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is it true, oscilloscope must reach at least 4x observed freq?
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The Electrician:

--- Quote from: nctnico on September 13, 2022, 08:08:16 pm ---The signal isn't sub-sampled. Just turn sin x/x reconstruction on and you'll see the actual waveform. Linear (vector) display is one of the most useless features of a DSO IMHO.


--- Quote from: 2N3055 on September 13, 2022, 07:44:02 pm ---As for linear/sinc interpolation, that is a matter of visual aesthetics in this case.. Both are showing nonsense...

--- End quote ---
No, not at all. Both methods will show a waveform that connects all the sample points by definition. Remember an oscilloscope is there to visualise a waveform. It makes little sense to turn sin x/x off when there are few sample points to work with; it is there to help your brains to see and interpret the signal. However the assumption is that the oscilloscope is setup in a way the signal is sampled correctly (either through a high enough samplerate or using ETS on a periodic signal).

--- End quote ---

That signal is quite sub-sampled.  The rise time of the square wave is 180 nS; no details of the rising and falling edges are visible.  The sample rate is 8 Ksa/s and there is substantial frequency content well above 4 kHz.

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.
nctnico:

--- Quote from: The Electrician on September 13, 2022, 08:42:44 pm ---
--- Quote from: nctnico on September 13, 2022, 08:08:16 pm ---The signal isn't sub-sampled. Just turn sin x/x reconstruction on and you'll see the actual waveform. Linear (vector) display is one of the most useless features of a DSO IMHO.


--- Quote from: 2N3055 on September 13, 2022, 07:44:02 pm ---As for linear/sinc interpolation, that is a matter of visual aesthetics in this case.. Both are showing nonsense...

--- End quote ---
No, not at all. Both methods will show a waveform that connects all the sample points by definition. Remember an oscilloscope is there to visualise a waveform. It makes little sense to turn sin x/x off when there are few sample points to work with; it is there to help your brains to see and interpret the signal. However the assumption is that the oscilloscope is setup in a way the signal is sampled correctly (either through a high enough samplerate or using ETS on a periodic signal).

--- End quote ---

That signal is quite sub-sampled.  The rise time of the square wave is 180 nS; no details of the rising and falling edges are visible.  The sample rate is 8 Ksa/s and there is substantial frequency content well above 4 kHz.

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.

--- End quote ---
First of all, I strongly doubt the older Agilent scopes don't have sin x/x reconstruction. This is a pretty basic functionality for a DSO.  Even my old CRT based Iwatsu DS-8617 DSO had sin x/x interpolation. And for a good reason, with 4 times oversampling you won't get a true representation of a waveform using vectors (linear) reconstruction.

Secondly, feeding a high frequency waveform and then using a too low samplerate is just 'user error'. It proves nothing.
The Electrician:
The Agilent DSO5054 does not have sin x/x interpolation.
The Electrician:

The Agilent DSO5054 does not have sin x/x interpolation. It's only at 500 MHz that the oversampling rate is 4x.  At lower frequenies, for example audio and low RF, the oversampling rate is much higher.
tggzzz:

--- Quote from: robert.rozee on September 13, 2022, 03:03:45 pm ---
--- Quote from: switchabl on September 13, 2022, 02:08:52 pm ---It is what happens if you just ignore the Nyquist criterion. And then apply sinc-based interpolation anyway. Did you have to turn that on manually or is that actually the default? If so, that would seem to be either a bug or a strange design choice indeed.
--- End quote ---

the default behavior of a Siglent SDS1204X-E. i'd imagine most other similar modern DSOs would default to the same.


--- Quote from: BillyO on September 13, 2022, 01:57:39 pm ---I wonder if the OPs question is well answered here?

--- End quote ---

he asked a simple question, and in return received a whole jumble of complex technical explanation that went straight over his head - and that was, indeed, irrelevant to the context in which his question sat. i'd imagine he is long gone, and will likely not be seen on the forums again.


to answer the OP's question: as the (broadly speaking) frequency of the signal you're sampling heads towards 1/2 of the scope's sampling rate, the image displayed by your DSO will approach a smooth undulation / sine curve. if you wish to observe fine detail in a complex waveform, you need to ensure that your DSO has a sampling rate at least several times the 'frequency' of that detail - as per the earlier posted screenshots.

--- End quote ---

That is misleading to the point of being false.

Replace "frequency of the signal" with "the highest component frequency of the signal" and that would be more nearly correct.


--- Quote ---if you are just wanting to observe a simple RF signal (such as AM modulation on an RF carrier), then you can get away with a sampling rate that is a tad over 2x the carrier frequency. just be wary that a DSO will 'smooth over' signal features that approach the scope's limits.[/i]

--- End quote ---

Completely wrong in several important respects, as noted earlier in the thread - and in more detail in standard textbooks.
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