To gain some FFT experience? I checked? the bandwidth of individual harmonics of a 14MHz square wave? They show as having a bandwidth of 1.5 MHz which is ridiculous I have tried many simulation settings but they all produce the same results?? What Pulse and tran settings should I be using ?
I don't use FFTs regularly, but some things you should be doing include:
The simulation must run for an exact integer multiple of cycles, and turn off Windowing in the FFT. ?Either of those can make the components wider.
More data points at finer time increments is usually better. ?Specify a Maximum Timestep in the .TRAN command.
Many circuits start with a burp, which you might not always notice. ?Remember, a transient simulation begins with the variable sources turned off, and then suddenly they are turned on. ?That can cause a bias shift if signals are AC-coupled anywhere ... which translates into a widening of the FFT components. ?To avoid that when it happens, don't start saving data until after the bias has settled or the transient has died out ("Time to Start Saving Data" in the .TRAN command). ?Remember to adjust the stop time accordingly so you still have an integral number of cycles saved.
Also, don't forget, you can't make perfect "square" waves. ?The Pulse waveform rise and fall times should not be zero.
