Help designing a low-noise TIA

[Jeroen Belleman]

Jan Panteltje wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

Yeah, well, you forgot about the h_oe output conductance,
which isn't zero. There is effectively a resistor of a
few 10 kOhm in parallel with your 10 MOhm collector resistor,
limiting the gain. Somebody already told you.

Jeroen Belleman

 

[John Larkin]

On Fri, 7 Jun 2019 09:13:29 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, June 7, 2019 at 9:35:47 AM UTC-4, John Larkin wrote:
On Fri, 7 Jun 2019 03:59:16 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all again.

Joseph Gwinn, I will take a look at the patent to see how they compensated for the op-amp drift.

Regarding the rotary connectors, there is not a lot of noise information on their website. We moved to a liquid metal rotary feedthrough (the liquid metal is mercury) to remove problems with noise that comes from brushes. I don’t know what the noise specs are. We are set up to run it all using wire that twist up, but that is hard to run tests like that. You get one try, and then have to replace the wires.

Jan Panteltje, several kOhm of input impedance will reduce our expected current by a factor of ten or more. So I am not sure your design will work. We’d drop our expected current down to 100 pA, or less.

John Larkin, we did have the rig inside mu-metal for a while, but that was hard to maintain and did very little for the static (Earth’s) field that we also have to get rid of. (If you don’t get rid of the Earth’s field, when the coil is spinning the centrifugal force makes the coil slightly bigger, and then when you brake it it gets smaller, leading to a signal that looks like the one we are trying to see.) So we abandoned that. But we haven’t tried electrostatically shielding the entire setup. We put our breadboard into a metal box to reduce noise – are you suggesting we box up the whole thing into an electrostatic box? We had a real one of those when I was in grad school, a screened room. Is that necessary for the coils as well?

Ideally, I’d like to be able to see the integrated pulse or the actual V vs t of the pulse on the oscilloscope screen for the students to be able to see it before digitizing it. But if I have to digitize and then play with the signal, I will.


This looks to be an extraordinally difficult experiment.

Magnetic shielding is probably not practical here. The compensating
and helmholtz coils can help with mag fields. If electrostatic
coupling is squirting noise into the system, it's easy to test that:
temporarily shield the area with grounded aluminum foil or something
and see if it makes any difference. Also look at the amp output with a
scope when nothing is spinning.

I will say, for the last time, that the basic physics makes voltage,
and measusing current was an artifact of hundred-year-old
instrumentation limits, and the TIA is a bad idea.
So how would you sense it? Say using opamps* on the front end?

AD8429. I'd spin it and bring out a big signal.

One issue with his compensating coil is that it has the same R and L
as the spinning coil (I assume)... I was going to say that cuts the
voltage signal in 1/2.. but that might not be true.

It won't. It would cut the current in half if you use a TIA and put it
in *series*.

Process the comp coil separately.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 7 Jun 2019 08:55:10 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Thursday, June 6, 2019 at 11:27:28 PM UTC-4, John Larkin wrote:
On Thu, 6 Jun 2019 20:05:11 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Thursday, June 6, 2019 at 12:48:53 PM UTC-4, ithacacoll...@gmail.com wrote:
Hi all,
Again, thanks for the great ideas. I will try and keep up with them all. The original Tolman-Stewart used this rotating coil and braking; they re-did their experiment about a decade later with a constantly rotating system and then everything moved to ac and was easier. The recent papers from the 80s used the ac system. The problem is, the physics is harder, and we want this to be for undergrads. So we like the braking experiment better.

Re: noise reduction. We can try and reduce the noise by turning off monitors and running at night. Ideally, we’d like to be able to run this as part of our undergraduate advanced lab, so making it an extremely low-noise environment is not ideal. But we can try. We’ve actually tried mu-metal, and it is not great at keeping out the magnetic fields. We’re currently considering just adding a 60 Hz notch filter.

We haven’t yet seen the signal we want, precisely because right now the noise swamps the TIA, and brings it to the rails.

