Hobby radar design - Page 1
Forum section: electronics / rf-microwave

author: sckzor
author_rank: Contributor
timestamp: August 12, 2022, 01:44:17 am
content: Hello forum,I am a software guy, recently I have become interested in writing software that relies on data that can only be supplied by a radar system. The issue is that every off the shelf radar I have seen has been very expensive and not really what I am looking for. I want a dead simple analog radar that I can aggregate data from without much work. When looking for something like this I found a project made by MIT (herehttps://ocw.mit.edu/courses/res-ll-003-build-a-small-radar-system-capable-of-sensing-range-doppler-and-synthetic-aperture-radar-imaging-january-iap-2011/pages/projects/) that is basically exactly what I was looking for. The issue with theirs is that it is rather large, clunky and still more expensive than I would like. For this reason I took the schematics made by MIT and created what (I hope) is a schematic for a similarly spec-ed one made from cheaper surface mount electronics. I have done some simple projects with analog electronics and a little bit of RF stuff but I have never made a project as complex and RF intensive as this before. Before I dump a large chunk of change on the parts for this board though I would like to get some input on whether or not it will work as I intend. I have placed a link to a picture of the schematic for the project as well as the five different uncommon ICs that I put in it. All other parts should be generic or common parts like 555 timers and op-amps.If anyone has time to check this I would really appreciate a fresh, more experienced set of eyes looking at it.Many thanks,sckzorLink to schematic:https://files.sckz.org/uploads/1660268499506579982.pngRaltron RQRA-2328-2536-CR VCO:https://www.raltron.com/webproducts/specs/VCO/RQRA-2328-2536-CR.pdfKYOCERA AVX AT 0603 Attenuator (3db):https://datasheets.kyocera-avx.com/AVX-AT-Series.pdfNXP BGU8052:https://www.nxp.com/docs/en/data-sheet/BGU8052.pdfAnalog Devices HMC213BMS8E:https://www.analog.com/media/en/technical-documentation/data-sheets/hmc213b.pdfAEROSEMI 2A Step Up Converter:https://www.olimex.com/Products/Breadboarding/BB-PWR-3608/resources/MT3608.pdf

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 12, 2022, 04:43:50 am
content: You have the RF and IF ports on U5 flipped.I'm assuming AE2 is the receive antenna, in which case U4 is backwards (you have its output connected to the antenna).Connect all the NC pads of the VCO to ground.Get some good antennas to go with this. L-com is one place to look for 2.4 GHz wifi gear.The LM324s will run on +5V, so ditch the SMPS. You could also consider a proper R-R opamp (LM324 only goes down to ground).Otherwise, it looks like a decent design, looking forward to seeing your results

author: Berni
author_rank: Super Contributor
timestamp: August 12, 2022, 05:27:50 am
content: Do watch out how you do PCB layout for the RF parts as parasitic effects and transmission lines are very important for RF. It can make the difference between this turning into an unstable noise machine or a well performing radar.This is likely the reason that the MIT students went with ready made RF blocks. You don't need to know anythyng about RF design to just plug them together and have them perform to what the datasheet claims (they are just SMD chips in a metal box with connectors anyway). But yes these RF modules can be pretty pricey. Most of the time they are used for prototyping things or in niche small volume RF equipment.If you just want a radar to play with these are some ready made radar modules that implement just the RF stuff on a board (LO, Mixers, Amplifiers, Directional patch antenna..etc) and operate in the frequency bands designated for radar like 10Ghz and 24Ghz. What you get out of them is the ready to go IF out of the mixer.Example of one:https://www.aliexpress.com/item/4001178786384.html?spm=a2g0o.productlist.0.0.7f052399TPu4Cy&algo_pvid=25a6a566-3d13-43e7-8a85-414307386cd7&algo_exp_id=25a6a566-3d13-43e7-8a85-414307386cd7-8&pdp_ext_f=%7B%22sku_id%22%3A%2210000015056303221%22%7D&pdp_npi=2%40dis%21USD%2134.16%2131.77%21%21%2111.62%21%21%40210318cf16602820166238419eaf5c%2110000015056303221%21sea&curPageLogUid=FRkCv64z6iAM

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2022, 07:21:49 am
content: Make sure you are legally allowed to transmit the radio signals, and that you won't "inconvenience" other users in the band or in other bands.Authorities tend to become ratty and punitive if they have to discover why people are complaining.

author: geggi1
author_rank: Frequent Contributor
timestamp: August 12, 2022, 08:42:49 am
content: I have seen presentations where cheep SDR hardware are used as radar on 2,4Ghz.Some radardesigns are passive radar type based on WIFI ewuipment.

