Put the pre-amp head together.

A project log for Fiber Optic Isolated Voltage Probe

Use cheap plastic fiber optics to make a useful Isolated Voltage probe for an oscilloscope or other instrument.

BotLawsonBotLawson 07/23/2016 at 00:220 Comments

While I'm waiting for the LM317 regulators, I decided to put the pre-amp head together. Managed to get 9-10nS rise and fall times. Works out to an approximate bandwidth of 35MHz. Roll-off past 35MHz is first order as best I can tell with a noise source and scope FFT. Might be able to compensate with the post-amplifier and get more bandwidth. I'll setup a better test bench when everything is assembled.

Had to play with the input capacitor values a lot as well. Ended up putting 1pf caps in C3 and C6. (parallel to the 5M input resistors) For the input capacitors to 2v5ref, C4 ended up at 100pf and C7 at 47pf. Had to remove trimmer C5 due to the unbalanced capacitive dividers. I'll see how much variation I get, but I might have to boost C3, C4, C6, and C7 by 5-10x to get a design that is repeatable. (Though the 0.5-1pF input capacitance might be worth trying several values of C4 and C7 after assembly)