The answers over there are quite wrong.
Am I going to be blamed for that, too?
FWIW I never even read the page, I just found the image on google image search.
It's a 5Hz sine wave sampled at 11Hz intended to show where the "AM" effect comes from.
The 450MHz signal in the 450MHz screenshot definitely is a fantasy signal. The amplitude suddenly gets higher (compared to the 408MHz screenshot) while you'd expect it to be lower.
I don't know what the signal amplitude is at the input but likely it is quite high to at least see some signal. When increasing the sensitivity, there will be more gain so likely the front-end is now distorting due to clipping / overdrive recovery / etc.
No, it's from here:
https://dsp.stackexchange.com/questions/60859/conclusions-of-sampling-around-nyquist-rate
The answers over there are quite wrong.
Am I going to be blamed for that, too?
FWIW I never even read the page, I just found the image on google image search.
The 450MHz signal in the 450MHz screenshot definitely is a fantasy signal. The amplitude suddenly gets higher (compared to the 408MHz screenshot) while you'd expect it to be lower.
The previous one wasn't 408, but 490 MHz (the frequency counter got confused). So the amplitudes are plausible and decrease with increasing frequency.QuoteI don't know what the signal amplitude is at the input but likely it is quite high to at least see some signal. When increasing the sensitivity, there will be more gain so likely the front-end is now distorting due to clipping / overdrive recovery / etc.The input amplitude was said to be 1.6Vpp, which is ~13 div at 120mV/div, or 1.67x full scale. The amp certainly has some headroom, but I don't know how much. Since Performa01 explicitly mentioned the potential overdrive issue in his post, I assume that he took care not to overdrive the amp
The answers over there are quite wrong.
Am I going to be blamed for that, too?
FWIW I never even read the page, I just found the image on google image search.
It's a 5Hz sine wave sampled at 11Hz intended to show where the "AM" effect comes from.I'm not blaming you but the people who posted the graphs on Reddit and commented on it. Drawing a line through sample points as-is is never a natural representation of a sampled signal. You'll need to put the samples through an analog or digital filter in order to reconstruct the resulting signal.
It's a 5Hz sine wave sampled at 11Hz intended to show where the "AM" effect comes from.
Perhaps a better question would be whether the Rigol's interpolation implementation is flawed and starts to suffer from aliasing too early.
There were some decent indications that the Siglent is better, but to properly compare apples to apples we would need to capture signals that have frequencies which are equal not absolutely, but as fractions of the sampling rates of both, and that, ideally, stay within the spec'd bandwidth, for which we would need to reduce the sampling rate artificially. Those could be say 10 frequency values from 1/5 to 1/2.5 the respective sampling rates. If someone's willing to do such a test with SDS800X HD (which goes to min. 500 MSa/s, right?), then I can do it with DHO800.
The answers over there are quite wrong.
Am I going to be blamed for that, too?
FWIW I never even read the page, I just found the image on google image search.
It's a 5Hz sine wave sampled at 11Hz intended to show where the "AM" effect comes from.I'm not blaming you but the people who posted the graphs on Reddit and commented on it. Drawing a line through sample points as-is is never a natural representation of a sampled signal. You'll need to put the samples through an analog or digital filter in order to reconstruct the resulting signal.
Just so.
Why did they decide to use a first-order hold reconstruction (linear interpolation) rather than a zero-order hold (step)? Typically that would be an unwitting default "looks right" choice.
Personally I hate both, and much prefer to see just the samples (i.e. dots).
If you look carefully at the 450MHz screenshot Performa01 posted, you'll see that the displayed signal can't be right. Just look how the histogram for the frequency measurement shows a wide spread instead of a single frequency! And also look at the amplitude compared to the 408MHz signal. IOW, the screenshot doesn't show what is actually happening. Just stick to testing fs / 2.5 + 2% or so. DSOs can show fantasy signals in such circumstances!
But I'm still wondering how 490 MHz @1GSa/s and 10ns/div look like?490 MHz is a bit nasty – but I’m sure you are aware of that and just want to be mean 😉
Thank you! Yes, of course I wanted to be mean
If you display the same with a single-shot, then I think we would see the same kind of "AM envelope" which was observed on the Rigol.
I think it was just unable to trigger on the envelope, therefore the trace looks so weird.
If we could do an FFT on the interpolated trace, then I'd expect to see peaks at 490 and 510 MHz (image).
[ If I think about it, maybe it is even possible with the Interpolate math function. ]
QuoteA less demanding configuration would be 450 MHz – this time at 2 ns/div, to show the sine wave better. There are no cyclic effects to see anymore. Reconstruction works as it should, but the amplitude is so low that I needed a little help from the vertical gain again – at 120 mV/div the trigger was fully working again, as can be seen from the trigger frequency counter.