author: sckzor
author_rank: Contributor
timestamp: August 12, 2022, 09:40:13 pm
content: Thank you to all that responded to help.I will address everyone's points:First, radar_macgyver, I will use a list like you.1) Upon further inspection I do indeed have the RF and IF ports reversed on the mixer. I don't really know what I was thinking there, good catch.2) You are also correct about the RF amp being backward too, I think I just copied and pasted it from the other RF amp and forgot to reverse it. I would have never noticed that.3) I was planning on connecting all the NC pins to GND, I just did not want to clutter the schematic or make it confusing. At one point I had a note in there but I must have removed it or something.4) The L-com antennas look very nice, I will seriously consider them for the project. Thanks for the advice.5) For the op-amps I copied MITs project exactly, they were using a discontinued part that probably only used 12v and I just dropped in the LM324, I didn't really pay attention and I will change it. That said I don't think I really need a R to R op amp and getting the -5v I need for the R to R would just add unneeded complexity. If there is any advantage to the R to R please let me know.I will keep you posted on progress in this thread.Secondly Berni,I know that these RF parts are really particular about their board layout. I did a little bit of reading about this before posing and I think the general consensus is that without all of the fancy lab grade equipment the best strategy is to use a 4 layer PCB, keep your RF stuff well spaced on the first of the four layers and separate it from the bottom layer using a ground plane and a voltage plane. Finally use the final plane for excess signals that wouldn't fit on the top and hope for the best. If you have any better suggestions then please let me know. To address your second point, I did look around a bit for some pre-made radar but almost all I found looked were either doppler radar or had conflicting information. I want to do SAR with this so I don't think that these systems would work. Even if they do work for SAR I have learned a lot a lot about analog electronics and RF already from just reading/listening to the technology surrounding this topic and now I am interested to see if I can do it my self, even though it will probably not perform as well as a store bought unit.Third tggzzz,The 2.4 GHz band I am using is within the US unlicensed spectrum, the most the parts could even possibly over shoot even if I did not tune it to be within that spectrum is like 100 MHz and with such low power I am operating at and the fact that I will only be turning it on intermittently I think using a microwave oven would be more destructive to things around me than this project. I don't intend on making this a commercial product of anything so even if it is technically illegal I don't think anyone will even notice, much less mind.Finally geggi1,I had never heard of using SDR hardware for radar before. I looked into it after your post and it looks really interesting. It looks like it is pretty undeveloped inaccurate though, I want something that has at least some chance of working, where I am not breaking all new ground. Even if it is accurate though, like I wrote in my address to Berni, I am having fun with this and I would like to continue.I want to thank everyone again for helping out here. I won't be able to revise the schematic until Sunday as I will be out of town until then and do not have access to my main computer. When I revise it I will send it here for anyone who is interested.- sckzor

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 12, 2022, 10:07:41 pm
content: Using a rail to rail opamp does not imply a negative supply - it just means that the output stage of the opamp is optimized to allow operation all the way to the supply rails. The LM324 can operate down to the negative supply, but not the positive (the data sheet says it can go to 1.2V below the positive rail). This limits the output voltage swing if you're powering it from +5V. A true RRIO opamp will give you a bit more dynamic range, that's all. An LM324 will work fine too.To Berni's point, you could also consider adding shielding cans to the board. Leadertech and Laird make some decent ones. A bandpass filter on your TX path (eg: Mini-circuits BFCG-252+) will prevent radiating harmonics.I got myself an ADI "Pluto" SDR kit, but found that there's no reliable way to time-sync the transmit and receive sections - but that was after only a couple of days of tinkering with GnuRadio - maybe I missed something.One last thing: make sure either that there's nobody around while you're testing outdoors, or that people are aware of what you're doing. Folks with strange antennas and "OMG, he's got wires!!!" get unwanted attention from law enforcement. Ask me how I know...

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2022, 10:26:09 pm
content: Quote from: sckzor on August 12, 2022, 09:40:13 pm(Deinterleaving your responses is a pain)What makes you think you are transmitting in only the ISM bands? Hardware non ideal behaviour means your transmissions can be splattered outside the ISM band. If you don't understand RF specifications (e.g. IP3), then please pause for thought.Whether or not it is a product is irrelevant. Disrupting another service it the point.How often the transmitter is on merely affects the hamce of interference, detection, prosecution.

author: Berni
author_rank: Super Contributor
timestamp: August 13, 2022, 07:02:06 am
content: Quote from: sckzor on August 12, 2022, 09:40:13 pmYep that is a good PCB strategy for RF.As for the radar modules, most of them are FMCW and this is mostly what you have built. CW is for Continuous Wave (as opposed to radars that emit pulses) and FM is for Frequency Modulation. This means that the LO is actually a VCO that can be controlled somehow. If you don't use FM and just use a static LO then you get a Doppler radar that will only see moving targets. If you feed a ramp waveform into the VCO then you get frequencies out of the mixer that correspond to the distance to the target (due to phase shift of distant targets moving faster at a given frequency sweep rate). Doing SAR radar then just means taking these FMCW snapshots and running them trough the math.Tho if you are looking to get long ranges out of your radar it might be best to DIY, since you can give it more transmit power and use lower frequencies that can travel farther. The modules tend to use high frequency because it makes them more accurate in the shorter distances they are designed to be used, while building your own radar that works that high into the GHz starts to get into quite some RF magic.

