Practical limits on amplification - Page 1
Forum section: electronics / projects

author: madladlabs
author_rank: Newbie
timestamp: March 02, 2021, 05:47:19 am
content: I'm working on a project and I am trying to determine how far away can you detect a weak light source. Ultimately the goal would be to build a rotating reticle optical sensor used to determine polar coordinates of a light source.The general idea is that I have a beacon (IR LED 940nm) that is flashing at a known frequency (right now I'm guessing I'll use 1kHz - 10kHz) and I would like to use a photodiode to detect the presence of said beacon.As an upper bound for the range I picked 100m - I don't expect it to work that far, but I needed to pick something to design the system against and thought that was a good stretch goal. The LED light source will produce on the order of 1 mW/sr. Typical photodiodes seem to have a listed effective area of 4 mm^2, but since I'm working with orders of magnitude at this point I'll call it 1 mm^2. At 100m away, the solid angle of a 1 mm^2 surface is 1e-10 steradians. So, we can expect the sensor to receive about 1e-13 W. Finally, the response of a typical photodiode is on the order of 1 A/W, thus the generated current is about 1e-13 A. Obviously, that's an (almost) insignificant current that may be indistinguishable from noise, but I'm hoping since I know that I'm looking for a constant signal with a known frequency that it might make things a bit easier. And the goal of the project is to determine the limits, not necessarily get the circuit working at the ridiculous extremes. If I move to 10m away from the beacon, the signal is increased by 100x making it 1e-11 A, which is a little less ridiculous.For the signal processing, I was thinking of starting with a high pass filter (to get rid of the part of the signal due to ambient light) and having several stages of amplifier circuits. There may be some additional filters between the stages - don't know how much noise to expect yet, but the general idea will be to band pass only the signal that I expect.For stage 1, I am currently looking at the circuits described in figures 16 (op amp based) and 17 (FET based) of this dochttps://www.hamamatsu.com/resources/pdf/ssd/si_pd_circuit_e.pdf. Maybe the answer is to build both circuits and see which one performs better. Combining the circuits from figures 17 and 16, results in something that looks like the compound amplifier circuit described at the end of this presentationhttp://edge.rit.edu/edge/P09051/public/photodiodeamplifers.pdf. I'm planning on using gains of 100 on stages 1 and 2. If I have a load resistor on the photodiode of 1 kOhm, I should be able to turn the 1e-13 A current into a 1e-10 V signal. After stages one and two, that should get amplified to 1e-8 V and 1e-6 V. Then, a few more stages to get a usable signal to compare against a reference signal. Maybe some sort of a multiplexer to sample the output of various stages, so that as the sensor gets closer to the source and the signal gets stronger I can use the outputs of the unsaturated stages.Anyone has experience with this much amplification and might give some pointers? There's more info I can share, but this post is long enough already.Anyone want to place bets on how far away the circuit will detect the presence of a source somewhere in front of it?

author: magic
author_rank: Super Contributor
timestamp: March 02, 2021, 08:04:08 am
content: It feels a bit like radio reception so maybe hams will be able to offer some practical advice.But in theory, you can drive noise arbitrarily low by increasing observation time and constraining receiver/amplifier bandwidth closely around the target frequency. Noise grows with square root of bandwidth and with square root of time, but signal grows linearly with time, so SNR decreases with time.It seems that for a 10x increase in distance, you get 100x decrease in power and only need to increase recording time 10000x

author: CJay
author_rank: Super Contributor
timestamp: March 02, 2021, 09:22:41 am
content: Quote from: madladlabs on March 02, 2021, 05:47:19 amIcarus is recorded as the furthest detected source of light, so 5 billion light years but it did take some rather specialised gear.Joking aside, wheel alignment systems I've worked on used to (may still) use a flashing LED and a CCD line sensor, the LED flashed at a known rate which allowed the system to 'null' out ambient light sources and local interference, maybe consider using something other than the photodiode, it will increase the complexity but perhaps a cheap CCD camera with an IR pass filter?

author: Zero999
author_rank: Super Contributor
timestamp: March 02, 2021, 10:41:08 am
content: What is the background illumination level?Adding a lens will increase the signal to noise ratio.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 02, 2021, 01:42:10 pm
content: Narrow bandwidth and long times will do a lot, but for mad crazy results, look at how lock-in amps work.

author: Kalvin
author_rank: Super Contributor
timestamp: March 02, 2021, 04:11:07 pm
content: Quote from: CJay on March 02, 2021, 09:22:41 amCJay is talking about synchronous detector here? Synchronous detector are very useful when one wants to increase the SNR of the detected signal, and one can control/modulate either the transmitted signal (mechanical or electrical on-off keying, for example a rotating disc with holes on it) or modulate the the detected signal (using mechanical on-off keying at the detector, for example a rotating disc with holes on it).See figure 10 (Chopper wheel) for better explanation:https://www.analog.com/en/analog-dialogue/articles/synchronous-detectors-facilitate-precision.html

author: CJay
author_rank: Super Contributor
timestamp: March 02, 2021, 04:38:27 pm
content: Quote from: Kalvin on March 02, 2021, 04:11:07 pmA very slow one on the equipment I was working on, but yes, essentially that.An optical filter helped a lot too.

