CRT oscilloscope horizontal sweep - Page 1
Forum section: electronics / testgear

author: taydin
author_rank: Frequent Contributor
timestamp: September 19, 2019, 07:20:47 pm
content: I started to read the Tek 2465B service manual to understand horizontal sweep, and so far I wasn't able to understand how horizontal sweep works on this scope.When a 400 MHz signal is present at the input and I have adjusted the timebase so that one period is displayed, the horizontal sweep speed must be 2.5 ns. If I enable X10 magnification, it will be 250 ps. I really can't believe that the cathode ray is swept at that speed. Even if it were possible, would the electron beam be able to illuminate the phosphor on the CRT with a contact time of picoseconds?So I'm thinking this scope divides the input signal down to a manageable speed, but that doesn't make sense, either. I would have to be storing the samples and then drawing the waveform like a DSO.Can somebody explain how horizontal sweep works with this scope?

author: taydin
author_rank: Frequent Contributor
timestamp: September 19, 2019, 07:39:24 pm
content: In fact, if the electron beam were to sweep at a speed of 250 ps across the CRT, that would correspond to a speed that is DOUBLE the speed of light. The electron beam would bend just like water would bend if you wiggle the garden hose from left to rigntI believe the sweep is happening at a much lower rate so that the electron beam can sufficiently interact with the phosphor to cause it to illuminate. But then, how does the scope know what to draw? I HAS TO sample the signal in real time and store it somewhere ... But this obviously isn't a DSO

author: taydin
author_rank: Frequent Contributor
timestamp: September 19, 2019, 08:13:56 pm
content: OK one correction. It isn't possible to have the 2465B display a single period with 400 MHz. The smallest timebase setting is 5 ns and there are 10 divisions, which means the smallest sweep is 50 ns. And when X10 is enabled, it is 5 ns. But still, 5 ns sweep speed? I'm having a hard time imagining the phosphor illuminate with an electron beam passing over it at a speed of about 1/10 th the speed of light.

author: PaulAm
author_rank: Frequent Contributor
timestamp: September 19, 2019, 09:40:37 pm
content: Couple things.That is not a one shot trace, it's a repetitive trace. At high frequencies the trace gets dimmer, but with enough overwriting it is visible. The jitter on the trigger is specd at < 50ps with 5 divisions of 400 MHz at 500ps/div so it will pretty much follow the same track with a repetitive signal.Also, the PDA potential is 14KV, so the electrons will be moving fast when they hit the phosphor.If you really needed to see fast signals, you got the 2467 with the MCP CRT. Now that is a piece of engineering.

author: 0culus
author_rank: Super Contributor
timestamp: September 20, 2019, 12:37:33 am
content: Page 30 onwardhas some interesting discussion about the physics of the mcp. I have a 7104 and it, too, is a magnificent piece of engineering.

author: nctnico
author_rank: Super Contributor
timestamp: September 20, 2019, 12:40:05 am
content: Quote from: taydin on September 19, 2019, 07:39:24 pmBut it still works. There are scopes which sweep the beam across the stream faster than light; your water hose analogy is right though. No rules of physics get broken.

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 09:27:02 am
content: Yes, 5ns has gotta be sufficient to cause phosphor luminescense, even if it doesn't seem intuitive to me ...So unless you have a coating that responds to even less time than that, it wouldn't have been possible to have multiple GHz CRO's