Bandwidth check of my oscilloscope with simple means ? - Page 1
Forum section: electronics / testgear

author: Frank_MV
author_rank: Regular Contributor
timestamp: December 10, 2020, 07:53:50 pm
content: Hello,I would like to check the bandwidth of my oscilloscope (RTB2000- 100MHz), the amplitude level at -3dB.Unfortunately, I do not have a sine wave generator with constant amplitude.There is only a simple signal generator available 100KHz - 150MHz - whose amplitude is unfortunately frequency dependent.Is there a possibility with simple means ?br,Frank

author: Kibabalu
author_rank: Regular Contributor
timestamp: December 10, 2020, 07:59:37 pm
content: you need a rectangular pulse with a slew rate as fast as possible. Then you measure the rise time T_r (time from 10% to 90% of the magnitude). The formula BW = 0.35/T_r gives you the bandwidth your oscilloscope at least has. If the slew rate of your pulse was not fast enough to exceed the bandwidth of your scope, than the real bandwidth could be even higher.

author: tggzzz
author_rank: Super Contributor
timestamp: December 10, 2020, 08:18:33 pm
content: Why do you want it measure thebandwidthof a scope? For a scope the important thing is to preserve the fidelity of the waveshape.The best way to verify that is to examine the rise time of a step function (not an impulse). A clean transition and flat top is important, unless you know the actual waveform and are prepared to do the deconvolution yourself.A 100MHz scope will probably have a rise time of 3.5ns, so a waveform with a rise time of less that, say, 1ns will give you a good idea of the scope's response.Of course you can never measure a waveform with a scope, only with a scope plus probes. There are many pitfalls for the unwary associated with poor probing technique.

author: 2N3055
author_rank: Super Contributor
timestamp: December 10, 2020, 08:52:04 pm
content: Quote from: Frank_MV on December 10, 2020, 07:53:50 pmAs for simple check, no simple answer. You need leveled signal generator (or at least one that you know has decent amplitude accuracy) and sweep.My best advice is to go to tggzzz's web site in his signature and have a read.. He has lots of great info on scope use, probes and such. Quite a lot actually, so it will keep you busy for a time..

author: Fungus
author_rank: Super Contributor
timestamp: December 10, 2020, 09:02:07 pm
content: Quote from: Frank_MV on December 10, 2020, 07:53:50 pmYes. Turn on the "Rise time" measurement on your 'scope, set it to single trigger mode and touch the probe to a 5V supply to get a fast rising edge.For 100MhZ the rise time displayed should be around 3.5ns.(or 3V supply, or... battery, or...some low voltage supply)

author: tggzzz
author_rank: Super Contributor
timestamp: December 10, 2020, 09:10:24 pm
content: Quote from: Fungus on December 10, 2020, 09:02:07 pmDon't forget to take account of how the inductance of the lead inductance (~1nH/mm) and probe tip capacitance will affect the waveform!

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 07:58:59 am
content: Quote from: tggzzz on December 10, 2020, 09:10:24 pmNot much - it has to work at over 100MHz!

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 08:30:34 am
content: Quote from: Fungus on December 11, 2020, 07:58:59 amA 6"/15cm ground lead (or equivalent) and 15pF tip capacitance will visibly resonate at ~100MHz.

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 08:38:47 am
content: Quote from: tggzzz on December 11, 2020, 08:30:34 ama) A properly terminated BNC cable would be better, yes, but OP is asking for "simple means".b) Resonance won't affect the "rise time" measurement shown on screen - they discard the top/bottom part of the signal when they calculate that.

author: BravoV
author_rank: Super Contributor
timestamp: December 11, 2020, 08:39:07 am
content: Quote from: tggzzz on December 10, 2020, 08:18:33 pmRe-emphasizing with red highlight, as these easily will be ignored or missed by beginners.Guess where did I knew/learned that.

