Electronics > Projects, Designs, and Technical Stuff
Why are oscilloscopes so inaccurate?
coppercone2:
scopes are good at telling you what is actually happening instead of giving too useful a measurement etc
its like being able to look at something to see if its doing the right thing then trying to break out and measuring tape
ANd I think high end unfriendly lecroys do a bit more then 'suck' (well they do suck balls but they are good)
its like, being able to look at something and then decide where you put the ruler to measure it. The cabinet looks crooked (not lets measure every part of the cabinet to see if its good.. carpenters would go insane).
David Hess:
--- Quote from: Alex Eisenhut on August 10, 2019, 04:48:45 pm ---
--- Quote from: David Hess on August 10, 2019, 03:16:17 pm ---
You use an oscilloscope that includes a differential comparator.
--- End quote ---
Sounds like a use case for a vintage analog scope...
http://w140.com/tekwiki/wiki/1A5
--- End quote ---
Or something a couple years newer:
http://w140.com/tekwiki/wiki/7A13
Those differential comparators are ancestors to the ones now made by Preamble/LeCroy:
https://teledynelecroy.com/probes/differential-amplifiers
An alternative method is to make an integrated measurement at a specific point in the waveform which gives the speed advantages of sampling with the low noise of integration:
http://w140.com/tekwiki/wiki/7D12
And that sort of thing went on to become gated integrated measurements in automated analog oscilloscopes like the Tektronix 4 channel 22xx series and maybe 24xx series.
A DSO's averaging mode is effectively the same thing but is still limited by the precision of their signal conditioning and digitizer. In practice, very few applications require better accuracy so the alternatives are no longer economical. They were used in the past because there was no better alternative.
dom0:
As an owner and not-so-infrequent user of a 7A13 I can say that it is a surprisingly useful tool to have in the bag, just like the 7A22.
Measurement functions of analog 'scopes also tend to be decent. For example, the DC VM in the 2246 has an accuracy spec of +- 0.5 % of reading. Peak measurements are implemented using precision rectifier, so relatively accurate as well. Only if you use manual cursors you are actually getting the full deflection and aberration errors.
Edit: On second thought I am not so sure about the manual cursors. My guess would be that those are implemented using S&H circuits, so deflection and aberration errors are zero since cursor and measurement position are identical. So even those might actually be quite good. Have to check.
David Hess:
--- Quote from: dom0 on August 11, 2019, 05:07:22 pm ---As an owner and not-so-infrequent user of a 7A13 I can say that it is a surprisingly useful tool to have in the bag, just like the 7A22.
--- End quote ---
I really like the 7A13's versatility but it comes at the cost of high noise. I am not sure there is anyway around that when a bootstrapped differential amplifier is involved.
I do not know about Tektronix but HP makes low voltage probes which include the voltage comparison function. The offset controls of a DSO do the same thing but usually operate over only a very limited range unlike a 7A13 which is a very useful +/-10 volts.
--- Quote ---Measurement functions of analog 'scopes also tend to be decent. For example, the DC VM in the 2246 has an accuracy spec of +- 0.5 % of reading. Peak measurements are implemented using precision rectifier, so relatively accurate as well. Only if you use manual cursors you are actually getting the full deflection and aberration errors.
--- End quote ---
Peak voltage measurements on the 22xx series are actually made using the B trigger which is how these oscilloscopes achieve full bandwidth peak voltage measurements. DC voltage measurements are made by inserting a very low pass trigger coupling filter which explains why they cannot be gated which is too bad.
Signals are measured by using the B trigger level comparator as a successive approximation ADC to determine the peaks or DC level of the applied signal.
--- Quote ---Edit: On second thought I am not so sure about the manual cursors. My guess would be that those are implemented using S&H circuits, so deflection and aberration errors are zero since cursor and measurement position are identical. So even those might actually be quite good. Have to check.
--- End quote ---
I think that is correct; cursor measurements should be more accurate than graticule measurements for that reason.
HendriXML:
I had some fun using a DSO as a decent precision voltage meter.
By doing some calibration steps, using the vertical offset (using multiple iterations narrowing down the measurement window) and a lot of averaging it is possible to get pretty descent precision.
https://www.eevblog.com/forum/testgear/using-a-awg-and-a-scope-do-a-low-voltage-level-characterization-of-a-1n4005/msg2476305/#msg2476305
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