Products > Test Equipment

Quality PC-Based oscilloscope?

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tinhead:

--- Quote from: tinhead on January 21, 2013, 01:34:03 am ---
--- Quote from: alm on January 21, 2013, 12:11:28 am ---Those responses look pretty terrible, do you have any data for a decent probe

--- End quote ---

yep, i do. And here is the less funny part, all my measurments are bad, the terminator i've used is broken. With different one
terminator and P6243 everything looks flat as in user manual - with the broken one even P6243 was 3dBm out at 400MHz.

I have to sleep, tomorrow i will provide corrected values

--- End quote ---


here we go, both TESTEC and TEXAS are definitely good enough

TESTEC TT-HF512 500MHz x10 Probe

  1MHz    0dBm
 50MHz   -0.1dBm
100MHz  -0.1dBm
150MHz  -0.2dBm
200MHz  +0.2dBm
250MHz  +0.2dBm
300MHz  +0.4dBm
350MHz  -0.3dBm
400MHz  -0.7dBm
450MHz  -1.7dBm
500MHz  -2.2dBm
550MHz  -2.8dBm



TEXAS 250/II 250MHz x10 Probe

  1MHz     0dBm
 50MHz  -0.3dBm
100MHz  -0.5dBm
150MHz  -0.1dBm
200MHz  -0.4dBm
250MHz  -0.1dBm
300MHz   0.0dBm
350MHz  +0.3dBm
400MHz   0.0dBm
450MHz  -0.5dBm
500MHz  -1.1dBm
550MHz  -2.4dBm


The Hantek probe, and i assume all the others almost identical probes on ebay (where only name and color seems to be random),
is still useless on Tek TDS7xx series. Probably with different trim cap insde it would work, but yeah, i don't give a s* on that.


Hantek P6200 200MHz x1/x10 Probe

  1MHz     0dBm
 25MHz     0dBm
 50MHz  +1.7dBm
 75MHz  +2.9dBm
100MHz  +3.8dBm
125MHz  +4.4dBm
150MHz  +4.5dBm
175MHz  +4.7dBm
200MHz  +4.9dBm

nctnico:

--- Quote from: JoeyP on January 21, 2013, 02:39:25 am ---
--- Quote from: nctnico on January 21, 2013, 02:21:32 am ---@JoeyP:
You got the numbers wrong. The signal is about 3 LSB high from the ADC's perspective, not the 10 bit fictional resolution. On top of the 10 bit resolution there is also the scope's sin(x)/x interpolation to make a neat line. Here are two other pictures at lower zoom level settings:

--- End quote ---

Actually you got them wrong too. I should have ignored your numbers and done all of the math from the start. Your signal was 1.2Vpp in the first image. You attenuated it 100X for the second image, so it is 12mVpp. 256 x 0.012 / 1.6VFS = 1.92 LSB referred to the 8 bit converter. That's 7.68 LSB referred to the 10 bit result. So the actual DNL in the image is 0.95 LSB referred to the 10 bit result. Still monotonic and happens to be exactly the DNL spec for the TI/National ADC08200, a typical 200MHz 8 bit converter.

--- End quote ---
You can reason all you want but the last images I posted clearly show that the lack of noise causes the interpolation algorithm not to work properly. In case of an oscilloscope you actually want the input stages to be as low noise as possible which contradicts the noise requirement for oversampling. I'm sure that when I add enough noise to the input signal the hi-res mode will show a triangle waveform with straight lines. I'll leave that as an exercise for the reader though. I think I clearly demonstrated that hi-res modes on scopes can and will produce false readings.

Wuerstchenhund:

--- Quote from: HiredMind on January 19, 2013, 04:19:04 am ---I know most PC-based oscilloscopes are pretty much garbage.  But I'm wondering, does anyone make a pc-based scope with pro-level electronics on par with Tek/Agilent/Rigol, etc?   To give an example: If I can't find an adequate PC-based one, my next choice would probably be a Rigol DS2072)
--- End quote ---

There are lots of external 'scopes' (digitizers), which generally can be divided into two categories: 'low-end/crap' (with often pathetic specs) and good quality ones. Unfortunately, if you look at the latter and take out digitizer modules for VXI, PXI and other embedded systems there is not too much left, and what's left mostly consists of (very expensive) highend stuff like Agilent's DSO90008 series.