author: tggzzz
author_rank: Super Contributor
timestamp: August 13, 2022, 07:42:11 am
content: This blog is interesting and might be relevant:https://hforsten.com/There are blessedly few entries; clearly he is someone to be respected for not succombing to the "look at me" and "I opened a box and it works as expected" syndromesMIMO radar antenna arraysShort introduction to multiple input multiple output (MIMO) radar antenna arraysBackprojection BackpropagationEven better focused homemade SAR images with timedomain backprojection algorithm and automatic differentiation based autofocus.Synthetic-aperture radar imagingDifferentiable synthetic-aperture radar image formation with Tensorflow. Including very fast image formation and autofocusing utilizing GPU.Third version of homemade 6 GHz FMCW radarNew and improved version of the 6 GHz FMCW radar with two receiver channels.TRL measurements with homemade VNA and open source softwareMeasuring transmission line parameters of 50 ohm microstrip on OSH park 4 layer process and S-parameters of 1 nF SMD capacitor.Improved homemade VNASecond version of the homemade 30 MHz - 6 GHz VNA with improved performanceRF power detector and scalar network analyzerNext on the list of homemade RF test equipment is RF power meter. I decided to make one to calibrate the output power of my VNA and to also measure output power of the previous radar projects.Cheap homemade 30 MHz - 6 GHz vector network analyzerVector network analyzers are used to measure high frequency circuits, unfortunately they are too expensive for student budget so I decided to make one myself for small budget.Heartbeat detection with radarPhase of the radar baseband signal is very sensitive to movement allowing to detect small movements caused by breathing and heartbeat.Homemade synthetic aperture radarSecond version of the 6 GHz FMCW radar now with SAR imagesHorn antenna for radarDesigning and fabricating better antennas for the radar6 GHz frequency modulated radarMaking a cheap and simple radar with a range of about 100 meters.

author: sckzor
author_rank: Contributor
timestamp: August 14, 2022, 03:21:57 am
content: radar_macgyver:QuoteI only recall seeing rail to rail opamps being used with negative supplies. I suppose that is where my misconceptions about them arose, I wrongly assumed that there were not advantages beyond the ability to go negative. Thanks for clearing that up, I am still a little fuzzy on everything surrounding op-amps and I am excited to read more about them next time I get a chance to. With regard to the circuit, knowing this, I don't see why I wouldn't go with a R to R opamp to get better dynamic range even if it is a few cents more money. Something like the LMC6484 seems like a good, popular choice. Thanks!Berni:QuoteI am happy to hear this should work! I would hate to have my circuit mysteriously not work for some reason I can't detect.QuoteThis is very interesting, I would love to read more about these topics, thank you for letting me know about this, as soon as I get a chance I will look more into this.QuoteThis makes sense, I would like a long-ish range device, at hundred feet of somewhat accurate range is my hope, since 2.4 WiFi is rated at about 300 outdoor this seems pretty reasonable to me. Thanks for your help!Tggzzz:QuoteThat blog is indeed very interesting from what I saw of it. I am a little short on time right now so I did not have a chance to read all of the entries you mentioned but I look forward to doing so in the future.QuoteI will read more into this, I do have somewhat limited knowledge of all RF specifications, as I continue designing I will take this into account. Your concern is appreciated.

author: sckzor
author_rank: Contributor
timestamp: August 14, 2022, 07:59:45 pm
content: Thank you very much for taking all the time to explain that evb149,QuoteI have a decent amount of experience working with hardware accelerated 3d graphics. I have worked with Vulkan and OpenGL somewhat extensively and while I am certainly no expert, I understand how it works. I watched the presentations about the 3000 series GPUs and even picked the 3060 Ti up a few days after launch in order to experiment with all of its features, so I have worked with RTX before. Since visible light is at the end of the day just EMF radiation I don't see why modeling 2.4 GHz radiation with RTX on a GPU would be all that much different than modeling light, as a matter of fact, because the antenna originates from (basically) a single point I would think that doing this would actually be easier. That said, I have not looked into using GPU hardware for modeling EMFs.QuoteI am sure you are correct in thinking that there exist tools to do this already that someone in the OSS community has created, and tying this to a higher level tool would likely not be super difficult.I did not consider simulation because I think for me hardest part of creating something like a SAR imager is the hardware due to my lack of experience. If I cannot figure out how to make this then there is no point in spending time writing all of the software to support it and do calculations for it. However, I think that something like this could come in extremely helpful when testing the software, not needing to turn something on or go to a location in order to test on a new data set or something. Thanks again.

