How to measure short times, low current, low ringing on oscilloscope? UGB? - Page 1
Forum section: electronics / beginners

author: hakunamatatarr
author_rank: Newbie
timestamp: February 16, 2022, 08:22:04 pm
content: Hi everybody,first of all: I am an absolute noob regarding electronics and especially HF techniques. So if I explain something weird please just ask! I hope you can help me!I am trying to measure short voltage pulses of solar cells which are excited by a 1 ns laser pulse. So electrically, there is a 0.8 - 1.3 V background voltage with a roughly 0.02 V pulse on top. The rise takes about 10 - 500 ns and the following decay 1 - 10 us. I am especially interested in the rise.The solar cell is contacted in a measurement box which can be connected to a BNC cable which goes directly to an oscilloscope (LeCroy Waverunner HRO 66Zi 600MHz analog bandwidth).My problem is that I see ringing/oscillations thatmake it hard to see the rise properly. The main frequency is at 23 MHz if that gives you any information. I understood so far that the ringing is caused by reflections due to the impedance mismatch between the 50 Ohm BNC and the 1 MOhm scope. Indeed, if I change the entrance of the scope to 50 Ohm the ringing disappears. However, I cannot measure with 50 Ohm because I need to measure close to open-circuit conditions, so I want to draw the least amount of current from the device as possible; ideally even with a higher impedance than 1 MOhm.I could reduce the ringing already by choosing a shorter BNC and avoid adapters which were installed before but now I am stuck.Is there any way to reduce it even further?I read something about a unity gain buffer / voltage follower, but I do not understand very well if that would help.Otherwise, people mention that for high frequencies one should use an active probe which already has a high impedance?(The impedance of the solar cells changes for every sample and the light intensity, so I cannot say much about that).I would be thankful for any idea!Thanks!Attached is an example of a pulse I would get, once in linear and once in logarithmic time scale.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2022, 09:38:18 pm
content: Not an easy task you have set out for yourself! Making a measurement like this illustrated the idea that the probe and scope become part of the circuit, so you either have to minimize that or figure out how to deal with it in your calculations. In this case, it isn't initially clear to me that your ringing is actually due to reflections and there are other possible causes. Could you tell us:The size of the solar cell and exactly how it is connected to the initial BNC connector or cable?The length and type of BNC cable that you are using to connect it to the scope?What your actual measured results are with the equipment you have? Screenshots of the scope would be best, including some with the pulse and rising edge spread out over the whole screen, so 5-10ns/div.How you manage the offset? Your pulse is ~20msV, so that would imply a scope setting of 5 or 10mV/div--but the 1.2V starting voltage would push that off the screen. So do you add an offset somehow, use AC-coupling, what?Also, if you can, the theory behind why a 1ns laser pulse yields a much longer, spread-out electrical pulse. Is that simply due to the electrical characteristics of the panel, such as capacitance and inductance of the leads, or something else? Also, why 1.2V? That implies 2 cells or something other than a typical silicon cell. The laser hits both?

author: KT88
author_rank: Frequent Contributor
timestamp: February 16, 2022, 10:15:54 pm
content: To test for reflections of the cable you could use different leghts. It would create different ringing frequencies due to different propagation times.The challenge I see is to match the impedance of the solar panel to the cable. A 10X probe would already help although it comes with more noise due to the 10X smaller signal voltage. A slight improvement would be achieved through averaging. As the frequency limit for passive 10X probes is 500MHz ( I'm not aware of faster ones at least). This would not show a very steep edge, unfortunately.An active probe is the best way if you don't intend to build your own probe...

