'scope probes and 100MHz EMI

[George Herold]

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.)

 

[keithw86@gmail.com]

On Apr 8, 3:41 pm, George Herold <ggher...@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 couldn’t 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 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.)

 

[John Larkin]

On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), George Herold
<ggherold@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

 

[George Herold]

On Apr 8, 5:11 pm, "keith...@gmail.com" <keith...@gmail.com> wrote:

On Apr 8, 3:41 pm, George Herold <ggher...@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 couldn’t 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.

 

[David L. Jones]

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

 

[George Herold]

On Apr 8, 7:45 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), George Herold





ggher...@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?

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.

 

[George Herold]

On Apr 9, 3:21 am, "David L. Jones" <altz...@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.

George H.

 

[John Larkin]

On Fri, 9 Apr 2010 07:13:11 -0700 (PDT), George Herold
<ggherold@gmail.com> wrote:

On Apr 8, 7:45 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), George Herold





ggher...@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.

 

[John Larkin]

On Fri, 9 Apr 2010 07:59:43 -0700 (PDT), George Herold
<ggherold@gmail.com> wrote:

On Apr 9, 3:21 am, "David L. Jones" <altz...@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

 

[George Herold]

On Apr 9, 6:36 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 9 Apr 2010 07:13:11 -0700 (PDT), George Herold





ggher...@gmail.com> wrote:
On Apr 8, 7:45 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 8 Apr 2010 13:41:49 -0700 (PDT), George Herold

ggher...@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.

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.

 

[George Herold]

On Apr 9, 6:43 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 9 Apr 2010 07:59:43 -0700 (PDT), George Herold





ggher...@gmail.com> wrote:
On Apr 9, 3:21 am, "David L. Jones" <altz...@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- 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.

 

[George Herold]

On Apr 8, 4:41 pm, George Herold <ggher...@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.)

Hi guys, Well a bit of screwing around with the LT1016 comparator,
that I thought I’d report on.

The simple circuit now looks like this

+--R4R4--+
| |\ |
Vin---+--R2R2--+--R3R3-+-+ \ |
| | | >---+--->to 74HC14
R1 C1 +--- /
R1 C1 | |/ LT1016
| | |
GND GND Vref

Vin comes from a PMT and is a ~100mV pulse with several hundred ns
width.
R1 = 50 ohms
R2 = R3 = 100 ohms
C1 = 100 pF
R4 is 50k ohms, (though this may get tweaked a bit to set the
hysteresis level)

R2 and C2 form a lowpass filter that gets rid of switching crap on the
input signal.
Without R3 the capacitor C1 caused the 100 MHz of ‘stuff’ in the room
to become coupled into the input. I assume this was due to the small
stray capacitance between the output and the (+) input. If I put a
few (2.2 – 10pF) of capacitance in parallel with R4 then there would
all sorts of oscillations during the transition. (1 pf in parallel
seemed to make things a bit cleaner..???)

I don’t really understand
1.) Why does the stray capacitance from output to input cause the
100MHz ‘stuff’ to couple in better?
2.) Why does R3 reduce this effect?

(I guess the real problem is I don’t know how the 100 MHz ‘stuff’ gets
into the circuit.)

Thanks,

George H

 

[John Larkin]

On Tue, 13 Apr 2010 09:08:59 -0700 (PDT), George Herold
<ggherold@gmail.com> wrote:

On Apr 8, 4:41 pm, George Herold <ggher...@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.)

Hi guys, Well a bit of screwing around with the LT1016 comparator,
that I thought I’d report on.

The simple circuit now looks like this

+--R4R4--+
| |\ |
Vin---+--R2R2--+--R3R3-+-+ \ |
| | | >---+--->to 74HC14
R1 C1 +--- /
R1 C1 | |/ LT1016
| | |
GND GND Vref

Vin comes from a PMT and is a ~100mV pulse with several hundred ns
width.
R1 = 50 ohms
R2 = R3 = 100 ohms
C1 = 100 pF
R4 is 50k ohms, (though this may get tweaked a bit to set the
hysteresis level)

R2 and C2 form a lowpass filter that gets rid of switching crap on the
input signal.
Without R3 the capacitor C1 caused the 100 MHz of ‘stuff’ in the room
to become coupled into the input. I assume this was due to the small
stray capacitance between the output and the (+) input. If I put a
few (2.2 – 10pF) of capacitance in parallel with R4 then there would
all sorts of oscillations during the transition. (1 pf in parallel
seemed to make things a bit cleaner..???)