author: sckzor
author_rank: Contributor
timestamp: August 14, 2022, 08:09:15 pm
content: QuoteI am familiar, I was making a general guess as I was writing the post, as the signal propagates it is inevitably going to become harder to detect because of the affects of propagation but also, and probably more importantly, the background noise on that band. I think it would be hard to expect more range than my original estimate unless I was in a field in the middle of nowhere or somewhere else with minimal background noise.

author: sckzor
author_rank: Contributor
timestamp: August 19, 2022, 09:51:50 pm
content: First I apologize for taking so long to do this, I got super busy during this week. In the spare time I did have though I was able to make the corrections to the circuit and rework some things, most notably the triangle wave generator. I made the ramp speed variable with a potentiometer and switched from using a 555 timer to another quad op-amp for the design. If someone could quickly glance over that circuit, I would appreciate it. Normally I would just order a couple of op-amps and try it out but I don't want to pay shipping on everything twice, I am not ready to order the rest of the parts yet and I have run out of op-amps. I also did some reading/video watching about the things that everyone mentioned, it was all very interesting.I have attached to this post a link to the updated schematic and I will begin working on a PCB shortly. Any other comments, suggestions or critiques are welcome!Thanks.Schematic link:https://files.sckz.org/uploads/1660945702684486670.png(higher resolution now too)

author: sckzor
author_rank: Contributor
timestamp: August 21, 2022, 03:45:43 am
content: QuoteYes, probably JLC, I have used that one in the past. I would like to try making my own pcb at some point but I think impedance matching the traces would be too hard and PCBs from a fab are really high quality and pretty cheapQuoteI think assembly will be the hardest with lots of small SMD components. I plan on using one of those SMD soldering plates. I have made one in the past for other things, and while it looks a bit janky it works good enough.

author: Yusuf Alsaedi
author_rank: Newbie
timestamp: September 29, 2022, 07:44:39 pm
content: I may have to make one of those janky SMD soldering plates

author: sckzor
author_rank: Contributor
timestamp: November 30, 2022, 03:36:25 pm
content: Firstly, I apologize for necro-ing this thread but this doesn't really belong anywhere else so I am putting it here.Secondly, after doing some more design work, ordering parts and manufacturing PCBs I have finally built the radar system outlined above. It took longer than I thought it would because I got busy and there was an error in the first revision of my PCB but it did finally happen.I have not done any analysis of the performance of the system yet but it does appear to be working. Once I do do this I will post my findings here again.I have attached a photo below.Thank you all for your help,sckzor

author: sckzor
author_rank: Contributor
timestamp: December 12, 2022, 08:46:35 pm
content: Good afternoon,I have started to test my radar system, to do this I have connected the sync pulse and the signal from the video amp to a Raspberry Pi Pico that I have performing snapshot captures like a rudimentary 50 KHz oscilloscope. This data is then sent over the serial port to my PC. I then take the data that I capture and feed it through a Fourier Transform to see the frequency breakdown of the data. This system all works well, however, the problem is that the received data seems very odd to me.Attached are photos of the full screen data capture, the data capture and the data after the FT and an image of the data, the data after FT and a colored historical spectrogram.Attached to this post is an image of the scope recording the same wave form to show it's relation to the sync pulse and to prove it is not some sort of fluke with the Raspberry Pi.Starting about halfway up the chirp waveform there seem to be some response waveform. This doesn't really make sense to me because the signal being transmitted is continuous I should be seeing a constant response wave form with a relatively constant phase shift of the TX signal shown in the output.This said, it is pretty obvious that something is happening. When I move a surface back and forth in front of the device, something happens to the wave. Mainly it seems that the large lobes on the graph move up and down and change their size. I really do not know what to make of this change. There appears to be no appreciable change in the FT breakdown of the frequency when this happens. I have tried everything I can to help remove interference in the system with shielding, isolated power supplies and such so I am fairly confident that this is not being caused by interference. I have also beeped the entire circuit so I do not believe that it is being caused by shorts or something either.I have attached an image of the schematics of the final circuit that was printed on the board. A couple of small changes were made but nothing large (I think). The only part that has changed since the original BOM was the Raltron VCO which was switched with an equivalent from Crystek because the Raltron part was out of stock. The link to that datasheet follows if you are interested.https://www.crystek.com/microwave/admin/webapps/welcome/files/vco/CVCO55CC-2230-2430.pdfAny insight anyone can provide about this problem would be extremely helpful to me. I do not have any equipment capable of measuring such high frequencies nor do I know some place that does so intuition is the only debugging method I have for the high frequency component. Hopefully the issue is with some low frequency component as that is much easier to work with.If there is any extra information that you need or anything you would like me to try, let me know. I would be happy to do it. Things have slowed down for me a bit now so I should be able to do it in a relatively timely manner.Hopefully I do not have to write this project off as a failure.Thanks,sckzorEdit: Embeds from attachments don't seem to be working. All of the images are attached. Which one is which is pretty self explanatory.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: December 13, 2022, 06:06:19 am
content: My hunch is that something's fishy with the IF amplifier.I would try to probe the output of the mixer, possibly after disconnecting the 1 uf capacitor coupling cap. Another point to note is that the mixer output is not properly terminated to 50 ohm. Finally, to minimize the number of variables, connect the TX output to the RX input (without an antenna), but use an in-line attenuator. With no antenna, you should really only see a DC output from the mixer, maybe with a small discontinuity at the ramp transitions.Could you post a scope shot of the mixer output, both with and without a 50 ohm termination? You may need to crank up the gain of the scope quite a bit to see anything.