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 09:03:54 am
content: Quote from: BravoV on December 11, 2020, 08:39:07 amI wonder if there's a reason why oscilloscopes measure "rise time" by discarding the top/bottom 10% of the signal?

author: Berni
author_rank: Super Contributor
timestamp: December 11, 2020, 09:18:01 am
content: No the 10% 90% is because you can't possibly accurately tell where exactly that edge begins and ends. If you want to cheat the rise time then you define it as 20% to 80% (some do use this trick)Getting bandwidth from rise time is also not always completely accurate because it depends on what shape the frequency response has at the cutoff point. But the shapes that optimize a faster rise time out of the same bandwidth will also generally have more overshoot and ringing. Still the resulting bandwidth number is going to be about in the ballpark of +/- 20% so its good enough.As people have said square waves are better for testing oscilloscopes because it not only shows the approximate bandwidth but also shows if the cutoff response of the input introduces extra overshoot. A lot of scopes will have a little bit of overshoot and that is fine, but if the overshoot is something like 20% of the signal amplitude there is definitely something wrong.As a source for generating these fast rise time square waves its easiest to use a fast digital logic chip driving the coax trough a 50 Ohm termination. One popular circuit for doing this uses parallel Hex invertershttps://www.epanorama.net/circuits/tdr.htmlto produce a fast clean square wave (Typically used for TDR measurements of cables, but is also an excellent oscilloscope test signal)

author: nctnico
author_rank: Super Contributor
timestamp: December 11, 2020, 09:34:03 am
content: In reality using risetime is a very poor method because it assumes a certain filter response for the oscilloscope. This isn't standard so using the risetime may give false results. The only way to be sure is to use a levelled sinewave generator (together with a 50 Ohm coax cable and 50 Ohm terminator on the oscilloscope input if the oscilloscope doesn't have a 50 Ohm input mode).

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 09:53:54 am
content: Quote from: Fungus on December 11, 2020, 08:38:47 amOn most scopes it won't be correctly terminated, of course: it would be 50ohm//15pF. Whether that has an effect depends on the length of the cable - the standard standing wave stuff that I won't repeat here.The resonancewillaffect a risetime measurement, to some extent. The sharper the resonance the greater the overshoot - and correspondingly shorter time taken to reach 90% of the final amplitude.Another issue, especially with scopes that digitally process the samples, is the scope's internal filter response. That can be optimised (or pessimised!) in different ways for different purposes.

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 10:05:36 am
content: Quote from: Berni on December 11, 2020, 09:18:01 amCircuit with imperfect but useful measurements (<300ps risetime):https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 10:31:55 am
content: Blah, blah.I just did it with a capacitor shoved into an Arduino and I got very consistent times around the 3.5ns mark.(on my 100MHz Micsig)

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 10:38:26 am
content: Quote from: Fungus on December 11, 2020, 10:31:55 amWhat did you do with the capacitor?I'm not surprised about the risetime; modern jellybean CMOS is faster than most people imagine. Especially those that toggle the output at 1Hz and think they have a 5Hz bandwidth waveform (5*fundamental frequency, cough).

author: 2N3055
author_rank: Super Contributor
timestamp: December 11, 2020, 10:42:56 am
content: This is second topic with exactly same question from OP.This one also devolved into discussion about creating fast pulses, like most of them..Again, answer to OP question, no, there is no simple improvised way to measure -3dB point of scope front end without leveled signal generator.If you had 200-500 signal generator with decent amplitude accuracy across the frequency (like SDG6000X or similar) you could fairly well estimate what the curve is.OP never explained what he wants to accomplish or why he asked the question. Without that it is hard to help him.

author: Berni
author_rank: Super Contributor
timestamp: December 11, 2020, 10:50:18 am
content: Quote from: tggzzz on December 11, 2020, 10:05:36 amYep it works great for testing oscilloscopes up to about 500MHz, for higher up it becomes too slow.For going faster i would recommend just buying Leo Bodnars fast pulse generator:http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=295I have two of them (SMA and BNC version) and they are great little generators for making ridiculously fast pulses. Well worth the pretty low price of them. Being small is also a nice thing since you can bring the generator to where it is needed rather than using a cable (the regular garden verity coax cable really degrades the rise time of pulses that are this quick)EDIT:Also one way to level a wobbly freqency response signal generator is to use one of these:https://www.keysight.com/en/pd-1000001913%3Aepsg%3Apro-pn-8472B/low-barrier-schottky-diode-detector-10-mhz-to-18-ghz?cc=SI&lc=engThis takes in a high frequency RF signal and AM demodulates it into a DC voltage while having a pretty flat and stable frequency response. So you can use this DC voltage as feedback to adjust the signal generator to the same level at various frequencies.These RF detectors can sometimes be picked up cheep on the used market.

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 10:53:54 am
content: Here's the results from a coin cell battery held like this:The capacitor was 100% necessary to get a good time. Without it I was getting results in the uS range.Results are much less consistent for some reason (maybe because I'm touching the wires and both sides of the battery) but it can be done.Here's my best one:

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 10:56:29 am
content: Quote from: tggzzz on December 11, 2020, 10:38:26 amI stuck it into the +5V and GND pins as a place to clip the probe onto.(no software needed!)The Arduino was the closest thing I had to a power supply on the table.

