#70: Digital Oscilloscope Bandwidth Measurement - direct and using risetime

This is a followup video to my last one which discussed the basics of oscilloscope bandwidth. This video answers a viewer question about digital scope BW and usability above the rated bandwidth, as well as making the 3dB bandwidth measurement two ways - directly, and through a risetime measurement. In each case, one has to be careful that the signal source is "known" to be flat over the frequency range of interest, or that the risetime of the test signal is either known, or significantly faster than the risetime of the scope response.

[@famossfla]

Another wonderful video. So easy to understand when you explain it. Wish I had 1/10th the knowledge you have. I was asleep in Tech school, but still learning in retirement. Thanks

[@conconmervin]

Man, you really are great in simplifying the explanation on complex concepts. I wish my professors during college were as good as you. I they were, I will be working in NASA by now. :) I like all your videos, keep it coming. Thank you.

[@scytales2548]

It's a pleasure de watch your very informative videos. Thanks.

[@davisgrier5162]

This is a great video that illustrates these concepts very well. Thank you.

[@spectralcodec]

Thanks for another great video! I have a 2465B and can't wait to see how high it goes!

[@joesooobob8034]

always useful at anytime!!!!!!!!

[@igsaturation]

As always, great video. Thanks for taking up the suggestion for the rise time measurement. I recall an old video of yours utilizing a higher wideband scope, maybe that scope doesn't have a Gaussian roll off and could serve to show were the 0.35/tr becomes less accurate.

[@Pwaak]

Very informative and interesting, Thank You!

[@RossYoungblood]

Thanks, needed to dust the cobwebs out of my brain. Was very useful. Nice equations, thanks!

[@opalprestonshirley1700]

Great video. Thanks.

[@sureshlingabathina]

Excellent

[@johnchung6900]

Thanks for the video :)

[@greekmovie]

Great demonstration. Keep up the excellent work. Thank you. de SV1OBT

[@mandigit]

Thank you for greate teaching video. What reason is for summing time as square root?

[@eclecticneophyte2581]

This is a very informative video, thank you for taking the time to produce it. How much error is significant if the generator frequency is 100, 200 or more hz off + or - ? Can I say, hope for reasonable results if I use one of the low-cost-from-China synthesized generators (allegedly good up to 4.4 Ghz), or even a small handy-talkie intended for use on say 440 Mhz? In my case, I need something in excess of 500 Mhz to test a TDS 3052B...not likely doable with the 440Mhz HT, but perhaps something else? Thanks.

[@rbmwiv]

My siginal gen only goes to 25mhz. What can I measure with that? Thanks excellent video

[@jimmyc451]

this is an awesome video but is there a difference between measured values vs displayed values on the scope? i have found in some scopes, the meter is better than the display

[@2bitornot2bit73]

How high of a square Wave frequency can this 100Mhz scope measure reliably?

[@36r37]

Hi, Awesome video(s)! [Q] I have a 70 Mhz AWG and a 200 Mhz DSO. I as trying to do some measurement of my own. Unfortuantly i can't reproduce you "first" approach (since the scope exceeds the gen.). So I Was looking at the second way, but i don't have the actual (real) Rt of my generator, the scope reports a Rt of 9ns. Is there anyway to acquire that number, or do i need some better gear (the AWG manual reports Rt should be < 10ns, but that tells me next to nothing .. :-) ? Thanks!

[@w2aew]

36R Given the fact that a first order estimate of your scope's risetime is 0.35/200MHz = ~1.8ns, and that this is substantially faster than the 9ns risetime you measured on the signal generator - the likelihood is that this is pretty close to the actual risetime of the generator. So, it's not fast enough either. Two other things you can do.. You can build one of the schmitt trigger oscillators using a 74AC14 as I've shown in other videos. This gives you a signal with about 2ns risetime that you can use for testing. Or, even easier, since you said the scope is a DSO (digital) scope, it is highly likely that the scope has an FFT function. If you simply run an FFT on the noise of the scope (just turn on FFT using the most sensitive vertical setting), and look at the bandwidth that is shown in the resulting FFT plot.

[@36r37]

***** Thanks! I built the circuit, and indeed it got a 2 ns rise (what a difference in signal when looking at the AWG). Awesome! Now i was checking your FFT option. I tried it, but i got lost. I disconnected all input cables, set the scope up as 2 ns / 500uV (thats most sensitive i guess). Then i turn on FFT, it show "20.0dB 625Mhz/div. Sa 25Ga/s and Center 4.375Ghz". I have not a clue what to look at! :-) I did had a look at your FFT video (very nice), but related to this i'm missing it.

