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Aligning Measured and Simulated Data
This example demonstrates aligning simulated and measured time-domain data when using a Pulse Generator.
The HP 8130 has a delay between its trigger output and its signal output. This delay induces a minor difficulty when attempting to align simulated and measured data, because simulators have no such delay. This delay has some dependency on pulse parameters, especially Period. This was accounted for during driver design by adding the `Pulse Delay Offset' instrument option.
The value specified for `Pulse Delay Offset' is added to the TDR or PULSE input's Delay value. Positive values will shift the waveform to the right, and negative values will shift the waveform to the left. This option enables you to align the simulated and measured waveforms. The option may need adjustment if the Period is changed. The default for this option is zero.
A fundamental problem with time-domain measurement is that the instruments do not have infinite speed, so you cannot measure everything you can simulate.
There are two situations when this is really important:
| • | The 54120 series cannot digitize any data until at least 16ns after a trigger edge is received. For this reason, IC-CAP imposes a lower bound on the T-mode Start time, to protect you from obtaining invalid data. |
| • | The HP 8130 emits a trigger pulse approximately 18ns before its signal pulse. It was designed this way because oscilloscopes may need some `setup' time to capture the first edge appearing on the pulsed signal line. In plots, this causes the measured waveform to shift to the right, relative to the simulated waveform. |
To obtain good time-axis alignment between simulated and measured data, we recommend these steps:
| • | Increase the Delay in the PULSE input in the setup. This should shift the simulated waveform to the right. |
| • | Decrease the HP 8130's `Pulse Delay Offset' Instrument Option. This should shift the measured waveform to the left, inducing alignment. |
By following the above steps, there should be no problem obtaining proper alignment between simulated and measured waveforms.
Note that if you try to set `Pulse Delay Offset' to less than zero, you may get a measurement error message. To avoid such errors, first increase the Delay field in the PULSE or TDR type input. The instrument's `Pulse Delay Offset' option plus the Delay in the PULSE input must add to greater than or equal to zero.