George, The dc offset when it is just sitting there usually sits around a volt or so. We’re working with low-noise op-amps, and so they are not trimmable. We were using the LT1792 and that one we could trim and that worked well. We can and have tested the circuit with an actual honest-to-goodness current source, and with two coils, where we send a pulse into one coil and pick it up in the second coil. The first experiment works well, the second is inconclusive so far.

As for connections to the coils, to contact the rotating coil we use a rotary feedthrough that one of my students found on the web: http://www.mercotac.com/html/110.html . The stationary coil, we just bring the contacts out and onto the bench. None of the electronics is attached to the coils, it is all on the bench. We connect our two coils in series when the wires come to the breadboard.

Breadboard? At 200 nV?


Huh, have other people used these (rotating contacts) for low noise
measurements? Is there a spec? I'd want to try and measure the noise,
low currents might be different than high.. (mA) currents.


The mercotac things use "liquid metal" contacts. What metal is
liquid??
Mercury? Gallium if you heat it a little, I wonder if there is a temperature
spec.?

"mercotac" is a subtle hint.

I'd expect some healthy thermoelectrics.
Oh dear, don't throw another monkey wrench at it.

One more hardly makes a difference.

Maybe the thermal times are long enough so that he can look at
differences between the two spin directions.

And no one's mentioned 1/f noise. :^)

I have, several times.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[George Herold]

On Friday, June 7, 2019 at 9:35:47 AM UTC-4, John Larkin wrote:

On Fri, 7 Jun 2019 03:59:16 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all again.

Joseph Gwinn, I will take a look at the patent to see how they compensated for the op-amp drift.

Regarding the rotary connectors, there is not a lot of noise information on their website. We moved to a liquid metal rotary feedthrough (the liquid metal is mercury) to remove problems with noise that comes from brushes. I don’t know what the noise specs are. We are set up to run it all using wire that twist up, but that is hard to run tests like that. You get one try, and then have to replace the wires.

Jan Panteltje, several kOhm of input impedance will reduce our expected current by a factor of ten or more. So I am not sure your design will work.. We’d drop our expected current down to 100 pA, or less.

John Larkin, we did have the rig inside mu-metal for a while, but that was hard to maintain and did very little for the static (Earth’s) field that we also have to get rid of. (If you don’t get rid of the Earth’s field, when the coil is spinning the centrifugal force makes the coil slightly bigger, and then when you brake it it gets smaller, leading to a signal that looks like the one we are trying to see.) So we abandoned that. But we haven’t tried electrostatically shielding the entire setup. We put our breadboard into a metal box to reduce noise – are you suggesting we box up the whole thing into an electrostatic box? We had a real one of those when I was in grad school, a screened room. Is that necessary for the coils as well?

Ideally, I’d like to be able to see the integrated pulse or the actual V vs t of the pulse on the oscilloscope screen for the students to be able to see it before digitizing it. But if I have to digitize and then play with the signal, I will.


This looks to be an extraordinally difficult experiment.

Magnetic shielding is probably not practical here. The compensating
and helmholtz coils can help with mag fields. If electrostatic
coupling is squirting noise into the system, it's easy to test that:
temporarily shield the area with grounded aluminum foil or something
and see if it makes any difference. Also look at the amp output with a
scope when nothing is spinning.

I will say, for the last time, that the basic physics makes voltage,
and measusing current was an artifact of hundred-year-old
instrumentation limits, and the TIA is a bad idea.
So how would you sense it? Say using opamps* on the front end?

One issue with his compensating coil is that it has the same R and L
as the spinning coil (I assume)... I was going to say that cuts the
voltage signal in 1/2.. but that might not be true.

George H.
*as a fellow physicist, discrete front end design is not our forte.
(Excluding Phil H. :^)

Good luck getting this to work.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[George Herold]

On Thursday, June 6, 2019 at 11:27:28 PM UTC-4, John Larkin wrote:

On Thu, 6 Jun 2019 20:05:11 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Thursday, June 6, 2019 at 12:48:53 PM UTC-4, ithacacoll...@gmail.com wrote:
Hi all,
Again, thanks for the great ideas. I will try and keep up with them all. The original Tolman-Stewart used this rotating coil and braking; they re-did their experiment about a decade later with a constantly rotating system and then everything moved to ac and was easier. The recent papers from the 80s used the ac system. The problem is, the physics is harder, and we want this to be for undergrads. So we like the braking experiment better.

