Improving 200ns LED Pulse Shape With DS1040z and MOSFET Drain-Source Oscillations

Im trying to pulse an LED on for 200 nanoseconds. Im using the DS1040z One shot pulse generator to send a pulse width of 200 nanoseconds to the gate of a MOSFET. Using an Oscilloscope, I was able to get that 200 nanosecond pulse width. But when I probe from drain to source, I get this dampening or dissipative oscillating signal at the Drain to source. Im not too sure why or whats causing this to happen.  Also, my 200 nanosecond pulse is not a pretty square wave. What can I add to the circuit to improve the shape of the square wave?

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This appears to be indicitive of excessive inductance in your circuit, either in the circuit wiring or on the scope probe earth lead. You have to be aware that your pulse probably has rise and fall down near a nSec, which means that you are dealing with extremely high bandwidths, so VHF/ UHF design, layout,  and measurement techniques are called for. Additionally, the MOSFET needs a high gate drive current to overcome the static and dynamic gate capacitances, if the gate drive is insufficient expect your output waveforms to have  excessive rise and fall times. Are the MOSFET and LED capable of handling the rise and fall times desired. Power supply decoupling and very short leads (low inductance) around the LED will be essential. Additionally, constant current drive of the LED may be better. You have not supplied enough information to help much further in my mind, a schematic and the actual circuit layout and measuring technique (an image) would be helpful. Jim Willians, who is no longer with us, had a lot of useful tips on these sorts of issues, his app notes are all available on line, and may be of some helpCheers,Richard

If you are probing a point that can stand adding a 5040-ohm resistor to ground, you can build a 100X resistive probe that will not ring: Connect a 4990-ohm resistor between the test point and the center conductor of a 50-ohm coaxial cable, using short leads. Ground the coax shield nearby. Connect the other end of the cable through a 50-ohm terminator to  the oscilloscope input. If the scope has a 50-ohm input, the terminator is not needed

If you are probing a point that can resist the addition of a 5040 ohm resistor to ground, you can build a 100X resistive probe that will not sound: connect a 4990 ohm resistor between the test point and the center conductor a 50 ohm coaxial cable, using short cables. thanks