author: sckzor
author_rank: Contributor
timestamp: December 14, 2022, 02:23:16 am
content: Hello,Unfortunately I do not currently own an RF attenuator. I will order something like thishttps://www.amazon.com/Digital-Attenuator-DC-3-8GHz-Parallel-Attenuation/dp/B0828GQ672/and some extra SMA patch cords assuming none of the information I have below shows some sort of glaring fault in the design.Now that you mention it, back when I first began investigating this problem a week or so ago I do remember taking the hook attachment thing off of my scope probe and pressing it up to the IF leg of the mixer. When I did that I was just looking to ensure that there was some sort of output and I was not amplifying noise or something. At the time the output looked correct but now that I take a closer look into it I am not sure that it is. I would think that because the distance to the object is not changing, the reading should have some sort of standing wave that is not correlated to the ramp frequency, however, it seems that in all cases it is correlated to the ramp. Stranger yet, both with and without the coupling capacitor (i.e. regardless of the state of the video amp connection) I see the wave form is relatively stable and has a very noticeable and somewhat predictable change when I point the device at different distances. When I added the 50 ohm terminator I no longer saw a stark difference and the wave was much less stable. Perhaps the load of the terminator is destroying the useful signal or maybe my cheap probes are attenuating the signal before it ever reaches the instrument? I have no idea what to make of that change.As an aside I suppose that when I was designing I assumed that the mixer output was not a 50 ohm signal line hence the lacking terminator, in hindsight it makes perfect sense why it would need one but I never made that connection in the design phase. I used the 50 ohm terminator built in to my scope for the readings.Thanks for the help,sckzor

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: December 14, 2022, 05:21:33 am
content: So it looks like there's way too much gain on the video amp, causing it to saturate and hit the rails. You may want to breadboard a gain stage that's more appropriate, using the oscilloscope to verify that the output is an amplified version of the input.The termination resistor needs to be applied to the output of the mixer. Just turning on the 50 ohm termination while you're using a probe is no good, that's meant for if you're using a coax signal fed directly into the scope. Solder a 50 ohm resistor (preferably SMT) to the mixer output, and probe as before. I would then design the video amp to amplify the output of the terminated mixer output.For an attenuator, get something like this instead:https://www.amazon.com/dp/B091KGWJ2M/If possible, look for used fixed SMA attenuators on ebay or other places. The Amazon ones may work at 2.4 GHz, but will likely have poor VSWR.

author: LM21
author_rank: Regular Contributor
timestamp: December 14, 2022, 08:06:57 am
content: I wouldn't use LM324 anywhere. It is slow and its output voltage range is limited. Like others have said, there are opamps that work with +5V and ground power supplies. LM324 is maybe 50 years old design, and it shows. At 5V and ground as supplies, its output can be only between 1-3.5V or less.Some PCB houses also offer PCB assembly at reasonable cost.NanoVna costs only about 60-1500£$€, depending on how high frequencies you want to use.Tiny Sa and Tiny Sa ultra wont cost much, either.

author: sckzor
author_rank: Contributor
timestamp: December 15, 2022, 08:36:44 pm
content: The LM324 is not used anywhere. The two op-amps used on the board are Texas Instruments LMC6484s. To my knowledge these have better performance than the LM324. I was testing the chirp VCO and video amp using the LM324 because I had them on hand and that is what others who have done a similar project used (if my memory serves), however I removed them earlier in the design process at the recommendation of the others here.The NanoVNA and TinySa are very interesting. If I were to get one I would probably build a NanoVNA because it is probably cheaper and I have all the tools to do it. If I am still having issues debugging this or I want to go even deeper into RF once this is over I will definitely keep it in mind.Thanks!

author: sckzor
author_rank: Contributor
timestamp: December 16, 2022, 01:46:21 am
content: I have just terminated the signal from the mixer and measured it on the scope. It does appear that the signal is hitting the rails of the op-amp when it crosses over zero volts. I will have to redesign the circuit to accommodate this like you said radar_macgyver. If you have any input on the best way to implement the redesign of the video amp let me know I am open to any and all suggestions. The photos of the scope readout with the 50 ohm termination are attached to the post.I also ordered the attenuators and extra coaxial cable so I should be able to test without the antennas by sunday-ish. I will update the thread at that point.Thanks!Edit: Photos weren't there for some reason

author: profdc9
author_rank: Frequent Contributor
timestamp: December 16, 2022, 02:41:44 am
content: You might be interested in this development board:https://omnipresense.com/product/ops243-c-fmcw-and-doppler-radar-sensor/It is a 24 GHZ band FMCW radar. Even has built in FFT, but it will return the raw quadrature demodulated return data.