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 10:59:35 am
content: Maybe the simplest method of all is simply to believe what a reputable manufacturer prints on the front of the device.(nb. it'll be a bit more than the labelled value - because they want to be sure...)

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 11:05:55 am
content: I'd be interested to see what people can get using this cowboy technique on higher bandwidth scopes.

author: Elasia
author_rank: Frequent Contributor
timestamp: December 11, 2020, 12:26:42 pm
content: Someone needs to stay out of the special candies...Case in point to not always go by what the mfg says... Siglent SDS2100x+.. can ONLY be legitimately raised to 350MHz but actually has a front end on all units that goes to 500MHz+, typically -3db of around 580-620MHz.The best way to measure just the scope itself is to get one of leo's pulse gens you can directly attach and once you have a reference you can then attach your probes and see what they are rated for. You can also check things like bnc cables... half the time people dont know just how horrible they are. On the other hand most of this is moot if you are dealing with 100MHz and lower

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 12:40:32 pm
content: Quote from: Elasia on December 11, 2020, 12:26:42 pmSure, we all know what thebestway is but OP wants "simple means".(and his 'scope is 100Mhz)Quote from: Elasia on December 11, 2020, 12:26:42 pmIt worked, didn't it?

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 12:48:21 pm
content: Quote from: Elasia on December 11, 2020, 12:26:42 pmDirect attachment also avoids the VSWR issues associated with the mismatch to 50ohms//15pF. There's a reason probe cable is very lossySuch step generator tools are (arguably) more valuable in that they can show you the consequences of poor probing technique.QuoteCoax (and other) transmission lines have a well-defined attenuation-vs-frequency characteristic, and the attenuation rises quites steeply with frequency. Pity the engineers that had to span the Atlantic ocean with coax cable; the repeaters had a lot of high frequency pre-emphasis. (Plus the TAT7 cable's impedance was unavoidably 61.8ohms, IIRC)

author: vk6zgo
author_rank: Super Contributor
timestamp: December 11, 2020, 12:57:41 pm
content: Quote from: 2N3055 on December 11, 2020, 10:42:56 amIf you have a decent VTVM with an RF probe, you can look at spot frequencies across the band of interest, adjusting the signal generator to the same level for each spot, which is how we did things back in the day, when absolute accuracy of sig gens was more the exception than the rule.The beauty of it is that the device you use to check the level doesn't need to have high absolute accuracy, just good frequency response accuracy.Of course, if you had an RF probe for use with a DMM, with a known accuracy, you could do the same.PS:-You could also use the precision detectorBernireferred to in the edit to his post 18.

author: radiolistener
author_rank: Super Contributor
timestamp: December 11, 2020, 01:00:15 pm
content: Quote from: Fungus on December 11, 2020, 10:53:54 amHere is my result. I connected lab PSU with 12.9 V to oscilloscope BNC with 1 meter wires with banana jacks.Lab PSU has some uF range capacitor on the output.I think this happens due to wire inductance.

author: radiolistener
author_rank: Super Contributor
timestamp: December 11, 2020, 01:06:58 pm
content: Quote from: Berni on December 11, 2020, 10:50:18 amNice device, I want it, but very expensive.

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 01:18:56 pm
content: Quote from: Fungus on December 11, 2020, 10:53:54 amHow do you use your scope to measure conductance?(That's a pet peeve of mine)

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 01:34:30 pm
content: Quote from: vk6zgo on December 11, 2020, 01:31:57 pmYes, as I asked the OP in reply #2.It has only been a day; let's hope he will return with some comments.

author: nctnico
author_rank: Super Contributor
timestamp: December 11, 2020, 01:36:43 pm
content: Quote from: Elasia on December 11, 2020, 12:26:42 pmBig fatNOSee my earlier reply.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 11, 2020, 01:46:19 pm
content: Quote from: tggzzz on December 11, 2020, 01:18:56 pm1 uSiemens would be about 1M, so you could use the 'scope as the null detector for a bridge.

author: 2N3055
author_rank: Super Contributor
timestamp: December 11, 2020, 02:01:35 pm
content: Quote from: Elasia on December 11, 2020, 12:26:42 pmThat is absolutely wrong. You can officially buy 500MHz bandwidth, but officially you can upgrade only from 350Mhz:Siglent SDS2000XP-4BW50 Upgrade 500 MHz 4 channelBandwidth Upgrade from 350 MHz to 500 MHz for the SDS2354X Plus. Only in interleaving mode For this upgrade the latest firmware is required.Nr: SDS2000XP-4BW50EAN: SDS2000XP-4BW50Also, by using Leo's pulser, you can absolutely make a great verification of scope's risetime. Bandwidth is measured by sweep with leveled gen. Period.If you upgrade the scope, you can use fast pulser to check whether upgrade went through, because you will see faster risetime. And that is all.Sometimes, some people don't really need to know bandwidth, they only need to check how fast is their scope in showing fast digital edges. That is what pulser is born for.

