As a follow-up question. Is there really such a thing as a square wave?
Hi,
Thanks for the sanity check everyone.
I have done a screenshot of my scope, I have used both channels of the signal generator, one of the channels is using a BNC cable with mini-grabbers (yellow) and one channel is using a scope probe set to x1 (blue)
The results seem wildly different, I'm assuming you really shouldn't use a scope probe for the sig gen at higher frequencies?
michaeliv, the scope I'm on is a Rigol MSO1104Z-S, im using the built in Signal Generator.
Hi,
Thanks for the sanity check everyone.
I have done a screenshot of my scope, I have used both channels of the signal generator, one of the channels is using a BNC cable with mini-grabbers (yellow) and one channel is using a scope probe set to x1 (blue)
The results seem wildly different, I'm assuming you really shouldn't use a scope probe for the sig gen at higher frequencies?
And for a 15 MHz square wave you kinda sorta need about 150 MHz analog bandwidth and about 1.5 GSa/s, otherwise even a very good signal will start to look deformed.
And for a 15 MHz square wave you kinda sorta need about 150 MHz analog bandwidth and about 1.5 GSa/s, otherwise even a very good signal will start to look deformed.
Your other points about the probes are correct, but those points are far too simplistic.
The scope bandwidth you need is completely unrelated to a square wave's frequency; it depends only on the risetime/falltime. The traditional rule of thumb is tr=0.35/BW. Simple thought: when looking at one transition, the scope neither knows nor cares when the next transition will come along.
Secondly, the relationship between samples/s and bandwidth is also too simplistic. Many scopes have an "ETS" mode where the sample rate is much less than the signals' bandwidth; many years ago I used a 25MS/s to observe sub-nanosecond risetimes. First google result for a contemporary scope: https://www.picotech.com/library/oscilloscopes/equivalent-time-sampling
And for a 15 MHz square wave you kinda sorta need about 150 MHz analog bandwidth and about 1.5 GSa/s, otherwise even a very good signal will start to look deformed.
Your other points about the probes are correct, but those points are far too simplistic.
The scope bandwidth you need is completely unrelated to a square wave's frequency; it depends only on the risetime/falltime. The traditional rule of thumb is tr=0.35/BW. Simple thought: when looking at one transition, the scope neither knows nor cares when the next transition will come along.
Secondly, the relationship between samples/s and bandwidth is also too simplistic. Many scopes have an "ETS" mode where the sample rate is much less than the signals' bandwidth; many years ago I used a 25MS/s to observe sub-nanosecond risetimes. First google result for a contemporary scope: https://www.picotech.com/library/oscilloscopes/equivalent-time-sampling
OK, granted, I guess that's why it's called "rule of thumb" ;-)
Of course I assume a square wave has "reasonably" (for some definition of that) steep edges, needing about enough bandwidth for the 7th to 9th harmonic to show a reasonable (...) image of the signal. Since there is a relation between maximum possible rise/fall time and base frequency of a square wave the base frequency also sets the lowest allowable bandwidth limit.
As for ETS mode, hmm, I think that's largely a (historical) kludge, although if properly implemented it should work for suitable signals. IOW - usually not.
You just can't observe a not strictly repetitive (over many periods of the timebase) signal using ETS. And if you already know that much about the signal there are only a few things left to find out about it by using a scope.
For anything non-repetitive, glitchy, not-quite-known-in-advance there is no way around having enough bandwidth and sample rate to capture the actual signal - at first go.
And for a 15 MHz square wave you kinda sorta need about 150 MHz analog bandwidth and about 1.5 GSa/s, otherwise even a very good signal will start to look deformed.
Your other points about the probes are correct, but those points are far too simplistic.
The scope bandwidth you need is completely unrelated to a square wave's frequency; it depends only on the risetime/falltime. The traditional rule of thumb is tr=0.35/BW. Simple thought: when looking at one transition, the scope neither knows nor cares when the next transition will come along.
Secondly, the relationship between samples/s and bandwidth is also too simplistic. Many scopes have an "ETS" mode where the sample rate is much less than the signals' bandwidth; many years ago I used a 25MS/s to observe sub-nanosecond risetimes. First google result for a contemporary scope: https://www.picotech.com/library/oscilloscopes/equivalent-time-sampling
OK, granted, I guess that's why it's called "rule of thumb" ;-)
Of course I assume a square wave has "reasonably" (for some definition of that) steep edges, needing about enough bandwidth for the 7th to 9th harmonic to show a reasonable (...) image of the signal. Since there is a relation between maximum possible rise/fall time and base frequency of a square wave the base frequency also sets the lowest allowable bandwidth limit.
The second sentence is wrong, pure and simple: the transition time is independent of the period.