I don’t really understand
1.) Why does the stray capacitance from output to input cause the
100MHz ‘stuff’ to couple in better?
2.) Why does R3 reduce this effect?

(I guess the real problem is I don’t know how the 100 MHz ‘stuff’ gets
into the circuit.)

Thanks,

George H

Sounds like a flakey layout. Does the PMT signal come in on a nice
coax? How is the shield grounded relative to the comparator circuit?

Is there a good ground plane? Are you still breadboarding on
copperclad?

Is Vref bypassed close to the comparator? I'd use an RC for Vref, both
close to the comparator.

Is comparator Vcc bypassed?

External hysteresis may not help. 50K certainly won't. I prefer
comparators with internal hysteresis, if any.

Got a picture?

John

 

[George Herold]

On Apr 13, 5:01 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 13 Apr 2010 09:08:59 -0700 (PDT), George Herold





ggher...@gmail.com> wrote:
On Apr 8, 4:41 pm, George Herold <ggher...@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.)

Hi guys,  Well a bit of screwing around with the LT1016 comparator,
that I thought I’d report on.

The simple circuit now looks like this

                      +--R4R4--+
                      | |\     |
Vin---+--R2R2--+--R3R3-+-+ \    |
     |        |         |  >---+--->to 74HC14
     R1       C1     +--- /
     R1       C1     |  |/ LT1016
     |        |      |
    GND      GND     Vref

Vin comes from a PMT and is a ~100mV pulse with several hundred ns
width.
R1 = 50 ohms
R2 = R3 = 100 ohms
C1 = 100 pF
R4 is 50k ohms,  (though this may get tweaked a bit to set the
hysteresis level)

R2 and C2 form a lowpass filter that gets rid of switching crap on the
input signal.
Without R3 the capacitor C1 caused the 100 MHz of ‘stuff’ in the room
to become coupled into the input.  I assume this was due to the small
stray capacitance between the output and the (+) input.  If I put a
few (2.2 – 10pF) of capacitance in parallel with R4 then there would
all sorts of oscillations during the transition. (1 pf in parallel
seemed to make things a bit cleaner..???)

I don’t really understand
1.) Why does the stray capacitance from output to input cause the
100MHz ‘stuff’ to couple in better?
2.) Why does R3 reduce this effect?

(I guess the real problem is I don’t know how the 100 MHz ‘stuff’ gets
into the circuit.)

Thanks,

George H

Sounds like a flakey layout. Does the PMT signal come in on a nice
coax? How is the shield grounded relative to the comparator circuit?

Is there a good ground plane? Are you still breadboarding on
copperclad?

Is Vref bypassed close to the comparator? I'd use an RC for Vref, both
close to the comparator.

Is comparator Vcc bypassed?

External hysteresis may not help. 50K certainly won't. I prefer
comparators with internal hysteresis, if any.

Got a picture?

John- Hide quoted text -

- Show quoted text -

"> Got a picture?"

OK airing my dirty laundry....

http://img708.imageshack.us/gal.php?g=dscf0004r.jpg

Yeah this is still the same messy piece of copper clad.

I've got bypass caps everywhere I can put them... on Vref there's a
220pF and a 0.1uf ceramics.

Since the input pulses are about 100mV tall, I wanted about 10mV of
hysteresis.. which is about what I've got.

Without R3 in place the amount of 100MHz 'stuff' during the transition
depends on the value of C1. Changing the R2 feeding it doesn't do
much. (Well of course it changes the RC time constant and amount of
hysteresis) Once I found that C1 was the issue I was just flaying
around trying to understand it. What I don't get is why putting R3 in
place kills of this effect. Since there is no current flowing into
the (+) input why does a wee bit of resistance help things?

Perhaps I should stick this in a metal box and see if that gets rid of
the 100 MHz stuff.

The circuit seems to work just fine now... But I worry that not
understanding why it works will come back and bite me sometime in the
future.

George H.