No, she later hiked out on her own. Never date lawyers.
I guess he could spin a diffamp and a VCO (4046, or a purchased RF VCO) and couple the signal out RF-wise or magnetically or electrostatically or something.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Any electronics would have to be along the spin axis, tall and skinny, probably potted, somewhere on the drive shaft. With a bit of care (and bandwidth limiting) it wouldn't couple into the coil. Some physicist could do the math!
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The electronics should not be a fundamental limit to the measurement; the physics is bad enough, what with the motion, ambient fields, centrifugal growth, thermoelectric and other effects in coupling the nanovolt signal...
The best signal conditioning would be two voltage-input diffamps, one per coil, one spinning.
May as well lowpass some as soon as possible, even before the diffamp, to keep junk from railing the amp and wrecking any integration.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
On Jun 8, 2019, snipped-for-privacy@gmail.com wrote (in article):
I assume that the wheel is dynamically balanced in two planes using a commercial balancing system then.
The trick is to keep the electronics near the axis of rotation, connected to the coil using a radial twisted pair (discussed later). Radial so there is no contrary acceleration signal there. TP to reduce interference.
.I think that it was you that mentioned that the radial breathing of the coil caused a signal due to the variation in the area of the coil through which
but it seemed to me that if one could arrange for two counter-rotating coils in parallel and almost touching, wired such that the electron deceleration signals sum, the interference from area variation would cancel. I phrased this in terms of oscillating coils (in a reply to whit3d?), but the principle is the same.
.This should have worked, and not required a one-meter separation (versus 0.1
loop of wire supplying the led. If one used twisted pairs to carry the led drive and photo currents, magnetic effects would be sharply reduced. If
from the paper chopper wheel on the shaft. (One gets a far larger signal if the chopper wheel is built for use in transmission.)
One can purchase integral 3-channel rotation sensors with considerable
best to buy the code wheel from Broadcom as well. These components are widely used in motion-control systems. This approach will allow the tracking of shaft angle versus time in great detail during a braking event.
Given the sensitivity of the apparatus, all circuits carrying significant current should be done with twisted pairs. Voltage only circuits will not cause magnetic interference, but can pick up magnetic signals, and so should also be twisted. And shielded, to control electrostatic interference.
.Joe Gwinn
On Jun 8, 2019, snipped-for-privacy@gmail.com wrote (in article):
Joe has suggested a JFET at the coil, powered via the same twisted pair as carries the amplified signal back to the instrumentation. The JFET turns the voltage signal into a current signal, which is turned back into an amplified voltage signal well away from the rotor. .
It also sounds like a chopper amplifier would help greatly. The electronic ones chop at 20 KHz or above, so low-pass filtering is needed to prevent aliasing et al. Given a signal in the Hertz, a simple RC filter should suffice. This will greatly slow amplifier offsets from causing drift into the rails, making drift mitigation or cancellation easier to accomplish.
.As for power-line noise, if one locks the sampling window to the local power-line frequency such that an integral number of whole power-line cycles will fit into the sampling window, the power line frequency and all its harmonics will be profoundly suppressed. It is not required that the window be synchronized to the power-line fundamental, but that will work. This filter is widely implemented in digital multimeters - look for the ability to set the sampling aperture in NPLCs (Number of Power Line Cycles). Handheld instruments use a crystal clock to make the aperture a multiple of either 50 Hz or 60 Hz cycles - a tenth of a second works for both, but does not adjust for the actual current power frequency, which varies by a Hertz or two. Benchtop instruments of high precision will synchronize to the actual power line cycle, this being essential to achieve the needed degree of cancellation of power-line interference to support such precision. For DC measurements, I often set NPLC to 100, and live with the 5-second settling time.
And you will need the electrostatic shields, which can be made of copper window-screen fabric soldered to copper skeleton wires with a big soldering iron such as those used when fabricating copper gutters.
Joe Gwinn
On a sunny day (Sat, 08 Jun 2019 08:39:49 -0700) it happened John Larkin wrote in :
Yes, she would have sued you after hare death...
