Generating Pulses 3D View

General Instructions

This spectral simulation is an interactive Java applet. You can change parameters by clicking on the vertical arrow keys. The five control buttons at the lower right are used to start (triangle) and pause (square) the simulation, to skip forward or back one section at a time (double triangles), and to change speed (+ and -).

After the simulation is complete, the start button takes you back to the beginning of the simulation. You may experience a delay at this point.

This applet shows the huge number of harmonics that are necessary to reproduce a low duty-cycle pulse train. The applet will simulate up to 35 harmonics and a duty cycle as low as 0.05.

Practically speaking, this shows that if you want to reproduce a very low duty-cycle pulse train, you need an amplifier that has a large bandwidth. Notice that with a duty cycle of 10%, it takes more than ten harmonics to produce a good pulse. The more bandwidth the amplifier has, the more harmonics it can pass, and the faster the "rise time" of the pulse can be.

It's not always wise to make the fastest pulse. A mechanical switch or a fast IC can cause a fast turn-on time. That makes for a good pulse or step function, but it produces harmonics at very high frequencies-harmonics that can create radio frequency interference with sensitive circuits. That's the reason, for example, that airline pilots ask you to turn off electronic devices during takeoff- digital devices have fast rise times, and pilots are concerned about interference with the aircraft's navigation electronics.

Design engineers have to study the effects of "EMI" (Electromagnetic Interference) on their circuits and their designs have to pass international standards to make sure they don't interfere with other electronic devices. A high percentage of EMI problems can be directly related to fast rise-time pulses.