Products > Test Equipment
is it true, oscilloscope must reach at least 4x observed freq?
gf:
--- Quote from: Fungus on September 14, 2022, 04:14:14 pm ---Some people even turn the screen persistence way up then enable dot mode and pretend they can see the real signal building up on screen. :)
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If the fundamental frequency is not an exact integral fraction of the sample rate and if the trigger point is interpolated between the adjacent samples, then it is not impossible.
tggzzz:
--- Quote from: Fungus on September 14, 2022, 04:14:14 pm ---
--- Quote from: tggzzz on September 14, 2022, 02:59:07 pm ---Personally I like having the option to turn off interpolation and simply see dots representing samples.
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Some people even turn the screen persistence way up then enable dot mode and pretend they can see the real signal building up on screen. :)
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Why "pretend"? In some cases that is what happens.
Linear (r higher order) interpolation is a fiction designed to help those that never mentally completed "join the dots" puzzles as a kid :)
Someone:
--- Quote from: nctnico on September 14, 2022, 02:40:01 pm ---
--- Quote from: EEVblog on September 14, 2022, 12:02:37 pm ---
--- Quote from: Someone on September 14, 2022, 10:21:23 am ---Cool, now go and read what the OP actually wrote. Nothing about single frequency sine waves and ideal sampling/reconstruction. Again I keep saying it, 2.5 is some vague compromise with unspecified criteria. What is actually important? amplitude accuracy? relative phase? waveform shape?
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Remember that the input wave shape gets changed by the input bandwidth response of the scope front end. Different scopes and models have different input antialiasing filters and responses.
This is why the shape of the input filters matters in these dicsussions. Gaussian response is usually assumed unless otherwise specificed.
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Gaussian response is long gone. The typical anti-aliasing filters are much steeper. If you want a Gaussian response, then you'll need to set the bandwidth limiter. IMHO this is one of the reasons higher end scopes have multiple bandwidth settings so the user can choose to have maximum bandwidth or a slow (first order) roll-off.
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Gaussian analog anti-aliasing filters aren't disappearing (as they are cost effective), the work is being shifted to digital. Tek front end ASIC comparison below.
A modern scope has multiple different sampling rates with all sorts of filtering going on along the chain, its far removed from the simple models people keep trying to attach to it.
David Hess:
--- Quote from: tautech on September 13, 2022, 12:34:14 am ---
--- Quote from: David Hess on September 13, 2022, 12:15:14 am ---
If you want to see anything other than the fundamental sine wave component of the 100 MHz signal, then the oscilloscope bandwidth needs to be much higher to cover the harmonics. Otherwise a 100 MHz oscilloscope viewing a 100 MHz signal will only display the 100 MHz fundamental sine wave component, and possibly with a 3dB amplitude error.
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FTFY.
It need be noted label BW can be substantially different from actual -3dB BW as is the case with the 100 MHz rated SDS2104X Plus which the 1st we received I tested with 3 yes 3 signal sources to convince myself the ~185 MHz result was actually real !
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Some oscilloscopes are weird. Even old high bandwidth instruments often break the Gaussian response -3dB rule, and the 0.35 rule does not apply to them either. For other instruments, the specified bandwidth was more of a guideline than a rule, and the instrument was *at least* that fast. Modern examples of weirdness include the Rigol 1000Z series where full power bandwidth varies with the V/div setting.
--- Quote ---Supporting that result is they are also supplied with 200 MHz probes.
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Tektronix specified their oscilloscopes for -3dB bandwidth *at the probe tip*, so a 200 MHz Tektronix oscilloscope with a 200 MHz Tektronix probe yielded 200 MHz. At least for them, the probe bandwidth specification represents the highest frequency which the probe will reproduce faithfully and not the -3dB bandwidth.
--- Quote from: Fungus on September 13, 2022, 10:41:11 am ---
--- Quote from: David Hess on September 13, 2022, 12:15:14 am ---As pointed out by tggzzz, a bandwidth limited signal can be reconstructed with a sampling rate greater than twice the bandwidth no matter where in the frequency spectrum it is, within the bandwidth of the sampler.
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In theory? Yes.
In practical terms? Not so much. The reconstruction filter would become very unwieldy as you approach Nyquist.
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In a subsampling bandpass application, the input signal bandwidth is tightly constrained by a high selectivity bandpass filter, so the difficulty of reconstruction is considerably relaxed. Ideally aliasing products will be close to or below the noise floor.
--- Quote ---ie. I've sat a potentiometer and manually dialed a frequency where I no longer observe the AM modulation effect mentioned earlier. The frequency I ended up with was right there in the 2.5x ballpark. Maybe it could have been 2.4x but it's definitely not as low as 2.2x.
2.5x may be a "non-specific/vague figure" but it works in practice.
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I find 2.5x times to be optimistic when sin(x)/x reconstruction is required. The effect you mention can be modeled as the result of non-linear mixing between a pure sine wave and the sampling clock, with aliasing of some of the mixing products. The reconstruction then has multiple solutions and each pass shows one of those solutions. When a fast edge is applied, the same thing happens even when the reconstruction filter correctly filters the higher frequency aliasing products because the mixing products are still there close to the aliasing frequency. I find the result annoying in the extreme.
The solution is to use a faster sample rate. Sometimes there is no substitute for high bandwidth and fast sampling rate.
--- Quote from: The Electrician on September 13, 2022, 08:42:44 pm ---This scope is old enough that it doesn't have sin x/x interpolation. As switchabl mentioned the older Agilent scopes typically use linear interpolation, and it isn't at all useless. If your choices are dot mode and linear interpolation, linear interpolation is just fine. When your signal is oversampled, you can't tell the difference anyway. This scope has rated max sample rate of 4 Gsa/s, and analog bandwidth of 500 MHz, so it's 4 times oversampled at rated bandwidth, and much more than that for lower bandwidth signals.
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For Tektronix you have to go back to 1990 and the 2232 series of DSOs for the last of their DSOs which did not support sin(x)/x reconstruction, and those are what I consider to be the first "modern" DSOs, at least from Tektronix. Their 2430 series first made in 1986 had sin (x)/x reconstruction and I think all of their instruments after that, other than the 2232 series, did.
Honestly I have never missed sin(x)/x reconstruction on the 2230 and 2232. Usually equivalent time sampling removes the need, and cases where single shot acquisitions need to be made with full bandwidth signals are rare.
radiolistener:
--- Quote from: oxy on September 12, 2022, 04:43:14 pm ---some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.
Is it true?
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No, for 100 MHz signals, the oscilloscope bandwidth should be about 1 GHz (10 times).
This is because you're needs to see at least 10 harmonics of square wave in order to see waveform distortions. Otherwise you will see just a sine and cannot see signal distortions and glitches.
You can still use 400-600 MHz bandwidth oscilloscope, but it's ability to see 100 MHz waveform details will be very limited. For example, you can see if signal exists, and can see if it's clean sine. But cannot see if it has glitches or distortions.
Note, that oscilloscope bandwidth is applied for a clean sine wave. If you're dealing with other kind of waveform it consists of high order harmonics and oscilloscope should be able to see it, otherwise your scope input will works like low pass filter and all what you can see is just a 100 MHz sine despite the fact that signal waveform is actually square wave, triangle or other type.
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