HP 54753A User Manual page 219

Improving Time Domain Network Measurements
Removing Measurement Errors
Generating the Digital Filter
The second part of the normalization process generates the digital filter. Unlike
the errors removed by subtracting the first normalization signal, the errors
removed by the filter are proportional to the amplitude of the DUT response.
For the second part of the TDR normalization process, the DUT is replaced by
a short circuit. The frequency response of the test system is derived from the
measured short cal signal. Note that a short circuit should be used rather than
an open circuit. When a step hits an open circuit at the end of a real-world
transmission line, some of the energy is lost due to radiation rather than being
reflected. Of course there is no such thing as a perfect short either, but the
energy lost due to resistance in the short has a much smaller effect.
It is important that a good quality short be used, because the normalization
process assumes a perfect short circuit termination. Any non-ideal components
in the measured short cal signal are attributed to the test system. If any of the
non-ideal components are, in reality due to the short itself, the filter will attempt
to correct for error terms which do not exist in the test system. By attempting
to correct for errors which do not exist, the filter can actually add error terms
into the normalized measurement results.
In the second part of the TDT normalization process, the transmission through-
path is connected without the DUT. The frequency response of the test system
is then measured with the aid of the step stimulus. With this information, a
digital filter can be computed that will compensate for errors due to anomalies
in the frequency response of the test system.
Correcting for Secondary Reflections
Secondary reflections caused by the impedance mismatch between the test port
and the transmission media can also be corrected. In step TDNA, airlines can
separate the primary reflection from the secondary reflection. Time windowing
can then be used to remove the secondary reflection. In CW TDNA, a third
normalization is used.
The impedance mismatch between test port and transmission media reflects a
portion of the primary reflection back towards the DUT. A secondary reflection
from the DUT may then be measured. Secondary reflections are usually very
small.
Figure 10-10 shows the relative size of primary and secondary reflections. The
lower waveform is a copy of the upper waveform with the voltage scale greatly
expanded about the baseline to show more clearly the shape of the secondary
reflection. The DUT is a short circuit connected to the oscilloscope through a
10-13