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 02:49:16 pm
content: Quote from: 2N3055 on December 11, 2020, 02:01:35 pmUsually if you are interested in bandwidth/frequency, then it is better to use a frequency domain tool such as a spectrum analyser.(Or, say it quietly, a scope with an FFT)

author: Elasia
author_rank: Frequent Contributor
timestamp: December 11, 2020, 03:10:48 pm
content: Quote from: 2N3055 on December 11, 2020, 02:01:35 pm

author: 2N3055
author_rank: Super Contributor
timestamp: December 11, 2020, 03:11:14 pm
content: Quote from: tggzzz on December 11, 2020, 02:49:16 pmSometimes amplitude is important...But for strict frequency domain measurements SA is primary instrument... That is not questionable.Although, i managed to some nice quick checks with a scope on occasion, at 432 MHz and 868 MHz telemetry transceivers.

author: 2N3055
author_rank: Super Contributor
timestamp: December 11, 2020, 03:16:03 pm
content: Quote from: Elasia on December 11, 2020, 03:10:48 pmOK so I didn't understand what was your point here.. Sorry, still don't, but I see that I responded to something else. Sorry for that.

author: Bud
author_rank: Super Contributor
timestamp: December 11, 2020, 03:41:31 pm
content: There is a neat method how to measure your oscilloscope's bandwidth that Keysight published in a couple of their Blogs. What you do is connect a fast pulser to input, apply Math "differentiate" function, then apply FFT to the result. This will produce a plot in frequency domain. Now use cursors to read the frequency point where level on the FFT plot drops to -3dB from the top.https://community.keysight.com/community/keysight-blogs/oscilloscopes/blog/2016/09/01/how-to-measure-your-oscilloscope-and-probe-s-bandwidth-yourself

author: JDubU
author_rank: Frequent Contributor
timestamp: December 11, 2020, 04:24:42 pm
content: I have a Rigol DS2000 scope that has a trigger output signal with a surprisingly fast risetime (faster than the scope's bandwidth).To measure the risetime of the scope, I set the scope to auto trigger on one channel and connect the trigger out to the input of the other channel with a BNC cable. Since it's auto triggering, the scope will free run and the trigger out pulse shows up, in sync, on the second channel. Set the second channel to measure risetime and calculate the bandwidth from it (BW= 0.35/risetime).Don't know if the RTB2000 has a fast trigger output risetime.

author: Fungus
author_rank: Super Contributor
timestamp: December 11, 2020, 05:04:27 pm
content: FWIW I just did another test using the little probe spring instead of the clip-on lead.As expected, the ringing was much reduced.(Note for newbies: The "ringing" on the display probably isn't in the signal, it's coming from the probe clip. Use the spring whenever possible! BNC is even better!)The measured rise times also seemed a teeny bit smaller, but there's not much in it.PS: I just noticed that the crocodile clips on these Micsig probes aren't detachable. That's annoying.Edit: Re-did the screen captures after I remembered the 'scope can measure overshoot.nb. For those people just joining the thread, the capacitor is just shoved into the GND and +5V on the Arduino. I'm just using it as a 5V voltage source.

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 05:12:01 pm
content: Quote from: 2N3055 on December 11, 2020, 03:11:14 pmMy 350MHz Tek 485 triggers nicely on a 1GHz signal. The beam deflection leaves something to be desired, thoughYou can also estimate a signal generator's risetime using a spectrum analyser:https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 05:19:22 pm
content: Quote from: Fungus on December 11, 2020, 05:04:27 pmThe ringing is the consequence of theinteractionbetween the scope probe's tip capacitance and a ground lead's inductance.You can shift the frequency higher by reducingeitherthe capacitance or the inductance.https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/BNC leads have a mis-match between the 50ohm cable impedance and the 50ohm//15pF scope input. That will form a comb filter at frequencies determined by the cable's length.

author: Berni
author_rank: Super Contributor
timestamp: December 11, 2020, 05:19:40 pm
content: Quote from: Bud on December 11, 2020, 03:41:31 pmYep that method works too. Here it is done using Leos fast pulser on a MSO9204HFirst trace is the Ch 1 input, next trace is the differential of that signal (Showing a peak slewrate of about 4 GV/s) and then the FFT of that showing the bandwith.And yes i know the cutoff does not look all that nice, but it is only a 2GHz scope so getting about 3.2GHz out of it at -3dB is not bad.

