CRT horizontal scan at speed of light (or more!?) - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: May 30, 2019, 07:04:58 am
content: The speed of light is about 33 picoseconds per centimetre. Assuming it were possible to have a phosphor with a fast enough response time, would it be possible for a CRT to scan horizontally at SOL? Obviously the electrons would not be travelling that fast, but could the position they impact on the phosphor be made to change that quickly? Or would the electrons in transit have too much of an arc in their path?

author: Ampera
author_rank: Super Contributor
timestamp: May 30, 2019, 07:20:56 am
content: Presuming you could speed up electrons to the speed of light (or near the speed of light), you could bend the beam of electrons at the speed of light, so long as you also speed up the electrons to move at the speed of light. If you can actually do that, there's likely a cutoff bend to how fast you can speed up an electron. I'd say yes.

author: ruffy91
author_rank: Regular Contributor
timestamp: May 30, 2019, 07:28:00 am
content: It's no problem to reach more than the speed of light for the apparent "speed" of the beam. Nothing is moving at SOL even in this case.

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2019, 10:09:05 am
content: You don't need a CRT: you can do it with a bright torch.Shine a torch at a target 1m away, and move the spot sideways at 1m/s. Now move the target to 2m, and the spot will move at 2m/s. Move the target to 1e9m away, and the spot will be moving at 1e9m/s.More importantly, noinformationcan be carried in the spot, of course.Also, have a look a Cherenkov radiation: it is caused by sub-atomic particles moving faster than the speed of light.

author: dzseki
author_rank: Frequent Contributor
timestamp: May 30, 2019, 10:12:07 am
content: Quote from: Circlotron on May 30, 2019, 07:04:58 amThe Tektronix 7104 1GHz realtime oscilloscope was famous for this, and it operates that way in practice!

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 02:34:41 pm
content: Quote from: dzseki on May 30, 2019, 10:12:07 amThe speed of light is 33 picoseconds per second. The 7104s fastest sweep is 200 picoseconds per division. Combined with the vertical response of 350 picoseconds over an over-scanned display, this is an apparent speed faster than the speed of light but barely.Quote from: Circlotron on May 30, 2019, 07:04:58 amWhat is even more remarkable is that the Tektronix 7104 can do this with a single shot waveform that is visible to the eye.

author: apis
author_rank: Super Contributor
timestamp: May 30, 2019, 02:42:32 pm
content: Quote from: tggzzz on May 30, 2019, 10:09:05 amFaster than the speed of light in a medium such as water, but not faster than the speed of light in a vacuum, i.e. not faster than the physical constantc.

author: wraper
author_rank: Unknown
timestamp: May 30, 2019, 02:51:22 pm
content: You can say shine with a laser towards the Moon. Then start rotating it sideways. Speed of the moving light dot on the Moon will exceed speed of light easily.https://youtu.be/lR4tJr7sMPM

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 03:36:53 pm
content: Quote from: wraper on May 30, 2019, 02:51:22 pmWe know it happens and how it works. But on a Tektronix 7104, you can personally see it.

author: wraper
author_rank: Unknown
timestamp: May 30, 2019, 03:43:03 pm
content: Quote from: David Hess on May 30, 2019, 03:36:53 pmYou just need to buy it first.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 04:01:10 pm
content: Quote from: wraper on May 30, 2019, 03:43:03 pmPft, details!

author: duak
author_rank: Super Contributor
timestamp: May 30, 2019, 07:24:43 pm
content: Now that's just the H sweep. What if you observe a fast pulse with a vertical channel? and aren't the vertical channels generally faster than the horizontal?An interesting bit of engineering. I used a 7104 once within a year or two of its introduction to trap a transient pulse. We had a storage scope too, but it didn't show the shape of the pulse anywhere as well as the 7104 did.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2019, 04:28:33 am
content: Almost all of the speed comes from the vertical movement of the beam. 200 picoseconds per division of horizontal movement is not that fast. The vertical is much faster.Although something to keep in mind is that the 7104 is unusual in having a very high horizontal bandwidth. In most oscilloscopes, the horizontal bandwidth is only 1 or 2 MHz. The 7104 uses a horizontal amplifier and CRT structure which is faster than most vertical amplifiers and CRT structures in other oscilloscopes at 350 MHz. In X-Y mode, the horizontal speed could be at least doubled.Note that horizontal bandwidth relates to how quickly the beam can start and stop and ultimately has nothing to do with slew rate which is why a 500 picosecond per division oscilloscope like a 7904 can get away with only having 1 MHz of horizontal bandwidth.

author: golden_labels
author_rank: Super Contributor
timestamp: May 31, 2019, 04:54:23 am
content: Circlotron:It’s not thatnothingcan move faster thanc. Only information can’t. Since no information travels between points hit by the beam(1), thec“speedlimit” doesn’t apply. Another, much more interesting example isphase velocity.____(1) To be precise: the beam will produce — depending on the medium being hit — local potential change, heat spike or burst of radiation, which will propagate. But none of those will exceedcitself. Imagine an airplane moving above the speed of sound.

author: dzseki
author_rank: Frequent Contributor
timestamp: May 31, 2019, 06:20:03 am
content: Quote from: wraper on May 30, 2019, 03:43:03 pmNow tell me it is cheaper to look at the moon's surface from a spaceship waiting on the (strong and focused enough) laser beam to appear.