How Can I estimate the numerator and denominator from the transfer function?

hi all,...

first I want to estimate the transfer function from the Output and Input of my system

my code

Fs=1/(4*10^-8);

NFFT=2^9;

len=512;

w=logspace(0,6,512); %setup for x-axis

Y1=fft(CH1,NFFT)/len; %input

Y2=fft(CH2,NFFT)/len; %output

ratio=Y1.*Y2./Y1.^2;

MagH=abs(ratio);

%transfer function

MagHdb=20*log10(MagH); %transfer function converted to dB

PhaseHRad=angle(ratio); %phase done in radians

PhaseHDeg=PhaseHRad*180/pi; %phase done in degrees

subplot(2,1,1)

semilogx(w,MagHdb,':b') %semilog plot

xlabel('frequency [rad/s]'),ylabel('20 log10(TF) [db]'),grid %xaxis label

subplot(2,1,2)

semilogx(w,PhaseHDeg,':b')

.............................................................

the output plot will be attached >

now i saw that this out is wrong or i follow bad rules .

Also how can i find the numerator and denominator from the transfer function? . . .

many thanks!

[belal]

this is the plot output