Help designing a low-noise TIA

Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspin, he says you are the people to help me out. I understand a decent amount about circuits, but not enough to design the low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart experiment https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of the first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked rapidly. The electrons keep moving and create a small pulse of current. Originally, Tolman and Stewart used a ballistic galvanometer to act as a charge amplifier and integrate the current to find the total charge.

I’d like to use a TIA to convert the small current pulse into a voltage, then record that voltage as a function of time. The problem is that the coil acts as a giant antenna and picks up all sorts of unwanted noise, so I’d like to get rid of that noise. In particular, it is really good at finding 60 Hz signals in the room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a feedback resistor and capacitor in parallel. We attach the coil (about 200 Ohm resistance, 500 mH inductance) to one input and put the other input across 200 Ohms to ground. The large amplification leads to huge amplification of the noise, and it is hard to see our signal. We expect the current pulse to be 1 nA of current, almost square wave in shape, and it should last the duration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

 

[Phil Hobbs]

On 6/4/19 3:28 PM, ithacacollegephysics@gmail.com wrote:

Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspin, he says you are the people to help me out. I understand a decent amount about circuits, but not enough to design the low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart experiment https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of the first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked rapidly. The electrons keep moving and create a small pulse of current. Originally, Tolman and Stewart used a ballistic galvanometer to act as a charge amplifier and integrate the current to find the total charge.

I’d like to use a TIA to convert the small current pulse into a voltage, then record that voltage as a function of time. The problem is that the coil acts as a giant antenna and picks up all sorts of unwanted noise, so I’d like to get rid of that noise. In particular, it is really good at finding 60 Hz signals in the room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a feedback resistor and capacitor in parallel. We attach the coil (about 200 Ohm resistance, 500 mH inductance) to one input and put the other input across 200 Ohms to ground. The large amplification leads to huge amplification of the noise, and it is hard to see our signal. We expect the current pulse to be 1 nA of current, almost square wave in shape, and it should last the duration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

One approach would be to put another winding around it,
electrostatically shielded inside and out using copper tape, and hang
the TIA on that (inside the shield). You could get the coupling
coefficient up to probably 0.5, and it's easily measured, so it wouldn't
hurt the accuracy much.

The shield would look like a shorted turn , so you use two half-turn
shields with a nice big overlap , insulated with kapton tape. Some
capacitors going across the gap would help too.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Miles, KE5FX]

On Tuesday, June 4, 2019 at 1:46:13 PM UTC-7, George Herold wrote:

How big is your coil (diameter)? Do you see signals from the coil
spinning in the earth's B-field? I was browsing this,
https://authors.library.caltech.edu/3372/1/TOLpr16b.pdf
(from your wiki link) and it looks to be a big coil. But hard to tell.

Great paper. It really gives you a feel for how physics was done
back in the days before physics was "done."

I wonder what was responsible for the rapid variations in the local
geomagnetic field that they had to compensate for. They say that they
could detect the effect of cars driving by, but they also say that
separating the compensating coil(s) from the experiment had little
effect, suggesting that the fluctuations were coming from a significant
distance.

I guess they could have been seeing the effects of solar activity,
measuring the K index a couple of decades before the term caught on.

-- john, KE5FX

 

[George Herold]

On Tuesday, June 4, 2019 at 3:28:55 PM UTC-4, ithacacoll...@gmail.com wrote:

Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspin, he says you are the people to help me out. I understand a decent amount about circuits, but not enough to design the low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart experiment https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of the first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked rapidly. The electrons keep moving and create a small pulse of current. Originally, Tolman and Stewart used a ballistic galvanometer to act as a charge amplifier and integrate the current to find the total charge.

I’d like to use a TIA to convert the small current pulse into a voltage, then record that voltage as a function of time. The problem is that the coil acts as a giant antenna and picks up all sorts of unwanted noise, so I’d like to get rid of that noise. In particular, it is really good at finding 60 Hz signals in the room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a feedback resistor and capacitor in parallel. We attach the coil (about 200 Ohm resistance, 500 mH inductance) to one input and put the other input across 200 Ohms to ground. The large amplification leads to huge amplification of the noise, and it is hard to see our signal. We expect the current pulse to be 1 nA of current, almost square wave in shape, and it should last the duration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

Hi Matt, Thanks for asking here... I guess I'm hoping that this will give
you more ideas, than just you and I going back and forth on email.

