How is pico second measured, when definition of second is ~1 bn Ce-133 cycles? - Page 1
Forum section: electronics / beginners

author: pndv
author_rank: Newbie
timestamp: September 17, 2022, 04:04:47 am
content: A second, by definition is 9,192,631,770 cycles of cesium-133 atom. One cycle can give us a resolution of at beat 1ns. Then how do some oscilloscopes offer picosecond resolution? Even in ophthalmic surgery femto-second laser is so common.

author: ataradov
author_rank: Super Contributor
timestamp: September 17, 2022, 04:23:08 am
content: Partial cycles exist. They picked Cesium-133 because it is stable over time, not because it it the fastest thing out there. This is just a definition, it does not have a lot of impact on the short-term measurements.Instruments existed before Cesium standard was adopted.

author: MrAl
author_rank: Super Contributor
timestamp: September 17, 2022, 11:46:40 am
content: Quote from: pndv on September 17, 2022, 04:04:47 amHi,Actually one cycle is nearly 0.1 ns so you only have to get around 100 times shorter than that.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 17, 2022, 07:33:56 pm
content: Quote from: pndv on September 17, 2022, 04:04:47 amThey use oscillators (mostly phase locked loops, locked to very precise oscillators), and fractional delays.Let's say you have constructed a device with high-frequency oscillators, and delays you know (can measure relative to the oscillator, by examining the phase difference between the original oscillator signal and the delayed signal) are some specific fractions of the oscillator period.You use a fast counter to calculate the number of cycles in a specific measurement interval, often one second (but possibly much longer), because there are extremely precise 1 pulse per second (PPS) sources. Let's say you do some measurements, and find that your oscillator generates 100,009,217,415 pulses per second. This tells us that the oscillator frequency is 100,009,217,415 Hz = 100.009217415 GHz.Because there are 1,000,000,000,000 picoseconds in each second, each oscillator period is pretty much exactly 1,000,000,000,000 / 100,009,217,415 = 9.999078343453 picoseconds.Let's say you have a fractional delay of 1/16 of the oscillator period (again, easily measured by comparing phase change after delay to original delay, if it is shorter than the oscillator period). Based on the previous paragraph, we know it is 9.999/16 ≃ 0.625 picoseconds.So, it is not about making measurements on stuff that takes only a single picosecond, or even femtoseconds; it is all about having a regular repeated phenomenon, like oscillator cycling, that we can count. We do things repeatedly, in a manner that each of them takes as precisely the same amount of time as possible, and when we know both the count and the duration (measured along a slower but very precise clock), we can tell how long each cycle took.It is possible to do the same with e.g. analog to digital converters. Let's say each conversion takes 31 cycles. One thing you can do then, is use 32 converters in parallel, where each gets triggered one cycle later than the previous one. They each probably have their own storage capacitor, but the trigger is simply a 32-bit shift register, and the data bus has a latch that reads the conversion result during the cycle preceding the triggering of the next sample. These tend to be specialized devices, but what matters, is the idea: careful timingin sequence, and counting those sequences, and making the hardware such that each step in the sequence has the same properties (duration etc.) as the other steps.Similarly, when you examine a laser (the wavelength of the emitted laser pulse) or laser pulse duration, you do not need to measure it directly with a stopwatch-type timing device or femtometer ruler. You can use diffraction and optical refractions to measure the wavelength, and phase difference of the two signals (pulse start and pulse stop, measured at the same point but not necessarily by a single sensor) with respect to a much slower clock, to determine the pulse duration. There are a lot of tricks in Physics you can use to measure things indirectly: for example, we basically measure weight/mass by balancing it with a spring, and measuring the deflection. When the precision of each measurement is known, we can define things at surprising accuracy. Just consider how Henry Cavendish determined thegravitational constant Gin1797-1798using a torsion balance, in theCavendish experiment.

author: radiolistener
author_rank: Super Contributor
timestamp: September 18, 2022, 10:08:27 am
content: Quote from: pndv on September 17, 2022, 04:04:47 amOscilloscope just use interpolation in order to get picosecond or femtosecond resolution.Devices which are needs to measure real picosecond or femtosecond intervals usually using some kind of tapped delay line which contains a number of cells with known delay time. Each cell has some fixed delay time and when you know it, you can calculate actual delay between two events. And when cell delay time is about picosecond, you can get picosecond resolution for time delay measurement between two events.Such approach to measure picosecond intervals with tapped delay line is known as Time to Digital Converter (TDC) and often used on a high speed FPGA in order to measure extreme short picosecond intervals. It requires calibration in order to know actual delay for each cell in a tapped delay line implemented on FPGA and allows to measure very short delay between two pulses with picosecond resolution.See details here:https://en.wikipedia.org/wiki/Time-to-digital_converter

author: jpanhalt
author_rank: Super Contributor
timestamp: September 18, 2022, 10:28:20 am
content: You seem to be confusing a unit of measure with a quantum of time. For example, a unit of length is the meter, yet you can easily measure to 1 millionth of a meter.A quantum of time has not been demonstrated, but if it is quantized, it is very small. See: "chronon" in Wikipedia or elsewhere.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2022, 10:35:07 am
content: Planck time = 5.39 × 10^-44 second (the time a photon travels at the speed of light a distance equal to the Planck length).