'scope probes and 100MHz EMI
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From: George Herold on 8 Apr 2010 16:41 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.)
From: keithw86 on 8 Apr 2010 17:11 On Apr 8, 3:41 pm, George Herold <ggher...(a)gmail.com> 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. How long was your ground wire? One handy trick is to wrap a paperclip around the ground contact on the end of a probe to use as the "ground probe". You can then bend the paperclip to hit a ground while the probe tip sits on the point of interest. This minimizes the ground length (and loop area). > 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. You could be picking that up in your probe ground. It will be everywhere. > 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.)
From: George Herold on 8 Apr 2010 19:15 On Apr 8, 5:11 pm, "keith...(a)gmail.com" <keith...(a)gmail.com> wrote: > On Apr 8, 3:41 pm, George Herold <ggher...(a)gmail.com> wrote: > > > > How long was your ground wire? One handy trick is to wrap a paperclip > around the ground contact on the end of a probe to use as the "ground > probe". You can then bend the paperclip to hit a ground while the > probe tip sits on the point of interest. This minimizes the ground > length (and loop area). > I was using about a 3" length of wire w/alligator clip. I did try winding the wire around the tip, but nothing changed. The paper clip is an idea I've never tried before. Thanks. What I should really do is find the bag of changable tips that came with the probe. (They should add a 'parts bin' to plastic DSO's, ...you could keep the probes there too.) My guess is I'll still see the 100Mhz emi. So when I was just looking at the ground signal of the piece of copper clad, I cut pieces off of it and made it smaller and smaller. I cut the 6x6 to 1.5x6 and not much changed. When I made it 1.5x4 the signal was perhaps 70%, (my notebooks at work), at 1.5x1.5 the signal was below 10 mV (10%) and hard to see. I was wondering if it's some resonance in the probe/scope. I need to hit the thing with a fast pulse or step. > > 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. > > You could be picking that up in your probe ground. It will be > everywhere. > Yeah I ground both sides of the probe and there's 100mV at 100MHz. Thanks, George H.
From: John Larkin on 8 Apr 2010 19:45 On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), George Herold <ggherold(a)gmail.com> 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 couldn�t find any strong source, but there where nodes >where the signal was much smaller. > >So can anyone help me understand what I�m seeing? I assume it�s some >sort of capacitive pickup. (Without the large piece of copper clad I >don�t 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.) Probes tend to pick up ambient noise, like FM stations, bus activity, whetever. Fet probes are a lot better than passive ones. For really clean waveforms, get a piece of coax with a BNC on one end. On your copperclad breadboard, solder the shield to ground and run your input signal into the center lead, directly or through a resistor. If your scope can be set to 50 ohms, or you use a feedthru terminator, use a 450 (or 470) ohm resistor at the breadboard end of the coax. That will form a super-clean, super-wideband 10:1 probe. I do this well into the GHz range. You can also solder a coax connector directly down to your copperclad. I like SMBs or SMAs for really fast stuff. The connector center pin goes through a resistor to whatever you want to probe. Then use a standard cable to run to your scope. Now you can save the breadboard without a dangling cable. ftp://jjlarkin.lmi.net/BreadBoards.jpg ftp://jjlarkin.lmi.net/BreadBoards2.JPG What's a PMT cost these days? John
From: David L. Jones on 9 Apr 2010 03:21
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 couldn�t find any strong source, but there where nodes > where the signal was much smaller. > > So can anyone help me understand what I�m seeing? I assume it�s some > sort of capacitive pickup. (Without the large piece of copper clad I > don�t 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-oscilloscope-phenomenon/ and don't forget Part 2: http://www.eevblog.com/2009/07/26/eevblog-20-the-unusual-oscilloscope-phenomenon/ and Part 3: http://www.eevblog.com/2009/07/28/eevblog-21-the-unusual-oscilloscope-phenomenon-part3/ Dave. -- ================================================ Check out my Electronics Engineering Video Blog & Podcast: http://www.eevblog.com |