author: sckzor
author_rank: Contributor
timestamp: December 16, 2022, 03:56:01 pm
content: That is a really cool board. Thank you for letting me know about it, if I ever need anything that is accurate and reliable I will seriously consider that system.My goal with this project though was to get a grip on RF and analog electrical engineering both of which have appeared as dark sorcery to me for a while. I have done some digital things like embedded Linux and micro controllers but I never had a good grip on analog electronics. I find that I learn best by jumping in an trying things, only issue is that it gets pretty expensive pretty fast.

author: sckzor
author_rank: Contributor
timestamp: December 21, 2022, 10:52:46 pm
content: I received the attenuators yesterday, they came in late due to some shipping delays (I am assuming winter related). Once I had them I did some testing and it appears that your hunch is correct radar_macgyver. It looks like something is going wrong with the RF mixing. I have once again attached labeled images from my testing. I also tried the circuit with the coupling capacitor disconnected and the results were almost identical. Unfortunately I forgot to take a photo of it and I do not want to disconnect it again unnecessarily.My next step in troubleshooting would be to make sure that it is the mixer causing issues. My best guess for how to do that with the limited resources I have would be to disconnect the RF from the receive signal amplifier and connect it to the LO signal on the mixer using a small bodge wire. By doing that the output off the mixer should be close to DC, otherwise the mixer is having issues. This is probably not the best option ever because of impedance matching an whatnot but without a full blown RF lab I cannot think of a better method that would get close enough to at least tell me if I am entirely wrong. I don't see why a test like this would hurt the RF mixer so at the very least I feel like it is worth a shot.The only other explanation I can think of for this behavior would be the circuit traces for the ramp (which is clearly seems to be somehow mixed in with one of the signals) run about .7 mm from the received RF signal after amplification. I suppose that the ramp trace could be inducing a current into the trace that carries RF to the mixer, causing it to behave strangely, however, I would think that the ramp would have to be oscillating much faster to create that kind of change current in that trace because the mixer is mixing different frequencies not amplitudes. The difference in frequency between the ramp and the RF signal is enough that the change in frequency would look basically like a small, constantly changing DC offset as far as the mixer is concerned. I could be wrong though.I don't know if I mentioned this before, but I do believe the mixer is working to some extent because in the past the upper and lower lobes on the output with the antennas connected (highlighted in yellow and teal respectively on the corresponding image) predictably changed when I changed the distance of a surface from the radar.Hopefully this information gives some more ideas...Thanks again for any and all help.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: December 22, 2022, 12:14:58 am
content: The pix don't mention of the conditions that the measurements were made under. Is the mixer IF port terminated? It should be. With the TX looped back to RX, aside from a DC shift, the mixer output and video amp output seem fairly well aligned. The DC shift is due to the coupling capacitor, and is required since if you note the mixer output is negative-going with respect to ground. The video amp design gets around this using the coupling capacitor, and the amplifier end is tied to +5V. This seems to tell me that the video amp is working OK, though the last picture shows it saturating with the antenna connected. Note how the part highlighted in teal is showing up at the video amp output, but the part in yellow gets clamped to the upper rail. The video amp's first stage includes a gain control (RV2 in the schematic), does adjusting it help prevent the saturation from occurring?I may have missed it, but did you post the board layout? On any RF board, that's kinda critical. From the one pic in reply #19, it looks like everything's 'spread out' over the board which makes me think that the traces are going to be lossy. Since you seem to be somewhat new to RF, consider replicating the MIT-OCW setup with connectorized parts first before taking a stab at doing it on a PCB. Without test instruments, this is going to be difficult to debug. Consider getting a USB receiver that can be used as a spectrum analyzer (Analog Devices Pluto, for example).Having said that, I think you have a nearly working radar, just needs a couple of tweaks to make it usable.

