### Author Topic: Pulse generator, rise time and Rigol DS1054z bandwidth  (Read 20075 times)

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#### JacquesBBB

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##### Pulse generator, rise time and Rigol DS1054z bandwidth
« on: December 01, 2015, 07:16:36 pm »
This is a question following my earlier post

https://www.eevblog.com/forum/beginners/a-few-question-on-jw-pulse-generator-based-on-2n3904/

directly related, but I prefer to start a new thread uniquely (if possible ) devoted to  the question of the
measure of the bandwith of the Rigol DS1054z with a  Jim Williams pulse generator. The other post is devoted to the construction of  pulse generators and I will continue to post there my various experiments on the matter.

https://www.eevblog.com/forum/beginners/bandwidth-calculation-caveats!/msg807413/#msg807413

and I still do not understand.

The problem is that I find a rise time r_t of 1.2 ns on the  Rigol DS1054z that corresponds
to  a bandwidth  BW =  0.35 / r_t = 292 Mhz , way off the  advertised 100 Mhz.

Tim proposed that i this is due to sin(x)/x interpolation, or vector display.

I have  done it again with dots display, and I have  the same  results.

Suppressing sin(x)/x interpolation is more difficult, as it is only possible (as far as I understand)
with 3  or 4 channels.

In this case, the sampling rate falls to 250 Ms/s.  According to
http://m.eet.com/media/1140862/19209-263113.pdf
it is thus normal to have  a smaller bandwidth

At this  sampling rate, I found nearly the same  curve, whether or not I use the
sin(x)/x interpolation.
The figure given by the Rigol  measure is
4.05 ns  without  sin(x)/x   - >  86 Mhz

and
3.75 ns   with sin(x)/x         ->   93 Mhz

but this is only due to the difference of  maximum, not to a difference in the slope, and this
is  minor compared  to  the huge difference at 1 Gs/s.

When I  increase to  500Ms/S  I get  2.8 ns  - >  125 Mhz  ( sin(x)/x on)

and                    at   1Gs/s              1.4 ns  ->   250 Mhz  (sin(x)/x) on).

Edit :

So my question remains :  what is the real bandwidth of the Rigol DS1054z at full  sample rate  (1Gs/s) ?

How to relate the rise time to the band width ?  All the notes I have seen are minor differences, of a few (max 20)  %,
here we speak of a factor of  2-3.
« Last Edit: December 01, 2015, 09:16:27 pm by JacquesBBB »

#### T3sl4co1l

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #1 on: December 02, 2015, 08:20:44 pm »
Have you used a -20dB (or more) attenuator yet?

Tim
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Bringing a project to life?  Send me a message!

#### Howardlong

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #2 on: December 02, 2015, 08:46:00 pm »
A liberated one is about 2.3 to 2.4ns.

I did a video on it here with a liberated DS1000 at 16:26.

http://youtu.be/mS3sCJd_GPk

#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #3 on: December 02, 2015, 10:51:04 pm »
@Tim :
Yes,  as you can see the amplitude of the pulse is  now very much reduced  to about 1.5 V.
I  made an atenuator  with   two  68 R  and  160R = 150R +10R.

according to http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm
The atenuation should be 16.33 db  with no reflection.

@Howardlong : I see you have 2.4ns on the Rigol DS1054z,  at 1Gs/s . In the same setting, I have 1.4ns . You can see it in the picture above.

How do you explain that ?  My pulse is certainly  not as good as yours, but this should make my raising time even worse.
How can I get 1.4 ns ?

It would be interesting to have some other measures on the rise time of the Rigol DS1054z.

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #4 on: December 03, 2015, 02:27:55 am »
Get a leveled RF signal generator and increase frequency until amplitude drops to 70% of original amplitude. Then you know -3dB bandwidth. Simpler than using a pulse generator with unknown specs

#### Electro Fan

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #5 on: December 03, 2015, 02:57:44 am »
Thanks for the tip - that works simple enough.  On a MSO2072A I got about 130 Mhz at -3dB (based on 70% of the original amplitude measured at 70 MHz).

