Electronics - Measuring impedance with a signal generator and oscilloscope

Note: This experiment produced flawed results. Fortunately it was looked in to deeper by WB0SMX who published his results here:
www.wb0smx.net/?p=2215
I welcome peer review and am grateful to him for his work on this.
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[@amirb715]

as Carmine noticed, the measurement is completely wrong when the impedance is complex (when it has some reactive element) simply because V1 and V2 will have a phase difference as well as amplitude difference. you cannot just use the amplitude of V1 and V2 in that formula. The formula is correct but V1 and V2 must be phasor (amplitude and phase together). The result for pure resistor was also off not because the scope was cheap, because those long wires at 10MHz add enough inductance to create phase difference between V1 and V2

[@carllattimore8227]

Amir B: More video, less comments.

[@ferrabras]

I was about to write a comment about it. You need to also measure the phase between current and voltage. Actually, the precision resistor shall be characterized too. You can use two "identical" ones. 10MHz in the air has a wavelength of 33cm. So 8cm is enough to go from a valey to a peak. So everything shall be contained within a very small area so that the propagation of voltage and current waves doesn´t need to be taken in to consideration. If you measure the voltage at reference resistor over DUT, the impedance comes out of the ratio between the two waves, because Vdut/Vref=Zdut/Zref. (Same current passing through both, if wave length is greater than 1m, for instance).

[@researchandbuild1751]

It must have been above the self resonant frequency of the inductor at 10mhz and thats why the impedance actually decreased at higher frequency (the self capacitance of the inductor came into effect). Something to watch for on large inductors like that

[@MrTepidFusion]

This method is the core of Lab 2 from the "Learning the Art of Electronics" textbook. I've been banging my head against it for hours, using phase shifts and trig and blah blah blah, mostly because the prompts are a bit misleading. This is so much simpler. You've sent me on the path of righteousness. A+.

[@microsoft2k]

You could replace Rref with a pot. Adjust the pot to until Va2 was 1/2 of Va1 then the measure the resistance of the pot.

[@ElectroRestore]

To get a more accurate reading, substitute the scope with a DVM (or DMM) and then multiply the voltage reading (which on a DVM is RMS voltage) times 2.8284 to get the peek to peek voltage with a DVM or DMM.

[@rjinnh3933]

Hi Kevin Another great vid. I now know another way of measuring Inductance besides using an L/C Meter. Haha By including your referenced 'Peer Review', we're now all armed with enough info to qualify our test results and maybe try to re-remember some basic electronics that almost all of us once knew like the back of our hands. Thanks for this vid and the Peer's Review.

[@fantuznet]

very educational and "visual". well, this is how we learn to build our own equipments, by also using them !! having antenna and LCR analyzers can simplify the job, but we need to have good basis on the theory behind. this one video is an example of theory explained also through practice. nice if next time you could add capacitors, demonstrate the inverse formulas, resonance etc. well done !

[@cosmicazur]

Very interesting. I see this being very useful in RC drone society, measuring BLDC motors. I suppose the rare earth magnets will cause a big issue reading inductance of windings. Measuring inductance and impedance of a motor with a signal generator and an oscilloscope will be pretty good. If we figure out how to do it right.

[@unklewink]

I do something a bit different. I adjust the frequency until the device under test reads 1/2 the total applied voltage. At that point, the resistance (reactance) of the reference and DUT should be equal.

[@hongbinbin24]

Very helpful to me. Much appreciated!

[@joshbarnard3003]

Thank you so much that was exactly what I needed.

[@robertpendergast2620]

Old fashion was is both more DIY and reminds one of history and the math. I like that better than button generated readings.

[@friedmule5403]

Really great video, I was thinking of an other methode. First I am a totally noob so it is properly wrong but:-) What if you connected your generator directly to the DUT and right after that a decade-resistance box. and treat it as a voltage divider, when you reach exactly ½ of your output, you can read the impedance on your decade box. Would that work?

[@steveo4233]

Good video on the theory and some practical applications. But, is your Rref a wirewound resistor? It doesn't like a carbon-comp. That could explain the errors you are seeing.

