Stanford Research Systems SR620, Руководство пользователя

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Specification Guide 23
SR620 Universal Time Interval Counter
SR620 SPECIFICATION GUIDE
This section provides a guide to understanding the SR620's specifications and their effect on the accuracy
and resolution of a measurement. First a little terminology-
TERMINOLOGY
LEAST SIGNIFICANT DIGIT (LSD)
The LSD is the smallest displayed increment in a measurement. The SR620 has a 4ps single-shot LSD and
thus the smallest amount that two single-shot time interval measurements may differ by is 4ps.
RESOLUTION
Resolution is the smallest difference in a measurement that the SR620 can discern. That is, the smallest
statistically significant change which can be measured by the SR620. Resolution is of primary interest in
comparing readings from the same instrument. The instrument resolution is limited by many things including
short-term timebase stability, internal noise, trigger noise,etc. Because these processes are random in
nature, resolution is specified as an rms value rather than a peak value. This rms value is the standard
deviation of the measured value. The SR620's single-shot resolution is typically 25ps rms. This number can
be improved by averaging over many measurements, or in the case of frequency and period measurements,
increasing the gate time. The single-shot LSD is always smaller than the single-shot resolution.
ERROR
Error is defined as the difference between the measured value and actual value of the signal being measured.
The error in a measurement is of primary concern when the absolute value of the parameter being measured
is important. Error consists of the random factors mentioned above and systematic uncertainties in the
measurement. Systematic uncertainties include timebase aging, trigger level error, insertion delay, etc..
Systematic errors may always be measured and subtracted from subsequent measurements to reduce the
error. The SR620's absolute error is typically less than 0.5ns for time interval measurements less than 1ms.
DIFFERENTIAL NON-LINEARITY
Absolute error is of interest in determining how far a value is from the actual value. Often only the relative
accuracy (the difference between two measurements) is important. Differential non-linearity is a
measurement of the relative accuracy of a measurement and is specified as the maximum time error for any
given relative measurement. The SR620's differential non-linearity is typically ±50ps. That means if the time
interval is changed by some amount the SR620 will report that change to within ±50ps of that change.
Graphs 1 and 2 show the SR620's typical differential non-linearity as a function of time interval. Graph 1
shows the non-linearity over the time range of 0 to 11ns. The deviations are due to the residual non-linearity
of the time-to-amplitude converters. This curve repeats every 11.11ns- the period of the time-to-amplitude
converters. Graph 2 shows the non-linearity over the time range of 0 to 11ms. For times greater than 11ms
the non-linearity is dominated by the timebase error.