Why do oscilloscopes have bandwidth limits? - Page 1
Forum section: electronics / testgear

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 04:48:36 am
content: “… because an oscilloscope can be considered as a low-pass filter and this is the -3db cutoff frequency”, yes but why should it be the case that an oscilloscope is a low pass filter?“… because it’s good for the market if manufacturers can segment buyers according to need”, yes but that’s a way of taking advantage of bandwidth limits - not a reason for themIf my scope can sample at, say, 8GSa/S and I am happy to sample my waveforms with 8 sample points each, why do I not have a scope capable of measuring (some aspects of) a 1GHz digital signal regardless of the scope “bandwidth”?

author: ataradov
author_rank: Super Contributor
timestamp: November 02, 2022, 05:07:57 am
content: The fundamental reason is that designing a high bandwidth front end is not easy and there are physical limitations. There are further limits set by marketing, of course, but usually they are not too far away from the physical limits, especially for higher end models in the same range.And of course it is possible to design 1 GHz front end, but that would put the whole device into much higher price bracket.

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 06:36:08 am
content: From experience how far can you use an oscilloscope beyond it's bandwidth limit? Specifically, I'd like to compare the phase difference between in and out DDR2 data busses between an FPGA and a DRAM chip. This is important because the input signals will need to be captured inside the FPGA using a clock with a phase delay to the output clock. The slowest speed of the DRAM is 125MHz, which is starting to imply a 1GHz 'scope for many thousands of dollars. Just for one measurement! Will a 350MHz (soft upgraded) scope be any help to me at all?

author: ataradov
author_rank: Super Contributor
timestamp: November 02, 2022, 06:57:38 am
content: 125 MHz square wave will look like a sine wave with 350 MHz bandwidth. The rest of it would be filtered out. Remember the bandwidth of the scope is stated for sines, and if you look at the spectrum of the square wave, only the first harmonic would fit.And if you want to preserve the phase to that extent, you would need a way better equipment. This is not going to happen on consumer-level gear.

author: jonpaul
author_rank: Super Contributor
timestamp: November 02, 2022, 07:04:46 am
content: Not a simple question!SR, Bandwidth, risetime, aberration are all different but interrelated specifications.Digital scope vertical BW is a spec for the amplifiers.Read about the theory of ADC, SR, BW.Sample rate is not directly related, and there is a limit to the BW for a particular max SR, the Nyquist rate. BW < 1/2 SR.The filter shape for antialiasing is the determinant.See the Tektronix Measurement and circuits concepts book on vertical amplifiers.Finally the best analog scopes are TEKThe most unreliable specs are on the Chinese knockoffs.We use Yokogawa DL and DLM series digital scopes, and TEK 2465/7/B, 7000 analog.Jon

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 02, 2022, 09:22:45 am
content: Quote from: Anding on November 02, 2022, 04:48:36 amWhy does my tape measure has length limit. Mine is 20ft but why not unlimited?

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2022, 09:28:48 am
content: Why can't my car go faster than sound?

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 10:05:13 am
content: Quote from: Fungus on November 02, 2022, 09:28:48 amNo reason why it shouldn't but that would require an engine sufficiently powerful to accelerate the car against the restive forces of friction and air resistance which increase with velocity (reference Newton II: F=ma), and sufficient fuel onboard (mass which itself needs to be accelerated) to complete the acceleration. In the case of a car, it would require rolling gear that maintains it's mechanical integrity at the high RPM for travel above 330 m/s.Can you put the equivalent physics for a 'scope in two sentences?

author: Brumby
author_rank: Unknown
timestamp: November 02, 2022, 10:08:05 am
content: Quote from: Anding on November 02, 2022, 10:05:13 amThere is more that can be said about the subject, but I'll focus on one...."Do you want to pay for it?"