Just after posting that, I realized I HAVE a much better receiver:
That USB stick will go as low as 20 MHz, as high as 1.7 GHz, but 74HC4046 does not go that high, so 20 MHz or so is nice. Then you can also run my spectrum analyzer, it uses that stick:
Now I am curious, just for fun, maybe in the coming week I will set that up simple transistor amp, 4046 with a piece of wire as antenna, that rtl-sdr stick as NBFM receiver, then apply a 200 nV 1 kHz square wave to the input, see what we can hear in audio and see on the analyzer.
Antenna should be in the middle of the coil, else you get the moving away moving closer sort of Doppler. One could use capacitive coupling too, 2 metal plates, these sticks are extremely sensitive.
Cracked it?
Ticket to Maui ;-)
Three items that aren't on the list:
(1) gating the sensor to an exact multiple of 1/60 second (alas, that only compensates 60 Hz currents into non-active loads, though)
(2) there's a possibility that q/m for charge carriers in different metals is different. So, you could do a one-rotor experiment with counter-wound dissimilar metals as the coils, and get much better rejection (same temperature, glued adjacent wires, so very good geometry matching). Bill Sloman has some experience with matched paired windings.
(3) a remote island, with pleasant weather, and all cellphones etc. in a box far from the experimental pavilion. Also, comely attendants bearing chilled beverages...
I'm thinking that motion (even just vibration) of the box will be an issue. Small circuit, low current, attention to the opamp bias source/sink wiring, and a capacitor rather than a battery (batteries commonly have nickel or steel cases) can at least minimize that issue, so it could calibrate out.
The whole circuit (op amp, capacitors, analog switch) would have to weigh a gram, but not much more. Less, if one goes chip-on-board.
Consider this:
Take two identical flat coils very close together (which you have already) and connect them with a torsional coupling. Set up an angular oscillation, one against the other; the mechanical drive is trivial. The electron-slosh potentials add (twice the signal!) and most everything else subtracts.
It's easy to put diffamps (and MEMS accelerometers?) on the coils, easy to get the wires out.
At, say, 3 Hz, you get over 20,000 acceleration peaks per hour, 250K overnight, with no drama, all serene and quiet. Signal average.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
One other thing is vital. Reduce the gain in the TIA, big time. There is always a chance of noise getting in especially since you want to run this during the busy times when your students are there.
When the amp rails it's almost all over, can't use the signal anymore. So the TIA should only have as much gain as needed to get you over the non-man-made nosie floor (Johnson noise, noise in the opamp, et cetera).
Now filter the heck out of the signal, preferably using a digital system such as a laptop run on its battery. The other nice side effect would be that your students will also learn about how to use a nowadays simple tool such as a laptop for signal pickup and filtering. Most universities have LabView. Us "regular folk" who don't have access to academia often use more pedestrian methods :-)
-- Regards, Joerg http://www.analogconsultants.com/
It sounds good, BUT the laptop has bunches of DC/DC converters between the active stuff and the battery. You want a small laptop (tablet, maybe?) with 'airplane mode' and you want it rather far distant from the active region. Distance should be large compared to the separation of rotating and stationary coils. Try to find a recent enough chromebook that the display backlight isn't CCFL type.
The easy 'filter' is just to gate the integration of signal over an exact multiple of
1/60 second, and that doesn't take any computing. it won't help much, though, against the high voltage transformers for a fluorescent-tube backlight.
I bet most modern laptops are LED backlit. Even my more than 10 year old Samsung NC-10 is. However, they might employ cheap PWM for brightness control.
Generally laptops are quiet when not fed by their little power brick. My Gammatech Durabook (semi mil-spec) is quiet while on mains power but that's a different quality of engineering. I have found sources of very faint noise where fancy analyzers from Standford Research and others have failed, using a laptop.
Yes, that, and VFD drive in buildng elevators, A/C systems in the ceilings, and so on.
Main thing is, don't cram all that gain into the very first stage with no filtering whatsoever. It's not going to work in situations like Matt's.
-- Regards, Joerg http://www.analogconsultants.com/
On a sunny day (Sat, 08 Jun 2019 08:39:49 -0700) it happened John Larkin wrote in :
OK, so I grabbed the soldering iron and made a simple 4046 VCO at about 25 MHz. C 100 pF, R 10k trimpot near minimum, pin 9 at 2 V or so,
5V supply, antennas 1 inch pieces of wire, distance about 1 feet (30 cm duh)Receiver is a rtl-sdr stick... I will think about this a bit and then add a TIA or maybe transistor pre-amp.