author: hakunamatatarr
author_rank: Newbie
timestamp: February 17, 2022, 12:27:21 am
content: Thank you both for the fast answer!The size of the solar cell and exactly how it is connected to the initial BNC connector or cable?Also, if you can, the theory behind why a 1ns laser pulse yields a much longer, spread-out electrical pulse. Is that simply due to the electrical characteristics of the panel, such as capacitance and inductance of the leads, or something else? Also, why 1.2V? That implies 2 cells or something other than a typical silicon cell. The laser hits both?About the solar cell: It is actually not a Si cell but a perovskite solar cell which we produce in-house. It is a very small pixel with the size of 4 x 4 mm2 and produces directly a voltage of 1.2 V with this light intensity (comparable to the amount of sun light a cell would get).Regarding the time difference in laser and voltage pulse: The perovskite absorbs the light and creates an internal voltage. But the charge carriers need some time to reach the electrodes, depending on their transfer through the different layers. This is exactly what we want to study, the faster the better (more or less).I am not sure about the connection to the BNC. There is definitely some internal wiring because one can choose between 4 cells with a rotary switch (which obviously doesn't help). I will ask tomorrow about more info.The length and type of BNC cable that you are using to connect it to the scope?The length is about 15 cm. We cannot go shorter just because of the measurement setup. I don't know what different types of BNCs there are... Could you explain me more about this? I am pretty sure that it has 50 Ohm, but otherwise no idea. We actually tried with different cables. I attached a figure comparing a BNC and a LEMO cable with roughly the same length. I can ask tomorrow as well.What your actual measured results are with the equipment you have? Screenshots of the scope would be best, including some with the pulse and rising edge spread out over the whole screen, so 5-10ns/div.The figures that I attached are measured with the equipment we have currently setup. Sorry, I don't have any screenshots right now; again, I will update tomorrow.How you manage the offset? Your pulse is ~20ms, so that would imply a scope setting of 5 or 10mV/div--but the 1.2V starting voltage would push that off the screen. So do you add an offset somehow, use AC-coupling, what?Yes, I used scope settings of 5 mV/div typically. I just offset 0 V with the position control to -1.2 V. Is there something wrong with that?In this case, it isn't initially clear to me that your ringing is actually due to reflections and there are other possible causes.I can try this tomorrow by using cables with different lengths as KT88 suggested. But why do you doubt that?Btw. as trigger we use a photodiode that also gets hit by a small part of the laser. It shows a pulse amplitude of about 45 mV.Could you also explain me easily why an active probe would be better? Where is the difference?

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2022, 12:54:16 am
content: Quote from: hakunamatatarr on February 17, 2022, 12:27:21 amI'm not sure how you are using a LEMO cable, but sticking with BNC (assuming the actual cable is a suitable 50-ohm type like RG-58) a 15cm cable gives you about 0.75ns each way. So we'd have to see your ringing to see if that might correlate. Otherwise, the inductance of the wiring at the cells combined with any capacitance in the system might cause ringing as well. And some small amount of ringing won't destroy your measurements--what you have doesn't look that terrible at first glance.QuoteNo, not at all. If your scope and/or probe support that, it's the way to go.QuoteOne of your problems is that it is impossible to match impedances passively anywhere in your system without causing other problems. An active probe will have 'high enough' input impedance and very low input capacitance--and capacitance is where you have issues with high frequencies and fast edges. If your wires are very short, you don't need to worry about matching impedances like you do with anything long enough to be a transmission line. So the active probe will have the high impedance/low capacitance input and amplifier/buffer/offset source right there at the test point, eliminating all of those issues with impedance matching and reflections. Then it will have a 50-ohm buffered output that can drive a transmission line and termination (scope set to 50R) without any distortion or losses. One issue to look for, though, is that most active probes are 10X attenuation, but your signal is so small that you really should have a 1X probe. They do exist, and there are even some inexpensive AC-coupled versions that might work, given the timeframe of the pulse you are looking at.Edit: Just to be clear, I was explaining what an active probe would accomplish, not specifically recommending one for your application. Noise may be an issue, for example. What you are trying to do is fairly challenging based on the signal level and bandwidth required.

author: jmelson
author_rank: Super Contributor
timestamp: February 17, 2022, 02:12:15 am
content: "Solar cells" are terrible for measuring fast pulses. What you want is a small-area PIN diode, and set it up to be reverse-biased (photoconductive mode). You do need to set the scope input to match the cable impedance. If you can't get enough signal that way, then you need to use a transconductance amplifier. Smaller photodiodes have less capacitance, and reverse biasing them reduces capaciance by at least an order of magnitude.Jon

author: KT88
author_rank: Frequent Contributor
timestamp: February 17, 2022, 07:00:30 am
content: To my understanding it's about charactzerizing the solar cell - not the laser.For that purpose measuring the voltage is not wrong. The problem is the impedance matching as bdunham7 is explaining....

