Frequency Modulation for varition of pitch frequency

I need to varied pitch frequency 125 by frequency modulation ,I need to get from freqmod positive numbers ,How can I do that?PP is pitch period in seconds and should be positive,

for freqmod(m) = 1*cos (2*pi*(fc/fs)*m + (fd/fm)*cos (2*pi*(fm/fs)*m));

 %PP=1/freqmod(m);

end;