I designed an optically isolated oscilloscope probe - Page 1
Forum section: electronics / projects

author: electron_plumber
author_rank: Regular Contributor
timestamp: August 09, 2025, 10:17:17 pm
content: Finally, I can measure those elusive high-side gate drive waveforms! I've always wanted an optically isolated oscilloscope probe but they cost a fortune. E.g., the Lecroy HVF108 gives you 150MHz of bandwidth and retails for >$7k, last I checked. So, I decided to try to design my own from first principles with no previous optics experience. I learned a lot. And there's lots to learn still. But I'm pretty amused that it works as well as it does and wanted to share the good news.And for comparison, the same HS gate drive measured using the expedient but atrociousprobe mathmethod:I was aiming for 500MHz, but there's a resonance in my laser control circuit that I need to sort out. So, for the meantime, I've reduced bandwidth a bit and settled for ~160MHz (-3dB), which is still quite usable.For the probe head, I'm reusing a design I previously describedhere. It's similar to James' design, although I think he did a slightly better job at handling the composite amplifier crossover than I did.In addition to this ^, I designed a PCBA to handle laser modulation and a PCBA to handle demodulation (not shown). I haven't decided whether or not I plan to productize this, so I'm holding off on posting the electro-optics for now but I thought I'd share for general inspiration for my fellow tinkerers.Cheers!

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2025, 10:46:32 pm
content: Quote from: electron_plumber on August 09, 2025, 10:17:17 pmWould you give some idea of the various stages? Is the laser PWM or FM modulated?I am not even sure how I would approach this. For lower bandwidths transformer and optical isolation can be combined. At higher bandwidths, I would survey the various RF to fiber laser converters which seem to operate without digital conversion.

author: Psi
author_rank: Super Contributor
timestamp: August 09, 2025, 10:55:22 pm
content: Nice work.A 100:1 version with 300V input would be useful.+/- 30V might not be enough for some 12-21V high side drivers switching inductive loads with 40/60V fets. Especially badly working ones that need probing to fix them12V is obviously common for SLA batteries and 19-21V for laptop power.If it had protection and would clip instead of blow up then +/-30V might be enough for 12V SLA stuff at least.If I could build a 100:1 version with USD$200 of parts I would do it.

author: moffy
author_rank: Super Contributor
timestamp: August 09, 2025, 10:58:08 pm
content: Congrats, looks very nice, would certainly be interested if you published the details.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 10, 2025, 12:14:08 am
content: Interesting, does the probe shown only act as the optical transmit side, and there's a separate receiver somewhere else?I actually would be happy with an input range of only +/-10V, especially if that helped keep the input referred noise low.

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: August 10, 2025, 03:14:05 am
content: Quote from: mtwieg on August 10, 2025, 12:14:08 amFrom the picture it looks like it outputs electricity. It probably uses a chip that contains both the transmitter and receiver.Is it necessary to use an optically isolated probe to measure something this slow? From the screenshot, it looks like the math method only worked poorly because someone either forgot to configure the skew setting, forgot to configure the bandwidth setting to one much slower than the probes, or used ground springs that were too long.Don't get me wrong. Such a device is good to have. Especially when there is a lot of common mode voltage. I am just not sure if someone who needs to measure a high side driver that sits on top of only about 31.5 V will want to buy or make one.

author: electron_plumber
author_rank: Regular Contributor
timestamp: August 10, 2025, 04:38:56 am
content: Quote from: mtwieg on August 10, 2025, 12:14:08 amFor Rev 1, I kept it very modular. So there are actually 3 PCBAs. In addition to the probe head above, there is a laser modulator / fiber optic cable / receiver that which are not shown. In rev 2, I’ll combine the laser into the probe head so there will be two PCBAs.Quote from: 44kgk1lkf6u on August 10, 2025, 03:14:05 amIf only it were that simple. See previous response.Quote from: 44kgk1lkf6u on August 10, 2025, 03:14:05 amIt's just an arbitrary demo. I agree that by throwing away some of the probe bandwidth and being very careful with skew and ground placement you could achieve abetterresult with two SE probes than I showed above. But unless they're very low capacitance (active?) probes, the result won't be accurate. Even with careful deskew and ground clip placement, the optical probe injects an order of magnitude less capacitance than two passive SE probes. It also allows you to utilize your oscilloscopes full dynamic range. It's just better in every way.

author: electron_plumber
author_rank: Regular Contributor
timestamp: August 10, 2025, 09:01:07 pm
content: One more fun test. Here's the same high-side gate loop with and without Rgate shorted. When shorted, a 244 MHz is excited on turn-on. Probe input impedance should be decently high at that frequency, so I suspect this result to be fairly accurate. To test, I made a little near-field probe (I guessed at the loop diameter) and measured emissions. Seems to corroborate the ~244MHz energy content. Neat."If you can’t measure something in two different ways, you can’t be sure you’re measuring it at all."