[@w2aew]

36R You probably want to slow down the time/div setting until the Center frequency shows something like 200MHz or so. Then, you should see the noise floor be higher on the left side, starting at DC, then at some point it will drop lower - at a frequency approximately equal to the scope bandwidth.

[@mickeyaslam5272]

Any suggestion on the 50 Ohms coax cable with flat response from Dc to up to 1GHz and known insertion loss?

[@w2aew]

Depends on your needs/purpose. For interconnecting of test equipment, the very popular and inexpensive RG-58 will work fine as it only has about 0.2dB loss per foot at 1GHz.

[@gearstil]

At minute 10.05 you inverted 2.7372 (that means 1 / 2.7372) and you obtained 365.33 (multiplied by 0.001). That means that the formula for the BW should be (0.35 / trScope ) * 1000

[@w2aew]

Yes, there are a few "factors" assumed: Bandwidth (in MHz) = 0.35 / risetime (in ns), or risetime (in ns) = 0.35 / bandwidth (in MHz). Since 1/1MHz = 1000ns, that's where your factor of 1000 comes from. To simplify things with this formula, we use MHz and ns and ignore the 1000x factor.

[@sandeepkumar-xn3ed]

sir i have a basic rc circuit which will be used as photo-diode detector circuit, how can i measure its bandwidth using oscilloscope?

[@w2aew]

You'll need more than just a scope - you will need some sort of input signal to your circuit so that you can measure its response.

[@ernestb.2377]

Hi. I have now (for the first time) my own scope 🙂 It is a Kenwood CS-5165. It has A and B sweep, and I try to examine a fast rising edge. My problem is that when working at high speed Time Base positions of the Sweep Time that my trace intensity is very weak. I have just opened the case and adjusted the intensity according to the Service manual. It is better now but still weak light. The same thing when using the x10 MAG time base. Then I turned the Adj. pot to it's max. It gets some better but still not satisfy for those fast signals. I believe the same problem yesterday with very small signals, like something about a few tens of mVpp. Maybe you have some tips on why and how this happening, and maybe it is to expect. I can not tell us very little to no experience with analogue scopes. Maybe the scope got deteriorate after what is it >30 years? ...

[@w2aew]

Definitely to be expected on an analog scope. The faster the sweep, the dimmer the trace. And of course, 30 yrs of aging and use will also cause some degradation.

[@ernestb.2377]

@@w2aew No remedy?? I know, it is the technology, the beam has to sweep fast and has not enough time to leave a high intensity trace/mark... Even though I like those analogue scopes. Yesterday I was playing with an analogue video signal (monochrome and color), and if you know how (with B-sweep) you can examine every Line of the Frame. The trigger was really very stable. The trace is visible just enough to work with it. Tektronix MDO3014 triggers also nicely on video, you can even choose the line number to trigger on.

[@w2aew]

@@ernestb.2377 The nature of the beast of an analog scope with 30 years of age on it.

[@juliand1053]

I get a weird behavior when I measure bandwidth on TEK Scopes (TDS, TPS 1000/2000 Series): the PK2Pk Amplidtude at the guaranteed BW end (50, 100, 200 MHz) is even HIGHER than the start frequency (usually 50 KHz). I use a Fluke 6061A as generator. Of course, I also use a 50 Ohm termination. Does anyone has a clue why this is happening? thanks

[@w2aew]

This is odd behavior. While it is certainly possible that a scope could be misaligned and cause this problem - it is highly unlikely that all three of your scopes suffer the same problem. Thus, I think you need to carefully examine your test setup, equipment and methodology.

[@satrah101]

Hello all, if i push a scope past the range of what it was designed to do will this reduce to life of the equipment. I know this was only for educational information. Taking this reason from a car it can start in second gear but best result in to put it is first gear. this would be to same for oscilloscopes when testing. Just having an educational guess here may be wrong. have never owned an oscilloscope. Thanks in advance.

[@w2aew]

No damage. Just might not trigger properly, or display all the correct details of the signal.

[@w2aew]

No damage. Just might not trigger properly, or display all the correct details of the signal.

[@satrah101]

Thank you for the quick reply going to buy an oscilloscope soon i had to check first. still deciding on type at this stage, got three months, looking at a price about 1000 t0 1500 dollars. All the best for your channel.

[@freddiemortos8519]

Just a newbie question sir regarding oscilloscope probe bandwidth. How many Mhz of probe (@ X1) do I need for a 50Mhz analog bandwidth oscilloscope?

