Which bandwidth should I choose? - Page 1
Forum section: electronics / beginners

author: EdEditz
author_rank: Contributor
timestamp: May 26, 2017, 07:38:38 pm
content: Hello, My name is Eddy and this is my first post on this forum. I'm new to the world of digital oscilloscopes (I have an old Tektronix 545 with 102 vacuum tubes in it but it's broken) and am looking to buy my first scope. Now I work a lot with RF circuits and I want to be able to see a 100MHz sine-wave on my scope with reasonable quality. Now I have my eye on either the Rigol DS1054Z that Dave says is the best bang for buck you can get, but that's 50MHz (and I don't trust my capability to hack it) or the Siglent SDS1202X which is 200MHz. My question is, if I go for the Rigol, will I be able to view a 100MHz signal with some reasonable resolution?

author: sokoloff
author_rank: Super Contributor
timestamp: May 26, 2017, 07:43:41 pm
content: The hack is dead-easy. It literally took me 15 minutes to read the thread and 5 minutes to execute. (I did it over ssh, which I think is easier/faster than doing it via the keypad.)Neither of those scopes will be suitable for 100MHz work. In general, you want a scope bandwidth spec of at least 5x the highest frequency you will use.

author: tautech
author_rank: Super Contributor
timestamp: May 26, 2017, 07:49:59 pm
content: Quote from: EdEditz on May 26, 2017, 07:38:38 pmWelcome to the forum.There's a 100 MHzdifferencein the 3dB point between these DSO's, no surprise which one will display a 100 MHz signal more accurately.BTW, don't you mean SDS1202X-E?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 26, 2017, 08:04:39 pm
content: Just wondering why you would like to see a 100MHz sine on your scope? If you are working with RF a spectrum analyzer would make a lot more sense, because you will never be able to for instance tell the harmonics content just by looking at your scope screen.

author: Shock
author_rank: Super Contributor
timestamp: May 26, 2017, 09:06:52 pm
content: I don't own a Siglent so I cannot endorse them, Tautech can send me one, but he has never offered.The Rigol DS1054Z is upgraded by putting in one code, with that code you not only unlock the bandwidth but also all the other advanced software options Trigger, Decoder, Mem Depth, Recorder. It's trivial to do but entirely up to you. If you want to use the $1000 or so of free software options for more than a few days you can and if you change your mind you can reset it. It has that hardware capability for 100MHz built in so it's purely a code that enables the software.But yeah the DS1054Z is a nice scope, it's not perfect and no oscilloscope is. When you're making comparisons take note that the DS1054Z is a 4 channel oscilloscope.

author: yada
author_rank: Frequent Contributor
timestamp: May 27, 2017, 01:18:51 am
content: So if you are doing RF stuff in the 0-999 MHz range you wnt something that can display 5 times that?I always thought the band width was a measure of how much information was contained in the signal you were feeding into it, but it literally means what frequency the scope will go up to?What happens if you put a 100MHz sine wave into a 50MHz scope? Or what would happen to the trace on the screen if you adjusted 50MHz signal up to 51MHz? Would it just stop displaying a signal?

author: sokoloff
author_rank: Super Contributor
timestamp: May 27, 2017, 02:01:01 am
content: The scope doesn't "know" what frequency the signal is you're sending it. (And in fact, a square wave contains multiple harmonics higher than the frequencies that we measure the square wave to be. You want to be able to see the 3rd and 5th harmonic to get a good representation of the square wave.)A scope's bandwidth rating is generally the point at which the measured amplitude of a sine wave of that frequency is down -3dB from the truth. It doesn't suddenly "stop working" like you might be thinking of an old CRT monitor or something.

author: EdEditz
author_rank: Contributor
timestamp: May 27, 2017, 07:15:31 am
content: I obviously am confused about the meaning of bandwidth. Thanks for your replies.