Re: noise reduction. We can try and reduce the noise by turning off monitors and running at night. Ideally, we’d like to be able to run this as part of our undergraduate advanced lab, so making it an extremely low-noise environment is not ideal. But we can try. We’ve actually tried mu-metal, and it is not great at keeping out the magnetic fields. We’re currently considering just adding a 60 Hz notch filter.

We haven’t yet seen the signal we want, precisely because right now the noise swamps the TIA, and brings it to the rails.

George, The dc offset when it is just sitting there usually sits around a volt or so. We’re working with low-noise op-amps, and so they are not trimmable. We were using the LT1792 and that one we could trim and that worked well. We can and have tested the circuit with an actual honest-to-goodness current source, and with two coils, where we send a pulse into one coil and pick it up in the second coil. The first experiment works well, the second is inconclusive so far.

As for connections to the coils, to contact the rotating coil we use a rotary feedthrough that one of my students found on the web: http://www.mercotac.com/html/110.html . The stationary coil, we just bring the contacts out and onto the bench. None of the electronics is attached to the coils, it is all on the bench. We connect our two coils in series when the wires come to the breadboard.

Breadboard? At 200 nV?


Huh, have other people used these (rotating contacts) for low noise
measurements? Is there a spec? I'd want to try and measure the noise,
low currents might be different than high.. (mA) currents.


The mercotac things use "liquid metal" contacts. What metal is
liquid??

Mercury? Gallium if you heat it a little, I wonder if there is a temperature
spec.?

I'd expect some healthy thermoelectrics.

Oh dear, don't throw another monkey wrench at it.
Maybe the thermal times are long enough so that he can look at
differences between the two spin directions.

And no one's mentioned 1/f noise. :^)

George H.

George H.


Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to integrate the current pulse. Our thought was that integrating would allow us to integrate over the noise and be left with only the slower 0.5 s pulse. But in order to get a large current, the input impedance of the measurement system has to be small. This is why the TIA, with its virtual ground at the negative input is nice, acts as zero input impedance for the current pulse. What would be the input impedance of your transistor circuit be?

Whit3rd, We started first using a charge amplifier exactly as you described, but without the switch. So to prevent pinning the op-amp we added a feedback resistor also. The system we had was very unstable, even for short periods of time. But we did not try it with a switch to keep the capacitor for charging. You are right that the math is easy if we can integrate the current to get the charge. Then we only need to know the starting speed and the total charge.

John Larkin, I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

If the 60 Hz is from a real magnetic field thing, no. If it's
electrostatic common-mode voltage, yes. 60 Hz can be filtered out as
long as it's not so big as to drive things nonlinear.

I think your rig needs an electrostatic shield. I've seen foamy sheet
stuff covered with aluminum foil.

In most rooms, the 60 Hz e-field is huge. Plus lots of other stuff.

Why not amplify and digitize the outputs of both coils?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jan Panteltje]

On a sunny day (Fri, 07 Jun 2019 07:09:14 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<nvrkfehekobkftdu6sdvvhi44lap5qk65v@4ax.com>:

On Fri, 07 Jun 2019 15:49:26 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

Jan Panteltje wrote:
On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).


Yeah, well, you forgot about the h_oe output conductance,
which isn't zero. There is effectively a resistor of a
few 10 kOhm in parallel with your 10 MOhm collector resistor,
limiting the gain. Somebody already told you.

Jeroen Belleman

Spice does transistors pretty well.

What spice?

 

[Jan Panteltje]

On a sunny day (Fri, 07 Jun 2019 15:49:26 +0200) it happened Jeroen Belleman
<jeroen@nospam.please> wrote in <qddq14$12ml$1@gioia.aioe.org>:

Jan Panteltje wrote:
On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).


