Trigger Level and Delay Issues

The 89600 VSA relies on the input hardware to do the triggering. The input hardware must use a trigger level supplied by 89600 VSA, and return back to the VSA a delay value indicating exactly where the trigger occurred.

However, once the VSA gets this data, it passes through the 89600 VSA software resampling filters and time corrections before the data is displayed. These two steps can cause unexpected effects on the trigger level and delay.

Resampling effects

The 89600 VSA resampler acts like a variable-bandwidth low-pass filter, which is used to reduce the bandwidth of the raw input data to match the bandwidth desired by the user. If there is very much energy in the region that gets filtered away, the signal level and shape will change as a result of the resampling, resulting in a time waveform that does not match the original waveform that the triggering took place on. This effect is most pronounced if the input hardware has widely spaced "cardinal" spans, which is the case for the Infiniium and 6000/7000 Series oscilloscopes. The effect is also more pronounced when the signal is far from the 89600 VSA's center frequency.

If the user bandwidth matches the hardware cardinal span, and the hardware produces data at the native 89600 VSA oversample ratio a non-triggered measurement will not use the resampler. However, when triggering is turned on, the resampler is always used because the resampler is used to get exact sub-sample trigger delays. Note that the resampler will attenuate out-of-band signals even if the bandwidth is not changed, so the signal shape can change as a result.