Building a filter file
As pointed out in the previous section, all stages have an associated “filter”, even if no filtering is performed. The filters specified in StationXML are:
PolesZeros - Any analog filter specified by its poles and its zeros. Use this for Laplace transforms and IIR filters.
FIR - Finite Input Response digital filter
Coefficients - A more general way of expressing a digital filter. Can be used to express a FIR or an IIR filter.
ResponseList - A filter whose response is expressed as a list of frequency, amplitude and phase.
Polynomial - A filter expressed as a polynomial.
All except Polynomial are available in obsinfo.
obsinfo also offers some streamlined filter types for stages which have a simpler or different representation than the StationXML. They are stored as StationXML filters, but you have less (or more pertinent) information to enter:
ADCONVERSION - Analog to digital conversion stage. Acts as a Coefficients filter without any coefficients, and allows you to enter the maximum and minimum voltage and counts values, calculating the gain for you
ANALOG - gain-only Analog stage
DIGITAL - gain-only Digital stage
Below we provide a link to the template file for each filter and show the
result of obsinfo plot on each of them:
PolesZeros
FIR
Coefficients
ResponseList
See the instability possible if you specify a response list!
ADCONVERSION
AD_Conversion filters have no frequency response. They are implemented as ``Coefficients``objects with only one coefficient, so you get the coefficients plot as well.
ANALOG
Analog filters are implemented as pole-zero objects with no poles and no zeros.
DIGITAL
Response is the same as the analog filter, but it also shows off its single “coefficient” (1 at offset=0)