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2022, 08:23:39 am
content: Bonjour: Your setup and question indicate the wrong sensor, and poor transmission sustem.A matched Zo system is reqiuired, Zo source = Zo cable = Zo load 50 Ohm (or 75 etc)Use scope on 50 Ohm.Exactly which generic "solar cell" is used?You need a fast rise photodetector from USA firms like EGG, Untied Detector, or Japanese Hamamatsu.These have special low capacitance and controlled area diodes. Some requote DC bias. Most have 50 Ohm BNC terminations. Expect t6op pay $30..500 each.An generic low cost "solar Cell" has inherent high capacitance and current is proportion into a short circuit, not high Zo.Finally study the transmission of fast signals as in Tektronix Probe Circuits Concepts books.Nuclear physics photodetectors are run at much faster speeds than you seek,Bon Chance,Jon

author: KT88
author_rank: Frequent Contributor
timestamp: February 17, 2022, 09:45:25 am
content: How would you characterize a specific solar cell with another PD?

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2022, 10:44:55 am
content: Reccomend UDT OSI PIN diode high speed high sensitivityPIN-10Dhttps://docs.rs-online.com/0aa7/0900766b8137f701.pdfJon

author: KT88
author_rank: Frequent Contributor
timestamp: February 17, 2022, 12:29:35 pm
content: The solar cell is the DUT, not the laser...

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2022, 12:41:44 pm
content: spec on solar cell DUT?Normally the output current is proportional to light intensity,over,surface into a,short,circuitPerhaps OP needs a transconductance amp, current >>>voltage converter?Jon

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2022, 12:12:16 am
content: Why not just put your expensive scope into 50 Ohm mode?I'd personally do this kind of measurement with a capacitively coupled transimpedance amplifier so you don't have to compensate for solar cell capacitance, but I'm not a solar cell scientist (and not following a consensus is rarely a good idea in science, so just keep it fully 50 Ohm and measure the capacitance and compensate for the RC time).

author: jmelson
author_rank: Super Contributor
timestamp: February 18, 2022, 01:37:04 am
content: Quote from: KT88 on February 17, 2022, 12:29:35 pmOK, so why are we testing solar cells with 1 ns laser pulses, then?Seems like the wrong test.Jon

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2022, 01:56:40 am
content: From a quick google I would guess it's a "time of flight" measurement (with a special meaning for solar cell science) :https://aip.scitation.org/doi/am-pdf/10.1063/1.4948344

author: Electro Fan
author_rank: Super Contributor
timestamp: February 18, 2022, 06:08:12 am
content: Any chance something in here might be useful?

author: jonpaul
author_rank: Super Contributor
timestamp: February 18, 2022, 09:33:37 am
content: To the OP:Both photodiodes and photomultipliers for fast light pulse measurement were developed decades ago for nuclear research and are readily avcailble.Photodiodes for nanosecond pulses are specially made with low shunt C and Zo 50 Ohm, and usually a BNC connections.Any low cost commercial "solar cell" will not work.I suggest to first study the equiv circuit and physics of photo diodes to learn more.Kind Regards,Jon

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2022, 10:38:00 am
content: They are characterising the solar cell, not the original light pulse. Solar cells have huge transit time, which is part of what is being measured. The light pulse kicks lose electrons, the shape of the resulting current pulse gives information about the solar cell.The standard way of doing these measurements is with a fully 50 Ohm load, so all they have to do is use coax and put his scope input to 50 Ohm mode (which it does support).

