Products > Test Equipment

Bandwidth limit on Siglent SDS2000X Plus oscilloscope

<< < (6/8) > >>

rf-loop:

--- Quote from: Performa01 on July 20, 2023, 04:22:46 pm ---
--- Quote from: pdenisowski on July 20, 2023, 10:08:34 am ---Another problem is that the "0.35" factor only applies when the scope has a Gaussian frequency roll-off.  A scope with a more flat or brick wall type frequency response would require the use of a higher number factor (usually 0.4 - 0.45).

--- End quote ---
You should avoid the term "brick wall" here, as no serious scope frontend will have a frequency response that is even remotely brick wall.

--- End quote ---

But still "brick wall" is commonly used (for Flat-response meaning in oscilloscopes) when professionals talk because they know context.
It have widely used in old HP-Journals, and HP-Agilent-Keysight appnote etc papers. All we know (I hope) what it mean when we talk oscilloscopes and brick-wall type freq response and still we do not mean (in any cases) ideal Brick-wall filter aka rectangle filter because it do not exist in practice... just as all we know that there is not perfect square waves or true DC but still we talk using these...of course.

Or is it only allowed for engineers from A-group manufacturers. At least they quite often use that term.
 :) ;)  :) ;)

----
Common sidenote... HP-Journal, Oct 1993 is still good to read. The basics don't expire (mostly).

pdenisowski:

--- Quote from: rf-loop on July 21, 2023, 12:57:19 pm ---But still "brick wall" is commonly used (for Flat-response meaning in oscilloscopes) when professionals talk because they know context.

--- End quote ---

Keysight:  "Responses that have brick-wall filters are desired as they produce less noise"
https://www.keysight.com/us/en/assets/7018-04317/application-notes/5991-4088.pdf

LeCroy:  "the tailored brick-wall frequency response"
https://cdn.teledynelecroy.com/files/whitepapers/enhanced-sample-rate-whitepaper.pdf

R&S:  " The user can choose between Gaussian or brick wall filter characteristics to optimize the oscilloscope step response"
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/pdm/cl_brochures_and_datasheets/product_brochure/3608_6919_12/RTO6_bro_en_3608-6919-12_v0502.pdf

Tektronix:  "Most real-time oscilloscopes today have a rather sharp roll-off (e.g. "brick wall")"
https://www.tek.com/en/datasheet/dpo7oe-series-optical-probes


--- Quote from: rf-loop on July 21, 2023, 12:57:19 pm ---Or is it only allowed for engineers from A-group manufacturers. At least they quite often use that term.  :) ;)  :) ;)

--- End quote ---

Well, I've worked at both HP/Agilent and R&S, so that might explain it  :-DD

BillyO:

--- Quote from: Performa01 on July 21, 2023, 10:29:43 am ---You can tell that I've never worked with that formula, always used accurate frequency response measurements instead.
--- End quote ---

That formula is based on the physics of the system.  Provided you can mathematically characterize the system's response you can work out a version of that formula for it.  It should be quite accurate.   Gaussian response is well characterized and the resulting 0.35/risetime is accurate.  For moderately flat response systems (the top end of most mid-range PGA/VGA) the response is also well characterized and 0.4/risetime is also accurate. 

Using a sine wave generator has it's perils.  Unless you have calibrated it and are confident in it's performance it can also give very skewed results.  Ideally you would measure it's output and normalize it before taking each data point of the DUT to ensure it is flat at the frequency being tested.  The rise time is a single measurement and is accurate as long as the character of the response of the DUT is known.



--- Quote from: Performa01 on July 21, 2023, 10:29:43 am ---There were several reasons that got my thinking wrong - ridiculous bandwidth claims being one of them.
--- End quote ---
Like the over 600MHz for a un-corked SDS2000X-P?  As an example mine has a rise time of 660ps, not the 800ps claimed by Siglent.  Using the formula (0.4/risetime) we get a BW of 606MHz.  Using a sinewave sweep @ I got 650MHz .  That 606 MHz does not look like a ridiculous claim to me.  The difference is probably got to do with my not being able to determine if the sine wave generator was flat.


--- Quote from: Performa01 on July 21, 2023, 10:29:43 am ---It looks very different for a higher bandwidth scope like the SDS6000A. The 0.35 factor fits for the 1 GHz model, but nothing else matches.

--- End quote ---
I imagine the 500MHz license on the 6000 series has much better performance than advertised.  As for the 2GHz version, it is most likely using more advanced PGA/VGA with more agressive flattening.  Tektronix claim .45 for their "maximally flat" response so 0.46 seems very much in line with that.

mawyatt:
Amplitude Brickwall Filters are not realizable but the analog IIR high order Chebyshev, Inverse Chebyshev and Elliptical filters produce a good approximation at the expense of highly non-linear phase response, FIR digital filters do even better. One issue for scope use of one of these analog filters would be the highly non-linear Group Delay (derivative of phase with frequency) associated with these type filters, whereas the less Brickwall like Bessel and Gaussian have a much better Group Delay which is very important for scope use and why they are usually employed. Of course the Digital FIR filters can disconnected the phase/delay and amplitude responses, which is a huge benefit in some applications.

Seems the swept frequency is the overall best way to accurately characterize a DSO. Since one doesn't know the exact relationship between the frequency response and step/impulse response, one uses the usual fudge factors, i.e. 0.35, 0.4 and so on, if that's important! The issue with the swept frequency response; Does the signal generator remain "flat" across the entire band in question? This is easily resolved with a quality SA, where the amplitude variation can be accurately measured across the band of interest. The tracking generator in our SA is pretty good and we've used such to "characterize" our uncorked SDS2000X+, which demonstrated over 600MHz BW :-+

This is not to say the DSO inherent rise time and channel characteristics aren't important, it is and one must always consider the displayed waveform rise time/pulse waveform characteristics as being distorted by the DSO, how much so depends on the waveform and DSO. One must consider that the scope and waveform measurement at hand are non-coherent (generally a good assumption), so the display is the result of the root-sum-squared (RSS) response if the DSO is moderately well behaved. So for a 10% DSO displayed contribution/degradation the DSO rise time needs to be better than ~3.2 times the waveform under consideration, similar concept applies for waveform bandwidth measurements.

Best,

pdenisowski:

--- Quote from: BillyO on July 21, 2023, 02:56:57 pm ---Using a sine wave generator has it's perils.  Unless you have calibrated it and are confident in it's performance it can also give very skewed results.  Ideally you would measure it's output and normalize it before taking each data point of the DUT to ensure it is flat at the frequency being tested. 

--- End quote ---

Agree completely that you would want to be sure your signal generator amplitude is either "flat" or has a known frequency response (output level as a function of output frequency).

Most "professional" sig gens should have very good spectral flatness over typical scope bandwidths (100s of MHz up to low tens of GHz), especially as many monitor their own output level and can internally adjust output level as needed to maintain good "flatness" over their specified operating range.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod