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Metrology / Re: Update of my Keithley 2001
« Last post by eurofox on Today at 02:58:04 am »
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I contacted a member on this board always promoting he's 3458A ultra precise  :blah: :blah: :blah: and he simply said that is equipment is not available for other users ....  :horse:

There is no such thing as a free lunch ... - did you offer to compensate these people for their willingness to provide you access to their expensive lab or is your assumption that this would be free and you can later make money out of it selling refurbished & calibrated T&M gear ?

Yep of course, I offer to pay for the service and time.
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Test Equipment / Re: Advice on buying Multimeter in India
« Last post by arif_senpai on Today at 02:55:29 am »
What about looking into the Rishabh Multi 18s , or the Multi 20(might be slightly over your budget).

Haven't actually heard of them before. Couldn't find any reviews on the internet too.  :(

Design seems to be inspired from Gossen Metrawatt meters with automatic terminal blocking. But looking at the price, that might be the only thing they copied. I'll search for it a bit more.

Thanks for your comment.
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Yesterday, I got a replacement for the video DAC I had blown up by connecting the red channel to 12V instead of the adjacent pin on the ribbon cable where it was meant to go.
I snipped out the broken DAC which was the easiest way to remove it due to its PLCC44 package.
With the new one in place I got the exact same problem, say what?
Turns out that my replacement protection diode I had put in earlier wasn't a BAV99 as its bag label suggested, but a common cathode dual, so the red channel was now shorted to the 5V protection rail.
Just goes to show that you have to check everything... Luckily, this did not kill the DAC since its output is a current source and the part runs off of the same 5V.
After replacing the diode with a real BAV99 I finally got my red channel.
Now all looks good. Still waiting on getting the LED strips for the backlight.

I'm a bit shocked that the little 75 Ohm source termination resistor survived having 12 V across it, i.e. almost 2W of power... There wasn't even any discoloration.
Interesting side note, the VGA to LCD converter goes through some kind of calibration to measure the incoming video stream and centering the image. It also seems to be doing some black and white level adjusts. You can actually see the image quality improve over the first few seconds after turn-on.
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A police report released Thursday would seem to imply that the driver was looking at her mobile phone and only looked up 0.5s before the crash. Wouldn't be surprised if the driver faces charges of vehicle manslaughter. It's illegal in the UK to drive whilst using a mobile phone and it's probably illegal in a lot of other countries. Probably illegal in the state of Arizona as well, but people still do it.
https://eu.azcentral.com/story/news/local/tempe-breaking/2018/06/21/uber-self-driving-car-crash-tempe-police-elaine-herzberg/724344002/

Well colour me surprised. Who on earth would have ever conceived something like that would happen? Certainly not a software engineer, since they infamously respond to any problem with a car by turning it off and expecting it to have disappeared when they turn it back on.

There are two "safe" and one "dangerous" designs.

Safe: no automation, driver knows they are in control
Safe: full automation, driver can "switch off" and relax
Dangerous: partial automation with shared responsibility and quick responses required. If the driver can't relax, why have the automation in the first place.

Military maxim: what is everybodys' responsibility is nobodyd' responsibility.
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Beginners / Re: Problem with first lab power supply
« Last post by schmitt trigger on Today at 02:50:46 am »
Periodic And Random Noise.

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Sorry, yes, DHL have returned all Russians imports due to customs not allowing them to deliver anything to personal addresses.
They are being returned and will have to be re-shipped by DHL eCommerce.

This has been going on with my FedEx deliveries for a few years, since new sanctions began being imposed in 2014. As a result, Russia joined Brazil as the only countries I ship to using the standard postal service rather than an expedited time definite fully tracked service.

Brazil is on my list because too many customers refused to pay the horrendous customs charges levied, and the total cost to me of return shipping and dealing with claiming back the the local tax exceeds the total value of the goods, and I still give the customer a full refund.

