During this shutdown, I did a little pcb layout for my triggered common-collector Colpitts oscillator, and had some boards fabbed. A manufacturing person came in today and built a couple for me.
It's a triggered 125 MHz oscillator that will be used to time delays in a laser system. We want minimal time jitter so I tried to keep the Q up. The inductor is a Coilcraft Midi-Spring.
I used a BFT25 super-fast transistor, but it oscillates at more frequencies than I intended. Tons of jitter. The choice was to add a base resistor or go with another transistor. A BFS17 seems to work fine. That's a great little npn, fast but not too fast.
Here's the board
and here's the roughly 250th rising edge after it's triggered to run
at 100 ps/div. I haven't figured out how to get that scope to measure the RMS jitter on that edge; it's obviously smarter than I am, and lets me know. If I eyeball the p-p jitter and divide by 5 for RMS, I'm estimating 4 ps RMS jitter at 2 us out from start, which is a ratio of
500K:1. That's unheard of, so I may be doing something wrong.We plan to phase-lock this to a good OCXO, but it will take a while to lock, so the better the open-loop Colpitts behavior, the less frantic we need to be about the DPLL. 2 microseconds is plenty of time to do the math.
It's out on the bench, so I'll put it in a metal chocloate box with some feed-thrus for better EMI shielding. Gotta empty the box first.
I need to temperature compensate it and play with the active guard idea.
Phil H helped me think about this. Thanks.