Low-Cost USB Oscilloscope

[T.Jackson]

But without complex software algorithms (that I don't know enough about yet) -- I estimate the usable bandwidth to be about 1MHz @ 20M samples / second.

[T.Jackson]

This statement has a lot to do with it I feel ...

The fundamental AC waveform is the sine wave, a square wave is made up of a multitude of many, many, sine waves. We certainly have a square wave @ 500MHz w/ 2g sample / second.

Question is; how do we extract it?

[T.Jackson]

Qualified engineers like; Melanie & Jerson should have little difficulty in answering this.

[T.Jackson]

Maybe trade secret? :|

[Jerson]

Qualified engineers like; Melanie & Jerson should have little difficulty in answering this.

Trent,

I got into this thread a little too late. However, I do not understand the question you're asking here.
BTW : I have used a Parallax USB based scope in one of my designs and more recently a Velleman scope. Their bandwidth was stated to be 20MHz, but the highest frequency we could reasonably see would be around 100KHz IIRC. Its been a while since I did this, so my information may be of the mark. The 20Msamples/sec happened to be the sample rate. For repetitive signals, it becomes easy to reconstruct the waveform with higher resolution by staggering the sample points wrt the zero crossing of the waveform. However, the low cost PC scopes that I have used did not seem to have this feature and just took snapshots.
This seemed to be the pattern for acquisition
Pre Samples -> Trigger -> Samples -> Ready is signalled -> display is refreshed -> repeat the cycle

[T.Jackson]

Trent,

I got into this thread a little too late. However, I do not understand the question you're asking here. For repetitive signals, it becomes easy to reconstruct the waveform with higher resolution by staggering the sample points wrt the zero crossing of the waveform.

BitScope manages to pull off measuring one shot events @ 20MHz with only 40M samples. See; http://www.bitscope.com/design/

So, I guess this is ultimately my question; how do they do it? How in the world can a complex waveform be captured at 20MHz with only 40M samples?

Bitscope can measure a sinusoidal waveform well & truly beyond its sampling rate. Apparently, the waveform is actually all there as far as the ADC & software is concerned. This leaves me with a lot of question & much confusion.

[Jerson]

BitScope manages to pull off measuring one shot events @ 20MHz with only 40M samples. See; http://www.bitscope.com/design/

So, I guess this is ultimately my question; how do they do it? How in the world can a complex waveform be captured at 20MHz with only 40M samples?

What they are trying to state is the Nyquist criterion; where a frequency can be represented by at least 2 points on the wave. So, 40 mega samples can actually capture a frequency of 20MHz max; with a caveat - the frequencies must be in sync or you will see aliasing. And no matter what, you cannot *fill in the gaps* if you have only 2 samples in one cycle.

I would reckon that the bitscope could capture with reasonable faithfulness an aperiodic wave @40Ms/sec uptil about 40/8 = 5MHz. Obviously, it will be better to have more samples to fill the gaps; thats where the sub-sampling term comes in - this is suitable only for periodic waveforms. The nearest possibility without sub-sampling is B-spline interpolation for single shot data.

[Luciano]

Hi,

See this application note.

Agilent (HP) Application Note 1587

Evaluating Oscilloscope Sample Rates vs.Sampling Fidelity:
How to Make the Most Accurate Digital Measurements
http://cp.literature.agilent.com/lit...989-5732EN.pdf

Best regards,

Luciano