author: sckzor
author_rank: Contributor
timestamp: December 22, 2022, 06:29:12 am
content: Firstly, thanks for your encouraging words.I should have been more clear about this, but the photo that you mentioned with the video amp saturating was an old image I posed earlier, I have since fixed it, I just wanted to show the lobes that change with distance in the image and I believe they were the most prominent in that photo. It no longer saturates when connected to the antennas or the attenuators. The mixer IF is 50 ohm terminated into the video amp like before, nothing changed there.I have attached an image of the board layout. The whole board is very nearly the width of a standard credit card (85 mm) and a little bit shorter than one. I read a bunch of different materials and was under the impression that so long as the traces were less than the wave length of the signal (i.e 300/2.4 = 125mm) then things would be generally okay (probably not super great but still okay). I believe that the longest trace on the board is the one that runs from the power divider to the transmit antenna at 71ish mm. I also remember reading that you do not really need to worry about traces being angular at 2.4 GHz and you can just route them like normal. All of the traces are 50 ohm impedance controlled.Please let me know if you see any issue with the board. I could order another one if you think that it is a likely cause of the interference, although, even if the traces are wrong I would think that the interference would not look like it does. Alternatively, could the mixer possibly be heat damaged or something? I did have to reheat the board because the first revision had a footprint error, I think the mixer was one of the last components to come off which means it was on my somewhat poorly temperature controlled soldering plate for probably upwards of 5 minutes. I don't know if these parts are generally super sensitive?I am very new to RF, this is my first from-scratch project. I seriously considered trying the MIT OCW approach with the mini-circuits parts, however, after some consideration, I came to the conclusion that I see the mini-circuits route like I see building a desktop computer, it is interesting and you will almost certainly end up with something that works well but there isn't all that much to be gained from doing it learning wise. It is building expensive legos, definitely cool and I understand why MIT would do it that way, but I think I get the same value from watching their series about it as I would from doing it myself (I did watch their whole video series about it before starting this project.) I would have tried it both ways, comparing the results (I think some UC Davis students did this) but I don't have a whole lot of money to work with, a max of about $400 USD so I can't really afford to do both. I figured if I did it the mini-circuits way I would get something that works and a little bit of extra learning value. If I did it the PCB way I might end up with a cool looking, expensive paper weight but I would learn a whole lot about how RF systems are designed in the real world either way. To me, it was worth the risk of the expensive paper weight to have the ability to learn a lot more about this.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: December 22, 2022, 07:27:08 am
content: Here's the saturation I was referring to:Note how the rest of it looks like a scaled version of the mixer output. Try adjusting the gain control to see if you can lower the video amp gain to eliminate this effect.As for the layout, yeah there are certainly some issues. The RF traces are long, and run parallel to other traces, what you have then is a coupler (see for example the three traces under 'R17'). As a rule of thumb, they should be as short as possible, so in your case, the VCO and mixer should be close to the antenna connectors, rather than on the other side of the board. There are also way too few vias to the ground plane around, for example, the VCO - it has so many ground pads for a reason.tggzzz had posted a link tohttps://hforsten.com/, take a look there for examples of a more typical RF layout. Sure,his FMCW radarhas a lot more parts than yours, but much of it is bringing the signal processing on board; the basic design is almost identical. See also his discussion on the video amp design.

author: sckzor
author_rank: Contributor
timestamp: January 12, 2023, 06:18:13 am
content: My apologies for again taking forever to respond to this thread. Things again got unexpectedly extremely busy for me toward the holidays and the beginning of the new year. I did some further testing with some bodge wires on the PCB and I think that the interference explanation that radar_macgyver gave is still the most likely to be the case in this instance. To solve this problem I redesigned the PCB with a lot more grounding and I moved the components much closer to one another. I have not yet done impedance control on the traces (because I might switch PCB houses for some reasons) but barring that I think this design is much better. I have both attached photos of the design and made them available athttps://github.com/sckzor/RadarProject/tree/master/AnalogRadar/MkIIif you would like to load them into kicad (sorry if there are random extraneous files I am rapidly running out of time to work on this). If others think it is good too I will send it into production and hopefully make this radar work much better.Thanks to all that have helped (especially radar_macgyver),sckzor

author: Berni
author_rank: Super Contributor
timestamp: January 12, 2023, 06:30:24 am
content: All i know is that i need to put "Behold my latest fire hazard" on more of my boards.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 14, 2023, 06:59:03 pm
content: Much better layout, things are more compact. I would get rid of the 'thermal spokes' connecting the surface-mount pads to the ground plane, if you have a suitably rated soldering iron (or a board preheater) because it will make the soldering operations a pain. If you're going to stick with JLC's 7628 process, consider using a transmission line calculator program to design a 50 ohm microstrip (or coplanar waveguide) for all the traces that carry RF.It does look like you're using some rather large passives, 0805 maybe? Their pads will present a large parasitic capacitor to the nearby ground plane. If you can, drop the size to the smallest one you can. Ideally, one would pick a stackup and transmission line geometry whose trace width is very close to or equal to the passives used, so there's very little impedance mismatch when the traces meet a passive components' pads. With coplanar waveguide (CPW), one can tweak the trace width (by adjusting the spacing to the nearby ground plane) without changing the stackup. CPW does have some drawbacks - mainly that there can be multiple propagation modes depending on the spacing and width chosen. This is minimized by not deviating too far from the width used for microstrip.Finally, if you're open to a bit more layout tweaking, what I found is that laying out RF boards is a bit different in that one lays out the parts to minimize jogs and turns in the traces - everything's straight lines or 90 degree turns. Take a look at Henrik's boards for inspiration - note how all the RF parts are lined up neatly so the traces are all straight lines. He went the extra mile and made the 90 degree turns use miters to avoid impedance mismatches - I prefer to use swept (curved) traces instead. The pic below shows his implementation of a Wilkinson power divider:He also placed a solder mask opening over the the transmission lines so that the solder mask doesn't change the Zoof the line.Quote from: Berni on January 12, 2023, 06:30:24 amHaha, I might just do that on the one I'm working on now (48V/10A H-bridge)