The OPA140 looks fine. 1G ohm and 10 pF is a 10 ms time constant..

But your big problem is the 60 Hz pickup from your coil?

How big is your coil (diameter)? Do you see signals from the coil
spinning in the earth's B-field? I was browsing this,
https://authors.library.caltech.edu/3372/1/TOLpr16b.pdf
(from your wiki link) and it looks to be a big coil. But hard to tell.

We do a trick in our Earth's field nmr where there are two coils in
series (but wound in opposite directions.) A many turn inner coil that
picks up the nmr signal. and a much bigger outer coil, with the same
turns*area as the inner coil. And this picks up the same emf from the
AC 60 Hz as the signal coil... and cancels it. I think JR will take a turn
or two off the inner coil to try and match the cancellation.

George H.

 

[John Larkin]

On Tue, 4 Jun 2019 12:28:49 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspin, he says you are the people to help me out. I understand a decent amount about circuits, but not enough to design the low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart experiment https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of the first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked rapidly. The electrons keep moving and create a small pulse of current. Originally, Tolman and Stewart used a ballistic galvanometer to act as a charge amplifier and integrate the current to find the total charge.

I’d like to use a TIA to convert the small current pulse into a voltage, then record that voltage as a function of time. The problem is that the coil acts as a giant antenna and picks up all sorts of unwanted noise, so I’d like to get rid of that noise. In particular, it is really good at finding 60 Hz signals in the room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a feedback resistor and capacitor in parallel. We attach the coil (about 200 Ohm resistance, 500 mH inductance) to one input and put the other input across 200 Ohms to ground. The large amplification leads to huge amplification of the noise, and it is hard to see our signal. We expect the current pulse to be 1 nA of current, almost square wave in shape, and it should last the duration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

The source impedance of the voltage pulse will be low, so you may not
want a high-impedance TIA. The coil is more of a 200 ohm voltage
source, about 200 nV.

If the braking lasts a half second, a lot of the noise could be
filtered out. Or brake faster and get more signal.

I don't understand your circuit, where the 200 ohm things go. Can you
post the schematic?

You'll have two sources of noise, electrostatic and magnetic. The
electrostatic can be shielded.

Is it possible to split the coil into two sections, wired so that the
voltages add for the deceleration but cancel for external fields?



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Joerg]

On 2019-06-04 13:46, George Herold wrote:

On Tuesday, June 4, 2019 at 3:28:55 PM UTC-4, ithacacoll...@gmail.com
wrote:
Hi all,

I am posting to this forum at the suggestion of George Herold from
Teachspin, he says you are the people to help me out. I understand
a decent amount about circuits, but not enough to design the
low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart
experiment
https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that
was one of the first proofs that electrons inside metals carry the
current.

In this experiment, a coil of wire is spun to high speeds and then
braked rapidly. The electrons keep moving and create a small pulse
of current. Originally, Tolman and Stewart used a ballistic
galvanometer to act as a charge amplifier and integrate the current
to find the total charge.

I’d like to use a TIA to convert the small current pulse into a
voltage, then record that voltage as a function of time. The
problem is that the coil acts as a giant antenna and picks up all
sorts of unwanted noise, so I’d like to get rid of that noise. In
particular, it is really good at finding 60 Hz signals in the
room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a
feedback resistor and capacitor in parallel. We attach the coil
(about 200 Ohm resistance, 500 mH inductance) to one input and put
the other input across 200 Ohms to ground. The large amplification
leads to huge amplification of the noise, and it is hard to see our
signal. We expect the current pulse to be 1 nA of current, almost
square wave in shape, and it should last the duration of the
braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan Ithaca College Physics

Hi Matt, Thanks for asking here... I guess I'm hoping that this will
give you more ideas, than just you and I going back and forth on
email.

The OPA140 looks fine. 1G ohm and 10 pF is a 10 ms time constant..

But your big problem is the 60 Hz pickup from your coil?

How big is your coil (diameter)? Do you see signals from the coil
spinning in the earth's B-field? I was browsing this,
https://authors.library.caltech.edu/3372/1/TOLpr16b.pdf (from your
wiki link) and it looks to be a big coil. But hard to tell.