author: hakunamatatarr
author_rank: Newbie
timestamp: February 18, 2022, 11:25:54 am
content: Thanks everybody already for the answers!I guess I need to clarify a few things:As KT88 and Marco mentioned already a few times, the solar cell is the DUT. The measurement technique is called transient photovoltage (TPV). There is a continuous-wave laser shining on the solar cell to provide the background voltage and then the pulsed 1ns laser to provide the small signal pulse that I am interested in. One wants to measure at specific cw light intensities and will get different voltages depending on the quality of the cell and then one adjusts the pulsed laser intensity to get a pulse of about 20 mV. Typically, people look at the decay of this pulse but I want to study the rise. We do not apply any external voltage/electric field, this makes it different from a time-of-flight experiment. The whole measurement is done in the hope to understand the physics better that happen in the solar cell (recombination, diffusion, ...).We cannot use the 50 Ohm impedance because this would extract a lot of current from the solar cell but we want to test it at an open-circuit situation, the physics change in between these cases. That's why I suggested the unity gain buffer (UGB) because it seems that it can have a very high impedance at the entrance (generating the open-circuit situation) and a 50 Ohm exit, allowing to use the scope with 50 Ohm as well. If we could put the UGB close to the sample we could avoid most of the reflections? And the UGB apparently does not change the voltage signal, making it possible to measure the actual voltage values which is important as well (or at least be able to calculate it easily).Quote from: bdunham7 on February 17, 2022, 12:54:16 amI am using a LEMO cable just by inserting two LEMO to BNC adapters. I was quite wrong about the length though... The LEMO cable is 30 cm and the shortest BNC is 50 cm. I tried the same measurement with different BNC cables lengths and attached the scope screenshots below. At least one of the cables (the 100 cm one) was an RG-58, about the others I don't have any information. The frequency and amplitude of the oscillations does change. I also attached an image where I blocked the pulsed laser to just see the background with very small oscillations. Not sure if this give you any info.In general, the ringing doesn't look too bad in the images and we could probably live with this but it would be nice to be able to reduce it further.QuoteBut could you maybe recommend one? :D I understand that this would be probably better than using a BNC or maybe SMA cable (I heard that these are supposed to be good for HF) for us?Regarding the measurement setup, I realized that I didn't explain it very well and that probably a lot of problems come from the whole sample box. I am still not able to explain it properly because I don't know the exact wiring. But I made some pictures, so that you get an better idea. Each substrate of our DUT has 4 pixels on it (each with size 4x4 mm2) and is placed on some pins for electrical contacting (see Measurement setup 3.jpg), all having a common ground. The pins are mounted on a PCB which is wired to a 14 pin plug (Measurement setup 4.jpg). From there, a cable (not sure what kind of cable) goes to another box which is made to choose the pixel under test (just 1 of the 4 is illuminated) with a rotary switch (Measurement setup 2.jpg). In principle, one could apply a voltage to the cell here ("wire" connection) but we just measure and connect to the "sense" the cable (here BNC-to-Lemo and Lemo cable) which goes to the scope (Measurement setup 1.jpg).It is clear that all these electrical elements cannot help. So we thought about making a new box in which the substrates lies still in the pins, but the pins from each pixel are directly wired to a SMA connector which can go directly to the scope. Does this make sense?We cannot get rid of the box because we need to measure under inert atmosphere (N2). And the problem is that this will take some weeks to be built...

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2022, 12:16:28 pm
content: With an open circuit you will suffer severe RC effects, isn't it enough to be open circuit at DC and low impedance at high frequency?

author: hakunamatatarr
author_rank: Newbie
timestamp: February 18, 2022, 12:20:38 pm
content: Quote from: Marco on February 18, 2022, 12:16:28 pmTo be honest, I didn't know this was an option. How would that work?I am also not sure if this would change the physics, I need to think about this.

