Simplest DIY Inductance measurement circuit for Oscilloscope? - Page 1
Forum section: electronics / beginners

author: Thomas Ballantine
author_rank: Contributor
timestamp: July 08, 2025, 03:38:39 pm
content: I'm about to start an attempt to build the Soldersmoke direct conversionreceiverand some other ham projects, and want to make sure I have the means to measure the inductors involved, as I'm going to be winding some of them. I have a Hameg HM203-7, which has a component tester integrated, but which is only useful down to 0.1uF according to the manual (660pF just about registers), and I haven't been able to wind a coil big enough to cause it to show on the tester yet. Some of this might be due to needing to have coils with a high enough impedance to register (a coil made up of one of the cores of an ethernet cable wrapped around a pencil might not suffice), but I don't think the Hameg's internal component tester will be very helpful for ascertaining inductances for this project.I've looked into the various cheap LCR meters from Bezos barn, but they don't seem to go low enough either, and handheld DMMs generally don't have capability of measuring inductance (I have need of a new DMM since my no name one died, planning to getthis one).So I've drawn the conclusion that I ought to try to come up with some means of measuring the inductance myself. I know that there'll be some sort of signal generation involved (likely of higher frequency than the 60Hz of the component tester), and it will be either measuring phase difference as in thisas in thisguide, or by using a reference capacitor to build an LC resonant measurement system, as recommended hererecommended here(which needs a variable frequency signal generator).I was wondering, which technique is superior, and what would be a good way to get the capability in place for low cost? What I have currently is the Hameg, an ATX PSU, PC and a bunch of arduinos, I'm going to get the aforementioned DMM when I do my digikey order for the parts for the project.Hameg manual:https://www.sm5cbw.se/hameg/hmosc/hm203-7-man.pdf

author: RoGeorge
author_rank: Super Contributor
timestamp: July 08, 2025, 05:11:47 pm
content: For radio builds, you may want to build (or buy) a "grid-dip-meter", sometimes called "dipmeter" or alike:https://en.wikipedia.org/wiki/Grid_dip_oscillatorIt allows to either measure L, C, LC, sniff or inject signal in a radio, a very simple yet pretty useful ham tool. You'll find plenty of schematics online, particularly if you search in ham radio places:https://duckduckgo.com/?t=ffab&q=dip+meter+site%3Aqsl.net&ia=webIt is not clear if you already have a generator or not. If yes, you can build a grid-dipadapterfor it:https://www.eevblog.com/forum/beginners/measuring-the-self-resonant-frequency-of-an-inductor/msg5113206/#msg5113206or you can improvise your own measuring method by frequency sweep while observing the output with an oscilloscope.The dip-meter is a tool of the past. If you want a nowadays tool, buy a nanoVNA. Can do a lot of many other things, including measuring S11 and S21 to tune antennas, measure small C or L, can be connected to a PC, etc. NanoVNA is an open project, there are a few versions already, each with a ton of clones. Don't know which one to recommend, but there are some topics in this forum discussing which nanoVNA to buy:https://duckduckgo.com/?t=ffab&q=which+nanoVNA+buy+site%3Aeevblog.com&ia=web