We do a trick in our Earth's field nmr where there are two coils in
series (but wound in opposite directions.) A many turn inner coil
that picks up the nmr signal. and a much bigger outer coil, with the
same turns*area as the inner coil. And this picks up the same emf
from the AC 60 Hz as the signal coil... and cancels it. I think JR
will take a turn or two off the inner coil to try and match the
cancellation.

If the 60Hz getting in is E-field there is also the trick of winding
with coax and grounding one side (and only one).

However, much of it will be magnetic and then Matt can only notch it
out. If the detection is done in software notch filters are easy. If
analog you'd almost have to use a switched-capacitor filter for that.
Very likely several notch filters are required because when 60Hz is gone
Matt will discover that there's also a lot of 180Hz and 300Hz. Also,
make sure there is absolutely no 60Hz gear that turns on and off at
randon, such as electric cooktops, because that's next to impossible to
notch out.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[George Herold]

On Tuesday, June 4, 2019 at 6:18:41 PM UTC-4, John Larkin wrote:

On Tue, 4 Jun 2019 12:28:49 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspin, he says you are the people to help me out. I understand a decent amount about circuits, but not enough to design the low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart experiment https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of the first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked rapidly. The electrons keep moving and create a small pulse of current. Originally, Tolman and Stewart used a ballistic galvanometer to act as a charge amplifier and integrate the current to find the total charge.

I’d like to use a TIA to convert the small current pulse into a voltage, then record that voltage as a function of time. The problem is that the coil acts as a giant antenna and picks up all sorts of unwanted noise, so I’d like to get rid of that noise. In particular, it is really good at finding 60 Hz signals in the room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a feedback resistor and capacitor in parallel. We attach the coil (about 200 Ohm resistance, 500 mH inductance) to one input and put the other input across 200 Ohms to ground. The large amplification leads to huge amplification of the noise, and it is hard to see our signal. We expect the current pulse to be 1 nA of current, almost square wave in shape, and it should last the duration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

The source impedance of the voltage pulse will be low, so you may not
want a high-impedance TIA. The coil is more of a 200 ohm voltage
source, about 200 nV.

Yeah, Matt wants to measure current, but I'm not sure a TIA is the
answer. (I think he has 2 x 200 ohms, but I'm not sure.)

If the braking lasts a half second, a lot of the noise could be
filtered out. Or brake faster and get more signal.

I was thinking of a gated integrator... which I haven't used
in years. You could also trigger a DSO and average.
(which is like a gated integrator, with more time information.)

George H.

I don't understand your circuit, where the 200 ohm things go. Can you
post the schematic?

You'll have two sources of noise, electrostatic and magnetic. The
electrostatic can be shielded.

Is it possible to split the coil into two sections, wired so that the
voltages add for the deceleration but cancel for external fields?



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[George Herold]

On Tuesday, June 4, 2019 at 5:24:53 PM UTC-4, John Miles, KE5FX wrote:

On Tuesday, June 4, 2019 at 1:46:13 PM UTC-7, George Herold wrote:
How big is your coil (diameter)? Do you see signals from the coil
spinning in the earth's B-field? I was browsing this,
https://authors.library.caltech.edu/3372/1/TOLpr16b.pdf
(from your wiki link) and it looks to be a big coil. But hard to tell.

Great paper. It really gives you a feel for how physics was done
back in the days before physics was "done."
Huh.. I'm going to have to read it.

I wonder what was responsible for the rapid variations in the local
geomagnetic field that they had to compensate for. They say that they
could detect the effect of cars driving by, but they also say that
separating the compensating coil(s) from the experiment had little
effect, suggesting that the fluctuations were coming from a significant
distance.

I've got optical pumping coils/ apparatus, that I take
'down home on the farm' to test.... In the living room
I can see the fish tank pump spinning, it's a permanent magnet.

I test 'em in a bedroom; I can totally see a car pull into the nearby
drive-way.

I guess they could have been seeing the effects of solar activity,
measuring the K index a couple of decades before the term caught on.

There's some daily variation in the B-field... I never tried to measure
it.
Heading back to read about B-fields in 1916, I can only think things have
gotten worse since then.

George H.

-- john, KE5FX

 

[John Larkin]

On Tue, 4 Jun 2019 17:38:39 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, June 4, 2019 at 6:18:41 PM UTC-4, John Larkin wrote:
On Tue, 4 Jun 2019 12:28:49 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspin, he says you are the people to help me out. I understand a decent amount about circuits, but not enough to design the low-noise TIA I would like to build.

