Question about sampling rate and bandwidth (regarding extremely quick events) - Page 1
Forum section: electronics / beginners

author: PhilippeASP
author_rank: Contributor
timestamp: July 25, 2017, 03:09:54 pm
content: I'm running an experiment that involves confirming the pulse width coming from a laser pulse. So if the laser claims it can pulse for 1 nanosecond, I want to confirm it can in fact pulse for that long. I'm using an Si photodetector that's plugged into an analog oscilloscope. It's a 2213 tektronix with a 60 MHz bandwidth (100 to 200$ on ebay) and using it I was able to measure a pulse of about 10 nanoseconds.So now I'm wondering, what am I looking for if I want to characterize pulses that are even faster than that? I know that as a rule of thumb, I want 10 times more sampling than bandwidth. But that means that just to match that 10 nanosecond pulse, I'll need my new oscilloscope to have a 1GSa/sec. If I want to improve on it, I'll need multiple Gigasamples. What's bothering me is that, for most of my signal nothing is going to be happening. I can pulse the laser at 100 Hz up to 100 KHz, but since my pulses will be so quick, we're talking about a duty cycle of 1 over a million, maybe smaller. Is there a way, with modern oscilloscopes, to tell it to focus it's accuracy on the actual pulse and ignore the rest of dead-signal? It seems wasteful to have a billion samplings, if for 990 million of those samples the oscilloscope just reads zero volts.Does the bandwidth and sampling rate need to match the quickest event you're observing, regardless of how infrequently that even takes place? So if I have even just a single nanosecond pulse, I still need that gigahertz bandwidth to properly see it?References:My current photodetector, limited by it's 1 ns rise time:https://www.thorlabs.us/thorproduct.cfm?partnumber=DET10AThis is what I'm considering upgrading to (if I can find an oscilloscope that can match it's response). It has claimed rise time of 25 ps:https://www.newport.com/f/time-domain-photodetectors

author: rstofer
author_rank: Super Contributor
timestamp: July 25, 2017, 05:23:43 pm
content: If that 1 ns wide pulse were repetitive, you could be looking at a 500 MHz square wave. To see that accurately would require the ability to display at least the 9th harmonic so a 4.5 GHz bandwidth would be required and the sampling rate would be astronomical. I don't think this is what you want.OTOH, if you are talking about 1 ns of rise time on a 10 ns pulse, the bandwidth required is 0.35/risetime so somewhere a front end around 350 MHz could do the job. The single channel sampling rate should probably be 2.5 times 350 MHz or 875 MHz - a 1 GHz/channel sampling rate would work. It's pretty cheap to get the sampling rate, it costs a bunch more to get the front end bandwidth.http://www.edn.com/electronics-blogs/bogatin-s-rules-of-thumb/4424573/Rule-of-Thumb--1--The-bandwidth-of-a-signal-from-its-rise-time

author: tggzzz
author_rank: Super Contributor
timestamp: July 25, 2017, 08:53:40 pm
content: The bandwidth and sampling aperture need to "match" the quickest event you are capturing. If you have a repetitive signal, the sampling rate can be arbitrarily low.I have a sampling scope that can record a 140ps risetime on a CRT sampling at ~37kS/s, or on a chart recorder by sampling much slower.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 25, 2017, 10:20:22 pm
content: Back in the days when fast ADCs and memory were very expensive, equivalent time (ET) sampling would get you the added bandwidth without increasing the sample rate.For any repetitive event, ET is fine.Modern scopes almost universally poo-poo ET, why I don't know, as they could easily get 1GHz bandwidth with commodity ADCs (imagine a cheap Rigol doing that) -- but what they do do, is sample at ~10x the BW, so a 1GHz scope will have around 10GSa/s.The added bonus you get is, all that data is buffered, so you get pre-trigger data as well, and you get the whole waveform in one shot, no need for multiple triggers.On a CRT, very short sweeps, at very low duty cycles, are almost impossible to see. The solution they used for this, back in the day, was either a photographic plate (literally to take a photo of the screen, with the shutter open during the "acquisition" period), or using a phosphor coating that had the same effect (i.e., a storage CRT).Here's an emulation of that, showing a few traces of a xenon photoflash discharge (induced current, I think) on a Tek 475. Rep rate was a few seconds, and the trace is only 50 microseconds long, so the duty cycle is similar to your case. Exposure was 15 seconds on a digital camera:Perhaps the most advanced CRTs of all time, were storage tubes with an insulating microchannel plate (amplifies the electron beam current, much like an image intensifier tube amplifies a photocurrent, enabling night vision). The plate stored charge (it wasn't able to leak away), allowing a raster to read out the exposed waveform at a slower rate, and digitize it. These were also the fastest ADCs of their time, capable of single shot analog bandwidths of several GHz.Tim