author: madladlabs
author_rank: Newbie
timestamp: March 02, 2021, 06:30:02 pm
content: Since the detector and light source are not together by design, I think the only way I can create a synchronous detector is to use radio to transmit a synchronizing signal, which might be doable. What do you think?This sensor circuit is part of a larger whole. The end goal is to come up with a video or a set of videos where we can make a very crude homing missile using COTS components, as a way to introduce people to flight dynamics and controls. Before people go all "OMGWTF A DIY HOMING MISSILE!?!?!", technically anything you throw at a target counts as a missile. In my case, it will probably be a foam glider outfitted with control surfaces and electronics. Maybe, I'll need to use a little ducted fan to add some thrust, but that may not be necessary.I've been going back and forth on different approaches to the homing sensor and control. One approach is to use a camera as a detector and a small SBC to process the images with OpenCV. This approach has quite a few benefits: all the components are easily available and you just need to attach them, no analog design necessary, commonly available software libraries, camera-based system may be easier to debug, etc. The other approach I'm considering is using a photodetector in a way that's inspired by the homing system on the AIM9 missiles, which is covered in this articlehttps://medium.com/@OpenSeason/1946-germany-has-been-defeated-and-its-military-technology-put-under-the-microscope-the-west-e60b82926b40. The basics of this approach is to use a photodetector (in AIM9's case it's FIR, which makes things easier since you're looking for a bright signal (thermally hotter) against a dim ambient background (thermally cooler)) behind an asymmetric spinning reticle (kind of like a chopper wheel, but used differently). If you know the position of the disc when you see the signal of interest, then you know the relative rotation of the source to some reference angular position. If using a single slot reticle, the cutout in the reticle can be designed to give you different pulse widths as a function of the angle to target from the axis of the reticle. Then you can measure the pulse width and map it back to an angle.I am analyzing both approaches in parallel. Camera-based approach uses a raspberry pi zero. I like this approach for its simplicity and user-friendliness (especially nice if others want to repeat the project), but based on my initial testing the feedback has about 0.3-0.35 seconds delay, which is a bit long when you are trying to provide guidance for something in flight. This isn't an issue for flight stability, since the controls handling the flight surfaces are running on a microcontroller and are controlling at 100Hz. But the slow response of the camera homing loop will probably affect how well it tracks, especially if the target moves or if there is wind that blows the vehicle off course.The photodetector approach has the benefit of being "real-time." Its maximum response time should primarily be limited by the rate at which the reticle is spinning, so getting 100Hz sample rate should be achievable. The downside is that it's a whole engineering project on its own. But it's a fun, challenging, technically-cool, and informative project, which is why I'm entertaining the idea. Even if I fail, there is educational value in it. Maybe I could do a video on "why you can't do X," so that others learn from my pain. Of course, if this project wastes too much time and distracts from the main effort, it's worth scrapping it.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: March 02, 2021, 09:51:10 pm
content: I suggest a comparison with off the shelf products : Honeywell manufactures photoelectric IR beam sensors with ranges up to 240m outside. But they may use a higher power led (Cf current 50...95mA in the datasheet) and higher surface detectors (Cf enclosure size) with optical ambient light filtering.http://library.ademconet.com/MWT/fs2/0-000-138-01/Intellibeam-Family-Comparison-Chart.PDFQuote from: madladlabs on March 02, 2021, 06:30:02 pmWith that signal level (Cf your original post) you're going to be close to the noise floor of the detection apparatus (Photodiode + op amp + feedback resistor). Reducing the bandwith to the strictest minimum around the signal of interest is the way to go. You should aim at BW < 10Hz. At several KHz, it requieres a very sharp bandwith filter, you won't achieve that with passives. The way to go is passing the signal through a mixer fed with an accurate local oscillator, and low-passing the output with a slow filter, like they do in rf receivers.

author: zl2wrw
author_rank: Regular Contributor
timestamp: March 04, 2021, 08:52:51 am
content: Quote from: madladlabs on March 02, 2021, 06:30:02 pmmadladlabs, it looks like you are in the USA - are you familiar with theITARrules?What are the odds that what you propose to do will count as "exporting missile technology"?Do FAA rules have anything to say about UAVs that are controlled by IR seekers?Assuming that you go ahead with your plans, I would not be surprised if "the feds" (your feds!) get upset about you providing the world with detailed instructions about how to turn a UAV into an IR seeking guided missile (by which point it wouldn't be much of a step for those with malicious intentions to take your design and add a warhead, fusing system...).Yes, something you throw may be a missile, as is a bullet fired from a gun, but both of those areballisticmissiles, notguidedmissiles.I think that we'd all hate to see you end up doing time in federal (pound me in the #$%) prison

author: tim_
author_rank: Regular Contributor
timestamp: March 04, 2021, 09:16:58 am
content: In practical terms there is the issue that your components will generate 1/f noise. This type of noise has the disadvantage that it does not reduce in amplitude when you integrated for a long time.To avoid this it is necessary to modulate the signal with a frequency where 1/f noise it negligile, e.g. >100kHz, and perform synchronous detection. This will also get rid of any ambient light. Of course, this adds a lot of other issues.There are some dedicated ASICs that could help, like the ISL29501.Generally the easiest way to improve detection is to improve your optical system. E.g. optical gain on receiver, increate power of emitter.