#### Howardlong

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #6 on: December 03, 2015, 08:39:42 am »
@Tim :
Yes,  as you can see the amplitude of the pulse is  now very much reduced  to about 1.5 V.
I  made an atenuator  with   two  68 R  and  160R = 150R +10R.

according to http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm
The atenuation should be 16.33 db  with no reflection.

@Howardlong : I see you have 2.4ns on the Rigol DS1054z,  at 1Gs/s . In the same setting, I have 1.4ns . You can see it in the picture above.

How do you explain that ?  My pulse is certainly  not as good as yours, but this should make my raising time even worse.
How can I get 1.4 ns ?

It would be interesting to have some other measures on the rise time of the Rigol DS1054z.

If there is ringing or overshoot on the signal (not visible due to other effects such as sample rate) that would produce such a scenario. Also I am not sure what construction techniques you have used. If you are simply using a few wire ended resistors dead bug style that might well explain any overshoot and ringing. We haven't sen the construction method for your JW pulse generator either. I am not saying they are wrong, but when you talked about stringing four resistors together for the attenuator I wasn't sure how it was done, and layout will be fairly crucial.

Does your JW pulser have a pulse stretcher on it such as a piece of coax? It looks like it does from the scope traces.

As an alternative to the attenuator, you could also try a single 51 ohm resistor at the scope end of a BNC cable. If you use right angle terminations, you can, if you're careful, place 0603 or 0805 resistors inside the RA plug.

FWIW I noticed the same effect you have some time ago on the Rigol and I never managed to explain it away fully. However I've learned a lot since then in the realm of high speed time domain measurements, so maybe now's the time to figure it out.

I am not sure why the Rigol doesn't allow you to switch off sin(x)/x, on 1 or 2 channels, it's annoying.

#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #7 on: December 03, 2015, 09:46:43 am »
Get a leveled RF signal generator and increase frequency until amplitude drops to 70% of original amplitude. Then you know -3dB bandwidth. Simpler than using a pulse generator with unknown specs

This is "botter en touche" (kick into touch, evade the issue, dodge the issue, pass the buck (translation from http://www.wordreference.com/fren/botter%20en%20touche)).

The question is  : what is the smallest rise time possible for the Rigol DS1054z ?

This is what I want to understand. The fact that the specs of the pulse generator are not precisely known should not be an issue. Whatever you do, as far as I understand, you will never be able to measure a rising time with smaller value than the rise time of the oscilloscope.

measured r_t > oscillo r_t  with  the approximate  value  measured r_t = sqrt( oscillo r_t^2 + pulse generator  r_t^2)

#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #8 on: December 03, 2015, 09:54:15 am »
@Howardlong :

The construction of my pulse generator is on
https://www.eevblog.com/forum/beginners/a-few-question-on-jw-pulse-generator-based-on-2n3904/
as said in the beginning of this thread.

Yes I put a coax, but  could improve it by changing to more adapted impedance.
Yes I terminated with a T and 50R plug on the scope.

FWIW I noticed the same effect you have some time ago on the Rigol and I never managed to explain it away fully. However I've learned a lot since then in the realm of high speed time domain measurements, so maybe now's the time to figure it out.

I am pleased to see that someone else wants to understand.

Quote
I am not sure why the Rigol doesn't allow you to switch off sin(x)/x, on 1 or 2 channels, it's annoying.

I have verified that it is written so  in the  Rigol docs. (Edit) I agree that this is very bad. They should add the possibility to
remove this interpolation in a firmware upgrade.

In addition, I have a question :

When rise time is mentioned,   the amplitude of the signal is not mentioned. How does the rise time depends on the amplitude of the  step signal ?

