Automatic vs Manual Measurements

Question:  I work in a Metrology Laboratory calibrating Avionics test sets.  Much of the signal evaluation is done on pulse modulated carrier waveforms at around 1 GHz.  It is important that we maintain traceability to NIST.  I use a Tektronix TDS5104B to view waveforms and make Rise Time and other pulse modulated measurements.  I found the math and automatic measurement functions really useful.  However, I have been told that these functions are not traceable to NIST because of the many ways the scope can be set-up/configured.  I’ve known this for quite some time and decided to verify the automatic measurements against a manual measurement.  If the indications are very similar, I’ve decided to accept the automatic measurement as reliable when it comes to traceability.  Can you comment regarding the use of the automatic measurements in this way and possible traceability to NIST?

Answer:  Traceability implies that the uncertainty of a measurement is adequate for the purpose.  Automatic measurements can have uncertainties as low as or lower than the equivalent manual measurements.   This depends on having a displayed waveform that doesn’t vary too much from one unit under test (UUT) to another, and on having completely defined oscilloscope settings that are optimized for the automatic measurement.  If this is done properly, an automatic measurement may produce consistent results comparable to manual measurements.  

Tektronix often uses automatic oscilloscope measurements in automated and manual performance verification procedures, based on having consistent displayed waveforms and oscilloscope settings as described above, and backed by an understanding of the measurements and by experimental verification.  Tektronix issues certificates of traceable calibration based on these automatic measurements.

On the other hand, the limited flexibility of automatic measurements or excessive variability in the displayed waveforms from one UUT to another may make automatic measurements unsuitable for some measurements that should instead be made manually.

The adequacy of an automatic measurement may be evaluated as suggested in the question, by comparison with a manual measurement.   When comparing automatic and manual measurements it is important to use the same settings and methods as much as possible.  For example a rise time measurement requires determination of base and top lines.  The waveform may exhibit noise, or aberrations, or it may have a long settling time.  For the manual method note how the base and top lines are determined.   For the automated measurement, the base and top line methods (Histogram, Min-Max, or Histogram Mean) and Gating (if appropriate) should be selected to closely match the manual measurement method.

If there are excessive discrepancies between automatic and manual measurements, please contact us at Tektronix for assistance in determining the cause and if possible finding a way to improve the agreement between methods.

Explore posts in the same categories: Ask Scope Guru Q/A, Uncategorized

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