One could also frequency modulate a xtal oscillator, stability would be much better,
As to SNR... more signal is better.
Anyways, melted some lead... :-)
On Saturday, June 8, 2019 at 10:49:36 AM UTC-4, snipped-for-privacy@gmail.com wro te:
. Here is what my current take-aways are.
ated by the rotating and compensating coils.
as a current amplifier.
ep the feedback shorted to make sure the capacitor does not send the op-amp to the rails, then open the switch when we make a measurement.
rn of 60 Hz noise. Suggestions for that have included unplugging everythin g around the measurement system and running it at night (all good ideas), b uilding an electrostatic shielding box around the experiment, adding a 60Hz notch filter on the input, and finally digitally removing 60Hz, 180 Hz, an d 300Hz in the signal after we have collected the data.
Hi Matt, I'd say first thing is to figure out 'what' the 60 Hz is. Is it electrostatic or magnetic? Then see if you can figure out where it is coming from. I was looking at pickup from a pair of coils in my lab. Mostly electrostatic, till I turned the florescent light off. Then it looked to be magnetic... (has some directional/angle dependence. But no real sign of 60Hz B-field.. but mostly what I'm guess are gunk from switch mode power supplies. The gunk synced with the line, but much higher frequency. The only way I see 'real' 60 Hz B-fields in my lab is with a coil inside a metal box.
Re Notch filter: I'm not sure about this. I've never used a notch filter but you are looking at a transient effect... and the notch may not really help much. (I'd look at the transient/ step response of your notch.) A two pole 100ms time constant filter does a good job at killing 60 Hz stuff in our optical pumping. I don't know if that is too slow for you.
Good luck, try some stuff, and get back to us in a few months. :^)
George H.
On a sunny day (Tue, 11 Jun 2019 10:11:45 GMT) it happened Jan Panteltje wrote in :
Update, (after thinking) To get better center frequency stability and eliminate interference, I thought 'Oh I can PLL it to a 25 MHz xtal'. That is the usual system for a FM transmitter, done it before... used 4046 PLL as demodulator too, in a translation system. And then 'sh..t I have that, and no need as I have this:
So the link to the coil issue is solved :-) Also it is legal here, very low power FM.
So, now for that input amp... thing wants a few hundred mV, maybe those TCL274 as TIA, or as amp after the first transistor.
On a sunny day (Tue, 11 Jun 2019 14:28:41 GMT) it happened Jan Panteltje wrote in :
So.. after a night sleep... And thinking about the FM modulator and audio like bandwidth low noise thing a quick search with google for 'microphone preamplifier integrated circuit 80dB' found this as first hit:
I downloaded the datasheet from
This chip is made for 200 Ohm dynamic microphones, so low impedance inductive source. It had decent noise specs, and 60 dB gain (1000x voltage gain). So a 200 nV signal results then in 200 uV. Add an other normal opamp for an other 1000 x voltage gain, and we have 200 mV, enough to drive the FM modulator module. You could add any form of clipping and filtering for mains frequencies in that chain, or go digital and write to SDcard or EEPROM locally (on the coil, no need for link) but better to stay away from all that power stuff and use the deserted island solution, students will like that much better, combine with a survival training, and you will even be ready for Agent Orange's provoked wars. If you go digital here is a very good noise filter:
Hmm, I wonder about the input bias current and offset. ~1 uA =~ 200uV, x1000 is 200mV... or is that not a problem?
JL's low noise int amp had a bunch of bias current too... so maybe that isn't a problem.
George H.
Hi everyone,
Thanks for all the suggestions. Again, I definitely did not follow them al l. My current plan is to try the TIA or the integrator, or just the differ ence amplifier. I will also take heed of all the suggestions and try and r educe the noise as much as I can by turning things off, running on a laptop , and running at night. I will let you know how it goes!
--Matt.
On a sunny day (Wed, 12 Jun 2019 05:49:54 -0700 (PDT)) it happened George Herold wrote in :
I think for a reasonable temperature range subtracting a few hundred mV and setting zero before the measurement is not a problem.
One could look for better mike pre-amps chips. Also can we use AC ? We will see a pulse AFAIK?
I see that 1512 on ebay, 2 for 20 $, plus shipping from US source:
Right, I think it's a tiny pulse that lasts for a second, myabe a bit less.
GH
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