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65

EDIT updated (improved) description of phase detection circuit There are two principles used in these systems. The first is the time-of-flight principle. As you noted, if you wanted to get down to 3 mm accuracy, you need timing resolution of 20 ps (20, not 10, because you would be timing the round trip of the light). That's challenging - certainly not ...


24

The timing circuit doesn't have to run that fast. It just needs a time-to-digital converter which has a high enough resolution (0.1ns is nearly trivial with off the shelf CMOS technology) and then it can average many pulses (hundreds or thousands) to get the resolution improved by another order of magnitude. These are all fairly standard engineering ...


20

You don't have to run a clock that fast, and you don't need any new physical principles either, just some clever electronic design, mixing analog and digital components and making a few critical parts (switches, in essence) very fast. One simple technique, as described here on wikipedia, is a two-slope ramp. At the start of the time to be measured, you ...


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Instead of attempting to time the round-trip of individual pulses (which depends on a good way to separate reflected pulses from ambient noise), you can also build a phase-locked loop. Control the sending of outgoing pulses by a voltage-controlled oscillator, sending one pulse at each rising zero crossing. Whenever you see an incoming pulse just before the ...


6

Laser is coherent light, so with a technique called interferometry you can actually measure distance with a resolution of less than a micro-meter, regardless of your timing resolution. It should be noted that the measurement produced by interferometry has half-wavelength periodicity (e.g. 200-350nm for visible light). This means that in order to absolutely ...


6

It depends what you mean by "without an instrument". You need something to measure pressure with - your own senses won't be able to measure everyday pressure changes (though they could measure the change if for example you were thrown out of an airlock into space without a space suit). You can easily build a diy barometer. Just take a glass bottle turn it ...


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Surface coating of an integrating sphere is optimized for low losses. This white coating (barium sulfate or PTFE/Teflon) acts like an ideal lambertian scatterer. all light is scattered (Ok, not 100%, but a very high percentage like 99,5%. See ressources) it is emitted in the hemisphere following the cosine law: perpendicular to the surface it's highest. ...


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EM radiation, including light, is a spectrum of different wavelengths. Spectroscopy is the detailed analysis of a light signal by wavelength. Ordinary color images break up light into 3 channels (red, green, and blue), but spectroscopy is generally concerned with breaking up light into a higher number of bands (e.g. 10, 100, or more), and a spectrometer is ...


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A spectrometer does something similar to what a prism does: light goes in, and gets split up into a spectrum. If you shine white light through a prism, a rainbow comes out the other side. But not all things give off white light. In fact, each element, when excited gives off a unique set of wavelengths that act like its signature: these are called emission ...


4

An HBTI works in a very similar manner to a Fizeau / Michelson stellar interferometer. But in an HBTI, a correlation is made between fluctuations of amplitude (intensity) at points across a surface, unlike a Fizeau/Michelson which correlates fluctuations in phase. The timing of these fluctuations is much longer and this leads to a much larger tolerance in ...


4

Manganin, like constantin, are alloys invented in the late 1800s to solve a specific problem: resistance varies with temperature, and every resistor passing a current is subject to Joule heating. So if you are building precise electrical metering equipment their is a design advantage to using materials that show a stable resistance with temperature ...


3

The numbers on the dial generally represent either The largest value that can be read on that setting (from which you deduce the multiplier). or The multiplier. I believe almost all the digital examples I have seen lately are of the first type.


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The water in the air makes the difference. It coats the walls and then takes ages to pump off. For really good vacuum systems need to be baked to 100 degrees plus to drive off the water and pump it away. So if letting up a vacuum chamger to atmosphere it is best to use argon or dry nitrogen - but be careful not to use a cylinder and overpressurize the ...


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The electrons need to get from the top to the bottom without any interference from any gas molecules that might be in the channels. If nothing else, collisions with gas molecules will degrade performance. At atmospheric pressure, I don't think the device would work at all. You can blow a hole through an MCP with over-voltage, but I'm not sure how this ...


3

A car's speedometer typically measures the frequency of rotation for one of the wheels; in a two-wheel drive vehicle it's usually one of the non-drive wheels. The calibration to highway speed is based on the recommended tire size for that wheel. In many vehicles the speedometer counts only integer rotations of the wheel, and is therefore unreliable for ...


2

Application: A strain gauge is a device used to measured the strain (change in length as a proportion of the original length) in an object as a result of an applied load. Most strain gauges are designed to measure strains in only one direction. How it works: A common type of strain gauge consists of thin metallic foil cut into a pattern such that most of ...


2

In a sphere, any light emitted from the center will reflect off the sides at normal incidence come back to the center. In a cube, some rays never return to the center, so you aren't measuring all of the light emitted, which defeats the purpose of the device.


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To really understand the answer to your question we should make sure we understand how the ammeter works. Once we understand that we will see that neither of the resistors in the meter is "negligible", but rather that the total resistance of the meter must be negligible compared to the resistance of whatever you're measuring. How an ammeter works Suppose ...


2

I think it might be one of those things where people do something because everybody does. I agree with you, a figure of merit that includes noise would make more sense. But, as the circuit designer that I am, I could also say that that wouldn't be the end of it. For example, in the classic trans-impedance amplifier used for these kind of detectors the ...


2

The current doesnt pass through the voltmeter so the the resistance of load R is not affected.


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The height of the liquid in the manometer is independent of the diameter for the sizes that you will be using. The height IS dependent on the pressure drop across the manometer and the density of the fluid in the manometer. Since the differential pressure across the manometer "legs" is equal to the density of the fluid multiplied by the acceleration due to ...


1

Theoretically, these requirements arise from the way you connect the measurement devices to the rest of the circuit. A voltmeter is connected in parallel, as you said. Say that you are trying to measure the voltage drop across a resistor $R$ through which passes a current $i$. If the internal resistance of the voltmeter is comparable to $R$, then the ...


1

You can simply put the item in a centrifuge to simulate gravity.


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Only the last part of this question is amenable to a straightforward answer. All wavelengths are potentially observable using different techniques, from gamma rays through to very long wavelength radio waves, over at least 13 orders of magnitude in $\lambda$. Some of these wavelengths - Gamma rays, X-rays, UV, far infrared - require space-based ...


1

The answer would have to depend on a specific scale. i dont think there is an answer to this question without determining a scale. A scale will fix the dimensions and the relative wavelengths that are relevant to the spatial resolution of objects. Lets say one wants to observe a cubical object which radiates (very high at ultra-violet). At visible ...


1

There are indeed two honeycomb structures in the picture, but as suggested by the inset in the left bottom corner, the smaller one corresponds to the atoms. The graphene in the scanning tunneling microscopy (STM) picture you show is adsorbed on an Iridium surface with Miller indices (111). Since the lattice of the Iridium atoms at the surface and the ...


1

You can send sound waves of any wavelength into your instrument, but the trouble is that only specific wavelengths correspond to standing waves. The other waves will just die out because of destructive interference. When your sound wave collides with the closed end of a pipe for instance, it gets reflected back. If the wavelength is not right, the ...



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