The other day I hooked up a LT1016 comparator (live bug prototype on a
6=94 X 8=94 piece of copper clad) to trigger on pulses from a PMT. Pulses are ~ 300ns long and ~100mV high. I sent a low level sine wave into the input (50 ohm terminated to ground) and looked at the output with a X10 scope probe. (TEK TDS2022 200MHz scope.) There was 100 MHz =91stuff=92 visible during the transition. I hung extra caps where I could and added 10 ohm resistors to the supply lines, but nothing helped. I went out and found the 100MHz probe that came with TEK2022... no change. (I've just read Linear's AN13 on fast comparators... Thanks again Jon K.) And yes I compensated the probes. I finally noticed that when I hooked the probe to ground I could see 100MHz =91stuff=92 with about 100mV of signal p-p. I then put a Schmitt trigger inverter (74HC14) on the output of the comparator and when I looked at its' output everything was fine. If I hooked the probe to the input to the inverter (output of the comparator) I could see the 100 MHz =91stuff=92 again.... Though the inverter cleaned things up considerably.I took a new piece of copper clad, soldered a piece of buss wire on one corner. I hooked the probe and ground to the buss wire and walked around my lab/ office with it. There was 100MHz stuff most everywhere. I couldn=92t find any strong source, but there where nodes where the signal was much smaller.
So can anyone help me understand what I=92m seeing? I assume it=92s some sort of capacitive pickup. (Without the large piece of copper clad I don=92t see anything. I also put a few small coils across the probe, but could not see any magnetic pickup.)
Second do I need to look into getting a better probe? It would be nice to be able to look at the comparator output without the probe coupling all sorts of =91stuff=92 into the circuit.
Thanks,
George H.
(Oh the comparator circuit works great on the PMT pulses.)