By using the postal service, the customs seems to be rather more lax, the automatic returns don't happen.
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Metrology / Update of my Keithley 2001
« Last post by BorgheseRoman on Today at 02:48:56 am »
...
I contacted a member on this board always promoting he's 3458A ultra precise  :blah: :blah: :blah: and he simply said that is equipment is not available for other users ....  :horse:

There is no such thing as a free lunch ... - did you offer to compensate these people for their willingness to provide you access to their expensive lab or is your assumption that this would be free and you can later make money out of it selling refurbished & calibrated T&M gear ?


 
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Rather than precision analog over RF link, why not use just an isolated transformer ?
Because that only works for some definitions of "isolated". Transformers can be galvanically isolated and still have all sorts of coupling. Check out the capacitive coupling between primary and secondary, for starters.

For even more education, research the term "isolation capacitance" and set aside some time.

I too have been bitten by the "I can design a DC-coupled isolated differential probe" bug and I have some ideas. I don't know if they're novel ideas, or if they'll work, or what bandwidth I'll get, and someday when I have some free time  :-DD I'm going to experiment with them. But I'm tempering my expectations because I've had long and painful experience with trying to achieve isolation and it can be a serious PITA.

True isolation is a deep, dark rabbit hole.
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I assume that for what you would consider clocks or oscillators that gaussian noise isn't that dominant over period phase noise compared to my case??

Indeed, terminology can vary widely. I can see now why you used "phase noise" in the title, but before I thought you were meaning something else. So now, I'm not sure I understand your question. Also you seem to need the waveform of the photodector in order to find both the phase and the amplitude of each spectral component, yet you are saying you use a Spectrum Analyzer to read the photodetector. By Spectrum Analyzer I understand a device that is not aware of the signal's waveform, an SA does not know the phase of each spectral component, an SA knows only the amplitude information. An SA can not give I and Q.


When I started my PhD I inherited a spectrum analyzer along with the laser + optics mounted on a rail and balanced on motorbike innertubes. Really. After months of trying to get anything meaningful I tried a different approach which involved an expensive lock-in amplifier to demodulate the detector signal and, hence, get at the amplitude and phase. Today, I actually do both the spectral analysis and the phase analysis simultaneously on the same detector signal. Each data analysis method has its pros and cons and together can give a lot of insight into the properties of my sample.

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To avoid all these, I found another way of describing the accumulation of errors we didn't agree before. This time in just one small paragraph, without specialized terminology:

Imagine you want to walk on an alley. Straight line, one direction only, one step at a time. Your step is about one foot. To be more precise, 1 foot +/1 inch. Now, start walking 120 steps. How far are you now? In average, you will be 120 feet away from the starting point, but because of those +/- 1 inch at each step, you could be anywhere between 110 and 130 feet. The error is +/- 120 inch (+/- 10 feet). But if you walk 1200 steps, then the error could be +/-1200 inch (+/- 100 feet). That is the accumulation I was talking about.

1 feet, would be the equivalent of the average frequency (for my oscillator)
+/-1 inch, would be the equivalent of a phase noise of my oscillator
120 feet, for my clock (note clock is not the same as oscillator, a clock is made from an oscillator and a counter, so a time keeping device) would be value found in my counter (aka what hour shows your clock?)
110...130 feet would be the real time (the time indicated by an ideal clock)

Does this accumulation affect your measurement? I don't know, you tell me.  ^-^

My signal is quite similar to an audio signal. There is the modulation frequency due to the frequency difference between the laser "arms" - imagine a few kHz steady tone. The random diffusion is hiss and the phase oscillation is weak tremolo. I typically sample at a few kHz over a 1 second window and do this repeatedly for perhaps a minute. I only need say one part in a thousand accuracy which is quite a different realm than in the RF clock/oscillator world.

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In the meantime I spent about one hour reading your PDF thesis (I am only at 3.5.5. now - about half of the document). Very interesting reading. For now, the various techniques you described seem to converge to a sort of Laser Doppler Vibrometry scanning, and you want to study small moving particles during an electrophoresis process (in contrast with the study of a macroscopic vibrating object, like in vibrometry).


Exactly. There are a lot of techniques derived from the same fundamental principle.