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2021, 04:46:33 pm
content: Quote from: tim_ on March 04, 2021, 09:16:58 amUnfortunately modulating or chopping the signal isn't always a free lunch. If the phase noise coherence between the modulator and demodulator is good then things work well, however if the signal source modulator has poor phase noise and/or is displaced in time/distance from the demodulator the phase noise coherence suffers and so does the demodulated signal. Using a high quality low phase noise, including close in phase noise, modulating source is paramount for very low signal noise measurements, otherwise the signal gets contaminated by the noise from the modulation/demodulation process.Best,

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: March 05, 2021, 12:58:46 am
content: I'm not an expert in photodiode circuits design, but I did some readings, order of magnitude computations and simulations.Getting noise levels to detect 1e-13 A though an average photodiode seems to be achievable. As I said before, the bigger R, the better SNR. There's a poisonous effect : Bandwith*R = constant, because of the parasitic capacitances. So with all the other parameters fixed, the lower signal of interest frequency, the higher useable R, so the higher SNR. With limits : <100 Hz is the region of flicker noise, offset instabilities, 50Hz/60Hz mains and its harmonics, and the so.I optimized this using LTSpice, assuming a 1kHz signal to be measured, using a fast JFET op amp = LTC6244, with the biggest resistor value possible allowing a 1kHz bandwith.Optimization results : Rf = 35Meg, ~760nV/sqHz @ 1kHz at the output. This through the 35Meg resistor gives input noise current = 22fA/sqHz.Optimizing the same circuit for a 10kHz signal, I got Rf = 1Meg, noise @ 10kHz = ~130nV/sqHz, which gives noise current = 130fA/sqHz.Also : The bigger the parasistic capacitances, the lower SNR, because you have to use a lower value R to achieve the bandwith. I think it may not be advisable to use a larger area photodiode, because it brings of course more sensivity, but also more parasitic capacitance, which can lead to less SNR. This article supports this idea :https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7664252/On the other hand, using a lens in front of the photodiode bring a free photoelectric current amplifier effect, with no added parasitic capacitance.

author: mycroft
author_rank: Regular Contributor
timestamp: March 08, 2021, 06:08:48 pm
content: Maybe not directly pertinent but still interesting:https://electrooptical.net/static/oldsite/www/frontends/frontends.pdf.

author: mjs
author_rank: Regular Contributor
timestamp: March 08, 2021, 07:54:55 pm
content: Just detecting a light source far away is quite simple - several kilometers is rather easy with a 1mW modulated, collimated laser source, collecting lens, good aiming, dark sky, optical filter matching the source, photodiode, good amplifier and a narrowband receiver.It gets more difficult when you need to find out the location (i.e. without aiming nor tight collecting lens), background light and do it fast.I can't recommend enough the book Building Electro-optical systems: making it all work. In addition to frontends (the article in previous post) it discusses the whole system design and a lot of different details.

author: Medved
author_rank: Contributor
timestamp: March 09, 2021, 06:19:26 pm
content: Indeed, it is more a radio receiver...I would go for way higher frequencies, just to go as far away as possible from the 1/f noise.Then using very narrow band reception (that implies very slow scan speed, you have to make sure the detector is facing the source longer than 1/BW; 1s for the example below).Definitely use a superheterodyne, as having all the gain on a singke frequency domain is asking for feedback problems.The photodiode should drive a tuned LC circuit, which 1'st cancels out the diode capacitance and 2'nd provides DC supply path to bias the diode.Then some small amplification, just to get the noise down, but not to much, as at this point you do not have that much selectivity, so to not overload it by the out-of-band noise.Then mixer, converting it to some frequency where you may find some convenient band pass filters (like 455kHz, where you may buy 6-resonator LTM455xxx filters for really cheap on ebay/Ali/Bangood/similar), this will limit the bandwidth to about 10kHz wide slice. Enough to "swallow" some frequency drifts, but better to use good crystals on all frequencies. Again some amplification and mixing down to ~10kHz IF. Use some RC low pass, then sample by into a microcontroller (as an audio signal, something like RapsberryPi,...).Then sample ~1s chunk, do an FFT on the data and look for the spike around the frequency you are expecting it.Parallel to that evaluate the Vpp on the ADC input and in some way (some DAC,...) control the gain of the previous stages so you keep the ADC from saturating, mainly on noise, yet still maintain the level of the noise floor (you may get that from the FFT results) so you have sufficient sensitivity.With the last filtering in SW, you may easily made the bandwidth (detection, so scan speed) vs noise trade off adjustable...generally apply even a narrow band optical filter in front of the diode, so you restrict as much as possible all light outside of the working band, as the stray "background" light cause shot noise in the photodiode itself.