Document ID: chunk:federal_register_of_legislation:F2018C00341:body:0:p48
Version: federal_register_of_legislation:F2018C00341
Segment Type: other
Provision Reference: 
Character Range: 137109–140143

than 30 Hz. Amplitude errors shall be less than ±0.5 per cent in the relevant frequency range of 0 Hz to 30 Hz. All analogue signals shall be processed with filters having sufficiently similar phase characteristics to ensure that time delay differences due to filtering lie within the required accuracy for time measurement.

        Note: During analogue filtering of signals with different frequency contents, phase shifts can occur. Therefore, a data processing method, as described in paragraph 2. of this attachment, is preferable.

       2. Digital data processing

       2.1. General consideration

        Preparation of analogue signals includes consideration of filter amplitude attenuation and sampling rate to avoid aliasing errors, and filter phase lags and time delays. Sampling and digitising considerations include pre-sampling amplification of signals to minimize digitising errors; number of bits per sample; number of samples per cycle; sample and hold amplifiers; and time-wise spacing of samples. Considerations for additional phaseless digital filtering include selection of pass bands and stop bands and the attenuation and allowable ripple in each; and correction of filter phase lags. Each of these factors shall be considered in order to achieve a relative overall data acquisition accuracy of ±0.5 per cent.

       2.2. Aliasing errors

        In order to avoid uncorrectable aliasing errors, the analogue signals shall be appropriately filtered before sampling and digitising. The order of the filters used and their pass band shall be chosen according to both the required flatness in the relevant frequency range and the sampling rate.

        The minimum filter characteristics and sampling rate shall be such that:

            (a) Within the relevant frequency range of 0 Hz to fmax = 30 Hz the attenuation is less than the resolution of the data acquisition system; and

            (b) At one-half the sampling rate (i.e. the Nyquist or "folding" frequency) the magnitudes of all frequency components of signal and noise are reduced to less than the system resolution.

        For 0.05 per cent resolution the filter attenuation shall be less than 0.05 per cent in the frequency range between 0 and 30 Hz, and the attenuation shall be greater than 99.95 per cent at all frequencies greater than one-half the sampling frequency.

        Note: For a Butterworth filter the attenuation is given by:

                  and

        where:

        n is the order to filter;

        fmax is the relevant frequency range (30 Hz);

        fo is the filter cut-off frequency;

        fN is the Nyquist or "folding" frequency.

        For a fourth order filter

        for A = 0.9995: fo = 2.37 ∙ fmax

        for A = 0.0005: fS, = 2 ∙ (6.69 ∙ fo), where fS, is the sampling
       frequency = 2 ∙ fN.

       2.3. Filter phase shifts and time delays for anti-aliasing filtering

        Excessive analogue filtering shall be avoided, and all filters shall have sufficiently