We're trying to create a modern version of the Tolman-Stewart experiment https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of the first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked rapidly. The electrons keep moving and create a small pulse of current. Originally, Tolman and Stewart used a ballistic galvanometer to act as a charge amplifier and integrate the current to find the total charge.

I’d like to use a TIA to convert the small current pulse into a voltage, then record that voltage as a function of time. The problem is that the coil acts as a giant antenna and picks up all sorts of unwanted noise, so I’d like to get rid of that noise. In particular, it is really good at finding 60 Hz signals in the room.

Right now we’re using an OPA 140 with 1 GOhm and 10 pF as a feedback resistor and capacitor in parallel. We attach the coil (about 200 Ohm resistance, 500 mH inductance) to one input and put the other input across 200 Ohms to ground. The large amplification leads to huge amplification of the noise, and it is hard to see our signal. We expect the current pulse to be 1 nA of current, almost square wave in shape, and it should last the duration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

The source impedance of the voltage pulse will be low, so you may not
want a high-impedance TIA. The coil is more of a 200 ohm voltage
source, about 200 nV.
Yeah, Matt wants to measure current, but I'm not sure a TIA is the
answer. (I think he has 2 x 200 ohms, but I'm not sure.)

A ballistic galvonmeter measured current, but that doesn't mean we
have to. I think the physics produces voltage.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Tom Gardner]

On 05/06/19 02:05, John Larkin wrote:

A ballistic galvonmeter measured current, but that doesn't mean we
have to. I think the physics produces voltage.

At school we only ever used them to measure charge. They
integrated current over time of a current pulse, where
the pulse's duration was a short fraction of the
ballistic galvanometer's response time.

Never saw them being used to measure current; ordinary
galvanometers were sufficient for that.

 

TEst

 

[John Larkin]

On Wed, 5 Jun 2019 06:15:05 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

TEst

WOrks.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

Hi all,

Thanks for the replies! A lot to think about.

If you want to see our setup, here is a picture:
https://www.dropbox.com/s/is3vdxlcz24ohml/TS-expt.jpg?dl=0

Here is the simple circuit we are working with now:
https://www.dropbox.com/s/f9aik177qqellcj/TS-ckt.jpg?dl=0
In the simulated circuit, we've added a current source to act as the electron motion upon braking the coil.

Phil, I am not sure I follow your suggestions, sorry. We'd put another coil outside the rotating coil to measure the signal from the rotating coil? Elaborations appreciated!

George, our coil is about 8" in diameter. I remember the Earth NMR from the Food Truck when it came to Ithaca College. If you look at our setup, we have something very similar: we have a second, stationary, coil directly below the rotating coil, and this coil is counter-wound. This definitely helps to reduce the 60 Hz noise (if there is other noise, it is drowned out by 60 Hz noise). At the time I thought our methods were essentially identical. Do you think a larger stationary coil surrounding the rotating coil would work better than a stationary coil of the same diameter beneath the rotating coil?

Jeorg, right now we reduce the remaining 60 Hz noise by simply averaging our voltage signal over 17 ms, so we get one data point for every 60 Hz cycle.. This does reduce our noise. I planned to try to remove the 60 Hz noise via FFT and then fiter out the high frequencies, but I wanted to try and find an experimental solution first.

John Larkin, braking faster would be hard. The coil has about a pound of copper wire 8 inches from the rotation axis, and we spin it up to 6000 or 7000 rpm. It has a lot of angular momentum! Half a second is the best we've been able to do. And I am afraid we've worked pretty hard to get it spinning down even that quickly. As for the split coil, we have essentially done that with the rotating coil and the stationary coil. They can't both be rotating, or the signal we are hoping for cancels out (the electrons in the two coils are counter-rotating, so you get two pulses of current in opposite directions).

George, we did try a charge amplifier at first, which as far as I understand is the electronic analog of the ballistic galvanometer. But it was hard to get the op-amps to be stable and still be able to integrate over a whole second. So I figured we have much faster electronics now, so to get the total charge I can just integrate the current, so that's why we are trying a TIA now.

 

[John Larkin]

On Wed, 5 Jun 2019 07:42:37 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all,

Thanks for the replies! A lot to think about.

