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Best Brand Name for an Oscilloscope?
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MrAl:

--- Quote from: pdenisowski on December 02, 2022, 10:34:05 am ---Lots of really great discussion and comments here about measuring scope bandwidth!

We get so many questions about this topic, I actually made a video explaining the basic concepts of "bandwidth" in oscilloscopes.  Probably a bit too introductory for most of the people posting here, but it does cover a few things discussed in this thread such as calculating BW from rise time, the different frequency responses (Gaussian vs. flat), etc. 

One thing I didn't cover is how to measure the bandwidth of an oscilloscope (i.e. procedures, techniques), but it seems like this might be a good topic for a future video.  I've already learned a few things from this thread myself :)



--- End quote ---

Yes that is the way i have been doing it, sweeping a sine was up in frequency until you see the amplitude drop to 1/sqrt(2) which is the -3db point.
Since to do this test for a higher frequency scope like 100MHz you have to have a signal generator that can produce sine waves up to at least 100MHz.  I did not have that for testing a scope that was said to be 100MHz but was really 30MHz but my frequency gen only goes up to 24MHz anyway, so i had to use the pulse method and assume that the pulse rise time was what the spec said it would be, and calculate the 90 percent threshold for the rise time of the scope given a first order LP filter front end.
You can get this number by noting the charge time for an R and C filter:
1-e^(-t/RC)

and note that the 3db cutoff point for an RC LP filter comes from:
w=1/RC
which is:
2*pi*f=1/RC
and solving for RC i get:
RC=1/(2*pi*f)

then substitute that into the charge time above:
1-e^(2*pi*f*t)

and then given your frequency f and equating that to 0.9 the equation becomes:
1-e^(2*pi*f*t)=0.9

and solving that for any 'f' we get approxmately:
t=0.366/f

and many people use t=0.35/f which is ok too really because it's just an estimate anyway.
It is possible that the 0.35/f formula comes from the 10 percent and 90 percent thresholds but i did not look into it, but since the time comes out slightly shorter i would think it was because of that.
However, since the rise time of the test pulse is never perfect and often more like 3ns, i like the longer time of 0.366/f as that helps a little to include the rise time of the input test pulse which of course makes the rise time on the scope slightly longer.  The development of the formula for that is slightly more complicated because then we can not use a pulse with 0 rise time we have to use a ramp with whatever rise time we assume we will be working with.  Some here use 2ns i think from a digital logic circuit.

MrAl:

--- Quote from: 2N3055 on December 02, 2022, 10:59:47 am ---
--- Quote from: MrAl on December 02, 2022, 07:14:55 am ---
Oh so i guess we see some differences AGAIN between CRT and DSO scopes, that's a pain in the neck.

The settling time is the time to settle to within 99 percent of maximum, and that is an important spec of any ADC.  I think the percentage used can be different though, such as 95 percent or something.  So if you shoot a 1v high square wave into the front end and the scope display starts to rise, it's the time it takes to get to 99 percent of the full max which in this case would be 0.99 volts.  This could be a lot longer than the rise time.  This was a big concern for me when i was designing a 100MS/s scope way back in the early 1990's.

--- End quote ---

I will politely comment on that..
In my opinion it is not "between CRT and DSO scope"  but how industry was making and looking at scope usage (and design that stems from that) 30 years ago as opposed to today. And also technology gap.

Perfect impulse response (that was industry standard many years ago) proved to be suboptimal for many other things people do with scopes.
For example my Keysight MSO3104T has practically flat BW until it's 1GHz BW. If I'm looking at signal in frequency domain (FFT) or verifying amplitude of 868 Mhz oscillator it will show excellent amplitude accuracy. All while having "only" 450ps risetime.  If I wanted to do the same on scope with Gaussian response (that would yield perfect impulse response) i would need at least 2GHz scope for same amplitude flatness up to 1 GHz.

There are digital scopes that have Gaussian response. They are just not common anymore.

--- End quote ---

Yes when i said "CRT" i was talking about the old types back in the day before digital ones became more popular.

However, when i tested a friends scope and included the rise time of the input pulse, i got the same bandwidth as was tested in this forum somewhere that i read.  The test here was 30MHz and the measurement i did with the pulse was 30MHz, but that obviously was a low end scope.

pdenisowski:

--- Quote from: MrAl on December 02, 2022, 02:09:29 pm ---Yes that is the way i have been doing it, sweeping a sine was up in frequency until you see the amplitude drop to 1/sqrt(2) which is the -3db point.
Since to do this test for a higher frequency scope like 100MHz you have to have a signal generator that can produce sine waves up to at least 100MHz. 

--- End quote ---

Yes, this is how I measure oscilloscope bandwidth, although I realize that most hobbyists don't have signal generators that go up to the GHz range :)  If I were going to to a video on this  topic ("Understanding Oscilloscope Bandwidth Measurements" or similar), I would want to include approaches that don't require (very) high frequency generators.

I also think it's helpful to measure beyond the nominal 3dB bandwidth in order to graph out the frequency response (Gaussian, flat, something in between).  As some others have commented, a scope is sometimes useful (well) past its 3dB bandwidth -- such as when measuring a reasonably pure sinusoid -- as long as you take the decrease in level accuracy into consideration.  This also requires graphing the frequency response past the 3 dB point.

Another reason to graph this out is that the frequency response may have (non-trivial) ripples or other irregularities even within the 3 dB bandwidth.  One advantage of scopes with a Gaussian frequency response is that they tend to have lower passband distortion than a flat / brick wall response.
mawyatt:

--- Quote from: pdenisowski on December 02, 2022, 02:32:14 pm ---
Another reason to graph this out is that the frequency response may have (non-trivial) ripples or other irregularities even within the 3 dB bandwidth.  One advantage of scopes with a Gaussian frequency response is that they tend to have lower passband distortion than a flat / brick wall response.

--- End quote ---

Agree, any "ripples" in the amplitude response usually involves "ripples" in group delay, and why "Guassian" or Bessel (max flat Group Delay) responses are desirable. Since a scope is often employed to monitor digital signals, for example the well known "Eye Diagram", the instrument of measure should not introduce significant artifacts in the measurements, since the usual objective is to visualize the waveform of interest and not be significantly "colored" by the measurement instrument. This is where "equalization" comes into play, as folks do in digital channel communications to reduce ISI.

Since you are involved with R&S and knowledgable regarding DSO behavior, do you know if they employ "equalization" within the scope to help normalize the response? Would think this might be a post ADC FIR Equalization "Filter".

BTW excellent oscilloscope video :-+

Best,
pdenisowski:

--- Quote from: mawyatt on December 02, 2022, 03:24:08 pm ---Since you are involved with R&S and knowledgable regarding DSO behavior, do you know if they employ "equalization" within the scope to help normalize the response?
--- End quote ---

Yes, we use DSP-based correction filters to provide a "flat" frequency response.


--- Quote from: mawyatt on December 02, 2022, 03:24:08 pm ---BTW excellent oscilloscope video :-+

--- End quote ---

Thanks!  I've done several videos recently that explain various "intermediate" level scope topics (like ENOB, acquisition rate / blind time, etc.) and I have several more similar videos (on triggering, sampling rate, FFT, etc.) in the works. 

I cover most of our major T&M product lines (sig gens, spec ans, VNAs, scopes, power sensors, EMC, etc.) here at R&S, so unfortunately I can't spend all of my time on scope-related content. :)   For example, I just finished a webinar and whitepaper on EVM (error vector magnitude):

https://www.rohde-schwarz.com/us/solutions/test-and-measurement/rf-microwave-components/amplifiers/white-paper-understanding-evm_256149.html
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