« Last Edit: December 03, 2015, 11:40:59 am by JacquesBBB »

#### tggzzz

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #9 on: December 03, 2015, 10:26:31 am »
I am pleased to see that someone else wants to understand.

Others do too

Quote
In addition, I have a question :
When rise time is mentioned,   the amplitude of the signal is not mentioned. How does the rise time depends on the amplitude of the  step signal ?

One simple point is that if any of the front-end components are driven out of their linear range, then all bets are off. It is possible that a narrow strong pulse could exceed such limits.

That's true of analogue scopes as well, and Jim Williams has written good articles on how this has to be understood, and which (old) scopes are less susceptible to it. I can't be bothered to search for a reference, but it does everybody good to real all of Jim Williams output
There are lies, damned lies, statistics - and ADC/DAC specs.
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#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #10 on: December 04, 2015, 05:59:02 am »
From
https://www.eevblog.com/forum/testgear/new-rigol-ds1054z-oscilloscope/msg800232/#msg800232

Probably variation took the bandwidth safety margin a bit higher than usual. Mine with the "hack" tests well over 200 MHz with the risetime method, very close to the theoretical upper limit of their frontend design.

I see that other have found a resul for rise time (1.5ns)  that is very close to the 1.4ns  I got with my pulse generator.

Does anybody else has some  equivalent measure that he can post here ?
« Last Edit: December 04, 2015, 06:04:05 am by JacquesBBB »

#### Electro Fan

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #11 on: December 04, 2015, 06:31:14 pm »
From
https://www.eevblog.com/forum/testgear/new-rigol-ds1054z-oscilloscope/msg800232/#msg800232

Probably variation took the bandwidth safety margin a bit higher than usual. Mine with the "hack" tests well over 200 MHz with the risetime method, very close to the theoretical upper limit of their frontend design.

I see that other have found a resul for rise time (1.5ns)  that is very close to the 1.4ns  I got with my pulse generator.

Does anybody else has some  equivalent measure that he can post here ?

There is a thread that talks about Jim Williams pulse generators and related info here:
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/615/
You can find rise time measurements done with various scopes in the thread.

fwiw, I just measured rise time from a pulse gen on a MSO2072A and got about 3.85ns.  The Rigol spec (which is just based on a calculation) says that the 70 MHz scope should have a rise time of 5 ns and a 200 MHz version should have a rise time of 1.8ns.  Rigol scopes tend to outperform their rated bandwidth spec so it's believable that a 200 MHz version tests in the vicinity of 1.5ns.

Be careful of going too far down this road - you might get enticed into achieving picosecond rise times - which could lead to a new scope .  It starts with just wanting to see sub-nanosecond rise times (999 ps or faster - just so you can see ps instead of ns); then you want less than 500 ps, then less than 100 ps and so it goes.  Rise time measurements can be a gateway drug and the next thing you know you are on your way to having 20 oscilloscopes.

But this web site can not only create addictions it can also help cure or avoid them.  You can checkout the posts from Jay_Diddy_B, Howardlong and others and live vicariously by observing very fast rise times.

On the other hand if you want to measure really short duration stuff you could wind up with one of these:
« Last Edit: December 04, 2015, 07:01:19 pm by Electro Fan »

#### commie

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #12 on: December 04, 2015, 07:14:14 pm »
In addition, I have a question :

When rise time is mentioned,   the amplitude of the signal is not mentioned. How does the rise time depends on the amplitude of the  step signal ?

Hi Jacques,

The best way to understand this is first consider a rotating vector i.e., a sinewave, then ask yourself where do the highest slew rates(gradients) occur?. Naturally, this happens at the zero crossings, correct?The next question is, how to increase the gradient at the zero crossing? It turns out there are two ways of doing it, first is by increasing the frequency that one is obvious, then there is amplitude, i.e., if you increase a sinewaves amplitude the zero crossings(gradient(dV/dt) increase as well, which is not so obvious.
So, we have i=Isin(wt), we want to know what the gradient is at the zero crossings:
di/dt=I.w.cos(w.t), cos(0)=1
so di/dt=I.w.....where w=2.pi.f

Since a cpu performance throughput is  frequency only, it makes sense to characterize the silicon die to 1.1V instead of 5V and then we can run our pc's at 1.8GHz!