author: buta
author_rank: Contributor
timestamp: January 14, 2023, 09:36:53 pm
content: Given from datasheet:VCO, U5, Crystek has a lower output than Raltron, 0, 3, 6 dBm (min, typ, max)Attenuator, U1, AT0603T03ECATD, -3 dB, assume constantLNA, U3, BGU8052, gain, 17, 18.5, 20, dBLNA, U3, BGU8052, output power at 1dB gain compression PL(1dB), 18 dBm typCalculate:LNA, U3, BGU8052, output power, 14, 18.5, 23, dBmConcern:The LNA may operate in the nonlinear area as output power > PL(1dB), it may generate additional harmonics, noise or ....

author: sckzor
author_rank: Contributor
timestamp: January 25, 2023, 11:36:24 pm
content: I took your advice radar_macgyver and added a calibrated co-planar wave guide for the RF, and I swept all of the trace corners and removed the solder mask. I tried to make the CPW larger to match the size of the component pads but according to the calculator I used, the JLC stack up has pre-preg that is too thin to make the trace much larger than it is right now. I think I will use JLC again as I did all of the calculations and setup based on their stack up and they still seem to have the best price.I don't really want to change my SMD component sizes. I already have everything in 0805 size and I don't want to order new parts unless you think this will present a big issue later. Additionally, I am not the best at soldering and I start to struggle soldering anything smaller than 0805s with an iron. If I need to do any rework it is nice to be able to use an iron. If you think this is a big problem I will bite the bullet and change it but like I said, I don't want to.Finally, I made the thermal reliefs smaller to make the soldering easier.I attached more images of the new layout.Thank you for pointing out that discontinuity buta, I think I will just accept that it could be an issue and carry on unless it will completely break everything.ThankssckzorQuoteLast time I built an h-bridge one of the transistors I was using got overloaded and blew up in my face lol

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: February 07, 2023, 04:16:47 pm
content: @buta: the calculations don't include the LO to IF isolation of the mixer. The mixer, being double-balanced, will have some isolation between LO and IF ports. This will be degraded somewhat by coupling between the traces, but will still be higher than 0 dB.@sckzor: Sorry if this is coming in a bit late, but your layout looks much better with good coupling between ground planes. Consider rotating U6 so that the LO port (pin 2) faces up towards the board's TX port. This will eliminate a couple of bends from the RF traces. I would say that if you had to pick one part only for size reduction, it would be the RF coupling capacitors. You could easily make these 0402 or even 0603, with enough room around the parts, these aren't too hard to hand solder or rework. You could also drop the value to 1 nF or lower and make sure they are C0G dielectric, to minimize losses. On the layout surrounding U3 and U4 (the RF amps), you have a short stub on the RF traces connecting the amplifier outputs to the biasing inductor. You could make these "inline" to avoid the stub. Check out NXP'sAN11902to see how they designed a board for this part.AN11416also has more detail on the completed board, look closely at how the inductor is placed relative to the part. Also note the BOM shows the exact parts used.

author: buta
author_rank: Contributor
timestamp: February 08, 2023, 12:14:25 pm
content: Quote from: radar_macgyver on February 07, 2023, 04:16:47 pmThe calculation considers the TX path not the RX path, it shows the LNA U3 (use as a TX amplifier) operates close or above to its 1dB compression point in typ and max conditions. The TX signal will be distorted and will contain additional harmonics, noise or ...Usually amplifiers should operate few dB's below the 1dB compression point.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: February 09, 2023, 03:26:15 pm
content: Oops - missed that you were only looking at TX. The OP could replace U1 with a different part to bring down the gain. Then again, it's FMCW, so the amplifier can operate into saturation without sacrificing performance other than generating more harmonics.

author: sckzor
author_rank: Contributor
timestamp: February 14, 2023, 12:35:35 pm
content: You are not too late @radar_macgyver. I am still dealing with a tremendous amount of work and have stalled continued design unfortunately. If you think smaller coupling caps and inductors would be better I can probably spare the funds to replace them, and while I am at it I can order the larger attenuator. Later this week, probably Saturday or Sunday I will get around to making further revisions the PCB and and parts list and post about them.At the risk of redundancy, thank you again @buta and @radar_macgyver this would not be possible without your help.