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 05:25:01 pm
content: Quote from: Berni on December 11, 2020, 05:19:40 pmIt would look even better without that ringing/overshootBut yes, it is fun to be able to visibly apply the theory of time-frequency duality, to practically measure things indirectly.How about not differentiating, and comparing the FFT to that shown here:https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/especially:

author: Frank_MV
author_rank: Regular Contributor
timestamp: December 11, 2020, 05:37:01 pm
content: Thank you for all your contributions, I will read everything at my leisure.Background: I have upgraded my RTB2004 from 70MHz to 100MHz and would like to check if it was successful.Br,Frank

author: Bud
author_rank: Super Contributor
timestamp: December 11, 2020, 05:42:10 pm
content: Quote from: Berni on December 11, 2020, 05:19:40 pm

author: David Hess
author_rank: Super Contributor
timestamp: December 11, 2020, 07:34:35 pm
content: As mentioned earlier:If you use a probe, which is not a bad idea since what really usually matters is the bandwidth at the probe tip, then the connection should be made coaxially to the probe tip without the ground lead.Leo Bodnar's pulse generator is an inexpensive source for suitable fast edges:http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=295Also:Another source for clean and fast edges is the horizontal and vertical sync outputs on pins 13 and 14 on a VGA connector.

author: Berni
author_rank: Super Contributor
timestamp: December 11, 2020, 08:13:23 pm
content: One thing to keep in mind when testing probes like this is that at high frequency the >10pF of probe capacitance starts to really matter and can drag down a 50 Ohm source, but up to 100MHz it's still pretty reasonable.Quote from: Bud on December 11, 2020, 05:42:10 pmWell compared to other Infiniium family scopes it is pretty low.But mostly was saying in terms of the 3.2GHz measured bandwidth compared to the 2GHz rated bandwidth. If you turn on filtering to get actual 2GHz of bandwidth out of it the ringing goes away.

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2020, 08:24:03 pm
content: Quote from: Berni on December 11, 2020, 08:13:23 pm15pF at 100MHz is ~100ohms. So much for such a probe being "high impedance 10Mohm"!A 10:1 " low impedance" resistive divider Z0 probe will be 500ohms input impedance, and have a multi GHz bandwidth.

author: David Hess
author_rank: Super Contributor
timestamp: December 11, 2020, 10:00:50 pm
content: Quote from: tggzzz on December 11, 2020, 08:24:03 pmIndeed, probe bandwidth is specified with a 25 ohm source impedance and a coaxial connection to the probe tip, which is unrealistic for most real world usage up to 100 or even 200 MHz because of the inconvenience.

author: rifkum
author_rank: Contributor
timestamp: February 10, 2024, 04:15:00 am
content: I know this is a very old thread... but this is an interesting topic and new scope owners may be interested in the topic subject...Alan is a very good teacher and here is his video on measuring scope bandwidth.https://youtu.be/Ly5pmYopEfU?si=f5Q5FRmHvXZYNN_PCheers

author: vk6zgo
author_rank: Super Contributor
timestamp: February 10, 2024, 04:22:25 am
content: Quote from: tggzzz on December 10, 2020, 08:18:33 pmSo all those times I looked at signals coming out of the 50 Ohm or 75 ohm output of a DUT, (with the correct termination on the 'scope), I was doing it wrong?Thread too old for my "smart comment" & besides,tggzzzhas already addressed this point in a later posting.

author: David Hess
author_rank: Super Contributor
timestamp: February 10, 2024, 02:16:24 pm
content: Quote from: vk6zgo on February 10, 2024, 04:22:25 amIf I had noticed it, then I would have addressed it.I measure waveforms using an oscilloscope without a probe all of the time. Indeed, I have never used a probe on my sampling oscilloscope.Closer to the context of tggzzz's statement, many of my oscilloscopes return identical results when measuring the bandwidth of the instrument with and without a probe, but this will not be the case with all oscilloscopes.