author: EEVblog
author_rank: Unknown
timestamp: May 27, 2017, 07:19:21 am
content: Quote from: tautech on May 26, 2017, 07:49:59 pmThat one is brand new and would likely be the best bang-per-buck for 200MHz.But as others have said, the requirement to see 100MHz RF sine waves is a bit puzzling. But for sure you want a 200MHz scope for accurate 100MHz sine wave measurement. But the Riogl will display up to 150MHz or so IIRC with reduced amplitude.

author: EdEditz
author_rank: Contributor
timestamp: May 27, 2017, 08:40:42 am
content: That is the one I meant btw. the SDS1202X-E. I think that's the one I will go for. What I meant with being able to see a 100MHz sinewave is just to not have the signal be deformed by lack of resolution. You know, that the sinewave can't be accurately reproduced by the scope but I know now from your answers that that won't happen not even with a 50MHz scope. May sound like a stupid question to begin with but I'm just not familiar with digital scopes. Hopefully that will change soon

author: vk6zgo
author_rank: Super Contributor
timestamp: May 27, 2017, 02:09:00 pm
content: Quote from: EdEditz on May 27, 2017, 07:15:31 amBandwidth, like happiness in the old song, is "different things to different people".(1) Occupied spectrum of a signal--- for instance, if displayed on a Spectrum Analyser, an AM Medium Frequency Broadcast Station in Australia will occupy around 18kHz of spectrum around the Medium Frequency carrier ( other countries may vary), whereas a VHF Stereo Broadcast transmission will occupy around 200kHz.(2)The range of frequencies which may pass through a device, whilst the device maintains the same gain or loss-- the "bandwidth" of the device.(3)As Yada says above," a measure of how much information was contained in the signal".In simple amplifiers & modulation systems, there is a direct relationship between this & bandwidth as in (1), but with sophisticated modulation systems it is possible to send a lot more information with the same occupied spectrum.With the proliferation of computer equipment, the original meaning of (1) & (2) have been blurred due to the propensity of IT people to refer to bandwidth as"bit rate or throughput, measured in bits per second (bit/s)".Marketing people, of course, stomp across every one of these meanings with hobnailed boots!https://en.wikipedia.org/wiki/BandwidthEverybody leapt in with both feet to lecture the OP about the instrument bandwidth necessary to display 100MHzsquare waves.Apparently, they didn't notice the reference toRF.Far from needing to see harmonics of the fundamental, out to5f, in RF work, we want to minimise harmonics!

author: EEVblog
author_rank: Unknown
timestamp: May 27, 2017, 02:45:17 pm
content: Quote from: EdEditz on May 27, 2017, 08:40:42 amModern digital scopes are no different than analog scopes. The sample rate (along with SinX/X interpolation) is high enough not to cause the problem you are alluding to.So a digital scopes bandwidth roll off will basically be similar to that of an analog scope, as will be the trigger system as well.

author: rstofer
author_rank: Super Contributor
timestamp: May 27, 2017, 03:28:55 pm
content: Quote from: EdEditz on May 27, 2017, 08:40:42 amBased on the videos from The Defpom, that looks like a very nice scope. I have the DS1054Z so I am not looking for another scope but, if I was, that SDS1202X-E would be on the list. Alas, for my needs, I think 4 channels outweighs the benefit of the 200 MHz bandwidth.There is another thing about the SDS: It is brand new and there are teething problems. Siglent needs to put out a revision to the firmware but it hasn't happened so far. Well, to be fair, the scope has only been around for a month or so. It makes more sense to hold off a bit and wait to find out what happens in the field. Fix several things instead of just a couple.The DS1054Z has been around for a couple of years and has had a few revisions. The bugs are pretty much ironed out. That doesn't make the scope 'perfect' but it is vastly improved.You can search YouTube for 'TheDefPom SDS1202' to find the two review videos.

author: Electric2000
author_rank: Newbie
timestamp: August 25, 2017, 03:12:20 am
content: What is the 3db bandwidth of rigol ds1054z?