If you want to see our setup, here is a picture:
https://www.dropbox.com/s/is3vdxlcz24ohml/TS-expt.jpg?dl=0

Here is the simple circuit we are working with now:
https://www.dropbox.com/s/f9aik177qqellcj/TS-ckt.jpg?dl=0

R2 just adds Johnson noise. That opamp is fairly noisy too.

The DC gain of that circuit is about 4e6, and the typical input offset
of that opamp is 30 uV. Multiply those!

In the simulated circuit, we've added a current source to act as the electron motion upon braking the coil.

Phil, I am not sure I follow your suggestions, sorry. We'd put another coil outside the rotating coil to measure the signal from the rotating coil? Elaborations appreciated!

George, our coil is about 8" in diameter. I remember the Earth NMR from the Food Truck when it came to Ithaca College. If you look at our setup, we have something very similar: we have a second, stationary, coil directly below the rotating coil, and this coil is counter-wound. This definitely helps to reduce the 60 Hz noise (if there is other noise, it is drowned out by 60 Hz noise). At the time I thought our methods were essentially identical. Do you think a larger stationary coil surrounding the rotating coil would work better than a stationary coil of the same diameter beneath the rotating coil?

Jeorg, right now we reduce the remaining 60 Hz noise by simply averaging our voltage signal over 17 ms, so we get one data point for every 60 Hz cycle. This does reduce our noise. I planned to try to remove the 60 Hz noise via FFT and then fiter out the high frequencies, but I wanted to try and find an experimental solution first.

John Larkin, braking faster would be hard. The coil has about a pound of copper wire 8 inches from the rotation axis, and we spin it up to 6000 or 7000 rpm. It has a lot of angular momentum! Half a second is the best we've been able to do. And I am afraid we've worked pretty hard to get it spinning down even that quickly. As for the split coil, we have essentially done that with the rotating coil and the stationary coil. They can't both be rotating, or the signal we are hoping for cancels out (the electrons in the two coils are counter-rotating, so you get two pulses of current in opposite directions).

George, we did try a charge amplifier at first, which as far as I understand is the electronic analog of the ballistic galvanometer. But it was hard to get the op-amps to be stable and still be able to integrate over a whole second. So I figured we have much faster electronics now, so to get the total charge I can just integrate the current, so that's why we are trying a TIA now.

Is the amplifier and its batteries rotating on the coil? It would be a
lot easier to export volts than nanovolts.

Optical coupling would be fun to get the signal off the spinning coil.

I still think it would be better to use a voltage amplifier rather
than a TIA. Low noise diffamp.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

R2 just adds Johnson noise. That opamp is fairly noisy too.

The DC gain of that circuit is about 4e6, and the typical input offset
of that opamp is 30 uV. Multiply those!

We can remove R2.

Why is the gain only 4e6? Because we are using a current source, if we have a 1 nA pulse we should see 1V on the output, so a gain of 1e9 V/A.

What do you mean by multiply those?

Is the amplifier and its batteries rotating on the coil? It would be a
lot easier to export volts than nanovolts.

The circuit sits on the table -- we pull off the raw signal from the rotating coil. We'd have to very carefully balance everything to get the amplifying circuit and batteries rotating at 7000 rpm.

Optical coupling would be fun to get the signal off the spinning coil.

Pre- or post-amplification?

I still think it would be better to use a voltage amplifier rather
than a TIA. Low noise diffamp.

How would this work for such a low current? And if you put that current across any resistance, it drops the current generated by the spinning coil. So best case scenario, it's across, say, 10 Ohms, and then you have a voltage of 10 nV. A low noise diff amp can find that 10 nV signal and amplify it and not generate noise? Happy to try it -- are there any schematics I can look at, or a section in Horowitz and Hill?

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Wed, 5 Jun 2019 09:21:11 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

R2 just adds Johnson noise. That opamp is fairly noisy too.

The DC gain of that circuit is about 4e6, and the typical input offset
of that opamp is 30 uV. Multiply those!



We can remove R2.

Why is the gain only 4e6? Because we are using a current source, if we have a 1 nA pulse we should see 1V on the output, so a gain of 1e9 V/A.

You are not using a current source. You are using a coil of wire.

>What do you mean by multiply those?