[@martinalber5054]

very simple and interesting! can you do another example with a complex DUT impedance ? (R+jX)

[@geeess4685]

Might be old fashioned, but a great way to learn and understand.

[@shahzadnasir1052]

hey, according to reactance formula XL=wxL reactance should increase with frequency, where as your experiment is showing the opposite, why? and also i wanted to know your input voltage for the circuit, you have constantly talked about the frequency but i dnt think i could find you saying anything about the input voltage

[@kaskilelr3]

Hi great video, I have a question. I am using this method to measure the impedance of a piezo (i.e. a speaker), which is frequency dependent. However, I find that at some frequencies I get V2 > V1 (and greater than my source voltage). How is this possible and am I right to conclude I have a negative impedance (what does that even mean?). Thanks!

[@loughkb]

Hitting near resonance . Like a kid pumping his legs on a swing. He's not putting in enough energy to push the swing that high alone, he's adding some to it on the downward stroke.

[@cuban9splat]

Excellent demo. I still like the old methods better than the new LOL. 3 DE K7RMJ Frank

[@galileo_rs]

Qalculate for a better calculator ;) Arduino + AD9850 + 2x 1n5711 and you can build an impedance bridge. Also if you have a XY mode you can use Lissajous patterns to do the same.

[@JohnTheBrewer]

Nice demonstration. A question: is the 900 ohm precision resistor carbon or wire wound? If wire wound, it would have a reactive component and throw off your measurements. When you measured the first resistor, I was wondering if the error was a component of unintended reactance from using a wire wound. Anyway, good stuff! Thanks.

[@mehmetcuvelek709]

JohnTheBrewer This resistor has to be non-inductive.Ayrton-perry type wound resistors fit perfectly

[@DonDegidio]

Hi Kevin, Nice demonstration. 73 WB3BJU

[@jamescarrington9700]

Interesting result and thanks for the video. Btw I was surprised by the inductor impedance reading higher at 5 Mhz than at 10 mHz. An air core inductor should have the opposite impedance change with frequency. Perhaps the ferrite or iron core of your inductor is less effective at higher frequency ?

[@loughkb]

That core was out of a power supply, so who knows what the properties are. :-)

[@jamescarrington9700]

So it was likely optimized for use at lower frequencies. I think your measurements are right and they show the importance of measuring a component at the frequency you will use it at. Had someone measured that coil with an LCR meter say at 100KHz and then tried to use it at say in the hf range they would likely be scratching their head .

[@BEdmonson85]

Hi Kevin! Having watched most of your videos and building your 'DuinoVOX project a while back, I wander, why not build an inductance meter from an Arduino? I built one a long time ago using an op amp a couple reference capacitors and some other various discrete components. Just search Arduino inductance meter on Google. It's a very simple build. Ultimately if I remember correctly, the inductor is placed in parallel with the capacitors, a pulse is delivered, the circuit rings at the resonant freq, the op amp is set up as a comparator converts the wave form to a square wave and the Arduino measures the frequency and calculates the inductors value based on the known value of the capacitors. Maybe another video idea? 😊

[@loughkb]

Actually, I already started building something based on this vids idea. I have a Si5381 break out board that was donated to the channel, so I'm going to automate this measurement with an arduino nano. The board has three outputs, so it will have a few functions, including a CW beacon mode and signal generator. Doing the whole design myself, including the software.

[@carminecampo2045]

Your measurements are completely wrong. With inductance ,if you decrease the frequency you will decrease the impedances XL - 2 Pi F L (in Henry's).

[@ksb2112]

Maybe. After doing a tiny bit of research I ran across the following article and learned something I'd never heard before. Each inductor has a self-resonant frequency. Below it impedance increases with frequency, above it impedance decreases with frequency. If the demo is correct the SRF of that inductor is at or below 5MHz. www.everythingrf.com/community/how-does-the-inductance-and-impedance-of-an-inductor-vary-with-frequency

[@margaqrt]

In addition to the resistor and inductor tested, It would have been interesting to see this experiment performed with a known good manufacturer made antenna as a "proof of concept."