Yeah, well, you forgot about the h_oe output conductance,
which isn't zero. There is effectively a resistor of a
few 10 kOhm in parallel with your 10 MOhm collector resistor,
limiting the gain. Somebody already told you.

Jeroen Belleman

Yes Zout is not infinete, but depending on the tranny not as low as 10 k.

 

[Jan Panteltje]

On a sunny day (Fri, 07 Jun 2019 06:52:55 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<0bqkfe911doin8cudc5h8buq2cgpakno70@4ax.com>:

On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,

And some basic misunderstandings of how transistors work. Higher beta
does not increase the voltage gain of the usual transistor amplifier.

200 nV will not push 100 pA into the base of a transistor whose
collector current is around 1 uA.

He wrote:
<quote>

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.
end quote

He says 100 pA,
dunno why u use volts.

The impedance looking into the base
will be 10s of megohms.

Really?

Did you try that?

Bad specimen? O/C?

 

[George Herold]

On Friday, June 7, 2019 at 9:53:00 AM UTC-4, John Larkin wrote:

On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,

And some basic misunderstandings of how transistors work. Higher beta
does not increase the voltage gain of the usual transistor amplifier.

200 nV will not push 100 pA into the base of a transistor whose
collector current is around 1 uA. The impedance looking into the base
will be 10s of megohms.

I'm not sure how to calculate the input impedance..
(I know, look in AoE.)
But isn't the problem the emitter resistance. r_e = 25 mV/Ic
At 1 uA of Ic, r_e = 25 k ohm and the gain is R_c/ r_e = 400.

Jan, Do you know the Ebers-Moll model of the transistor?

George H.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 07 Jun 2019 17:47:54 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 07 Jun 2019 07:09:14 -0700) it happened John Larkin
jjlarkin@highlandtechnology.com> wrote in
nvrkfehekobkftdu6sdvvhi44lap5qk65v@4ax.com>:

On Fri, 07 Jun 2019 15:49:26 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

Jan Panteltje wrote:
On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).


Yeah, well, you forgot about the h_oe output conductance,
which isn't zero. There is effectively a resistor of a
few 10 kOhm in parallel with your 10 MOhm collector resistor,
limiting the gain. Somebody already told you.

Jeroen Belleman

Spice does transistors pretty well.

What spice?

LT Spice. It's free and fairly easy to learn and use. It includes a
lot of transistor and mosfet and diode models.

I rarely do much of the old classic EE math any more... just guess
then Spice it. I even Spice voltage dividers and RC timers and such.

I just did an RLC to go between an opamp and a 40 Ms/s ADC input. The
ADC data sheet wants an RC, but the time constant was too slow for my
purposes. A little inductance snaps it right up.

And I had another case: we're redesigning a laser controller, for IC
lithography, that we did in 2002. We used (at the customer's request)
a Maxim tapped silicon delay line to generate some asynchronous
delays. The part is naturally long gone, so I'll use a triggered
oscillator and some FPGA logic to do the same function.

This took minutes to simulate:

https://www.dropbox.com/s/nezshpka0eush92/Tplus_Trig_Osc_2.jpg?dl=0



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 7 Jun 2019 11:43:20 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, June 7, 2019 at 9:53:00 AM UTC-4, John Larkin wrote:
On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,

And some basic misunderstandings of how transistors work. Higher beta
does not increase the voltage gain of the usual transistor amplifier.

200 nV will not push 100 pA into the base of a transistor whose
collector current is around 1 uA. The impedance looking into the base
will be 10s of megohms.

I'm not sure how to calculate the input impedance..
(I know, look in AoE.)
But isn't the problem the emitter resistance. r_e = 25 mV/Ic
At 1 uA of Ic, r_e = 25 k ohm and the gain is R_c/ r_e = 400.

Dynamic resistance Re is 25K and Rb is beta times that.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Friday, June 7, 2019 at 3:13:09 PM UTC-4, John Larkin wrote:

On Fri, 7 Jun 2019 11:43:20 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, June 7, 2019 at 9:53:00 AM UTC-4, John Larkin wrote:
On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,

And some basic misunderstandings of how transistors work. Higher beta
does not increase the voltage gain of the usual transistor amplifier.

