

If a feedthrough attenuator is used or internal 50 ohm termination, then an oscilloscope's high impedance buffer sees 25 ohms.  I get practically identical results between using an external feedthrough termination, internal termination, and x10 probe attached to a termination (1) at 100 MHz and 200 MHz bandwidths.  Of course I should get identical results because of how probes and oscilloscope inputs are calibrated.  They are suppose to generate identical results with 50 ohm terminated sources providing a 25 ohm source impedance.

(1) They make or made special coaxial probe feedthrough terminations which place the 50 ohm termination as close to the probe tip as possible.  These are naturally expensive or unobtainable (2) but up to 200 MHz, a BNC to probe tip adapter connected to a BNC feedthrough termination works well enough.  A better home made solution would involve a probe tip to printed circuit board adapter and a coplanar feedthrough termination.

(2) Find them in the aisle next to the female BNC connectors which have a built in disk resistor feedthrough termination.
 

Online tggzzz

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Re: Rigol DS1054Z bandwidth
« Reply #58 on: December 26, 2016, 09:24:47 am »
IMHO there is not much use in trying to use an oscilloscope beyonds it's specifications especially a lower end one without dedicated 50 Ohm inputs. For starters there is the capacitance of the typical 1:10 probes which is a huge load for frequency components in the 100MHz region. And even if you use external 50 Ohm terminators the input capacitance of the scope will screw things up. At the end of the day you are just stacking errors on top of errors when trying to use an instrument beyond it's limits.

I wonder how anybody ever built and verified the performance of the worlds highest performance scope/SA/etc :)
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline nctnico

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Re: Rigol DS1054Z bandwidth
« Reply #59 on: December 26, 2016, 12:16:11 pm »
IMHO there is not much use in trying to use an oscilloscope beyonds it's specifications especially a lower end one without dedicated 50 Ohm inputs. For starters there is the capacitance of the typical 1:10 probes which is a huge load for frequency components in the 100MHz region. And even if you use external 50 Ohm terminators the input capacitance of the scope will screw things up. At the end of the day you are just stacking errors on top of errors when trying to use an instrument beyond it's limits.
I wonder how anybody ever built and verified the performance of the worlds highest performance scope/SA/etc :)
Apply math and physics like they use the 'Josephson effect' to produce a voltage reference.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline David Hess

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Re: Rigol DS1054Z bandwidth
« Reply #60 on: December 26, 2016, 03:53:26 pm »
I wonder how anybody ever built and verified the performance of the worlds highest performance scope/SA/etc :)

Apply math and physics like they use the 'Josephson effect' to produce a voltage reference.

It was more like applied engineering because the math and physics only go so far.

At least for time domain instruments like oscilloscopes and reference level pulse generators, they used *real* sampling oscilloscopes.  (1)  A sampling oscilloscope can also be used to calibrate a leveled sine wave oscillator and may be the best (least expensive) option for a hobbyist but typically a thermal RMS based meter would be used for this.

The original reference level pulse generator from NBS (now NIST), which shares a very similar design with the fast rise/fall outputs of the PG506 and earlier reference level pulse generators from Tektronix, still relied on a sampling oscilloscope (1) to verify its performance (instead of the reverse) and its design was already an order of magnitude too slow for the highest performance instruments of that time.

I think I read that at some point Tektronix sent a 70ps Type 284 tunnel diode pulse generator to NBS/NIST so they could certify it and it could become a gold standard at Tektronix but I have no idea how NBS/NIST went about doing that.  Then this gold Type 284 revealed that the original S-2 sampling heads were actually 75ps instead of 50ps.  I think the problem was that the big GR-874 RF connectors were too large to support TEM-mode propagation above 4 GHz.

These days NIST uses electro-optical sampling and an online search will reveal all kinds of information on this subject.

(1) Check out the calibration instructions for a Tektronix 067-0587-02 which is used to calibrate the Tektronix 7104 1 GHz real time oscilloscope.  They include using a 4 GHz sampling oscilloscope to calibrate the reference flat pulse generator output.  The 1 GHz 7A29 vertical amplifier for the 7104 also requires a 4 GHz sampling oscilloscope for calibration.

(2) I can tell from some of the photographs in the documentation that one of the sampling oscilloscopes NBS used was a 7000 mainframe with probably a 4 GHz S-2 sampling head.  Some of the other photographs show an older sampling oscilloscope made by HP and it looks like they had a different third sampling oscilloscope as well.
 


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