Any joy?

Cheers
Commie
« Last Edit: December 04, 2015, 07:59:47 pm by commie »

#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #13 on: December 05, 2015, 10:02:49 am »

There is a thread that talks about Jim Williams pulse generators and related info here:
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/615/
You can find rise time measurements done with various scopes in the thread.

I have read this thread, but did not find the information I wanted in it.

Quote
fwiw, I just measured rise time from a pulse gen on a MSO2072A and got about 3.85ns.  The Rigol spec (which is just based on a calculation) says that the 70 MHz scope should have a rise time of 5 ns and a 200 MHz version should have a rise time of 1.8ns.  Rigol scopes tend to outperform their rated bandwidth spec so it's believable that a 200 MHz version tests in the vicinity of 1.5ns.

OK, but my scope is supposed to be 100 Mhz, not 200 Mhz.

Quote
On the other hand if you want to measure really short duration stuff you could wind up with one of these:

If I find a good deal on these on ebay, why not ?

#### tggzzz

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #14 on: December 05, 2015, 10:11:06 am »

There is a thread that talks about Jim Williams pulse generators and related info here:
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/615/
You can find rise time measurements done with various scopes in the thread.
...
On the other hand if you want to measure really short duration stuff you could wind up with one of these:

If I find a good deal on these on ebay, why not ?

Make sure the quoted price includes shipping (and preferably installation) costs.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less

#### vk6zgo

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #15 on: December 05, 2015, 10:12:27 am »
I am pleased to see that someone else wants to understand.

Others do too

Quote
In addition, I have a question :
When rise time is mentioned,   the amplitude of the signal is not mentioned. How does the rise time depends on the amplitude of the  step signal ?

One simple point is that if any of the front-end components are driven out of their linear range, then all bets are off. It is possible that a narrow strong pulse could exceed such limits.

That's true of analogue scopes as well, and Jim Williams has written good articles on how this has to be understood, and which (old) scopes are less susceptible to it. I can't be bothered to search for a reference, but it does everybody good to real all of Jim Williams output

Yes,if you clip the top of the signal,the rise time is decreased,but,of course,the amplitude is decreased,too.
This is a legitimate way of decreasing the rise time of a signal,& is used,both to regenerate a signal with degraded risetime,& to produce a Quasi-square wave out of a sine wave.(in the latter case,both peaks are clipped).
The signal is clipped then amplified in a stage with better frequency response than that of the previous circuitry.
« Last Edit: December 05, 2015, 10:14:48 am by vk6zgo »

#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #16 on: December 06, 2015, 06:44:41 am »
In fact, Tim (T3sl4co1l) was right.

It  looks like the sin(x)/x, combined with some overshoot on the pulse generation is artificially increasing the slope of the step function on the Rigol DS1054z.

I have largely improved my pulse generator, closely following Jim William AN94
http://cds.linear.com/docs/en/application-note/an94f.pdf

I have now a much cleaner pulse. and find a time rise of  about 1.9 ns. This is still much better than normal specification for a 100Mhz scope, but quite in par with other measures mentioned above.

The measure is made with an home made 20Db attenuator and a 50R termination.

#### Fungus

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #17 on: December 07, 2015, 08:36:09 am »
Quote
I see that other have found a resul for rise time (1.5ns)  that is very close to the 1.4ns  I got with my pulse generator.