200 nV will not push 100 pA into the base of a transistor whose
collector current is around 1 uA. The impedance looking into the base
will be 10s of megohms.

I'm not sure how to calculate the input impedance..
(I know, look in AoE.)
But isn't the problem the emitter resistance. r_e = 25 mV/Ic
At 1 uA of Ic, r_e = 25 k ohm and the gain is R_c/ r_e = 400.

Dynamic resistance Re is 25K and Rb is beta times that.

Oh, thanks. That's fairly obvious, now that you've told me. :^)
GH

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[whit3rd]

On Friday, June 7, 2019 at 6:22:54 AM UTC-7, John Larkin wrote:

On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why not amplify and digitize the outputs of both coils?

Wrong question. Amplify, certainly, but... digitize? To what end?

To see if the signal is real and not an artifact. To *see* what the
artifacts are. To filter and signal process with matlab or something.

That means doing an UNCONTROLLED experiment, and sorting through lots
of data to compensate. The demonstrate-to-the-audience value is nil.

To signal-average many runs. To subtract out the stationary coil
signal with tunable math.

When a single 'run' takes twenty seconds and produces one number (or two,
one from the moving coil and one from a nearby stationary coil by way of nullifying
interference), signal-averaging just means making a histogram of a few dozen
trials. We all know what the interference is, don't we? Fluorescent lights
have AC voltages on the ionized gas in the tube, all the wiring in the walls makes
E and B fields because that's how the energy is transferred, and big metal moving
through Earth's magnetic field makes ripples in the distance. So, turn the lights off,
ground some chickenwire around the apparatus, don't let vibration act on
any ferromagnetic bits (you can get bronze nails and form lots of mechanism in
wood and copper).

And to demonstrate a deceleration-rate-dependant pulse. And to
illustrate the basics of signal processing, which does tend to use
computers nowadays.

Again, it is sometimes effective to compensate/correct/calculate, but
the deceleration-rate is easy to reject, it complicates the result, so
why NOT reject it? You can easily brake from multiple different
initial rotation rates to show the pulse is proportional, it does NOT
take a microsecond-by-microsecond breakdown to do that.

Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

If you do zero-velocity trial spin, you get a zero voltage at the opamp output.
You would certainly accept that. And for noise, the histogram of zero-velocity
trials should give a zero-centered histogram of outputs, while
high-initial-velocity trials would not. You can also reverse the spin direction.

Friction isn't noise-free, repeatable, easily modeled, so what does 'properly shaped pulse'
even mean?

 

[whit3rd]

On Friday, June 7, 2019 at 8:55:17 AM UTC-7, George Herold wrote:

On Thursday, June 6, 2019 at 11:27:28 PM UTC-4, John Larkin wrote:

The mercotac things use "liquid metal" contacts. What metal is
liquid??
Mercury? Gallium if you heat it a little, I wonder if there is a temperature
spec.?

I'd expect some healthy thermoelectrics.

If you put a battery and op amp into the spinning disk, the rotating
contacts can have the buffered integrator-capacitor voltage
on them, so a few microvolts won't matter. The nanovolt signals don't
need to pass through the mystery metals.

> And no one's mentioned 1/f noise.

Noise only makes a single run uncertain, multiple runs will solve that.
PMI's old designs used SiN passivation with SiO2 overcoat, and had
very low 1/f noise; those OP27s, and other offerings, are available from
AD nowadays. 80nV in 0.1 to 10 Hz bandwidth, and you only really
care about 0.1 to 1 Hz if you look at the brake-pulse integration. It might
hurt the take-a-thousand-readings schemes, however.

 

[John Larkin]

On Fri, 7 Jun 2019 12:45:44 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, June 7, 2019 at 3:13:09 PM UTC-4, John Larkin wrote:
On Fri, 7 Jun 2019 11:43:20 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, June 7, 2019 at 9:53:00 AM UTC-4, John Larkin wrote:
On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,

And some basic misunderstandings of how transistors work. Higher beta
does not increase the voltage gain of the usual transistor amplifier.

