HP 54753A User Manual page 112

Single-ended TDR Measurements
Single-ended TDR Features
in to the shorted end of the cable. Also, it allows the TDR response to be
measured as though it had been stimulated by a step with no preshoot or
overshoot rather than the actual step.
The final stage in Establishing Normalization is measuring the response of the
cable when a 50 ohm terminator is connected to the end of the cable in place
of the short. This response will be subtracted from all TDR measurements as
long as the Time base position has not been changed since the reference plane
was established. The normalization filter is applied to the resultant waveform.
This process removes the incident step from the normalized waveform and any
small synchronous noise that may be present.
TDT Establish Normalization and Reference Plane Feature
This feature performs the following:
• Establishes the Reference Plane.
• Measures the step transmitted to the TDT destination channel.
• Builds a normalization filter.
The Reference Plane is defined as the point in time that coincides with the 50%
point of the step that is transmitted when the cables used to connect to a device
under test (DUT) are connected together. Once the Reference Plane is
established, cursor measurements can be made in terms of propagation delay
rather than referenced to the trigger point.
The next stage in establishing the TDT Normalization process involves
measuring the step transmitted when the cables used to connect to the DUT
are connected together. As long as the transmitted step is allowed to settle to
its final value, this step height is equal to the incident step height. The sweep
speed must be slow enough to allow any slow tails caused by cable losses to
settle. This allows the oscilloscope to base its calculations of gain on the actual
measured height of the step instead of the nominal step height of 200 mV.
Also, from this information the oscilloscope builds a normalization filter which
can be applied to any transmitted signal. When applied to the step transmitted
through the cables used for TDT measurements, this filter removes any losses
or discontinuities caused by the cables. It also allows the TDT response of a
DUT to be measured as if stimulated by a perfect step rather than by the actual
step of the oscilloscope. The risetime of the perfect step is selectable allowing
TDT measurements of systems to be made at several different risetime values.
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