The trace is still a bit thick. Is this really just noise? Could also be the same effect (but with much lower amplitude and 5x higher frequency of the envelope). Even if it is hardly visible in the time domain, an FFT of the interpolated trace should still reveal the amount of attenuation for the 550 MHz image (ideally it drowns in the noise floor).
I’ve restricted the SDS824X HD to 500 MSa/s, hence a Nyquist frequency of 250 MHz even in single channel mode. The frequency response is the same as shown in an earlier post. The amplitude is 800 mVpp, so the signal would fill the entire screen height at 10 MHz. There is certainly no need for 10 different frequency values – I’ll show a few strategic ones only.
100 MHz = SR/5
200 MHz = SR/2.5
210 MHz = SR/2.38
220 MHz = SR/2.273
Yes, we should not forget what I’m demonstrating here: torturing a dirt-cheap entry-level instrument operating far outside its specifications, with a signal that is ~2.5 times the specified bandwidth. A heavily attenuated signal close to Nyquist, with a remaining amplitude barely high enough to get a solid trigger, sitting right on the edge of a steep AA-filter, is naturally not going to be a benchmark for the scope performance.
I guess this comparison can now be considered complete. Rigol's interpolation implementation is way worse than that of Siglent.
The answers over there are quite wrong.
Am I going to be blamed for that, too?
FWIW I never even read the page, I just found the image on google image search.
It's a 5Hz sine wave sampled at 11Hz intended to show where the "AM" effect comes from.I'm not blaming you but the people who posted the graphs on Reddit and commented on it. Drawing a line through sample points as-is is never a natural representation of a sampled signal. You'll need to put the samples through an analog or digital filter in order to reconstruct the resulting signal.
Just so.
Why did they decide to use a first-order hold reconstruction (linear interpolation) rather than a zero-order hold (step)? Typically that would be an unwitting default "looks right" choice.
Personally I hate both, and much prefer to see just the samples (i.e. dots).
The answers over there are quite wrong.
Am I going to be blamed for that, too?
FWIW I never even read the page, I just found the image on google image search.
It's a 5Hz sine wave sampled at 11Hz intended to show where the "AM" effect comes from.I'm not blaming you but the people who posted the graphs on Reddit and commented on it. Drawing a line through sample points as-is is never a natural representation of a sampled signal. You'll need to put the samples through an analog or digital filter in order to reconstruct the resulting signal.
Just so.
Why did they decide to use a first-order hold reconstruction (linear interpolation) rather than a zero-order hold (step)? Typically that would be an unwitting default "looks right" choice.
Personally I hate both, and much prefer to see just the samples (i.e. dots).zero-order hold you mean like stair steps plot? that will be awful in any circumstances in any scope. how do you come up with such an idea?
Here you go:
SDS824X HD_Sine_450MHz_1GSa_Single
A step (staircase) plot style is easier to see compared to a single dot and it doesn't pretend to plot a waveform shape that is completely wrong.
262.6 MHz:
It looks like a software flaw indeed.
...except that other software does the same thing, and I've seen the exact same effect using hardware reconstruction filters (from a DAC feeding into an RC filter).
What it means is that Siglent is doing something weird.
I'd like to see an actual AM signal fed into the Siglent to see what happens.
A step (staircase) plot style is easier to see compared to a single dot and it doesn't pretend to plot a waveform shape that is completely wrong.as sample rate is increased, say 10-50 points per cycle, stair step plot will be annoyingly disturbing imho, its not suitable to represent continuous analog signal, for statistics that maybe all right. the purpose of graph plot on dso is try to mimick as close as possible actual signal. highly undersampling such case above Sr / 2.2 Fungus demo'ed surely create distortion when plotted with so called first order (line) but thats not an excuse to use zero order. you can sim sampling at 10-20 points per cycle and try to plot it using stair step and look how bad it is.. i cant imagine how bad it is if lecroy is using such an idea for example in the attached plot
That's just the "AM" signal with persistence.
What it means is that Siglent is doing something weird. I'd like to see an actual AM signal fed into the Siglent to see what happens.
...except that other software does the same thing, and I've seen the exact same effect using hardware reconstruction filters (from a DAC feeding into an RC filter).
What it means is that Siglent is doing something weird.
I'd like to see an actual AM signal fed into the Siglent to see what happens.
I am afraid it's too late, you have already lost all credibility. Comparing Rigol's slightly flawed implementation with Mechatrommer's admittedly wrong first homebrew attempt -- and reaching the conclusion that Siglent's implementation must be "weird"?
Here you go:
SDS824X HD_Sine_450MHz_1GSa_Single
I'd like to see an actual AM signal fed into the Siglent to see what happens.