200 nV will not push 100 pA into the base of a transistor whose
collector current is around 1 uA. The impedance looking into the base
will be 10s of megohms.

I'm not sure how to calculate the input impedance..
(I know, look in AoE.)
But isn't the problem the emitter resistance. r_e = 25 mV/Ic
At 1 uA of Ic, r_e = 25 k ohm and the gain is R_c/ r_e = 400.

Dynamic resistance Re is 25K and Rb is beta times that.
Oh, thanks. That's fairly obvious, now that you've told me. :^)
GH

And in a voltage amplifier, if you double beta, Rb doubles, so you get
half the base signal current, times twice the collector current, which
cancel. So voltage gain is independent of beta.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 7 Jun 2019 13:11:03 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, June 7, 2019 at 6:22:54 AM UTC-7, John Larkin wrote:
On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why not amplify and digitize the outputs of both coils?

Wrong question. Amplify, certainly, but... digitize? To what end?

To see if the signal is real and not an artifact. To *see* what the
artifacts are. To filter and signal process with matlab or something.

That means doing an UNCONTROLLED experiment, and sorting through lots
of data to compensate. The demonstrate-to-the-audience value is nil.

To signal-average many runs. To subtract out the stationary coil
signal with tunable math.

When a single 'run' takes twenty seconds and produces one number (or two,
one from the moving coil and one from a nearby stationary coil by way of nullifying
interference), signal-averaging just means making a histogram of a few dozen
trials. We all know what the interference is, don't we? Fluorescent lights
have AC voltages on the ionized gas in the tube, all the wiring in the walls makes
E and B fields because that's how the energy is transferred, and big metal moving
through Earth's magnetic field makes ripples in the distance. So, turn the lights off,
ground some chickenwire around the apparatus, don't let vibration act on
any ferromagnetic bits (you can get bronze nails and form lots of mechanism in
wood and copper).

And to demonstrate a deceleration-rate-dependant pulse. And to
illustrate the basics of signal processing, which does tend to use
computers nowadays.

Again, it is sometimes effective to compensate/correct/calculate, but
the deceleration-rate is easy to reject, it complicates the result, so
why NOT reject it? You can easily brake from multiple different
initial rotation rates to show the pulse is proportional, it does NOT
take a microsecond-by-microsecond breakdown to do that.


Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

If you do zero-velocity trial spin, you get a zero voltage at the opamp output.
You would certainly accept that. And for noise, the histogram of zero-velocity
trials should give a zero-centered histogram of outputs, while
high-initial-velocity trials would not. You can also reverse the spin direction.

Friction isn't noise-free, repeatable, easily modeled, so what does 'properly shaped pulse'
even mean?

It means that it agrees with the physics, namely that the coil voltage
is proportional to acceleration.

Other experimenters instrument the angular position of the coil to
demonstrate that exact relationship. It's a good crosscheck in a
system that typically has a lot of deceptive things going on.

If the signal from the coil is *not* a reasonable pulse that
corresponds to the acceleration that the electrons see, something is
wrong. A simple integral measurement hides all that.

The pulse would look cool, too.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 7 Jun 2019 14:22:06 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, June 7, 2019 at 2:00:23 PM UTC-7, John Larkin wrote:
On Fri, 7 Jun 2019 13:11:03 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Friday, June 7, 2019 at 6:22:54 AM UTC-7, John Larkin wrote:
On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

... the histogram of zero-velocity
trials should give a zero-centered histogram of outputs, while
high-initial-velocity trials would not. You can also reverse the spin direction.

Friction isn't noise-free, repeatable, easily modeled, so what does 'properly shaped pulse'
even mean?

It means that it agrees with the physics, namely that the coil voltage
is proportional to acceleration.

The noise-impaired coil voltage won't be a clear indication.

It will be a clear indication of the noise. If, say, the opamp is
railing, or there's a big DC offset, or a lot of hum, the windowed
integral is less useful.

If the signal from the coil is *not* a reasonable pulse that
corresponds to the acceleration that the electrons see, something is
wrong. A simple integral measurement hides all that.