author: shabaz
author_rank: Frequent Contributor
timestamp: July 08, 2025, 05:17:56 pm
content: Hi,I agree with all of the above from@RoGeorge.Technically it is possible to measure down to at least a few uH using an oscilloscope and with not much else (well, a way of switching current through the inductor) however it's not the best tool or method for your use-case. This isn't really a recommended approach for your actual end goal I think.Besides, you'll be winding on toroids for your project, so you cannot easily tune the inductance anyway (short of pinching or stretching the wires a little bit).Instead, I'd recommend the following approach for your particular project (but the mentioned nanoVNA is the way to go if you're keen to actually measure these things)(1) Use the specified ferrites and iron cores, i.e. buy from a known good source. Don't just assume that any toroid of similar size will be OK.(2) Wind the specified number of turns, and fill up around 70-80% of the toroid for the main winding, i.e. don't squish it all tightly. If you're having trouble counting the precise turns, sometimes what I do is to take a photo with mobile, and then zoom in and count the turns.(3) For the band-pass filter, consider attaching a trimmer capacitor in parallel to the capacitors there, so you can trim them instead of the inductor, i.e. trim the filter portion of the system as a module, rather than measuring at a component level.(4) For the oscillator, be absolutely certain you're using the correct bolt (solid brass throughout, not just a brass-colored plating) and use the 'scope for confirming the output frequency range, rather than the inductance range, and then either add/remove one winding, or tweak the capacitors across the inductor, again using small trimmer caps, or by paralleling additional small fixed capacitances if necessary.(5) For the mixer, you don't need to tune anything. Provided you have the correct amount of turns, all will be OK.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 09, 2025, 01:55:58 am
content: With an external function generator, a breadboard and a handful of resistors you can expand the "component tester" range of your oscilloscope. If you use it often, making an adapter / measurement jig for it may be worthwhile.Another approach I quite like is the very common LC meter based around an PIC uC. I mostly rewrote and extended the software for use on an AVR, and there are many other ports too. The basic circuit is a (collpits?) oscillator based around an (most often) LM311, and then the microcontroller measures the oscillation frequency and calculates the DUT.https://duckduckgo.com/?t=h_&q=pic+lc+meter&ia=images&iax=imagesI never understood the "many digits accuracy" of expensive LCR meters. Induction of a coil can vary significantly, depending on the current it carries, parasitic components when it's used in an application introduce more inaccuracies. It's good to measure inductance to a few percent or so, but I've never been in a situation where being more accurate would have been beneficial.If you want to buy commercial meter, the DER-EE 5000 is a quite affordable handheld meter, and it has a pretty good performance to price ratio. But don't go for the cheapest, make sure you buy it together with the accessories you're interested in.

author: Thomas Ballantine
author_rank: Contributor
timestamp: July 09, 2025, 09:06:41 am
content: Thank you for all the helpful answers, I don't yet have a function/signal generator, I think that's probably something to get, either a new Chinesium one, or used $NAME_BRAND, but LC measurements seem a little less urgent now.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2025, 09:37:13 am
content: Just so you know, one can plenty enjoy building radios with nothing but a DMM.Most often there is no need to measure the exact value of the coils. Follow the number of turns, dimensions, type of wire, and the type of core material given in the design, and that's it. The exact value doesn't matter. The coil has to be in the ballpark of what the design says, then the fine tune is made in circuit, while aligning the radio.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 09, 2025, 11:32:46 am
content: Quote from: Thomas Ballantine on July 08, 2025, 03:38:39 pmAs a child (in 70s) I have build analog frequency meter.To use it to measure L I have found somewhere the LC generator schematic that it was told to generate practically with any LC set.What I remember of it - 2 transistors (those time germanium), practically with no other elements were used to generate and one as 'emitter follower' to not load the generating transistors by connecting meter.I think in first post here is the generator itself (get output from emitters):https://www.elektroda.pl/rtvforum/topic3198297.htmlFew years ago in one of our devices I have used this generator supplied from 3V3 (2x BC857 + 4k7).

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2025, 12:17:20 pm
content: That's a fun one, also known as the Peltz Oscillator. Same oscillator drawn differently:Apart from being very simple, it has the rare property of working even at very low voltage, at less than 1V!https://www.analog.com/en/resources/analog-dialogue/studentzone/studentzone-june-2025.htmlhttps://www.eevblog.com/forum/projects/simple-sinusoidal-oscillators/Later edit:https://www.eevblog.com/forum/projects/injection-locked-peltz-oscillator-with-bode-analysis/<--- with mistakeshttps://wiki.analog.com/university/courses/electronics/comms-lab-peltz-osc<--- same content from 2019, without the short circuit in the Q2 collector

author: PGPG
author_rank: Frequent Contributor
timestamp: July 09, 2025, 01:22:56 pm
content: Quote from: RoGeorge on July 09, 2025, 12:17:20 pmThis link is interesting - 3 schematics and no one will work

author: mawyatt
author_rank: Super Contributor
timestamp: July 09, 2025, 01:31:24 pm
content: If one has access to a DSO with Bode Function, impedance and admittance measurements are possible. Inductance or Capacitance can be derived from the plots.https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4341490/#msg4341490https://www.eevblog.com/forum/testgear/admittance-measurements-with-dso-awg-with-bode-function/Best