author: danadak
author_rank: Super Contributor
timestamp: July 26, 2017, 12:52:55 am
content: The tek microchannel plate solution.http://w140.com/tekwiki/wiki/Micro-channel_plate_CRThttps://archive.org/details/tektronix_062-0852-01_Cathode-Ray_Tubes_Jul69I think I read on a forum that some US government agency wanted torestrict Tek from using technology in its products.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2017, 01:32:30 am
content: Quote from: T3sl4co1l on July 25, 2017, 10:20:22 pmI agree. Given the repetition rate and triggering requirements, equivalent time sampling will work fine here.QuoteIt is just a matter of economics. Equivalent time sampling requires a separate trigger path and dedicated circuitry which comes at a fixed cost. Increasing integration makes higher sample rates and faster memory less expensive over time so eventually the real time sample rate becomes sufficient for equivalent time sampling to be dropped to save cost.Quote from: PhilippeASP on July 25, 2017, 03:09:54 pmThis will not be a problem at all. A 100 KHz repetition rate is more than fast enough for any oscilloscope to work with nanosecond wide pulses.Quote from: rstofer on July 25, 2017, 05:23:43 pmThis is basically the analysis I would do. Look at the instrument rise time instead of the bandwidth. But note that the timing resolution of a DSO is not limited to the sample rate because of interpolation and while a 100 MHz oscilloscope is limited to a 3.5 nanosecond rise time, it can still measure delay between two pulse with a resolution much smaller than that. Once you have sufficient bandwidth to support the needed rise time, the sample rate will have taken care of itself.

author: PhilippeASP
author_rank: Contributor
timestamp: July 26, 2017, 01:43:09 pm
content: Sorry about the delay, I’ve been getting the 502 Bad Gateway error every time I try and post on the forums.@almI don't mind buying used, I've been browsing for stuff on ebay.ca and they often have interesting discounts.I wouldn't mind doing a single event, but the laser's pulsing frequency doesn't go bellow 50 Hz, and I prefer not to go bellow 100 Hz because it makes it act funny. So I think an average pulse width would be fine.But yeah, as nice a deal as the old scopes were, in the end I would need something I can plug into a PC. So assuming I don't go down the sampling oscilloscope path, I will need those super high sampling rates in order to see my pulse? Maybe not the full 10x bandwidth, because as you pointed out I don't care too much about my waveform, I just need to know when the rising edge appears, and then how long until the falling edge appears. So even just... three samples over the duration of the pulse should theoretically suffice.@rstoferI managed to see the 10 ns pulse just fine with a 60 MHz oscilloscope, which is why I’m hesitant to upgrade to a much more powerful one if that “upgrade” will only allow me to see what I’m already seeing. Maybe the old scope I’m using was just lucky, and that’s why I was able to push it beyond it’s rated bandwidth.@tggzzzDo they still make sampling oscilloscopes? And if not, what was the technical term used to refer to them? When I'm browsing on ebay they seem to be labeled either as old analogs, or modern Digital Storage Oscilloscopes. Because, yeah, that sounds perfect. I can set the laser to pulse at 5 KHz and get tons of repetition which could feed plenty of data for the average sampling.@the Microchannel solutionThat sounds really interesting, but also... rather complicated. If it's available on used oscilloscopes that I can buy, cool. But I don't have the skill to dismantle an old scope and start performing rocket-surgery on it.EDIT:It doesn't say "Sampling" anywhere in the description, but this thing states it can reach 20 GHz, so I imagine it must be using one of the aforementioned tricks. And for this price it seems like quite a bargain. Is there some glaring issue with it that the keen eyed among you can spot?https://www.ebay.com/p/HP-Agilent-54120A-Digital-Digitizing-Oscilloscope-Mainframe/1103202317

