% Envelope transfer function of resonant inverters
% Assumptions: frequency modulation, voltage driven resonant tank
% See ECEN5817 course notes
% DM Spring 2012
%
global H; % Resonant-tank transfer function v/vs1
global fs; % Swithcing frequency [Hz]
%
% input voltage
Vg = 100;
Vs1 = 4*Vg/pi; % amplitude of the fundamental voltage at the input of the resonant tank
% VCO gain [Hz/V]
Km = 1e3;
%
scale = Vs1*Km*2*pi; % multiplicative scale factor in Genv(s)
%
%
fs = 110e3; % steady-state switching frequency fs0
%
% define resonant tank transfer function, series resonant example
% change definition of H(s) to get Genv for other resonant tanks
%
s = tf('s'); 
fo = 100e3; % resonant tank center frequency
Q = 10; % resonant tank Q factor
wo = 2*pi*fo; 
H = ((1/Q)*(s/wo))/(1 + (1/Q)*(s/wo) + (s/wo)^2); % series resonant inverter
%
% plot tank H(s) magnitude and responses
%
figure(1);
P = bodeoptions; 
P.FreqUnits = 'Hz';
bode(H,P,'b');
h = findobj(gcf,'type','line'); 
set(h,'LineWidth',2); 
grid on;
title('Magnitude and phase responses of the resonant tank transfer function H(s)');
%
% compute Genv
%
f=logspace(2,5,100);
G=f;
for fm=1:length(f)
    G(fm)=scale*Genv(f(fm));
end
%
% plot magnitude and phase of Genv
%
figure(2)
subplot(2,1,1);
semilogx(f,20*log10(abs(G)),'r','LineWidth',2);
grid on;
hold on;
title('Magnitude and phase responses of the envelope transfer function Genv(s)');
subplot(2,1,2);
semilogx(f,180*angle(G)/pi,'r','LineWidth',2);
grid on;
hold on;