function plotdb(w,mag,linetype)
%PLOTDB Plot Magnitude Bode diagrams.
%       PLOTDB(W,MAG,'linetype') plots 20*log10(mag) in
%       Bode format, with grids and labels.
%       W is the frequency vector.
%       MAG be either a vector or matrix.
%       The input argument linetype is optional. If it is present
%       it is used to set the plot line or point type.
%       See PLOT for valid line types.
%
%       PLOTDB(W,C,'linetype') plots the magnitude Bode plots of the
%       complex matrix or vector C. In this form UNWRAP is applied
%       to the phase before plotting.
%
%       See also PLOTBODE,PLOTNYQ,PLOTNIC,UNWRAP.

%       Dr M.P. Ford 4th August 1987,Revised J.M.Maciejowski, 13 Jan 1988
% Copyright (c) 1987,1993 by GEC Engineering Research Centre and 
% Cambridge Control Ltd. All Rights Reserved.
% History: MRN0015
%   `fixphase' replacedd by `unwrap', 29.5.93, JMM.

nargs=nargin;
error(nargchk(2,3,nargs));

[m,n]=size(mag);
if (length(w)~=m)&(length(w)~=n)
   error('Frequency vector does not match points, did you forget FGET?')
end

if nargs ==3
  if ~isstr(linetype)
      error('Third input argument must be ''linetype''')
  end
end
if nargs == 2
  linetype=' ';
end
mag=r2p(mag);


if linetype==' '  % do not have linetype
   semilogx(w,20*log10(mag)),grid,xlabel('FREQUENCY'),ylabel('MAG dB')
else
   semilogx(w,20*log10(mag),linetype),grid,xlabel('FREQUENCY'),ylabel('MAG dB')
end