Pulse generator, rise time and Rigol DS1054z bandwidth

[max-bit]

Pulse RT 255ps

[c4757p]

Did you read one bit of what I wrote?

[EV]

The pulse should look like this one if you want to get correct rise time.

[Fungus]

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.

Even if you could, what would it tell you about the 'scope that sine wave attenuation doesn't?

[c4757p]

Well, it would give you an inflated number for the size of your EE-peen, no?

JW pulse gen = EE viagra?

[JacquesBBB]

Here is what I got  using  a Tek TDS460A  normally 400Mhz, but limited to 100Ms/s.
It is not a very responsive machine, but its the highest BW I have.

I used it to calibrate the pot to  attenuate the  overshoot, following JW  AN94.

I have no other  way in my lab to generate signal of frequency higher than 20Mhz.

[T3sl4co1l]

And like I posted earlier,
https://www.eevblog.com/forum/beginners/bandwidth-calculation-caveats!/msg807413/#msg807413

And what does it matter?

Clearly, only signals with comparable waveforms can be compared with simple rules. (Duh?)

The circuit in my simulation has largely the same risetime, but reduced ringing/overshoot, if I tweak the frequency response to be softer: closer to a Bessel response, which will be more typical of oscilloscopes (e.g., Tektronix's famous vacuum tube distributed amplifiers from the early 60s).  However, I'm more interested in the frequency response, so I want those extra dB of flatness out to the cutoff frequency, which introduces ringing without improving the step response much.

(What I find impressive, really, is those Tektronix amplifiers must've been good for 500MHz of gain-bandwidth!  In tubes, no less!  The reason they topped out around 100MHz, at the system level, is a combination of factors: intentionally softened frequency response (for clean step response), required cascading of stages (two 100MHz LPFs cascaded do not have 100MHz BW, but more like 70MHz -- and they needed 3-4 stages in the oscilloscope to get enough gain!), and needing gain at all, GBW being what it is.)

Tim

[Howardlong]

From https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/msg775096/#msg775096, I think this picture paints a thousand words.

Hi,

Like Tim said, I think the pulse from your pulse generator is too short. Here is a simulation of a filter being tested with a short pulse and a long pulse:



You would think the short pulse indicates higher BW, but the filters are identical.

Try adding about 18 inches of 50 Ohm coax in parallel with the capacitor in your pulse generator. This will length the pulse to about 4ns.


Regards,

Jay_Diddy_B

[JacquesBBB]

I have seen this recent post

Below You will find step response of my hacked Rigol DS1054Z (100MHz). This test was performed using Tektronix 284 70ps pulse generator to check if there is any ringing.
Pictures for time base settings: 5ns/div and 50ns/div.
Results: quite good.
Edit: Especially for Fungus I'll read the rise time measurement shown on the upper picture: 3,2ns.

The observed rising time is here 3.2 ns. Much larger than the one I have of 1.9 ns.

The used pulse generator seems to be of high quality, and it would be nice to have other examples to understand if there is some variability  on the DS1054z scopes.

[macboy]

Pulse RT 255ps

Sigh.

[macboy]

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.

Even if you could, what would it tell you about the 'scope that sine wave attenuation doesn't?

Rise time for one thing.
Seriously though each is a valid test in different ways. One difference is that a levelled sine generator that goes to GHz is a very complicated and expensive piece of gear. A pulser is significantly simpler.

[Fungus]

Even if you could, what would it tell you about the 'scope that sine wave attenuation doesn't?

each is a valid test in different ways. One difference is that a levelled sine generator that goes to GHz is a very complicated and expensive piece of gear. A pulser is significantly simpler.

But the results are difficult to measure/compare.