Can't simplify that. Multiply them.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Joerg]

On 2019-06-05 07:42, ithacacollegephysics@gmail.com wrote:

Hi all,

Thanks for the replies! A lot to think about.

If you want to see our setup, here is a picture:
https://www.dropbox.com/s/is3vdxlcz24ohml/TS-expt.jpg?dl=0

Move the Apple monitor and the whole computer away from back there. They
could be a source of line frequency noise. Also a whole lot of other
noise (back light, long traces behind the screen). Nothing should be
plugged into wall outlets there either. As a test turn off the ceiling
lights in the room to see if that lowers the noise. Older style lighting
has magnetic ballasts which leak some magnetic field.

A brute force option would be to pack the whole coil setup into a
mu-metal box. If you bend that srt of metal you must anneal at the bends
or it can lose much of its shielding properties. Of course, if this is a
demo setup that would take away the "live" look and feel.

Here is the simple circuit we are working with now:
https://www.dropbox.com/s/f9aik177qqellcj/TS-ckt.jpg?dl=0 In the
simulated circuit, we've added a current source to act as the
electron motion upon braking the coil.

Like John suggested you could get rid of R2. 260ohms isn't going to make
much of a difference though.

The gain is a bit high, factor of 1e9 current-to-voltage. Make sure
noise does not rail the amplifier. I would try to get by with the
minimum in gain and go digital as soon as possible. I such cases I often
use the Behringer ECA-224 USB audio interface but not sure if it goes
far enough towards DC for you. Probably it does and you could
SW-compensate for the roll off.

Digital opens a lot of filtering opportunities and the use of pre-canned
C-library code. However, you can't go into that from an opamp that is
supplied +/-15V. Too much, and if you have an accidental big pulse the
audio input gets fried. The supply voltages would have to be reduced and
the gain as well.

Phil, I am not sure I follow your suggestions, sorry. We'd put
another coil outside the rotating coil to measure the signal from the
rotating coil? Elaborations appreciated!

George, our coil is about 8" in diameter. I remember the Earth NMR
from the Food Truck when it came to Ithaca College. If you look at
our setup, we have something very similar: we have a second,
stationary, coil directly below the rotating coil, and this coil is
counter-wound. This definitely helps to reduce the 60 Hz noise (if
there is other noise, it is drowned out by 60 Hz noise). At the time
I thought our methods were essentially identical. Do you think a
larger stationary coil surrounding the rotating coil would work
better than a stationary coil of the same diameter beneath the
rotating coil?

Jeorg, right now we reduce the remaining 60 Hz noise by simply
averaging our voltage signal over 17 ms, so we get one data point for
every 60 Hz cycle. This does reduce our noise. I planned to try to
remove the 60 Hz noise via FFT and then fiter out the high
frequencies, but I wanted to try and find an experimental solution
first.

I have seen such averaging. It sort of works but not very well. Too much
residual line frequency noise. If this has to be analog you can use a
switched-capacitor notch filter but if at all possible I'd do it on a
PC. For quick experiments the best would be a laptop running only on its
battery, no charger connected. This is because such chargers often
introduce ground loop noise.

John Larkin, braking faster would be hard. The coil has about a
pound of copper wire 8 inches from the rotation axis, and we spin it
up to 6000 or 7000 rpm. It has a lot of angular momentum! Half a
second is the best we've been able to do. And I am afraid we've
worked pretty hard to get it spinning down even that quickly. As for
the split coil, we have essentially done that with the rotating coil
and the stationary coil. They can't both be rotating, or the signal
we are hoping for cancels out (the electrons in the two coils are
counter-rotating, so you get two pulses of current in opposite
directions).

Listen to the ceiling when in that room. If you hear a muffled hum at
times or all the time there may be a big air diverter motor operating.
Those can spew 60Hz around. You could talk to maintenance if it's
possible to turn that off for a while. Same below if this isn't on the
ground floor and check what's on the other side of that white brick
wall. Such walls do not shield 60Hz at all.

George, we did try a charge amplifier at first, which as far as I
understand is the electronic analog of the ballistic galvanometer.
But it was hard to get the op-amps to be stable and still be able to
integrate over a whole second. So I figured we have much faster
electronics now, so to get the total charge I can just integrate the
current, so that's why we are trying a TIA now.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[John Larkin]

On Wed, 5 Jun 2019 07:42:37 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all,

Thanks for the replies! A lot to think about.