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2022, 12:26:57 pm
content: For a low impedance AC measurement you use capacitive coupling to the coax. So with a 3db point at 100 kHz you would use a ~30 nF blocking capacitor to the coax, then put the scope into 50 Ohm mode.Or to go even lower than 50 Ohm, directly capacitively couple into a transimpedance amplifier and only afterwards have coax (with a much larger capacitor and it will take a while to charge). But 50 Ohm seems low enough for time of flight, so probably here too.

author: hakunamatatarr
author_rank: Newbie
timestamp: February 18, 2022, 01:23:55 pm
content: But this would distort my pulse shape, no?I think it is better to stay at open circuit and try to minimize the RC effects

author: jonpaul
author_rank: Super Contributor
timestamp: February 18, 2022, 01:44:30 pm
content: https://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: February 18, 2022, 05:46:11 pm
content: Quote from: hakunamatatarr on February 18, 2022, 11:25:54 amThank you for describing your experiment in detail. I'll try to give you some ideas.For what you have, the signal actually doesn't look all that bad, but I have no way of really telling how accurate your results might be. If you are looking forrelativeresults, perhaps it is OK. But I think your precision, especially on the issues of time/decay, are probably not very good on an absolute basis. You are wanting to test the response of your cells, but I think what you are looking at is largely the response of your circuitry.It does not appear to me that what you are seeing is reflections in the BNC cable due to the impedance mismatch at the scope. There may indeed be such reflections, but a 50cm cable implies a round-trip time of about 5ns. I think what you are seeing is from a complex mess of LRC from your wiring between the samples and the probe box and the probe switching box itself. Different BNC cables will change the LRC equation by about 1pF/cm, so they will have an effect too. There are ways to try and deal with this passively, with either capacitive coupling asMarcosuggested or making an ad hoc 20X probe using a 950R resistor, but I think this will both load your cell in ways you don't want (an issue for your physics calculations) and also drive your signal down into the noise.A 1X active probe and a unity gain buffer would be effectively the same thing. I can't really recommend a probe because in your setup I think you need to do it differently, but as an example the ancient Tek P6201--1X, 900MHz, 300uVRMSnoise--would be a performance level to shoot for. Your issues include the need for low loading, low noise and high bandwidth and the usual answer to that question would be "pick any two". You can discuss it with an applications engineer from LeCroy, or perhaps someone with more knowledge will chime in here, but I think these will all prove difficult as you would need to run the probe all the way to the sample to get a good result. Otherwise you will still be looking at the response of your wiring in addition to that of the cell. What you need is to have your input amplifier/buffer right at the cell itself, and that implies that you would want four buffers, then you can run SMA-ended cables out to your scope. I would eliminate the probe switchbox and just connect the cables directly to individual scope channels. You could use a LeCroy AP034 or similar probe and one cell to make a calibration fixture so you could compare the result of your setup with a more established, commercial, calibrated result.For the UGB, I would suggest first trying a low-cost capacitively coupled FET-buffer. Presuming you don't need a DC response (and if you do, that can be handled separately with some care) this will allow a relatively simple circuit to solve all of your impedance matching issues. These are inexpensive and easy to build so that they could be integrated into your sensor array. They are also available on eBay in completed form if you want to be testing something in a few days. There are probably many other solutions as well, including some that might give you some gain rather than just being a buffer.https://cdn.instructables.com/ORIG/FQZ/1QZP/IPJTFO82/FQZ1QZPIPJTFO82.pdfhttps://www.ebay.com/itm/332393676082Edit: You also might want to look at something like the OPA855-Q1 which would allow you to make an isolated wideband transimpedance amplifier with a reasonably high input impedance (>> the cell impedance), 10X gain and decent bandwidth. This is would require more design work, but it is a solution that seems to be designed with applications similar to yours in mind.https://www.ti.com/lit/ds/symlink/opa855-q1.pdf?ts=1645209368066&ref_url=https%253A%252F%252Fwww.mouser.com%252F