author: tggzzz
author_rank: Super Contributor
timestamp: August 25, 2017, 07:59:09 am
content: If you are interested in "RF work", then a scope is probably the wrong tool. As a gross simplification, scopes are designed to look at the "rough" waveform shape of large amplitude signals - but RF circuits are dependent on the linearity of small amplitude signals including those that are far away from the frequency of interest. Much RF work is based on frequency domain analysis and specifications, but scopes operate in the time domain (exception: the baseband signal being transmitted/received).The concept of "you need 5*fc" to see a digital signal is misleading to the point of uselessness. For a scope operating in the time domain all that matters is the rise/fall time; the period is completely irrelevant. The rule of thumb is that BW=0.35/tr.

author: b_force
author_rank: Super Contributor
timestamp: August 25, 2017, 11:47:55 am
content: Quote from: EEVblog on May 27, 2017, 02:45:17 pmThat's what I used to think, but according to this post and video, the roll-off of digital scope is MUCH steeper.https://www.eevblog.com/forum/testgear/digital-oscilloscope-behind-the-scenes-presentation/What bandwith you need depends on the signals you want to look at.The 5*fc is very roughly a rule of dumb for square wave type signals, simply because of harmonics.Although a square wave with only 5 harmonics still doesn't really look like a square wave.You'd rather need the 9th harmonic or so.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 26, 2017, 01:47:09 am
content: Quote from: b_force on August 25, 2017, 11:47:55 amThey used to teach us that the7thharmonic was the bare minimum to get a reasonable square wave.

author: Brumby
author_rank: Unknown
timestamp: August 26, 2017, 03:29:52 am
content: Some quick charting in Excel....Pick your poison:

author: 2N3055
author_rank: Super Contributor
timestamp: August 26, 2017, 07:09:06 am
content: Quote from: Electric2000 on August 25, 2017, 03:12:20 amPeople measured around 120-130 MHz for -3dB...

author: djnz
author_rank: Regular Contributor
timestamp: August 26, 2017, 09:12:07 am
content: Quote from: Brumby on August 26, 2017, 03:29:52 amOr check this out:https://sara.ng/apps/square-wave/

author: b_force
author_rank: Super Contributor
timestamp: August 26, 2017, 10:45:53 am
content: Quote from: vk6zgo on August 26, 2017, 01:47:09 amThat's correct, the bare minimumSo you want to have just a little more than that.

author: VK5RC
author_rank: Unknown
timestamp: August 26, 2017, 12:41:31 pm
content: Quote from: PA0PBZ on May 26, 2017, 08:04:39 pm+1 re Spectrum Analyser for RF work; as a Ham I would use a Spec Analyser much more often. For a bit more money both Rigol and Siglent make a nice (up to 1-2GHz -edit 3GHz - Thanks tautech) Spec An with built in tracking generator. really useful for filters, testing amplifiers, antennas etc.As a fellow Tek 545 owner - look after that beauty!RobertEdit corrected freq range for SA

author: b_force
author_rank: Super Contributor
timestamp: August 26, 2017, 12:43:40 pm
content: Quote from: PA0PBZ on May 26, 2017, 08:04:39 pmRinging noise (and overshoot) of switching MOSFETs for example?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 26, 2017, 02:16:13 pm
content: Quote from: b_force on August 26, 2017, 12:43:40 pmAs TS said: "Now I work a lot with RF circuits and I want to be able to see a 100MHz sine-wave"

author: b_force
author_rank: Super Contributor
timestamp: August 26, 2017, 02:36:15 pm
content: Quote from: PA0PBZ on August 26, 2017, 02:16:13 pmOh, sorry, it was a specific questions towards the TSI thought you asked just a general question

author: rs20
author_rank: Super Contributor
timestamp: August 26, 2017, 02:42:20 pm
content: Quote from: djnz on August 26, 2017, 09:12:07 amBoth of these fail to simulate the phase roll-off; the oscilloscope behaves as if it has an RC filter. Hence why your pictures have post post- andpre-edge ringing, whereas a real scope only has post-edge ringing.Not to mention that real scopes have an amplitude rolloff as well; by plotting only the sum of the first N harmonics and completely leaving out the rest; you're showing what a scope with a perfect brickwall filter would look like, not the typical first-order RC.