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From now on, expect from me more question than answers.  ^-^

So far, the device looks like an interferometer to me. Since the paths for both rays of light are roughly the same length, I guess you won't care much about the phase noise in the laser source.

That's right. Purists say you need to calculate the phase coherence of the beam to figure out things like where to place focusing lenses etc. I don't bother. I just eyeball it and it works very well :)

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About electrophoresis, I know basically nothing, so I will assume your setup is with some calibrated gel and a DC passing current. In my understanding, this setup is to separate particles (or DNA chunks) by their sizes (the smaller particles moves faster through the gel, so smaller chunks of DNA will travel longer in a given time). Please correct me where I'm wrong.

Electrophoresis is a general term that just means migration of charged particles/droplets in a fluid toward an electrode of opposite charge. For me, it is microelectrophoresis - just particles in a liquid, e.g., milk, paint etc.

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The first question is why do you use AC instead of DC in the electrophoresis process? What advantages does the AC brings to you? Does the particles still migrate (with time) in one direction only, like in DC gel electrophoresis?


Gel electrophoresis requires a DC field since the goal is to separate the different species. For my method, there is a small volume that the detector observes, perhaps 1mm3. With a DC field the particles would migrate in one direction and eventually deplete the volume that the detector is looking at. Also, in the presence of salts, electrolysis can occur at the surface of the electrode causing highly undesirable chemical reactions, heating, turbulence etc. Using an AC field helps reduce these effects. The higher the frequency, the better the reduction but at a price. What I've built overcomes what many have been saying for decades is impossible. I can't go into the details in a public forum about that, though (IP and all that).

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I'm not sure yet what the photodetector reads: it reads only the beating effect between the reference light and the Doppler shifted light (you mentioned somewhere that the non-linearity of the photo sensor acts as a mixer in a heterodyne), or it reads the integral of interference fringes that are moving over the surface of the detector, or both?

So there are two optical geometries that can be used but are mathematically equivalent. One resembles holography in a way. Light is scattered by the moving particles and picked up by the detector. A diffuse second 'reference' source of light (from the same laser) is directed to the detector. Hence, both heterodyne to give a signal containing all the Doppler shifts from each particle. However, this doesn't yield the direction of the motion. Hence, one of the sources of light is frequency-shifted relative to the other so that know a stationary particle would be seen to be a particle moving with a Doppler signal equal to the frequency shift between the two light sources. This is basically modulating the signal just as you would do with a lock-in amplifier so that subsequent demodulation would allow you to recover weak signals buried in a lot of noise. So, the detector signal is basically a gaussian (due to diffusion) centered around whatever the modulation frequency is plus whatever motion is occurring due to the electrophoresis. The other optical arrangement is conceptually easier to understand and is the one I use. The two light sources intersect in the sample forming interference fringes. Due to the frequency shift the fringes move. They "sweep" past particles and the light from the particles goes to the detector.

My question/interest

Hopefully, it's evident that the magnitude of frequencies, noise etc are quite different for my experiments than in the world of RF oscillators etc. I'd like to know if IQ data generated using the various ways I read on other threads could be processed with my analysis methods and differentiate between different oscillators. If so, would it help pragmatic selection given a choice of oscillators? Or maybe provide a simple way to characterize long-term changes in a given oscillator's performance? I say simple because implementation of the analysis method is straightforward. Does anyone have such IQ data that I could play with?
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Test Equipment / Re: Advice on buying Multimeter in India
« Last post by arif_senpai on Today at 02:42:56 am »
If you are blowing up your meters after leaving them in amps range then comming back to check a voltage, something is wrong with your practice.
such a guy should unplug the red cable every time you finish using the meter, sure it'll wear the contacks down, but it'll still last longer than what it would if it were to blow up.

I never said I blew up my meter putting it on Amps range. My friend did. He borrowed it from me. As he said, he put the dial on current range and tried to check the voltage of mains. Lucky for him the circuit breaker popped.

But I will say, such a mistake could have happened by me too. To err is human.

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