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 10:08:39 am
content: QuoteBecause the length limit is set by the amount of material in the tape. Whilst there is no theoretical upper limit, above say 20ft it becomes hard to layout the tape and keep it rigid and inconvenient to rewind it. Above 20ft we can use surveyors tools or nowadays cheap laser meters to accomplish the measurement.Can you explain the engineering constraints on the bandwidth limit of a scope in two sentences?

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 10:10:33 am
content: QuoteThat is a financial constraint. What is the necessary physics that one needs to pay for?

author: Brumby
author_rank: Unknown
timestamp: November 02, 2022, 10:25:10 am
content: There is a reason electronics in the RF region is often referred to a voodoo.I know enough about the subject to know I understand extremely little - just a few principles and almost and next to no practical experience. I've tuned 27MHz mobile antennae and not much else.

author: Someone
author_rank: Super Contributor
timestamp: November 02, 2022, 10:34:10 am
content: Quote from: Anding on November 02, 2022, 10:05:13 amSame as the car needing more power and expensive fancy wheels/tires, fast amplifiers need more power and expensive components. Jamming in more power needs all the supporting expense or it blows up.

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 10:37:10 am
content: Quote from: Brumby on November 02, 2022, 10:25:10 amYes, this is bringing back memories of simulating RF circuits with a package called "Super Compact" many years ago (I was just running the scripts and printing out the logs ;-) . Answering my own question here butThe lumped circuit abstraction model starts to break down at RF frequencies and we can no longer treat components as discreet. The circuit board itself has to be modelled. At the same time as an oscilloscope is a measurement tool, a specification has to be given over which it maintains linearity of response and for which calibration is accurate. A convenient way to do that is to _impose_ (I'm guessing here) a low bandpass filter on the front end to exclude the region which the maker has not designed for (price point of the design considered)I'm curious, with ingenuity what use can be gotten "above the bandwidth limit". This can be experimented using a simple FPGA to generate fast (200MHz) clocks and waveforms. Ok there's a project!

author: Anding
author_rank: Contributor
timestamp: November 02, 2022, 10:39:08 am
content: QuoteThat is very helpful thank you

author: alm
author_rank: Super Contributor
timestamp: November 02, 2022, 12:05:05 pm
content: Read theTektronix Circuit Concepts book on vertical amplifiersthat Jon referenced, at least chapter one. Obviously no one makes vertical amplifiers with discrete transistors or tubes anymore, but the description about the relationship between bandwidth and transient response is still relevant.The bandwidth is limited by the design of the amplifier. In the case of an op-amp, it would be related to the gain-bandwidth product and the gain it is configured for in the circuit (very simplified). There might be additional limitations, like the configurable bandwidth limit (e.g. 20 MHz), or bandwidth limits added so they can offer lower bandwidth models without much additional engineering. That's why some scopes can be 'hacked' to a higher bandwidth, but this is generally limited to the highest bandwidth model in the same series, like 50 MHz Rigol DS1052E to 100 MHz (DS1102E equivalent), but not higher.One important distinction is if you want to look at the bus to decode what is being sent, or if you are verifying the signal integrity of the bus where you want to look at details like meeting setup-and-hold timing requirements. There's a rule-of-thumb chart postedherewith the necessary bandwidth for decoding and checking signal integrity for common buses.

author: jonpaul
author_rank: Super Contributor
timestamp: November 02, 2022, 12:14:30 pm
content: à moment pleaseanalog and digital scopes are completely different in the archetecure design and componentsAll blocks in the signals path affect the overall observed bandwidthFor example an analog scope CRT with a simple deflection plate has perhaps 10..50 MHz BWAt 100..300 MHz a dome mesh PDAFor 400 MHz to 1 ghz distributed deflection with controlled delay and ZoTektronix 2467/B and 7104 use that and microchannel intensifier platesee the Tektronix CRT concepts bookBon courageJon

author: robert.rozee
author_rank: Frequent Contributor
timestamp: November 02, 2022, 12:19:07 pm
content: i am quite certain that i will regret asking this, but...if you're just looking at the rise-time of digital (0-5v) signals, why do you need an amplifier in front of the ADC? most oscilloscopes are designed to handle input signal levels ranging from 10's of mV up to 10's of volts, but in the specific case outlined in this thread it seems that this sort of flexibility is not required.might you just need perhaps an impedance buffer, and possibly a differential driver (in the case of the ADC having a differential input). and i'm wondering if you even need more than ONE channel? yes, this might end up being a very specialized instrument, one that can barely be called an oscilloscope.cheers,rob :-)

