Campbell Scientific CDM-VW300 Series Dynamic Vibrating-Wire Analyzer System - Manual (Page 66)

CDM-VW300 Series Dynamic Vibrating-Wire Analyzers
and the Maximum Frequency (Hz) entered in DVWTool. The
CDM_VW300Config() instruction equivalent to Maximum Frequency (Hz)
is the HighFreq parameter. When a high-frequency limit is entered into
DVWTool or the CRBasic program, an error window is created that spans the
bandwidth between the frequency cut-off requested (Maximum Frequency or
HighFreq) and the frequency cut-off the analyzer can actually establish
(Actual Max Freq). If a measured frequency falls within the error band, the
high-frequency warning flag is set true. For example, if a high-frequency limit
of 1000 Hz is entered, and the closest the analyzer can come to complying is to
set the limit at 1049.07 Hz, any frequency above 1049.07 Hz is discarded, and
any frequency below 1049.07 Hz is used. However, any frequency between
1000 Hz and 1049.07 Hz will activate the high-frequency warning flag,
indicating that the data include at least one measurement above the user set
limit. See Section 8.5.1.2, Calculating Low- and High-Frequency Boundaries,
for more information.
8.5.1.2 Calculating Low- and High-Frequency Boundaries
The easiest way to minimize the size of the low- and high-frequency error
windows is to enter estimated limits into the Minimum Frequency and
Maximum Frequency columns in the DVWTool main screen, record the
resulting values displayed in the Actual Min Freq and Actual Max Freq
columns, then turn around and enter those values back into the Minimum
Frequency and Maximum Frequency columns or the LowFreq and
HighFreq parameters in the CDM_VW300Config() CRBasic instruction.
However, to calculate the actual-minimum and actual-maximum frequencies,
use the information in TABLE 8-1 to size the value. For example, to calculate
the actual-minimum frequency, assuming a measurement is to be taken at a
scan rate of 20 Hz, use an integer multiple of 47.68 to calculate the actual-
minimum frequency. If the low-frequency limit is estimated to be 150 Hz,
enter 3 x 47.68, or 143.04 Hz, as the Minimum Frequency in DVWTool or as
the LowFreq argument in the CDM_VW300Config() CRBasic instruction.
To calculate the actual-maximum frequency, assuming a measurement is to be
taken at a scan rate of 20 Hz, use an integer multiple of 47.68 to calculate the
actual-maximum frequency. If the high-frequency limit is estimated to be 500
Hz, enter 10 x 47.68, or 476.80 Hz, as the Maximum Frequency in DVWTool
or as the HighFreq argument in the CDM_VW300Config() CRBasic
instruction.
TABLE 8-1. Scan Rate and Boundary Resolution
Scan Rate (Hz) Boundary Resolution (Hz) 1
20 47.68
50 95.37
100 190.73
1 The low-frequency limit or the high-frequency limit must be an integer multiple of this frequency.
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