In texts the Fourier transform of a single pulse is explained as below:
They always create the pulse from -T to T, and they always show the sinc function symmetric around the origin.
But in reality there is no -T(there is not negative time on a scope screen) and even when I take the FFT of a single pulse in a circuit simulator I get a sinc function only for positive frequencies not anything symmetric with negative frequencies.
Below the example circuit which outputs a single 1V pulse with a total ON time of 1ms:
And when I take the FFT of the above pulse I obtain the following plot in linear scale:
Mathematical description of the pulse and the Fourier transform is different than what I get in the simulation. What is the difference between these two?