function Genv = Genv(fm)
% Computes envelope transfer function at modulation frequency fm [Hz]
% Tank transfer function H(s) and swithcing frequency fs must be defined
% externally as global
% See ECEN5817 course notes
% DM Spring 2012
%
global fs; % switching frequency
global H; % tank transfer function
%
wm = 2*pi*fm; % modulation frequency [rad/s]
ws = 2*pi*fs; % switching frequency [rad/s]
s = (1i)*ws; 
Ao = evalfr(H,s); % compute Ao
s = (1i)*(ws-wm); 
Al = evalfr(-H,s)/2/wm; % compute Al
s = (1i)*(ws+wm);
Au = evalfr(+H,s)/2/wm; % compute Au
%
Genv = (conj(Ao)*Au+Ao*conj(Al))/abs(Ao); % compute Genv
%
end