If you want to see our setup, here is a picture:
https://www.dropbox.com/s/is3vdxlcz24ohml/TS-expt.jpg?dl=0

Here is the simple circuit we are working with now:
https://www.dropbox.com/s/f9aik177qqellcj/TS-ckt.jpg?dl=0
In the simulated circuit, we've added a current source to act as the electron motion upon braking the coil.

R1 and L1, in series with a current source, have no function in that
circuit.

A coil is not a current source. If you short it with a low-impedance
device, a galvo or a TIA, you can pretend that it is.



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[George Herold]

On Wednesday, June 5, 2019 at 12:21:19 PM UTC-4, ithacacoll...@gmail.com wrote:

R2 just adds Johnson noise. That opamp is fairly noisy too.

The DC gain of that circuit is about 4e6, and the typical input offset
of that opamp is 30 uV. Multiply those!



We can remove R2.

Why is the gain only 4e6? Because we are using a current source, if we have a 1 nA pulse we should see 1V on the output, so a gain of 1e9 V/A.

What do you mean by multiply those?

So at DC (ignoring inductor and 10 pF cap) you've got 1 G ohm feedback R,
and ~250 ohms to ground on the inverting input. A (voltage) gain of 4e6.
You can think of the offset voltage (V_os = 30 uV typ) as being at the
non-inverting input to gnd. With the amp just sitting there, what's the
DC level of the output? Do you try and trim that?

Is the amplifier and its batteries rotating on the coil? It would be a
lot easier to export volts than nanovolts.

The circuit sits on the table -- we pull off the raw signal from the rotating coil. We'd have to very carefully balance everything to get the amplifying circuit and batteries rotating at 7000 rpm.

Have you seen anything that looks like a signal yet?
(or just noise so far?) Sometimes it's hard to know if something
fundamental is wrong.. or if you are just making a bone headed
circuit (or other) mistake.
You might try putting a larger dummy signal through the coil and
see if you can see anything. To test the circuit. (maybe you've done
that already.)

George H.

Optical coupling would be fun to get the signal off the spinning coil.

Pre- or post-amplification?


I still think it would be better to use a voltage amplifier rather
than a TIA. Low noise diffamp.

How would this work for such a low current? And if you put that current across any resistance, it drops the current generated by the spinning coil. So best case scenario, it's across, say, 10 Ohms, and then you have a voltage of 10 nV. A low noise diff amp can find that 10 nV signal and amplify it and not generate noise? Happy to try it -- are there any schematics I can look at, or a section in Horowitz and Hill?





--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Joerg]

On 2019-06-05 10:19, John Larkin wrote:

On Wed, 5 Jun 2019 07:42:37 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all,

Thanks for the replies! A lot to think about.

If you want to see our setup, here is a picture:
https://www.dropbox.com/s/is3vdxlcz24ohml/TS-expt.jpg?dl=0

Here is the simple circuit we are working with now:
https://www.dropbox.com/s/f9aik177qqellcj/TS-ckt.jpg?dl=0
In the simulated circuit, we've added a current source to act as the electron motion upon braking the coil.

R1 and L1, in series with a current source, have no function in that
circuit.

AFAIU Ipd0 represents a current induced in the coil L1, for simulation
or illustration purposes. It isn't a physical current source. R1 is
probably the DC resistance of the coil because it is made from thin
wire. At 446mH in air it has to be thin wire. At least at today's copper
price

A current pulse forms in L1 but it's wimpy, so Matt is trying to amplify
the heck out of it by setting the TIA conversion ratio to 1e9. Which I
think is a bit much. A mere 13-14nA of 60Hz noise from the coil could
rail the opamp.

A coil is not a current source.

Ahm, but that's how pretty much all switch mode converters work. An
inductor is charged up to a peak current and then dumps that same
current into a capacitor, either until it has no more current or in CCM
not all the way down to zero. The energy to create that current being
stored in the core if it has one, else in air, sometimes in an air gap.

... If you short it with a low-impedance
device, a galvo or a TIA, you can pretend that it is.

Active magnetic antennas work that way. The coil is connected to a very
low (ideally zero ohms) input impedance amplifier that has a structure
similar to a TIA. That sort of "B-field" antenna can be beneficial in
situations with lots of man-made electrical noise.

--
Regards, Joerg

http://www.analogconsultants.com/