VTI Instruments EX1401, User Manual

Page: 86 / 114

VTI Instruments Corp.
86 EX1401 Instrument Drivers
FilterCoefficients
The normalized filter coefficients of the HighPerformance and MedLatency filters can be
programmed by the user to achieve optimal results for their particular application. These are each
32-tap FIR filters with 64 coefficients.
GroupDelay
The GroupDelay parameter specifies the delay of each stage of the FIR and CFIR filters used by the
HighPerformance and MedLatency filters, respectively, in units of undecimated samples. By
default, this value will be correct for the preprogrammed value of FilterCoefficients . If, however, a
custom set of coefficients is supplied, this property should be set with the correct group delay for
those coefficients. This allows the firmware to perform group delay compensation on measured data
correctly with the customized filter response.
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 5-9 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
F
IGURE
5-10: R
ECORD
S
IZE AND
N
UM
R
ECORDS
P
ER
T
RIGGER
The minimum RecordSize is 1 and the maximum value is 65527. The maximum value for
RecordSize that is also a power of two is 32768. Since each data packet transferred from the
instrument to the host contains one data record from a single ADC channel, the user can achieve
higher data transfer rates by using a larger
RecordSize to reduce overhead. Doing so will affect the
data update interval, as the record period (the amount of time to acquire one data record) is: T =
RecordSize / SampleRate .
Setting The Sample Rate
There are four key properties to configure when setting the EX1401’s sample rate. These are
ClockFrequency , FilterType , DownsamplingFactor , and SampleRate . ClockFrequency defines the
rate at which the instruments ADCs produce data. The other properties all define how that data is
digitally decimated and filtered. When possible, choosing higher
ClockFrequency and higher
amount of decimation for the same final sample rate is generally better, providing better signal