Spider DSA User’s Manual
311
Figure 177. Example computing high and low sampling rate with a decimation filter.
The Real Time Digital Filters option includes three types of digital filters: Finite
Impulse Response (FIR), Infinite Impulse Response (IIR) and decimation filters.
For the FIR and IIR filters you can specify: low-pass, high-pass, band-pass or
band-stop filters with several different methods.
This chapter first explains some filter design theory and then introduces filter
operations within the EDM and the Spider hardware.
The goal of filter design is to calculate a series of filter coefficients based on user
specified criteria. The criteria are often described by following variables:
Number of filter coefficients: this is also known as the order of the filter.
The filter order defines how many coefficients are required to define the filter. A
lower order filter consists has fewer coefficients. A low order filter responds faster
than a higher order filter so there are fewer time lags between the input and
output of the filter.
Cutoff frequencies: For low-pass or high-pass filters, only one cutoff frequency
is needed. Band-pass or band-stop filters require two cutoff frequencies to fully
define the filter shape. Figure 178 shows a typical band-pass filter design with the
two cutoff frequencies set to approximately 0.1 and 0.2 Hz.
Stop-Band Attenuation: This specification defines how much of the input
signal is cut out of the output at the rejected frequencies. In theory, the higher the
attenuation the better the filter. The stop-band attenuation is greater than 40 dB
as seen from the highest side lobe just below 0.25 Hz.
Pass-Band Ripple: Ripple is an unavoidable characteristic if a digital filter. It
refers to the fluctuation in the filter shape outside the transition frequencies. If a
very flat filter is required, then it can be specified by choosing a very low ripple.
The ripple is seen in the stop band, but no ripple is evident in the pass-band.
Ideally the pass-band should be very flat, and some ripple is tolerable in the stop-
band.
Width of transition bands: This refers to the filter shape between a band-pass
and a stop-band region. Ideally this transition band should be very small.
However, a very narrow transitional band requires a higher order filter which
affects the filter response time and can also affect ripple. The transition bands are
between 0.05 to 0.1 and 0.2 to 0.25.
To Next Page
Loading...