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 02, 2022, 12:20:24 pm
content: I'll risk going over board here, but as my dayjob is working against these limits, I'll give a brief insight in what is going on.For scopes, I think there are a multitude of factors: Cost, interconnections, and technology.Cost is simple: faster needs more design expertise, fancier technologies, more hardware, more storage, and at the end of the day things are more expensive.Interconnections: You can't just send 10 GHz signals over cheap connectors. The connector starts to have a big impact, as do the cables. Everything starts becoming part of the circuit and that requires more finicky interconnections. I wouldn't want to use our 110 GHz UXR scope to measure an arduino output, because it would load it with 50 ohm which the arduino probably wouldn't be very happy with. High speed scopes start using connectors like 3.5 mm, 2.4 mm, etc, which are very fragile, require care in connecting-disconnecting, and are very expensive - a 2.4 mm to 3.5 mm metrology-grade adapter might cost several hundred euros a piece. I don't want that on a scope I want to use to measure some SPI busses with, so there is little point making those scopes that fast, in a right-tool-for-the-job kind of way.At the upper limit is technology. There are two metrics with regards to the 'speed' of an active device, the 'transit frequency', or ft, and the 'maximum power gain frequency', or fmax. This is the point where, due to the fundamental physical limits (things like the mobility of carriers in semiconductors) make it so a device no longer produces current gain (for ft) or power gain (for fmax). At this frequency, you need more current into a device than you get out, so you no longer have the ability to make signals larger - let alone do more complex processing of this signal. This puts a first-order boundary on a scope. (actually above-fmax circuits are a thing but lets not open that can of worms)For discrete active electronics this is going to be in the few GHz (usuall) at best. It goes higher when you use integrated circuits - small CMOS goes to about 300 GHz (depends on the technology, but most people agree that it peaks at around 40nm CMOS nodes). SOI can go further, to 400 GHz, SiGe can do 500 GHz, InP has technologies that go up to 1 THz (but that technology is trash for anything but a very simple amplifier).On top of that, a significant additional factor is that designing these high speed 100 GHz circuits is by no means trivial. It is by many considered a black art, and us millimeter-wave and RF designers are sometimes treated as voodoo magicians, wrestling waves out of devices. Sparameters, matching, smith charts...An on top of that, to get to these points, you need to start canceling out poles with zeros, or as an RF designer would see it, tune out capacitance with inductance. This introduces a bandwidth limit - you only tune out the capacitance at a certain frequency. Make it more wideband? That requires more complex interconnections of inductors and capacitors, which in turn leads to lower gain, and at some point you lose all the benefits you were trying to get. Even though you can design a 230 GHz to 250 GHz amplifier in 45 nm CMOS, you cannot design a DC-200 GHz amplifier in the same technology

author: Brumby
author_rank: Unknown
timestamp: November 02, 2022, 12:33:25 pm
content: Quote from: TheUnnamedNewbie on November 02, 2022, 12:20:24 pmThank you for that. That was a brilliant mental picture!

