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A laser pointer at 2B FPS [video]

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615 points thunderbong | 4 comments | 19 Oct 25 06:42 UTC | HN request time: 0.206s | source
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estimator7292 ◴[21 Oct 25 01:15 UTC] No.45651413[source]▶
Tl:dw for how this works:

He scans one line at a time with a mirror into a photomultiplier tube which can detect single photon events. This is captured continually at 2MSample/s (2 billion times per second: 2B FPS) with an oscilloscope and a clever hack.

The laser is actually pulsing at 30KHz, and the oscilloscope capture is synchronized to the laser pulse.

So we consider each 30KHz pulse a single event in a single pixel (even though the mirror is rotating continuously). So he runs the experiment 30,000 times per second, each one recording a single pixel at 2B FPS for a few microseconds. Each pixel-sized video is then tiled into a cohesive image

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1. mrheosuper ◴[21 Oct 25 04:02 UTC] No.45652361[source]▶
Yup, this technique also allows oscilloscope capture signal with frequency higher than their Nyquyst bandwidth.

The downside is it only works with repeative signal.

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2. fecal_henge ◴[21 Oct 25 17:06 UTC] No.45658305[source]▶
Rare enough though that the Fs/2 is higher than the analogue bandwidth on an oscilloscope.
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3. generuso ◴[21 Oct 25 20:20 UTC] No.45661162[source]▶
Mainstream oscilloscopes typically have sampling frequency at least five times greater than the bandwidth of the analog front-end. For example 1 GHz bandwidth oscilloscope will have sample rate of 5 GSps.

https://www.tek.com/en/products/oscilloscopes https://www.keysight.com/us/en/catalog/key-34771/infiniivisi...

"Sampling" oscilloscopes are a much less common product -- they are useful for analyzing signals that are too fast to digitize in the ordinary way. They typically sample at a very slow repetition rate -- some hundreds of kilohertz, but each sampling aperture can be exceptionally short, allowing to record signals to 100 GHz frequency.

4. mrheosuper ◴[22 Oct 25 02:08 UTC] No.45664179[source]▶
The BW of analog front end is where the amplitude drops in 3db. So if all you care is whether the signal is present and what is the frequency of it, you may get away with it.