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2025, 01:40:27 pm
content: Quote from: PGPG on July 09, 2025, 01:22:56 pmGood catch, thank you!That's strange, because the small schematic in the upper right of my LTspice print screen schematic was captured from the analog.com website, and it has no dots at all. I guess Analog Devices added the dots recently, and added them wrong.Who would have thought they edited an old webpage from years ago, though, my bad for not double checking the content, sorry.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2025, 01:53:01 pm
content: Doh, there are 2 webpages of the same lab activity.One from 2019, then wrongly copied again in 2025:https://wiki.analog.com/university/courses/electronics/comms-lab-peltz-oschttps://www.analog.com/en/resources/analog-dialogue/studentzone/studentzone-june-2025.html<- copied with mistakes

author: PGPG
author_rank: Frequent Contributor
timestamp: July 09, 2025, 02:46:08 pm
content: Quote from: RoGeorge on July 09, 2025, 01:53:01 pmAt the end of this wrong version there is the information that author is the software engineer. Should we consider this as an excuse?It seems not possible to not notice the bug as soon as you take a look at it. First what you see is shorting collector with base, what is not widely used practice. Then you see that LC are shorted - also not typical. I believe no one at analog even looked at it.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2025, 04:07:59 pm
content: B.Eng. in electronics, then M.Sc. in software, so not a complete stranger with electronic, but I guess he inherited all these lab activity webpages from somebody else. If one suddenly has to port/update/manage a big collection of lessons written by somebody else, usually there is no time to pay attention to the content, just mass porting plus a face lift for the new website. Such mistakes as the one you just spotted can easily happen.(If you click repeatedly "Previously approved version" in thehttps://wiki.analog.com/university/courses/electronics/comms-lab-peltz-osceventually the first version of the lesson was written by somebody else.)

author: PGPG
author_rank: Frequent Contributor
timestamp: July 09, 2025, 06:02:58 pm
content: Quote from: RoGeorge on July 09, 2025, 04:07:59 pmThese days, quantity of content is more important than quality. And this is true everywhere, even in datasheets

author: radiolistener
author_rank: Super Contributor
timestamp: July 12, 2025, 01:00:21 am
content: Do you have camera with flash?It seems RoGeorge found nice solution for youhttps://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/

author: RoGeorge
author_rank: Super Contributor
timestamp: July 12, 2025, 03:54:02 am
content: I'm afraid that will work only for coils with many, many turns (to induce a big enough voltage), and it requires an oscilloscope that can memorize a single trace. The OP's oscilloscope is an analog Hameghttps://www.sm5cbw.se/hameg/hmosc/hm203-7-man.pdf

author: RFDx
author_rank: Regular Contributor
timestamp: July 13, 2025, 07:38:19 am
content: Quote from: Thomas Ballantine on July 08, 2025, 03:38:39 pmhttps://www.anengmultimeterstore.com/product-aneng-an870-multimeterCurrently less than 18€ on AliExpress from the official Aneng store.Quote from: Thomas Ballantine on July 08, 2025, 03:38:39 pmIn any case, you would need a signal generator, which you don't seem to have. You could get by with just a DC mutimeter instead of a scope if you add diode detectors to the I-V test method.A PC soundcard can also be used to build a simple but quite accurate LCR-meter. All that is needed is a modest external circuit made out of 3 op amps connected as buffers, some reference resistors and a means to supply the op amps. There is free software available that does all the (audio) signal generation, measurements and necessary calculations for you.

author: Thomas Ballantine
author_rank: Contributor
timestamp: July 15, 2025, 10:12:49 am
content: I don't currently have a proper signal generator, but I think I'm going to experiment a little using the IO of my microcontroller boards. Probably not going to get great results, but it'll be interesting.Did a bit of looking for the sound card version, foundthis onewhich looks fairly easy to get working, so I think I'll be looking to build it eventually