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% butfilt.m: Interpolates the butterworth filter impulse response function

% GUISDAP v.1.60 96-05-27 Copyright Asko Huuskonen and Markku Lehtinen

%

% from the values measured by Jussi Markkanen

%

% input parameters:

% BW     : desired bandwidth in kHz (EISCAT definition)

% ch     : specifies the channel numbers

% p_dtau : global time unit in us (global parameter)

% output parameters:

% impresp: the impulse response

% ch_p   : impulse resposes for channels given by ch (global)

%

% See also: lin_filt, REC_impresp

%

% function impresp=but_filt(BW,ch);

  function impresp=but_filt(BW,ch);



  global ch_p p_dtau



% 

% measured impulse response of a 25 kHz Butterworth filter.

% values given for each us.

  p_m=[ -0.002 -0.003 -0.002 -0.001  0.001  0.006  0.013  0.023  0.036 ;

         0.053  0.072  0.095  0.120  0.148  0.176  0.204  0.233  0.260 ;

         0.285  0.307  0.327  0.342  0.353  0.360  0.362  0.360  0.353 ;

         0.342  0.327  0.309  0.287  0.263  0.236  0.208  0.179  0.150 ;

         0.121  0.093  0.065  0.040  0.016 -0.006 -0.025 -0.041 -0.055 ;

        -0.067 -0.075 -0.081 -0.084 -0.085 -0.084 -0.081 -0.077 -0.070 ;

        -0.063 -0.055 -0.047 -0.038 -0.029 -0.020 -0.012 -0.004  0.004 ;

         0.010  0.016  0.021  0.025  0.028  0.030  0.031  0.032  0.031 ;

         0.030  0.029  0.026  0.024  0.021  0.018  0.014  0.011  0.008 ;

         0.004  0.001 -0.002 -0.004 -0.006 -0.008 -0.010 -0.011 -0.012 ;

        -0.012 -0.013 -0.012 -0.012 -0.011 -0.010 -0.009 -0.008 -0.006 ;

        -0.005 -0.003 -0.002  0.000  0.001  0.002  0.004  0.005  0.005 ;

         0.006  0.007  0.007  0.007  0.007  0.007  0.007  0.006  0.006 ;

         0.005  0.004  0.004  0      0      0      0      0      0];

  p_m=col(p_m');

  len = length(p_m);



  dt  = 25/BW;  % time step for the desired bandwidth



% interpolation

  p_mx= ((0:(len-1))*dt)';

  px  = (0:p_dtau:(len-1)*dt)';

  p = spline(p_mx,p_m,px);

  

% scale to get unit area and

% assign to all channels specified at input

  

  impresp=p/sum(p);

  if nargin==2,

   ch_p(1:length(px),ch) = impresp*ones(1,length(ch));

  end