TDR Example
After loading the model file (54120.demo.mdl), open the mdltdr model for this example.
In this example, the setup tdr_setup makes measurements using the scope's internal capabilities for sourcing a TDR pulse train. The acquisition of reflected and transmitted waveforms is demonstrated, and the waveforms are plotted.
Measurement/Instrument Setup
The instrument setup is an HP 54123T oscilloscope with a 1 meter 50-ohm cable, part no. 8120-4948, attached from the channel 1 port to the channel 4 port. (Be sure to issue the Rebuild command so the oscilloscope will be recognized by IC-CAP.)
Simulation
The circuit for this model contains a transmission line to simulate the cable, and it contains a 50-ohm resistor to simulate the loading offered by the channel 4 input.
The 50 ohms present as a channel 1 source impedance are automatically accounted for in the tdr_step input. See the Table 109 for details.
Setup specifics
Edit the tdr_setup setup (mdltdr) as described in the following input and output tables.
Table 109 Inputs
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time
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This input is necessary to specify the time window for which the measured and simulated data will be acquired. It uses the Mode T. Naming it time is not necessary.
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tdr_step
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This input describes the pulse stimulus. Note the following comments on some of the important fields:
Mode:
must be V
+ Node and - Node:
points at which the simulated pulse is applied
Unit:
scope channel from which TDR pulse is sourced. The scope can only source from its left-most port, generally designated as CH1.
Type:
must be TDR
Pulsed Value:
Note: 400m is used here. The manual states "a pulsed value of 200mv" is what the scope will deliver. This is true when the scope delivers its pulse across a 50-ohm load. The open-circuit value is actually 400mV, and that is what is required here for agreement between measured and simulated waveforms. This value will affect simulated data only—the scope provides no flexibility in its pulse magnitude.
Delay Time, Rise Time, Fall Time, Pulse Width, Resistance:
These are all selected to cause the simulator to use a signal specification roughly compatible with the 54120-family TDR pulse specification. Changing these values will affect simulated data only.
Period:
Values from 2usec to 65535usec can be chosen, which will affect both measured and simulated data. For simulation, the limits are actually much wider
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Table 110 Outputs
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tdtout
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This output will collect a waveform on the 4th input port of the scope. Note the use of CH4, and the Mode V. This is transmission data, in that it is what is transmitted through the cable, from CH1.
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tdrout
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This output will collect a waveform on the 1st input port of the scope. Note the use of CH1 to specify this. During simulation, the waveform is captured across the + and - Nodes shown. This is reflection data, because it is the waveform reflected from the cable and CH4 back to CH1, where the stimulus originates.
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