author: gf
author_rank: Super Contributor
timestamp: February 18, 2022, 09:41:25 pm
content: Quote from: hakunamatatarr on February 18, 2022, 11:25:54 amWhen you say "draw the least amount of current", do you only mean DC current draw, or also AC load?DC load is avoidable in any case, by simply adding a DC blocking capacitor in series (as already proposed in this thread).During the rise time interval (which is obviously of interest for you) I do not really see a load difference between 1) connecting the cell via a cable into a high Z scope input, or 2) connecting it via a cable into a 50 Ohm scope input. Note that the instantaneous impedance of a 50 Ohm transmission line at the begin of the pulse is 50 Ohm, even if the other end of the cable is high Z terminated. At the time when the pulse enters the cable, it does not know how the cable is terminated at the other end, and it also does not know how long the cable is. It just sees a 50 Ohm load at the first moment which is the characteristic impedance of the cable. Only when the reflected wave comes back from the other end of the cable a couple of ns later, it interacts with the source, changing the apparent load then. But then the rise has already finished, and the cell had seen a 50 Ohm load during the rise time, although the other end of the cable is high Z terminated. With 50 Ohm termination at the scope you can at least avoid reflections and get a constant 50 Ohm load over time.For no-load measurement with minimum disturbance, a high impedance low input capacitance buffer, located in close proximity to the cell, is likely the best solution, or an active probe (as proposed as well in the thread).If you can live with (say) 500 or 1000 Ohm load, then a 10:1 or 20:1 "Z0 probe" might be an option, too (which may sound complicated, but which can be as simple as putting a 450 or 950 Ohm series resistor between solar cell and center conductor of the coax cable, and connecting the other end of the cable to the 50 Ohm input of the scope).

author: hakunamatatarr
author_rank: Newbie
timestamp: February 19, 2022, 11:54:53 am
content: Thank you so much everybody and especially bdunham for explaining this so easy and in detail for me!So my consequences are foremost to rework the whole cell connection, that I just have the pins which go directly to a contact (distance probably 3-5 cm) which can be probed.Then I need ideally an 1x active probe with high input impedance and low capacitance. I understood that good values for this are around >= 1 MOhm and <= 2 pF, is that right?My stay here is rather short so I doubt that I will actually see the new setup but if yes I will let you know how it went.Thanks again!

author: Marco
author_rank: Super Contributor
timestamp: February 19, 2022, 12:12:27 pm
content: I find the standard TPV measurement a bit weird for your application.The microsecond domain capacitance of the solar cell combined with the effective resistance of the recombination rate form a leaky integrator. That's not a problem if you are trying to measure the recombination rate, which is what the TPV measurement seems to be for, you are trying to divine signal features way into the stopband of the system though.In fast photodiode amplifiers you try to minimize the capacitive effects of the photodiode through minimal voltage variation, with a transimpedance amplifier, you get less signal but you increase the bandwidth of the system.

author: KT88
author_rank: Frequent Contributor
timestamp: February 19, 2022, 01:55:17 pm
content: It would be interesting to have at least a crude estimation of the source impedance of the effects that should be observed.It depends a lot on whether it will be MOhms or kOhms or Ohms... From there one will get an idea how much the input impedance of the cable or probe affects certain signal features.Something else to keep in mind: if the fundamental frequency of the rising edge is about 500MHz the featurs will be (much?) higher in frequency which will attenuate them in addition to the attenuation due to the load the probe poses to the DUT.The repeatability of the measurements depends further on the repeatabilty of the mounting in the test fixture...Yet another one could be proper preconditioning (appropriate baking) as humidity could have a significant influence on the results.

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2022, 03:04:41 pm
content: Quote from: hakunamatatarr on February 19, 2022, 11:54:53 amSort of, with the caveat that I don't fully understand your experiment and I have some of the same reservationsMarcoexpressed below. I would think any measurements like this would be done with a carefully defined load (perhaps variable) determined after characterizing the cell's DC response. But assuming you do actually want to measure the open circuit voltage of your cell with as much bandwidth as possible, you need a somewhat high impedance active probe with as low a capacitance as possible. Typically in high-BW work you would worry more about the capacitance than the DC resistance, so 200k/1pF would probably be much better than 1M/2pF. However, without being able to characterize your cells, it would be very hard to advise you any better or to give any estimate of what bandwidth you might reasonably expect to be able to measure to. For example, it is possible that the capacitance of the cells is such that the probe capacitance doesn't actually matter much, in which case your 600MHz scope is hopelessly optimistic.