Now you aren't thinking like a scientist. 'Something is wrong' means you can test
a new hypothesis, but it doesn't tell you WHAT new hypothesis. There's no value
in a 'something is wrong' determination unless another hypothesis can be put forward.

I wouldn't submit a paper, or teach a class, if the phenom involved is
an artifact.

Seeing the coil voltage would be useful in understanding what's going
on. Hiding it as a single integrated number is keeping your head in
the sand.

Testing for 'unknown' is less than productive,
and demonstrates... nothing in particular.

Isn't that what science does, discover things?

I poke around with scope probes and discover all sorts of things.

A simple integral measurement is a controlled experiment; such simplicity is the
reason we have controlled laboratory conditions. We make simplicity, it's
productive to make simplicity, and counterproductive to destroy it.

You aren't welcome in my lab!

Do you have a lab? Do you have an oscilloscope?


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[whit3rd]

On Friday, June 7, 2019 at 2:00:23 PM UTC-7, John Larkin wrote:

On Fri, 7 Jun 2019 13:11:03 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Friday, June 7, 2019 at 6:22:54 AM UTC-7, John Larkin wrote:
On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

... the histogram of zero-velocity
trials should give a zero-centered histogram of outputs, while
high-initial-velocity trials would not. You can also reverse the spin direction.

Friction isn't noise-free, repeatable, easily modeled, so what does 'properly shaped pulse'
even mean?

It means that it agrees with the physics, namely that the coil voltage
is proportional to acceleration.

The noise-impaired coil voltage won't be a clear indication.

If the signal from the coil is *not* a reasonable pulse that
corresponds to the acceleration that the electrons see, something is
wrong. A simple integral measurement hides all that.

Now you aren't thinking like a scientist. 'Something is wrong' means you can test
a new hypothesis, but it doesn't tell you WHAT new hypothesis. There's no value
in a 'something is wrong' determination unless another hypothesis can be put forward.

Testing for 'unknown' is less than productive,
and demonstrates... nothing in particular.

A simple integral measurement is a controlled experiment; such simplicity is the
reason we have controlled laboratory conditions. We make simplicity, it's
productive to make simplicity, and counterproductive to destroy it.

You aren't welcome in my lab!

 

[John Larkin]

On Fri, 07 Jun 2019 18:06:21 -0400, Joseph Gwinn
<joegwinn@comcast.net> wrote:

On Jun 7, 2019, John Larkin wrote
(in article<8mjlfehmel8g49fbi6asb0tpmbkib7g3fb@4ax.com&gt:

On Fri, 7 Jun 2019 13:11:03 -0700 (PDT), whit3rd<whit3rd@gmail.com
wrote:

On Friday, June 7, 2019 at 6:22:54 AM UTC-7, John Larkin wrote:
On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why not amplify and digitize the outputs of both coils?

Wrong question. Amplify, certainly, but... digitize? To what end?

To see if the signal is real and not an artifact. To *see* what the
artifacts are. To filter and signal process with matlab or something.

That means doing an UNCONTROLLED experiment, and sorting through lots
of data to compensate. The demonstrate-to-the-audience value is nil.

To signal-average many runs. To subtract out the stationary coil
signal with tunable math.

When a single 'run' takes twenty seconds and produces one number (or two,
one from the moving coil and one from a nearby stationary coil by way of
nullifying
interference), signal-averaging just means making a histogram of a few dozen
trials. We all know what the interference is, don't we? Fluorescent lights
have AC voltages on the ionized gas in the tube, all the wiring in the
walls makes
E and B fields because that's how the energy is transferred, and big metal
moving
through Earth's magnetic field makes ripples in the distance. So, turn the
lights off,
ground some chickenwire around the apparatus, don't let vibration act on
any ferromagnetic bits (you can get bronze nails and form lots of mechanism
in
wood and copper).

And to demonstrate a deceleration-rate-dependant pulse. And to
illustrate the basics of signal processing, which does tend to use
computers nowadays.