author: pdenisowski
author_rank: Super Contributor
timestamp: November 02, 2022, 12:54:38 pm
content: Here's my answer

author: pdenisowski
author_rank: Super Contributor
timestamp: November 02, 2022, 12:59:48 pm
content: Quote from: Anding on November 02, 2022, 06:36:08 amIt depends on the input waveform. If you're looking at a pure sinusoid, you can often use a scope well past it's 3dB bandwidth without the waveform being distorted, although the amplitude of the displayed sinusoid will be reduced.On the other hand, if you're looking at a signal with high frequency components (e.g. a square wave), then these higher frequency components will be attenuated more than the lower frequency components and waveform distortion will result. This is why a square wave (fundamental and odd harmonics) ends up looking line a sine wave when you have insufficient bandwidth: the higher frequency components have all been significantly attenuated. (ataradov mentioned this above as well)

author: joeqsmith
author_rank: Super Contributor
timestamp: November 02, 2022, 01:15:28 pm
content: I attempted to make a virtual oscilloscope to demonstrate LeCroy's Digital Bandwidth Interleaving. The basic idea was to combine two channels to double the bandwidth rather than the time resolution. Rodger Delbue chimed in with some additional comments on the subject:A few of us were attempting to build a simple oscillator on a breadboard for the fun of it. Far from a scope or anything useful, but it my give you some insight to the problems.https://www.eevblog.com/forum/projects/challenge-thread-the-fastest-breadboard-oscillator-on-the-mudball/msg3081669/#msg3081669In this video I tried to measure some pulses that I sent down different media, from copper to optics.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2022, 01:43:58 pm
content: Why there has to be a limit?For oscilloscope it's easy to say, you have a given budget (price, power, thermal, etc.) than there is the sampling theorem, and for your 8GSa/s ADC the most you can reconstruct (for an unknown non-repetitive signal is 4GHz).The question is much deeper, as in "why there is no observable infinite in the physical world"?Nobody knows, but so far infinite only exists as a concept, i.e. in math.In the physical world (as far as we can observe) nothing is infinite.

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2022, 03:36:53 pm
content: Quote from: Anding on November 02, 2022, 10:05:13 amI can do it in two words: "Inductance" and "Capacitance"

author: bdunham7
author_rank: Super Contributor
timestamp: November 02, 2022, 03:51:55 pm
content: Quote from: Anding on November 02, 2022, 04:48:36 amBecauseeverythingis a ultimately a low-pass filter? The analog front end (input, amplifier, scaling) will have limitations that depend on what you are willing to spend on them.As BW goes up, it becomes increasingly expensive to make these analog components linear and coherent, especially in the case of DC-to-BW limit designs. Not only do you get loss of amplitude, you get group delay and and other distortions that make it pointless to continue beyond a certain point. Your inexpensive 8GSa/s 350MHz scope (gee, I wonder which one that is?) doesn't even have 50R inputs, so it is going to be very difficult to get a 1GHz signal cleanly to its inputs, let alone scaling and amplifying it within the scope. For the digital signals you want to look at, just one probe that could connect those signals to an appropriate input will cost more than your entire scope.

author: TimFox
author_rank: Super Contributor
timestamp: November 02, 2022, 03:55:37 pm
content: Even if we have a good noise figure in the amplifier, the fundamental thermal noise from a 50 ohm resistor is 0.9 nV/Hz1/2.If you had an infinite (or much too high for the application) analog bandwidth, the amplified thermal noise added to your MHz-bandwidth signal would be huge.This is why many analog oscilloscopes have a front-panel switch to limit the bandwidth to much less than the full bandwidth: you can see easily the reduction in trace width (at sensitive V/div settings) with the lower bandwidth.

author: Circlotron
author_rank: Super Contributor
timestamp: November 02, 2022, 08:40:22 pm
content: Quote from: Fungus on November 02, 2022, 09:28:48 amTractive effort goes up with the square of the speed.Power required goes up with the cube of the speed.So if your boxy, mid sized 1970s car needs 15 horsepower to travel at 100kph then it will need 28,255 horsepower to travel at 1235kph.Oscilloscope cost vs performance goes up similarly.

author: SL4P
author_rank: Super Contributor
timestamp: November 02, 2022, 10:15:16 pm
content: Read aboutdV/dT

