'scope probes and 100MHz EMI
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From: George Herold on 9 Apr 2010 22:26 On Apr 9, 6:43 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 9 Apr 2010 07:59:43 -0700 (PDT), George Herold > > > > > > <ggher...(a)gmail.com> wrote: > >On Apr 9, 3:21 am, "David L. Jones" <altz...(a)gmail.com> wrote: > >> George Herold wrote: > >> > The other day I hooked up a LT1016 comparator (live bug prototype on a > >> > 6 X 8 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 stuff 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 stuff 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 stuff 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 couldnt find any strong source, but there where nodes > >> > where the signal was much smaller. > > >> > So can anyone help me understand what Im seeing? I assume its some > >> > sort of capacitive pickup. (Without the large piece of copper clad I > >> > dont 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 stuff into the circuit. > > >> > Thanks, > > >> > George H. > > >> > (Oh the comparator circuit works great on the PMT pulses.) > > >> Your "stuff" sounds consistant, but the 100MHz figure reminded me of static > >> ringing:http://www.eevblog.com/2009/06/21/eevblog-14-a-rather-unusual-oscillo... > >> and don't forget Part 2:http://www.eevblog.com/2009/07/26/eevblog-20-the-unusual-oscilloscope... > >> and Part 3:http://www.eevblog.com/2009/07/28/eevblog-21-the-unusual-oscilloscope... > > >> Dave. > > >> -- > >> ================================================ > >> Check out my Electronics Engineering Video Blog & Podcast:http://www.eevblog.com-Hide quoted text - > > >> - Show quoted text - > > >Yeah, I saw those videos of yours a while ago and was reminded of it. > >This is some sort of capacitive pickup.. though I don't understand > >it. IIRC, most of your bouncing chair pick-up was via the magnetic > >loop formed by the probe tip and ground wire.... When you used a piece > >of Alum foil to short the probe tip the picked up signals decreased by > >80% or so... I saw nothing with just a loop, or when I used a few turn > >coil across the end of the probe. > > >Say the boss wants a monitor of the digital pulses. Can I use a > >74HC14 to drive a coax terminated with 50 Ohms? I don't care if the > >voltage doesn't make it up to 5 volts. Can I parallel a few of the > >inverters for more current? Or will that cause bad things to > >happen? Hey maybe I can use John L.'s divide by ten trick. (I'll > >put 450 ohms in series with the output).. They'll see 500mV pulses on > >the 'scope but that should be fine. > > If you want a clean step at the scope end, you can... > > 1. Source terminate. Drive the coax with a 50 ohm impedance, using a > 50 ohm resistor or a divider that's Thevenin 50 out. This works into a > hi-z scope input, no reflections. It also works into a 50 ohm scope, > at half the gain. > > 2. End terminate. If the scope is 50 ohms, drive the signal into the > coax through any resistor you like, forming a divider against 50 ohms. > Also clean and reflection-free. Resistors above a couple of K will > start to introduce overshoot from stray shunt capacitance, but most > scopes are too slow to notice. > > John- Hide quoted text - > > - Show quoted text - Cool, thanks John I'll try the source terminated idea too. At the Vanderbilt FEL we had setups where the PMT was 10+ feet away from the electronics. And we would put 50 ohm terminations on both ends of the coax cable. George H. |