author: tggzzz
author_rank: Super Contributor
timestamp: July 26, 2017, 02:18:59 pm
content: Quote from: PhilippeASP on July 26, 2017, 01:43:09 pmThey were called sampling scopes. They were very usable within the obvious limitations. You could also marry them with analogue storage tubes, if necessary.The old traditional technology was used a diode bridge to the sampling, and stored the sampled output on a capacitor until needed. The advantage of a diode sampling bridge is that when not taking a sample the input is "ignored". That means that they are not susceptible to being overdriven outside the sampling time. In contrast, bog-standard input amplifiers that are overdriven can exhibit "strange" behaviour after the overload is removed.There is an alternative sampling technology used when digital processing was easy but ADCs weren't fast enough. The input was captured as a charge/voltage in a CCD analogue shift register, and read out slowly. Personally I wouldn't buy one of those.Overall it sounds as if your requirements are easily met by a modern digitising scope, so I'd just buy one of those. Make sure the front-end bandwidth (and risetime) is adequate to capture the input and to locate the time with sufficient resolution.

author: macboy
author_rank: Super Contributor
timestamp: July 26, 2017, 02:23:05 pm
content: Quote from: PhilippeASP on July 26, 2017, 01:43:09 pmA 60 MHz scope has about 5.8 ns rise (and fall) time in response to a step input. It follows then that the shortest pulse you will be able to see will be much like than 12 ns. Shorter than that and the floor pulse to is completely lost between the rising and falling edges, which is what you are seeing. So your result of a 10 ns pulse is simply a limitation of the scope. I think you will want (at least) a 2 GHz scope which will have about 180 to 220 ns rise time spec. This will have fast enough edges to allow you to start to see what is happening in between them, instead of having the entire pulse turned into a smooth hump by the scope. Faster is even better in this case.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2017, 04:45:58 pm
content: Quote from: tggzzz on July 26, 2017, 02:18:59 pmThose were not sampling oscilloscopes. Their claim to fame was high real time sampling rates at a time when ADCs and memory of equivalent performance were either unavailable or uneconomical.A CCD based digital storage oscilloscope would work in this application but no better than any other digital storage oscilloscope that has the necessary bandwidth.The distinguishing feature of a sampling oscilloscope is that the sampling gate time is not sufficient for the storage capacitor to charge to the input signal. Typically it is not even close. So the output is generated by measuring the change in charge in the storage capacitor. (1) To take advantage of the extremely high bandwidths this allows, almost all circuits between the sampler and the input are left out; even a delay line would lower the input bandwidth although this was sometimes included for ease of use.QuoteI agree. I suspect the real problem is budget.(1) A separate storage capacitor is not even used. The input capacitance of the buffering transistor or even the lumped capacitance of a short length of transmission line provides the capacitance.

author: PhilippeASP
author_rank: Contributor
timestamp: July 26, 2017, 05:23:30 pm
content: Quote from: David Hess on July 26, 2017, 04:45:58 pmWould that help? I assumed that doing that would only draw out the response time for the signal travelling to the oscilloscope.As for budget, I'm ready to spend about 500$ on a scope for the lab if it can help in troubleshooting, and to demonstrate a proof-of-concept. While the final instrument that we'll put in our product can be about 4000$. I just want to make absolutely sure that 4000$ instrument will work for my needs before I dish out the cash.

author: tggzzz
author_rank: Super Contributor
timestamp: July 26, 2017, 06:38:25 pm
content: Quote from: PhilippeASP on July 26, 2017, 05:23:30 pmConsider hiring a scope for proof of concept.When choosing a $4k scope, see if you can get the manufacturer to loan one to you for a few days.