Offset Correction; Figure 10: Digital Filter Aliasing - Tektronix IsoVu TIVM Series User Manual

Offset correction

TIVM Series Measurement System User Manual
The measurement system uses state-of-the-art technology that allows the DUT
to be totally isolated from the oscilloscope. This results in a very large common
mode rejection ratio (CMRR) and lets you see small signals that would otherwise
be obscured by the high common mode interference.
The TIVM Series products implement an offset correction algorithm to minimize
any drift in the system due to changes in temperature or fiber movement. The
offset correction algorithm maintains a constant DC level for the displayed
on-screen signal.
One of the components used for offset correction is a digital low-pass filter.
Normally, the frequency of the signal is high enough that the operation of the
filter is transparent. However, at low frequencies special care must be taken to
prevent aberrations from being introduced.
When attempting to capture a signal that has a frequency near 43.5 Hz, 87.0 Hz, or
130.5 Hz, the display might show aliasing. If you cannot avoid these frequencies,
the offset correction should be disabled. The sample rate of the digital filter was
selected to not alias at 50 Hz or 60 Hz.
Figure 10: Digital filter aliasing
To disable the offset correction algorithm, hold down the MENU button on the
controller and momentarily press the CLAMPING button. The OVERRANGE
indicator on the controller should blink two times. To re-enable the correction,
repeat the operation; the OVERRANGE indicator should blink once. Turning off
the correction is temporary; when the measurement system is removed/re-attached,
the offset correction is re-enabled.
Operating information
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