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 03, 2022, 04:19:10 am
content: Series inductance, parallel capacitance, and resistance turns everything into a low pass filter. Any wires will have at least a bit of series inductance, parallel capacitance, and resistance.

author: glarsson
author_rank: Frequent Contributor
timestamp: November 03, 2022, 08:51:51 am
content: Quote from: TimFox on November 02, 2022, 03:55:37 pmI don't have anything infinite at hand to test with, but wouldn't infinite bandwidth result in infinite thermal noise?

author: TimFox
author_rank: Super Contributor
timestamp: November 03, 2022, 01:49:43 pm
content: Quote from: glarsson on November 03, 2022, 08:51:51 amYes. That was my point.In practice, where there is no such thing as infinite bandwidth, increasing the bandwidth increases the thermal noise voltage proportional to the square root of the bandwidth (all other things being equal).This is clearly visible on a good analog CRO, looking at the trace width at high sensitivity, when changing the bandwidth.

author: glarsson
author_rank: Frequent Contributor
timestamp: November 03, 2022, 03:30:09 pm
content: Quote from: TimFox on November 03, 2022, 01:49:43 pmYes, but huge is not infinite. You can't use an analog cro to prove what happens at infinite bw.

author: TimFox
author_rank: Super Contributor
timestamp: November 03, 2022, 03:36:34 pm
content: Quote from: glarsson on November 03, 2022, 03:30:09 pmYes, huge is not infinite. I wrote "infinite (or much too high for the application".Infinity only makes sense mathematically in "the limit as the variable goes to infinity".If you take an analog CRO with a very wide inherent bandwidth and look at the trace width as you vary the bandwidth of the vertical amplifier, you see the trace width increase in such a fashion that, using mathematical language, the width increases without bound as the bandwidth increases.Were an infinite bandwidth possible, that would give infinite energy in the output, which is obviously non-physical.In one of the simple derivations of thermal noise, the calculation is done for a resistor and capacitor connected together, in thermal equilibrium; the bandwidth for the noise voltage calculation is determined by that time constant.Your quest to measure something with infinite bandwidth will be fruitless.A related problem (the "ultraviolet catastrophe", q.v.) led to the introduction of Planck's constant in thermodynamics: seehttps://web.mst.edu/~kosbar/ee3430/ff/Wireless/noise2/index.html

author: glarsson
author_rank: Frequent Contributor
timestamp: November 03, 2022, 07:31:13 pm
content: Quote from: TimFox on November 03, 2022, 03:36:34 pmI have not expressed any intention to measure with infinite bandwidth.

author: BillyO
author_rank: Super Contributor
timestamp: November 03, 2022, 07:38:34 pm
content: I can explain in one sentence as follows:In real life we have things like stray capacitance, inductance and resistance coupled with power limitations.

author: TimFox
author_rank: Super Contributor
timestamp: November 03, 2022, 09:14:24 pm
content: Quoting from above: "Yes, but huge is not infinite. You can't use an analog cro to prove what happens at infinite bw."

author: David Hess
author_rank: Super Contributor
timestamp: November 04, 2022, 12:33:04 am
content: Linear circuits have gain-bandwidth product limits. Bandwidth can be increases by sacrificing gain, but then more stages are required and this quickly reaches diminishing returns.I say linear circuits because *sampling* oscilloscopes avoid this limitation by sampling before amplification, and the result is massive bandwidth determined only by sampling strobe width.

author: kcbrown
author_rank: Frequent Contributor
timestamp: November 04, 2022, 01:20:55 am
content: Quote from: David Hess on November 04, 2022, 12:33:04 amHow does the A to D converter play into the bandwidth characteristics? Surely it has some effect, no?

