The
Timebase Correction (TBC) software package , used with high-speed
sampling oscilloscopes, can correct both random and systematic
timebase errors using measurements of two quadrature sinusoids
made simultaneously with a waveform of interest.
A
simple illustration of our timebase correction concept is shown
in Fig. 1, which plots uncorrected measurements (circles) of a
reference sinusoid with an estimate of the distorted sinusoid
(solid curve). The estimated sinusoid is found by minimizing the
average "distance" between the samples and the sinusoid.
If we assume, for illustrative purposes, that there is no additive
noise, we can estimate the total time error due to timebase distortion
and jitter by drawing a horizontal line between each measurement
(circles) and the distorted sinusoid. The length of each line
represents the difference between the nominal (oscilloscope) time
at which the measurement was taken and the time as determined
by the distorted sinusoidal fit. The time that each line intersects
the distorted sinusoid is the corrected time for each sample.
Once
the timebase error is known, it can be applied to a simultaneously
measured signal of interest if the timing errors of the simultaneous
measurements are sufficiently correlated. Fig. 2 shows an actual
measurement of five sinusoids measured simultaneously with the
reference sinusoids that were used to calculate the timebase error.
The estimated jitter before the correction was about 3.3 ps and
the effects of timebase distortion are clearly visible at 4 ns.
After correction for timebase error the residual error for this
example is estimated to be about 0.2 ps.
This
work was supported in part by the NIST
Office of Microelectronics Programs.
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