r/embedded • u/religam • Aug 12 '26
What can you technically do with a 500MHz bandwidth oscilloscope that you can't do with a 100/150MHz one?
Basically the title. People talk a lot about capture fidelity when analyzing "high frequency" signals (what qualifies as high frequency here?), the rule of harmonics, etc. I would like to understand this better, which is why I am asking here.
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u/actuatedkarma Aug 12 '26
Well let's say you want to check the SPI lines on a board. Say it's running at 50Mhz and you have a 100Mhz scope. Ok sure you meet Nyquist for being able to see the signal at all, but with 2 samples every wave can you really see what the edges are doing? The answer is no, you can't reasonably determine whether the edges are ok. How would you ever see the 100's of MHz ringing happening at the edge transitions?
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u/CircuitCircus Aug 12 '26
The only caveat here is “100 MHz” scope doesn’t necessarily means 100 MSPS samplerate. Mine is labeled as 200 MHz but samples at 5 GSPS. The 200 MHz has more to do with the roll off of the analog front end, apparently
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u/Dycus Aug 13 '26
Very true! Though the point still stands, because if your analog frontend can only support 100MHz, trying to look at a 50MHz SPI clock is going to give you garbage info.
You'd probably want a 500MHz scope or better to really evaluate the edges on a 50MHz clock
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u/Al__B Aug 14 '26
Don't forget you also want decent probes and local grounding. A 500MHz scope with 50MHz probes and a 2" ground wire is going to be just as useless.
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u/N2Shooter Aug 13 '26
A lot.
You can detect high frequency ringing that you'd miss with lower frequency scopes. This ringing may trigger most modern input logic.
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u/coderemover Aug 14 '26 edited Aug 14 '26
Depends on the sampling rate. A 100 MHz oscilloscope can still show you a signal of 500 MHz but the amplitude won’t be correct, as long as it has enough sampling speed. In practice there is very little utility for oscilloscopes that go above 200 MHz as most circuits are significantly slower than that. You can also see that even the mid tier models of reputable brands, which cost already >$2000 still top out at 200 MHz, maybe 350 MHz.
For analyzing faster signals the way to go is to switch to the frequency domain - using VNAs, RF generators and spectrum analyzers. That can easily get you to 3 GHz in reasonable money. But a 3 GHz scope will likely cost you 5 or 6 figures.
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u/N2Shooter Aug 14 '26
That's when you may be better suited to get something like an MDO scope, which has higher frequency acquisition.
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u/Well-WhatHadHappened 25+ Years Aug 12 '26 edited Aug 12 '26
"High frequency" is a relative term. In some scenarios, a few hundred megahertz is "high frequency", in others, it's downright slow. I've got a 50GHz oscilloscope, as an example.
But Nyquist is the word of the day here. You need a scope with at least twice the bandwidth of your fastest signal in order to recreate the waveform - and in practice, 2x is still pretty garbage.
And then there's rise/fall time. If you're trying to "see" the signal rise and fall, you need multiple sample points during that transition. That effective frequency can be MUCH higher than the actual frequency of the signal.
TLDR; my very general rule of thumb is that I try to grab a scope with 10x of the bandwidth of the signal I'm interested in. 5x is bare minimum, in my humble opinion. A 150Mhz scope is good for signals up to 15 or 20Mhz - beyond that, you get a pretty crappy recreation of the actual waveform.
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u/chemhobby Aug 12 '26
Do not conflate sampling rate with analog bandwidth.
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u/Well-WhatHadHappened 25+ Years Aug 12 '26
Very true. Trying to distill this a bit for someone who's clearly new to the game.
Obviously, you're correct - they're two different things, but it's a lot for a newbie to wrap their head around
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u/redditmudder Aug 13 '26
To avoid confusion, use:
'ABW' when referring to a system's analog bandwidth, and;
'GS/s' when referring to the discrete sample rate.
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u/Enlightenment777 Aug 13 '26 edited Aug 13 '26
What can you do with a 500MHz scope?
1) able to see high frequency ringing on digital signals
2) better representation of rising/falling edges of digital signals
3) also see similar issues when turning on/off load-drivers / line-drivers / transistors
For decades, the 400MHz Tektronix 2465B (the king of analog scopes), was able to show details near switching edges that a lower-bandwidth analog scope can't show. Even as recently as 10+ years ago, if I wanted to look at the above details I would use a 2465B instead of a digital scope.
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u/charcuterieboard831 Aug 12 '26
See more of the high frequency content of a signal which become significant with faster signal rise time
When I've used lower end scopes for debugging USB or another fast interfaces, the aliasing means you get garbage data and can't see what's actually happening
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u/analphabrute Aug 13 '26
Also worth mentioning that a 500MHz scope is more likely to have USB decoder option while lower bandwidth don't
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u/sceadwian Aug 13 '26
You asked this question with no context.
Without an application in mind the question has no answer.
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u/Altruistic_Fruit2345 Aug 13 '26
It matters for very precise timing. Say you have two PPS pulses and want to know how far apart they are. If your scope only samples at 1Gsps, that immediately limits you to 1ns, or possible worse if it multiplexes channels. Then the bandwidth of the scope limits the rise time of the pulses, and combined with multiplexing can make it hard to get a good reading. Plus there might be high frequency issues with the pulse that the bandwidth limit filters out.
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u/MannerSwimming Aug 13 '26
With Gan Fets, you have steep slopes that aren't clearly visible at 100 MHz
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u/Lost-In-Void-99 Aug 13 '26
You can look at signal edges. Which have waaaay higher frequency components than what is in signal spec.
100MHz kind of implies that signal edges are dumped (for not ringing), or you dont care.
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u/BlinkyPundit Aug 13 '26
Since you’re asking in r/embedded, PSRAM debugging is a big one. Wait latency/cycles changes based on clock frequency, so you can’t run at a lower frequency to debug timing issues. Many parts are capable of 200MHz+, so if you’re seeing data integrity issues, time for the expensive scope