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
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