Again, it is sometimes effective to compensate/correct/calculate, but
the deceleration-rate is easy to reject, it complicates the result, so
why NOT reject it? You can easily brake from multiple different
initial rotation rates to show the pulse is proportional, it does NOT
take a microsecond-by-microsecond breakdown to do that.


Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

If you do zero-velocity trial spin, you get a zero voltage at the opamp
output.
You would certainly accept that. And for noise, the histogram of
zero-velocity
trials should give a zero-centered histogram of outputs, while
high-initial-velocity trials would not. You can also reverse the spin
direction.

Friction isn't noise-free, repeatable, easily modeled, so what does
'properly shaped pulse'
even mean?

It means that it agrees with the physics, namely that the coil voltage
is proportional to acceleration.

While we may not be able to put a battery in the rotating coil, it occurs to
me that we could put a zero-bias JFET preamp in the coil, power from outside
via a twisted pair that also carries the amplified signal. Feed from a
constant voltage source and sense the current draft. This way, capacitance in
the twisted pair has little effect. And the twisted pair cancels magnetic
fields from powering the JFET.

Joe Gwinn

I'd expect that a couple of button-cell batteries or a supercap could
ride on the coil. Put the diffamp in the exact center to minimize
forces on it.

Heck, put everything along the rotational axis, on a long skinny PC
board.

Short the coil and spin it to make sure the amp is not reacting to the
acceleration, Hall effect or something. You can make a lot of offset
by pressing on the top of an opamp.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Joseph Gwinn]

On Jun 7, 2019, John Larkin wrote
(in article<8mjlfehmel8g49fbi6asb0tpmbkib7g3fb@4ax.com&gt:

On Fri, 7 Jun 2019 13:11:03 -0700 (PDT), whit3rd<whit3rd@gmail.com
wrote:

On Friday, June 7, 2019 at 6:22:54 AM UTC-7, John Larkin wrote:
On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why not amplify and digitize the outputs of both coils?

Wrong question. Amplify, certainly, but... digitize? To what end?

To see if the signal is real and not an artifact. To *see* what the
artifacts are. To filter and signal process with matlab or something.

That means doing an UNCONTROLLED experiment, and sorting through lots
of data to compensate. The demonstrate-to-the-audience value is nil.

To signal-average many runs. To subtract out the stationary coil
signal with tunable math.

When a single 'run' takes twenty seconds and produces one number (or two,
one from the moving coil and one from a nearby stationary coil by way of
nullifying
interference), signal-averaging just means making a histogram of a few dozen
trials. We all know what the interference is, don't we? Fluorescent lights
have AC voltages on the ionized gas in the tube, all the wiring in the
walls makes
E and B fields because that's how the energy is transferred, and big metal
moving
through Earth's magnetic field makes ripples in the distance. So, turn the
lights off,
ground some chickenwire around the apparatus, don't let vibration act on
any ferromagnetic bits (you can get bronze nails and form lots of mechanism
in
wood and copper).

And to demonstrate a deceleration-rate-dependant pulse. And to
illustrate the basics of signal processing, which does tend to use
computers nowadays.

Again, it is sometimes effective to compensate/correct/calculate, but
the deceleration-rate is easy to reject, it complicates the result, so
why NOT reject it? You can easily brake from multiple different
initial rotation rates to show the pulse is proportional, it does NOT
take a microsecond-by-microsecond breakdown to do that.


Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

If you do zero-velocity trial spin, you get a zero voltage at the opamp
output.
You would certainly accept that. And for noise, the histogram of
zero-velocity
trials should give a zero-centered histogram of outputs, while
high-initial-velocity trials would not. You can also reverse the spin
direction.

Friction isn't noise-free, repeatable, easily modeled, so what does
'properly shaped pulse'
even mean?

It means that it agrees with the physics, namely that the coil voltage
is proportional to acceleration.

While we may not be able to put a battery in the rotating coil, it occurs to
me that we could put a zero-bias JFET preamp in the coil, power from outside
via a twisted pair that also carries the amplified signal. Feed from a
constant voltage source and sense the current draft. This way, capacitance in
the twisted pair has little effect. And the twisted pair cancels magnetic
fields from powering the JFET.

Joe Gwinn