Re: Calibrators vs Signal/Function Generators


 

The differences between calibrators and generators are profound.

1. the calibrator is defined in terms of steps that are *exactly the needed intervals for calibration.*
2. values are *peak to peak* on calibrators, *rms* in generators, a huge issue.
3. most generators cannot easily be set to the required intervals over the huge dynamic range needed (mV to V), or to the precise time intervals (uS to S).
4. calibrators do ONLY the required task, nothing else.

low end generators do not even have calibrated output voltage (sometimes not even any markings, just a knob), and may have only dial frequency calibration. if you are lucky enough to have a higher end digital generator you may have a better grip on the time and frequency values, but considering theĀ  1dB or worse amplitude cal of even high end hp generators, and the need for proper termination, it's quite easy to make some pretty big errors in scope calibration. also, generators are general instruments, and are not really optimized for any task, and have divergent performance in many settings, but adequate for their use. calibrators, do only a specific task, and often have design implicit accuracy that can be set instrument wide with only one or two adjustments.

the argument about ultimate calibration is a bit circular, literally *NOBODY* has truly accurate items at their disposal, only *adequate* ones. I am lucky enough to have some very end cal equipment acquired over many years, AND I send it to a good cal lab, which *in turn* sends their gear out annually to a national accredited lab for cal. but as any metrology guy will tell you, there always remains some uncertainty in all measurements, even with known good standards. I can be confident my work is good enough for the task at hand, but only because I have tools with performance far beyond the required need for the work, *AND* I regularly cross-check what I am doing between different standards

scope cal is a modest undertaking, and 1% is adequate literally for any function, in the world of calibration, that is NOTHING in terms of precision, where the real discussion is in terms of ppm (parts per million) or better.

to be clear, if you have a source of known good time and voltage, and you know the impact of the terminating impedance, you can cal a scope, but it may be excruciatingly painful with a generator, and quick and easy with a dedicated calibrator. on something like a 2465, you really can't fool around, you need exactly the right tools, or you are just damaging the scope to attempt a "cal". do not go there.

anyway, just my $0.02 worth, you can get my industry articles on calibration from our site on this page:
https://www.sphere.bc.ca/test/data.html
look in the yellow bar at the left, under the AEA logo.

all the best,
walter

--
Walter Shawlee 2
Sphere Research Corp. 3394 Sunnyside Rd.
West Kelowna, BC, V1Z 2V4 CANADA
Phone: +1 (250-769-1834 -:- http://www.sphere.bc.ca
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