Since you mention Rigol I assume the DSO90008 is not what you're looking for.


--- Quote ---I know what you're saying: Why not just get a regular DSO?   Well as I said I'm a programmer, so I spend most of my working life in front of 4 gigantic monitors and screaming fast development PCs.  And I'm much faster with a mouse than those buttons and knobs on a regular scope.  I'd love to have the quality capture abilities of a high-end scope without paying for the tiny scope display.

--- End quote ---

The problem is that you generally pay excessively for *not* having the tiny scope display. Most of the better external scopes/digitizers are for special applications and come with a price that reflects that. At the end of the day, the most economic way is to just buy a regular DSO which can be remote-controlled, ignore that it has local controls and a screen, and just use it with your PC as you would do with an USB scope.

JoeyP:

--- Quote from: nctnico on January 21, 2013, 04:30:27 pm ---You can reason all you want but the last images I posted clearly show that the lack of noise causes the interpolation algorithm not to work properly.

--- End quote ---
Your last images may "clearly show" -you- something, but you seem to be working very hard to contradict the obvious.

tinhead has posted a good example of oversampling/hires mode for a Tektronix scope here: https://www.eevblog.com/forum/reviews/quality-pc-based-oscilloscope/msg183740/#msg183740

I've posted an example from an Agilent scope below. The test signal was a 20mV p-p ramp. Each image was captured after a single-shot acquisition using real-time sample mode.

The "Signal x 1" image is the actual signal catpured at 128mV/div. I used the vernier to make the math more intuitive. With 8 divisions, full scale is 1024mV so the 20mV test signal is 2% of full scale. Note the indicated amplitude of 52mV due to internal noise that the scope has added to the test signal. If you magnify the image, you can see that each 8 bit LSB occupies 2 or 3 vertical pixels depending upon rounding error for each position.

The "Normal x 16" image shows the same captured signal zoomed vertically 16x (after the acquisition halted). The 8 bit resolution is quite visible, and as expected 255 x 52mV/1.024V FS yields 13 LSBs p-p.

The "Hires x 16" image shows the same signal captured with hires mode turned on, also at 128mV/div, and then zoomed vertically16x to 8mV/Div. No other changes were made to the scope or the signal. I used 16x because the scope has increased the converter's 8 bit hardwre resolution to a 12 bit oversampled result, so 4096/256=16.

The increase in resolution in the hires image speaks for itself. If you magnify the image, you can see that each LSB of the 12 bit result now occupies 2 or 3 pixels in height, exactly as expected. You can also see that hires mode has cleaned up much of the noise resulting in an accurate amplitude measurement of 20mV p-p. The result is 79 individual steps, which is very close to the theoretical 4095 x 20mV/1.024V FS = 80.

At the very least, the hires/oversampling feature cleans up noise and allows much lower level single-shot signals to be measured. And if you really need the extra resolution, it's there too.

So if nctnico wants to pretend that this feature doesn't work, he is certainly welcome to do so. But while disparaging an entire industry, or a particular manufacturer using a fabricated argument and "polite" language may be easy, it is not fair to those being disparaged nor to those who may be reading in an effort to learn more about a valuable feature they may desire in a scope. It needs to be challenged, I will be here to do so any time I see it happen.

nctnico:
Let me repeat myself: a scope input has to be low noise to be usefull to begin with. The oversampling used in the high-res mode however requires a certain amount of noise to work. There is a clear contradiction here.  To put that to the test I've demonstrated that the required noise level can actually be insufficient in a normal situation and cause the oscilloscope to show a difference signal than there is in reality without any warning. BTW, for my test I've used a simple RC generator which can be bought for $50 through Ebay nowadays. It most certainly is not some super high end ultra low noise generator.

To put it simple: if there is not enough noise the oversampling simply will not work. You can't change the laws of physics. Oscilloscope makers clearly cut a corner here.

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