Does anybody else has some  equivalent measure that he can post here ?
Be careful of going too far down this road - you might get enticed into achieving picosecond rise times - which could lead to a new scope .  It starts with just wanting to see sub-nanosecond rise times (999 ps or faster - just so you can see ps instead of ns);
But it's soooo close already. Only 0.4ns needed....
« Last Edit: December 07, 2015, 01:42:23 pm by Fungus »

#### max-bit

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #18 on: December 07, 2015, 12:11:41 pm »

#### JacquesBBB

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #19 on: December 07, 2015, 01:47:58 pm »
Thanks, but I am not interested in buying a pulse generator in a black box.
Where are your shematics, pcb picture,  gerber files (if any ) ?.

I am interested in building one with elementary means, and to share experience with others that would like to build one.

This is why  I describe here https://www.eevblog.com/forum/beginners/a-few-question-on-jw-pulse-generator-based-on-2n3904/
the various problems I encountered in building this simple, but delicate device.

In addition, your pulse generator does not provide a plateau, which is, according to JW, not the best way to evaluate the performances of a scope.
« Last Edit: December 07, 2015, 04:51:24 pm by JacquesBBB »

#### c4757p

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #20 on: December 07, 2015, 01:54:18 pm »
In addition, your pulse generator does not provide a plateau, which is, according to JW not the best way to evaluate the performances of a scope.

According to basic mathematics it doesn't work at all. I really wish people would stop pushing these useless impulse devices as a way to evaluate oscilloscope behavior. The results are garbage, typically making the oscilloscope look significantly better than it actually is. Without a square rising edge, and a long enough flat top to allow the oscilloscope system to fully settle before the falling edge, the results are completely meaningless.

But no matter how many times people howl about this on here, other people continue to just ignore them.

To scatter a bit of signal with the noise again:

159mHz cutoff filter with a full rise, 0.35/rt = 160mHz: full_rise.png

159mHz cutoff filter with a truncated rise, 0.35/rt = 250mHz: truncated_rise.png
« Last Edit: December 07, 2015, 02:07:14 pm by c4757p »
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#### macboy

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #21 on: December 07, 2015, 02:55:23 pm »
In addition, your pulse generator does not provide a plateau, which is, according to JW not the best way to evaluate the performances of a scope.

According to basic mathematics it doesn't work at all. I really wish people would stop pushing these useless impulse devices as a way to evaluate oscilloscope behavior. The results are garbage, typically making the oscilloscope look significantly better than it actually is. Without a square rising edge, and a long enough flat top to allow the oscilloscope system to fully settle before the falling edge, the results are completely meaningless.

But no matter how many times people howl about this on here, other people continue to just ignore them.
THIS
It boggles my mind that people still use these impulse generators to "measure" rise time. The only valid input for measuring rise time is a step, and it needs to be a well-behaved step too, flat-topped and without overshoot. Overshoot adds high frequency content that would artificially contribute to a faster rise time, because mathematically it is like an implulse added to a step. If you want to build a step-type pulse generator to measure the rise time of a 100 MHz scope, you will need a known-good 1 GHz scope to calibrate the step generator before you can use that to characterize the 100 MHz scope. There is no way around that; you need a fast, clean step.

These (im-)pulsers are a fun project to build and play with especially when a person has no other signal source on the bench which can push a scope to its limits. But understand, you can't measure rise time with them.

#### c4757p

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #22 on: December 07, 2015, 03:03:33 pm »
Overshoot adds high frequency content that would artificially contribute to a faster rise time, because mathematically it is like an implulse added to a step.

Along these lines, here's a third simulation. Same filter, but a step with overshoot. The overshoot "just happens"  to be calibrated such that it's not even visible at the output of the filter, and yet...

0.35 / rt = 642 mHz

Holy shit, my filter is fast!!

overshoot.png
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#### max-bit

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##### Re: Pulse generator, rise time and Rigol DS1054z bandwidth
« Reply #23 on: December 07, 2015, 03:03:47 pm »
My Pulse Gen based Jim W. Pulse Gen (very simple)

#### c4757p

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