How to Match 3 nF Capacitor Output With 16 MHz Ringing to 50 Ohm Digitizer Input?

A current source is in parallel with a capacitor (3 nF), looking on a scope the capacitor ouput is ringing ( 16 MHz ringing). How can I match this output to 50 ohm? capacitor output will be sent to a 50 ohm digitizer.Pulsed input of about 550 ps/s.
Thank you.

Your problem is not clear. Are you developing a voltage pulse using a current source to drive the 3nF capacitor? How are things connected (printed circuit board, wires, etc)? What is does 500ps/s mean?

Hi Todd, thanks for your reply.
Yes, a voltage pulse is charging a 3nF cap through a 50 ft RG 51 (integrator) by the cap.

If I understand your circuit correctly the ringing is caused by the reflection at the open end of the cable; the end where the high-Z oscilloscope is connected. Place a 50 ohm shunt at that node it will should stop ringing.

Hi David,
A 50 ohm shunt reduced the ringing. What left I think is due to the coaxial cable, the best match will be when the length is a multiple of half wave length ( I did check this on smith chart software).
Jn this case I've to double the capacitor value, to keep the same time constant.
Please let me know if you have a different idea.
Thank you very much David

The current source 'sees' the 3 nF capacitor in parallel with 50 ohms. Changing the transmission line length should make no difference if the 50 ohm load is ideal. But as you still have some ringing the load isn't ideal. There is a reflection from the oscilloscope/50 ohm that then travels to the current source/3 nF cap and reflects again.

To fix this - if you can tolerate a 50% reduction in signal amplitude, is to place a 50 ohm series resistor at between the 3 nF capacitor and the coaxial cable. The current source now sees 3 nF||100 ohms rather than 3nF||50 ohms. To retain the same time constant you will of course have to change the cap to 1.5 nF.