Division Approximation In Fm Demodulator

Hi,

Thanks for the link, however I can't get through this link. May be you can email it to me at:

kadhiem_ayob@yahoo.co.uk

I have added the upconversion/downconversion as well in the code below.

The recovered I/Q fluctuate between .2 ~ .4 even though the inputs are unitary, they don't go minus/plus as I thought first.

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clear all;

%******************* Tx ***********************

%generate random zeros and ones

symb = round(rand(1,50));

%convert symbols to two frequencies(f1,f2)

%f1 = 10/256,f2 = 20/256

for i = 1:50

if symb(i)==0 incr(i)=10; else incr(i)=20; end

end

%one cycle sine/cosine data

lut = exp(2*pi*j*[0:255]*1/256);

%modulate using f1,f2

%f1,f2 generated with lut pointer from modulo adder

k = 1;

n = 0;

for t = 1:length(incr)

for i = 1:64 %upsample symbols by 64

n = n + 1;

x(n) = lut(k);

k = k + incr(t);

if k>256 k=k-256; end; %modulo 256 addition

end

end

%check FM signal spectrum

figure;hold

f = linspace(0,1,length(x));

plot(f,20*log10(abs(fft(x))));

%upconvert to carrier = 100/256

carrier = exp(j*2*pi*[0:length(x)-1]*100/256);

x_up = real(carrier .* x);

%check FM signal spectrum

f = linspace(0,1,length(x_up));

plot(f,20*log10(abs(fft(x_up))),'r');

%******************* Rx ***********************

%downconvert

xr = x_up .* real(carrier);

xi = x_up .* -imag(carrier);

%filter off the high sideband

h = fir1(40,2*20/256+.05);

xr = filtfilt(h,1,xr);

xi = filtfilt(h,1,xi);

x = complex(xr,xi);

%check FM signal spectrum

f = linspace(0,1,length(x));

plot(f,20*log10(abs(fft(x))),'g--');

legend('baseband','upconverted','downconverted');

%demodulate

I = real(x); I_diff = [diff(I) 0];

Q = imag(x); Q_diff = [diff(Q) 0];

m = (I.*Q_diff - Q.*I_diff)./(I.^2 + Q.^2) ;

% decimate by 64

m = m(32:64:end);

%compare Rx symbols with Tx symbols

figure; plot(m)

str = sprintf('%d',symb);legend(str)

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Thanks