[@w2aew]

The answer is - you can not get a 1x probe that will operate at 50MHz. Any 1x probe will have too much input capacitance to work that high. It's easy with a 10x probe to get 50MHz.

[@freddiemortos8519]

I have just bought a Uni-T oscilloscope with a 50Mhz bandwidth and it came with a probe that have only 6 Mhz at 1X and 60 Mhz at 10X. I just want to ask sir,is this enough for this kind of oscilloscope bandwidth? Or do I need to buy another probe with higher bandwidth capacity? thanks for a quick response sir :)

[@w2aew]

Freddie Mortos That's all you'll be able to accomplish with a 1x probe - period. With the 60MHz BW in the 10x position, you'll get the full 50MHz BW that the scope offers. The probe will not increase the available BW of the scope. Be sure to view my video on the basics of 10x/1x probes. Basic 1X and 10X Oscilloscope Probe tutorial

[@freddiemortos8519]

Thank you so much sir. I am really newbie when it comes to oscilloscope. I've tested the scope that I have bought awhile ago with 1Mhz square wave signal and at 1X settings on the probe,and the signal is very much attenuated. It does not look as a square wave anymore. Maybe I have to study more about this. Anyway the oscilloscope model that I'm talking about is the UNI-T UTD2052CEL. Thank you again sir for your time responding my questions. I watched all your videos and they are all very informative.

[@w2aew]

Freddie Mortos 1X probes are only useful for low frequency signals, typically a few hundred kHz or less. What you're seeing is the low-pass filtering of the 1x probe, and the excessive capacitive loading affect on your circuit. Rule #1 - always use 10x probes unless there is a specific reason to use 1x, and your signal/circuit are appropriate (low frequency, etc.).

[@sambasivam3900]

Actually I am using 100Mz oscilloscope can I able to measure the rise time of the more than 100Mz signal

[@w2aew]

No, you can't. A 100MHz scope means that a signal at 100MHz will be measured as much as 3dB lower in amplitude, and higher frequencies will be attenuated even more. Since the rising and falling edges of digital signals contain high-order harmonics of the signal frequency - they'll be highly attenuated, thus will measure MUCH slower than their actual speed.

[@sambasivam3900]

Can you suggest the which range of scope will measure the 100mz signals which I have to use

[@w2aew]

General rule of thumb for measuring digital signals is that the scope bandwidth needs to be at least 5x greater than the highest digital clock frequency. This will let you get the 3rd harmonic will only a little attenuation. Also, be careful about mixing up digital clock frequency with digital bit rate. 100MHz clock rate is a 100MHz square wave and would ideally need a 500MHz BW scope or greater. But, a 100Mbit/sec signal is a maximum of a 50MHz square wave, which would need a 250MHz scope BW.

[@sambasivam3900]

why I can't measure 100Mhz rice time in 100Mhz oscilloscope it's there any reason for that explain me

[@w2aew]

Can you rephrase your question? The phrase "100MHz rise time" doesn't make sense. What is your question?

[@RossYoungblood]

Physics. The definition of 100Mhz bandwidth is the point where with a 100Mhz input signal the viewed amplitude is -3db down. (50% power 70% amplitude). The scope manufactures have a general rule of thumb that you need a scope with 3x the bandwidth of the rise time to avoid severe degredation.

[@sureshlingabathina]

I do not know rise time of my signal generator how do calculate

[@w2aew]

Measure it with the scope. If the risetime is slower than the scope's fastest risetime (predicted by scope bandwidth), then you have your answer. If the risetime is very close to the scope's risetime, then you really have no way of knowing the actual risetime of your generator unless you can use a faster scope or find the specification in the datasheet for the generator.

[@sureshlingabathina]

Thank you

[@abdulrahimevli260]

İ meausured -3db bw of the data on the matlab plot. İ get data from the oscilloscope.it is 170 hz but signal 8Vpp at the 100hz and 5.6 vpp at the 250 hz oscilloscope. İ get different bw.Why it is different bw in the matlab plot and oscilloscope half power frequency?

[@abdulrahimevli260]

But signal start to attenuate at the 170 hz on the oscilloscope and at the 250 hz decreased half power

[@w2aew]

@@abdulrahimevli260 It's difficult to say what is going on without knowing what MATLAB is calculating, and how it is doing it.

[@abdulrahimevli260]

@@w2aew i took data from the oscilloscope and plot it on the matlab. Which bw is true i could not determine

[@abdulrahimevli260]

But it could be calculated bw frequency(-3 DB bw) on the matlab, must be decreased half power at the same bw requency calculated on the oscilloscope

[@abdulrahimevli260]

But i took time interval in the matlab plot to measure bw(1/t)