Campbell-scientific CR9000X Measurement and Control System Instrukcja Użytkownika Pobierz pdf (Strona 120)

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Section 3. CR9000X Measurement Details

The digital implementation of the CR9052 FIR filters maintains a group delay

that is independent of frequency (linear phase response). In addition, the

digital filter performance does not change with time, temperature, or

component tolerances. The on-board DSP automatically chooses the

appropriate low-pass filter to anti-alias the input data for the user's desired

measurement rate. If desired, users may load their own coefficients into the

on-board DSP to tailor the FIR filter's frequency response to their own needs

(band pass, band reject, etc.).

CR9052IEPE DC Frequency Response

The CR9052IEPE module has two programmable time constants available: 5

seconds and 0.5 seconds. The advantage of the 0.5 second time constant is that

if you have a step in the voltage (either from a shock to the sensor or when

initially supplying excitation) it will only take 0.5 seconds for 63% of the

voltage step to discharge, while with the 5 second time constant, it would take

5 seconds. See Chart 3.3-2 Step Discharge Rate.

Chart 3.3-2 Step Discharge Rate

0.2

0.4

0.6

0.8

1.2

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SECONDS

V/V_STEP RESPONSE

Tau= 5.0 Seconds

Tau = 0.5 Seconds

Tim e Constant

= 5 Seconds

Tim e Constant

= 0.5 Seconds

The advantage of the 5.0 second time constant is that it will not result in lower

frequencies being attenuated as much (3 dB at 0.03 Hz) as the 0.5 second time

constant (3 dB at 0.3 Hz). See Chart 3.3-3 Frequency Response for

attenuation plot comparison.

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