I want to create a signal in time domain using gauspuls command in Matlab, and plot the same in freq domain to check if i am getting the associated frequencies?

I have got a code from (www.simsonic.fr) which takes input as fractional BW, time and centre frequency, and gives in phase pulse and envelope as output. But my concern is how can I check if the signal is correct. I tried to use the fft command using which I got 2 peaks(one for the negative and one for the positive side). and also can anybody explain what fractional bandwidth really is? I have taken the Fractional BW as 1, because this gives me a bandwidth of 500Khz if my centre frequency is 500Khz, What I desire: to get a signal with centre frequency at 500Khz with 500Khz extended on either side. FBW=BW/Fc, if I keep FBW as 1 and Fc as 500k I get a BW of 500k, is this correct? The code for the same is attached below. I also need to check the corresponding signal in frequency domain, if I am getting the desired bandwidth, but I am not getting the same. Can anybody please help?

f0=0.5; % central frequency, MHz

t0=1.5; % pulse center time

bndwdth=1; % pulse -6dB bandwidth

duration=2*t0; % signal length

timebase=(0:round(duration/dt)-1)'*dt;

[signalI,signalQ,signalE]=gauspuls(timebase-t0,f0,bndwdth);

signal=signalQ;

figure(9)

subplot(2,1,1);

plot(timebase,signalQ,'.-')

title('source signal')

xlabel('time (µs)')

figure(9)

midf=5e5;

L=1000;

fs=midf*2;

Mb = fft(signal);

%Mb = fftshift(Mb);

f = fs*linspace(-0.5,0.5,length(Mb)); %Frequency Vector

subplot(2,1,2);

plot(f,abs(Mb))

title('Gaussian Pulse in Frequency Domain in bone')

xlabel('Frequency (f)')

ylabel('|Magnitude(f)|')

The figure or plots are: