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From: George Herold on 9 Apr 2010 10:13 On Apr 8, 7:45 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), 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. > > >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.) > > 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. > Great, I'll try it and let you know what I see. (If my comparator can't put out 10mA I can always use a 1k resistor and make a it 20:1.) Say do I need to put a few pF of capacitance in parallel with the 450 ohm resistor to compensate for the ~30pf in the coax cable? > 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 Nice... Those are a lot neater looking than my hack jobs. > > What's a PMT cost these days? > > John- Hide quoted text - > > - Show quoted text - We get PMT's from Hamamatsu for ~$100 each in lots of 100. They also sell us the PMT sockets with built in high voltage supply. For about $200 each. (They kinda suck as there is a bunch ~80kHz ripple from the HV supply.) George H.
From: George Herold on 9 Apr 2010 10:59 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. George H.
From: John Larkin on 9 Apr 2010 18:36 On Fri, 9 Apr 2010 07:13:11 -0700 (PDT), George Herold <ggherold(a)gmail.com> wrote: >On Apr 8, 7:45�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), 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. >> >> >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. >> >Great, I'll try it and let you know what I see. (If my comparator >can't put out 10mA I can always use a 1k resistor and make a it >20:1.) Say do I need to put a few pF of capacitance in parallel with >the 450 ohm resistor to compensate for the ~30pf in the coax >cable? The scope end should be terminated in 50 ohms. If your scope can't do this, use a feedthru terminator. The input end of the coax will look like a 50 ohm resistor (ie, no cable capacitance will be apparent) so a 450 or 950 ohm resistor makes a theoretically perfect wideband 10:1 or 20:1 probe. Ordinary carbon film or carbon comp resistors will be fine up to a GHz or so. Caddock makes some axial resistors that make nice 8 GHz probes if you use them this way. They plug into an SMA female nicely. > >> 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 > >Nice... Those are a lot neater looking than my hack jobs. > >> >> What's a PMT cost these days? >> >> John- Hide quoted text - >> >> - Show quoted text - > >We get PMT's from Hamamatsu for ~$100 each in lots of 100. They also >sell us the PMT sockets with built in high voltage supply. For about >$200 each. (They kinda suck as there is a bunch ~80kHz ripple from >the HV supply.) Nice price on the tubes, steep for the kinda-sucks sockets. A little flyback thing and a C-W multiplier chain would have maybe $12 worth of parts. I visited the Hamamatsu PMT factory once. It was like a high-tech clean room in hell: gas flames on practically every bench. John ps - cheap LVDS line receivers make great fast comparators.
From: John Larkin on 9 Apr 2010 18:43 On Fri, 9 Apr 2010 07:59:43 -0700 (PDT), George Herold <ggherold(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 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-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
From: George Herold on 9 Apr 2010 22:19
On Apr 9, 6:36 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 9 Apr 2010 07:13:11 -0700 (PDT), George Herold > > > > > > <ggher...(a)gmail.com> wrote: > >On Apr 8, 7:45 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), 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. > > >> >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.) > > >> 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. > > >Great, I'll try it and let you know what I see. (If my comparator > >can't put out 10mA I can always use a 1k resistor and make a it > >20:1.) Say do I need to put a few pF of capacitance in parallel with > >the 450 ohm resistor to compensate for the ~30pf in the coax > >cable? > > The scope end should be terminated in 50 ohms. If your scope can't do > this, use a feedthru terminator. The input end of the coax will look > like a 50 ohm resistor (ie, no cable capacitance will be apparent) so > a 450 or 950 ohm resistor makes a theoretically perfect wideband 10:1 > or 20:1 probe. > > Ordinary carbon film or carbon comp resistors will be fine up to a GHz > or so. Caddock makes some axial resistors that make nice 8 GHz probes > if you use them this way. They plug into an SMA female nicely. > > I used metal film resistors. Seemed to work fine but there was a bit of weirdness on the end of the pulse response.... ringing 'gone bad'. Adding a pF of capacitance only made it worse. So the good news is the voltage divider through the coax reduced the 100MHz 'stuff' by about 1/2. But it's still there... I think this may be a circuit screw up on my part. After the 50 ohm termination on the input line, I've got a series 300 ohm resistor and then 33pF to ground in front of the non-inverting input. There are some ~GHz switching transiients on the input pulse, and I was adding a bit of low pass filtering so as not to trigger on them. I clipped out the cap today and the 100MHz stuff went away.... Maybe I can 'buffer' the cap between two resistors? or reduce the cap some. I was trying to get this circuit to oscillate in LT spice tonight, but I'm pretty much a LTspice newbie and it seemed to work fine in transient analysis. > > > > > > >> 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 > > >Nice... Those are a lot neater looking than my hack jobs. > > >> What's a PMT cost these days? > > >> John- Hide quoted text - > > >> - Show quoted text - > > >We get PMT's from Hamamatsu for ~$100 each in lots of 100. They also > >sell us the PMT sockets with built in high voltage supply. For about > >$200 each. (They kinda suck as there is a bunch ~80kHz ripple from > >the HV supply.) > > Nice price on the tubes, steep for the kinda-sucks sockets. A little > flyback thing and a C-W multiplier chain would have maybe $12 worth of > parts. We might save some money by redoing the electronics on this instrument. (there is even more 'low hanging' electronic fruit.) But it's always a question of how much and what else could I be doing with my time. I've not done much HV or fast pulse stuff, so I'm going to make lotsa mistakes. > > I visited the Hamamatsu PMT factory once. It was like a high-tech > clean room in hell: gas flames on practically every bench. > > John > > ps - cheap LVDS line receivers make great fast comparators.- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text - Some how gas flames and clean room don't mix in my brain. George H. |