VTI Instruments EMX-4016B, User Manual

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APPENDIX B 51
of ½ decimation filter stages. While the ClockFrequency is global, the SampleRate and Span value
can be different in each channel. The value must be the one derived from a global ClockFrequency .
Span
The Span is the nominal frequency range of the acquired signal. The value of Span is determined
by the decimation filter’s cutoff frequency. In non-oversample mode, the Span = SampleRate /2.56
in. When Oversample is true, the Span = SampleRate /5.12.
RecordSize
The RecordSize specifies the number of data samples read back from the instrument at a time. The
total number of data samples captured for each trigger is equal to the product of RecordSize and the
Measurement.NumRecordsPerTrigger property -- that is, NumRecordsPerTrigger records are
captured, and each is of size RecordSize . Figure 4-12 shows the case where NumRecordsPerTrigger
= 4. As a special case, when NumRecordsPerTrigger is set to 0, an infinite number of data records
are returned in RecordSize sample chunks after a trigger event until the measurement is aborted.
Trigger Level
Trigger Trigger
Record Record Record Record
Record Record Record Record
Figure 4-13: Record Size and NumRecordsPerTrigger
With EMX-4250/4350/4380, the minimum RecordSize is 1 and the maximum power of 2
RecordSize is 32768. Since each data packet transferred from the instrument to the host contains
one data record from each ADC channel, in general, the user can achieve higher data transfer using
a larger
RecordSize value due to reduced overhead, but this affects the data update interval, as the
amount of time to acquire one data record is: T = RecordSize / SampleRate .
Oversample
When the Oversample is set to true, the data is twice oversampled than Nyquist sample rate. The
SampleRate becomes 5.12 times of Span . The Oversample mode is currently not supported.
Multipass
When the Multipass property is set to true, the data record contains data samples from multiple
½ decimation spans below the current decimation span specified by Span property. The
RecordSize /2 samples is at Span, RecordSize /4 samples is at Span /2, RecordSize /8 samples is
Span /4, etc., until it reaches less than 1 sample or the lowest decimation stage. Multipass is currently
not supported.