author: cdev
author_rank: Super Contributor
timestamp: August 26, 2017, 03:16:24 pm
content: For me I guess the question of whether I bought a new scope would hinge on whether I was using it for work or just for my hobby.You can do a lot with a cheap RTLSDR dongle as far as seeing RF and its spectral content. You know that, right?Quote from: EdEditz on 2017-05-26, 13:38:38

author: tggzzz
author_rank: Super Contributor
timestamp: August 26, 2017, 04:36:15 pm
content: Quote from: b_force on August 26, 2017, 10:45:53 amConsider a 1000Hz signal coming from a modern logic gate (e.g. 74lvc1g*) with a rise/falltime of ~1ns. You want to see whether the signal integrity is valid. What bandwidth do you need? Hint: it isn't 7kHz!If you think that example isn't relevant to RF, then consider a cheap single-chip 4.4GHz frequency synthesiser with a VCO operating from 2.4-4.4GHz. If you tell it to generate a 40MHz output, that will be done by a digital divider, so the output will be a square-ish wave with edges that are sufficiently fast to generate a 4.4GHz sine wave.More anthorpomorphically, consider that when displaying a transition, the scope neither "knows nor cares" whether the next transition occurs in 100ns, 1us, 1ms, 1s, ...Summary: who cares about harmonics of boring square waves; all that matters is the transition time.

author: Brumby
author_rank: Unknown
timestamp: August 26, 2017, 04:39:22 pm
content: Quote from: rs20 on August 26, 2017, 02:42:20 pmThe charting was simplistic. I make no excuses about that.What Iamfascinated by is how few times I have heard people ask about the pre-edge ringing. After all - how does the system know precisely when the transition isgoingto occur?

author: tautech
author_rank: Super Contributor
timestamp: August 26, 2017, 08:59:31 pm
content: Quote from: VK5RC on August 26, 2017, 12:41:31 pmSmall correction.

author: Brumby
author_rank: Unknown
timestamp: August 27, 2017, 02:38:36 am
content: Quote from: tautech on August 26, 2017, 08:59:31 pmThat reminded me of a "small correction" needed in an article in Electronics Australia many moons ago...It talked about a magnetron operating at a frequency of 3Hz. I think they either missed a character in publication - or that's one heck of a magnetron!

author: rstofer
author_rank: Super Contributor
timestamp: August 27, 2017, 04:30:52 am
content: Quote from: Brumby on August 26, 2017, 04:39:22 pmThe Gibbs Phenomenon? Here's where math hits the screen!https://en.wikipedia.org/wiki/Gibbs_phenomenonI managed to get a great example of this on my DS1054Z the other day and I didn't write down how I did it. I was not expecting such a textbook display.

author: rs20
author_rank: Super Contributor
timestamp: August 27, 2017, 07:31:13 am
content: Quote from: rstofer on August 27, 2017, 04:30:52 amAgain, that page mentions nothing about phase delays or RC filters. The Gibbs phenomenon is observed in a mathematical study of Fourier series where causality is not a requirement. However, in the world of real oscilloscopes, I claim () we are slightly constrained by causality, anda zero-phase filter cannot be causal.Quote from: rstofer on August 27, 2017, 04:30:52 amAre you saying that you saw ringingbeforethe edge (as opposed to after the edge)? As Brumby has expressed, that's impossible unless a) the oscilloscope can predict the future or b) it's an artifact of (non-causal) signal processing in the oscilloscope, or c) it's not really pre-edge ringing, but some sort of strange echo artifact that looks exactly like pre-edge ringing but is actually from previous edges.