FM modulation of a sqarewave carrier with sinusoidal message in gnu-octave

[Kavya Manohar]

 Cross posting my question on stackoverflow: http://stackoverflow.com/q/23931723/3687167

I have used the code mentioned there in octave to implement frequency modulation with sinusoidal message and square wave carrier. But the resultant FM waveform has a frequency which is not proportional to the message signal amplitude.
See the attachment.

What could be the reason. How could it be resolved?

Thanks in advance,

Kavya Manohar.

fm.png
Description: PNG image