author: LaurentR
author_rank: Frequent Contributor
timestamp: November 04, 2022, 04:12:58 am
content: Quote from: Anding on November 02, 2022, 06:36:08 amIf you have an FPGA, you may find that the easiest way to measure what you want to measure (e.g. tDQSCK seen from the FPGA I/Os) is probably to do it from inside the FPGA by using on-board circuitry. I am not sure what you're trying to do, but to a first degree, you should be able to make your circuit work without having to measure anything on the board with an oscilloscope.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 04, 2022, 04:49:11 am
content: Quote from: glarsson on November 03, 2022, 08:51:51 amNo the plank law cuts in above a few THz and the thermal noise drops off exponentially and has a finite integral out to infinite frequency.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 04, 2022, 05:07:50 am
content: Quote from: Anding on November 02, 2022, 10:05:13 amSort of. You might add some car details such as suspension capable of maintaining stability with the bump frequency that will be encountered. So now your horsepower limit is representing the sampling rate, the running gear speed capacity is bandwidth and suspension limitations are rise time. These are not direct analogs, but perhaps an illustration that the performance of a device requires several attributes which may be loosely or tightly related. The car example is probably more relatable to a lot of people. Having a great excess of horsepower or sampling rate does not achieve all of the desired performance. (The reverse, however is true. An insufficiency of any one of the elements will prevent the desired performance.)

author: james_s
author_rank: Super Contributor
timestamp: November 04, 2022, 06:24:04 am
content: Quote from: CatalinaWOW on November 04, 2022, 05:07:50 amOne can look at the various land speed record cars to get an idea of the real world challenges. One of the issues you run into is making wheels that can spin as fast as they need to without the tires and even the rims themselves breaking up from the centrifugal force. Then you have to keep the car on the ground, not easy when it is more like an airplane than a car in the first place. Then as you actually approach the speed of sound you have the shockwave to deal with.

author: Fungus
author_rank: Super Contributor
timestamp: November 04, 2022, 06:43:36 am
content: Quote from: james_s on November 04, 2022, 06:24:04 amThat's the reason CD drives (remember those?) stopped getting faster at 56x.

author: David Hess
author_rank: Super Contributor
timestamp: November 04, 2022, 12:27:08 pm
content: Quote from: kcbrown on November 04, 2022, 01:20:55 amWith an exception which does not matter here, ADCs have the same limitation and contribute to the bandwidth like any other linear stage.Starting on page 24,Circuit Concepts - Vertical Amplifier Circuitsfrom Tektronix discusses how several cascaded single-pole responses result in close to a Gaussian frequency response.A point made a couple pages later is that this response is deliberately chosen because it results in little to no overshoot.

author: Black Phoenix
author_rank: Super Contributor
timestamp: November 04, 2022, 12:48:43 pm
content: Quote from: Fungus on November 04, 2022, 06:43:36 amAnd even some of the 52x would start to create cracks in the centre of the disc after some time. I have some Microsoft Windows CDs with airline cracks in the centre and fixed (at least one that I remember) drive who a CD just disintegrated inside.

author: TimFox
author_rank: Super Contributor
timestamp: November 04, 2022, 02:24:29 pm
content: Quote from: ejeffrey on November 04, 2022, 04:49:11 amYes, that was Planck's solution to the "ultraviolet catastrophe", keeping the total radiated power output from a black body finite. The rest is history.

author: james_s
author_rank: Super Contributor
timestamp: November 04, 2022, 08:30:22 pm
content: Quote from: Fungus on November 04, 2022, 06:43:36 amOf course I remember those, I still use them often.

author: Anding
author_rank: Contributor
timestamp: November 05, 2022, 04:20:10 am
content: Quote from: Anding on November 02, 2022, 06:36:08 amNot perfect but the MSO5000 with 100MHz bandwidth limit can help me measure the phase difference of 100MHz clock signals. The generated duty cycle is 50% and the phase difference is 90 degrees whereas the reading is 64% and 100 degrees - but this is close enough to be helpful to problem solving with entry-level equipment" alt="" class="bbc_img" />" alt="" class="bbc_img" />