FM modulation - all negative values somehow become positive

I'm having a weird issue. I'm testing a simple FM modulator - demodulator. I run the code below and this is the graph I get

As you can see all the negative values magically become positive. But I'm really just doing a simple modulation and demodulation. What is the issue here?

Fs = 20e6;
fc=200;(Hz)
fDev = 150000;

t = (0:1/Fs:0.2)';
x = sin(2*pi*30*t)+2*sin(2*pi*60*t);
txsig = fmmod(x,fc,Fs,fDev);
dataOut = fmdemod(txsig,fc,Fs,fDev);
plot(t,x,'c',t,dataOut,'b--');
xlabel('Time (s)')
ylabel('Amplitude')
legend('Original Signal','Demodulated Signal')

Answer 1

[Vidya Viswanathan]

Hi,

I notice that the value of frequency deviation that you have specified is very high. This could result in over modulation which is probably why you are not getting the original signal back. If you specify the value of frequency deviation to a lower value, say 50, you would be able to restore the original signal. For example,

Fs = 20e6;
fc=200;(Hz)
fDev = 50;
t = (0:1/Fs:0.2)';
x = sin(2*pi*30*t)+2*sin(2*pi*60*t);
txsig = fmmod(x,fc,Fs,fDev);
dataOut = fmdemod(txsig,fc,Fs,fDev);
plot(t,x,'c',t,dataOut,'b--');
xlabel('Time (s)')
ylabel('Amplitude')
legend('Original Signal','Demodulated Signal')

The plot obtained for the above code is shown below.

I hope this answers your query.

Regards,

Vidya