Er, no. Let me guess. Your background is either theoretical or audio or software; you don't appear to have used a scope for looking at medium speed logic signals (medium speed = any logic family introduced since 1980)
Very few people look at square waves; the primary place you find them is in textbooks, especially associated with introductory Fourier analysis.
Most people look at logic signals. There the key factors are signal integrity and logic value. Signal integrity requires ensuring the transition time does not violate the specs in the logic families' data sheet (valid voltages at all times, and minimum slew rate, and minimum pulse widths).
You just can't observe a not strictly repetitive (over many periods of the timebase) signal using ETS. And if you already know that much about the signal there are only a few things left to find out about it by using a scope.
If only things were that simple.
Repetitive or pseudo-repetitive signals are required. Often signals can be arranged to be repetitive. If not then you can still get important information from pseudo repetitive signals. For example, eye diagrams are also a current technique that won't disappear soon - especially with bit rates >1Gb/s.
And for a 15 MHz square wave you kinda sorta need about 150 MHz analog bandwidth and about 1.5 GSa/s, otherwise even a very good signal will start to look deformed.
Your other points about the probes are correct, but those points are far too simplistic.
The scope bandwidth you need is completely unrelated to a square wave's frequency; it depends only on the risetime/falltime. The traditional rule of thumb is tr=0.35/BW. Simple thought: when looking at one transition, the scope neither knows nor cares when the next transition will come along.
Secondly, the relationship between samples/s and bandwidth is also too simplistic. Many scopes have an "ETS" mode where the sample rate is much less than the signals' bandwidth; many years ago I used a 25MS/s to observe sub-nanosecond risetimes. First google result for a contemporary scope: https://www.picotech.com/library/oscilloscopes/equivalent-time-sampling
OK, granted, I guess that's why it's called "rule of thumb" ;-)
Of course I assume a square wave has "reasonably" (for some definition of that) steep edges, needing about enough bandwidth for the 7th to 9th harmonic to show a reasonable (...) image of the signal. Since there is a relation between maximum possible rise/fall time and base frequency of a square wave the base frequency also sets the lowest allowable bandwidth limit.
The second sentence is wrong, pure and simple: the transition time is independent of the period.
Not really. What I meant is the following: the sum of rise and fall time can't be larger than the period of the square wave. At the limit it will already be a triangle or sawtooth.Er, no. Let me guess. Your background is either theoretical or audio or software; you don't appear to have used a scope for looking at medium speed logic signals (medium speed = any logic family introduced since 1980)
What I currently do to earn money is software. What I got an education in - way back when - is electronics. So there. ;-)Very few people look at square waves; the primary place you find them is in textbooks, especially associated with introductory Fourier analysis.
Most people look at logic signals. There the key factors are signal integrity and logic value. Signal integrity requires ensuring the transition time does not violate the specs in the logic families' data sheet (valid voltages at all times, and minimum slew rate, and minimum pulse widths).
Everybody knows that strictly speaking a square wave is a mathematical abstraction not existing in the real world. At least people should know.
That doesn't dissuade anybody from talking about square waves in the real, physical world if the signal in question is close enough to the mathematical ideal.
[...]QuoteYou just can't observe a not strictly repetitive (over many periods of the timebase) signal using ETS. And if you already know that much about the signal there are only a few things left to find out about it by using a scope.If only things were that simple.
Repetitive or pseudo-repetitive signals are required. Often signals can be arranged to be repetitive. If not then you can still get important information from pseudo repetitive signals. For example, eye diagrams are also a current technique that won't disappear soon - especially with bit rates >1Gb/s.
OK, but as far as i'm aware not that many people actually deal with that kind of bitrates (not even in a professional context), and I didn't mean to talk about measuring that kind of high speed signals.
Certainly the OP didn't talk about Gbps signals either, so information about measuring these is unlikely to be helpful to him, at least in the short/medium term.
Are you aware that you can buy a 160GS/s scope?
Are you aware that you can buy a 160GS/s scope?My bank account would like to respectfully disagree with you.
As a quick optimisation hack: If you have a crappy pulse gen or function generator, you can improve the rise time by piping the output via a few gates in parallel from a 74AC14 schmitt inverter array properly terminated close to the scope. 1-2ns edges on them. That looks more like the real fourier series output for a square (is there such a thing?) wave as well: ringing. This trick has been around since the dark ages.
That's a fast edge! I'm maxed out on a 465B at 100MHz and I can measure ~1.5ns off an 74AC14. I would love to use 74LVC1G14 but I can't see the damn things
Is that 50 ohm termination inside the 485?
Quite surprised to see that level of ringing on a Z0 probe with spear. TBH at that level because my crappy old Tek probes taper off at about 70MHz, and I'm too cheap to buy some nice ones, I built my own probe with a bit of quality RG58U and a couple of hand picked resistors and soldered it in circuit. No internal termination on my 465B though. Worked like a dream however
For the OP, here's a thread on square waves from last year:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/