[@loughkb]

I'll be re-visiting this in the future when I get a better signal source.

[@margaqrt]

Cool, I'm looking forward to your next video about this test circuit.

[@RadioHamGuy]

Great info, thanks again. 73

[@wire7]

Why not using rms voltage? Why using Vpp

[@loughkb]

The scope measures peak to peak, I multiply it by 0.3535 to convert to rms for calculating power in another video.

[@naughtykidlavya1680]

@@loughkb Why Vpp? It should be Vpp/2.

[@MrWasim100100]

nice video

[@cdrive5757]

To start with, ..If you're short on patience don't click on that wb0smx link. I'm old and crotchety, so I fit in that category. OK, ..while the theory behind this video is sound the test setup is not. While working with high frequencies your test fixtures, cabling, connectors, component selection, lead length, component placement, etc...etc, etc, need to conform to *"Industry Accepted Practice"*. For any students that are uncertain about what I'm saying it simply means this. When working with high frequencies your fixtures and test instruments must keep *"Reactance"* to as close to zero as possible! As currently shown, this circuit should not be relied upon for frequencies above the audio spectrum. I welcome any discussion to the contrary. Chris WA2ERQ

[@arijitmajumdar3606]

Thank you...

[@samglaim4274]

Great demo.... maths does have it's uses ;-)

[@robertvandersanden]

Hi Kevin, I'm afraid I have to correct you on the way you used your math. When the unknown impedance Zx is non-resistive, then the voltages Va1 and Va2 are out of phase, and you can't simply subtract their amplitudes, without taking the phase angle into account. If you have a two channel oscilloscope you can measure these two voltages simultaniously and use a math subtract function of the oscilloscope (if it has one) which will give the correct answer. Or else, place the resistor in the ground line, and measure both the total input voltage, the voltage across the resistor and draw some vectors to calculate Zx. Play with a spice simulator like LTSpice which is free, to find a good solution.

[@loughkb]

The setup and math were out of a technical manual, so I trusted the source. As to Va1 and Va2 being out of phase... Wouldn't that require some kind of delay across the reference resistor? I can't imagine there would be enough delay to be measurable across a single resistor.

[@robertvandersanden]

Kevin Loughin The current through the resistor and the unknown impedance is equal. Let's say that Zx is a capacitor, then the whole circuit becomes a low pass filter, with Va1 being the input voltage and Va2 the output voltage. This low pass filter has a phase shift between input and output, because that's a normal behavior for a low pass filter. Let's pick an input voltage of 1V and a frequency where Zx equals Rref equals 1kOhm, but as it is a capacitor, this impedance Zx is 90 degrees out of phase with the resistor. The total impedance is 1k - j1k, the absolute value 1.41k (pythagoras). The current is 1/1.41 = 0.707mA. The voltage across Rref is 0.707V, the voltage across Zx 0.707V as well, but 90degree out of phase. With your math the voltage across Rref would have been 0.293V. Hope this explains it a bit...

[@loughkb]

It's a compelling argument for sure. I'm going to re-run the experiment today with a capacitor for Zx and both inputs of the scope and look for the phase shift.

[@tim46767]

Robert why are you talking about current? you confound power with voltage. Voltage is voltage times nothing. Power is voltage times current in this case (AC) at a specific point (the same phase) But in this experiment we neither measure current nor power. We only compare two voltages regardless of the phase. Of course the waves are out of phase. But if you measure the voltage PEAK to PEAK, you "adjust" automatically the right phase. Again we are NOT measuring the product of two out of phase signals! The way Kevin do the math is 100% right. My only complain is that we need neither "precision" resistor nor "precision" oscilloscope. We can use a voltmeter too

[@robertvandersanden]

tim46767 Hi Tim, thanks for your reply. I mentioned current only because it is the same through Rref and Zx. I don't use it to calculate power because like you wrote, power is irrelevant. But when subtracting two complex (vectors in X-Y plane) voltages like Kevin is doing, you have to take the phase between these voltages into account, otherwise you get a wrong answer. Subtracting two signals with a phase angle in between is like calculating the length of the third side of a triangle where the other two sides are the two voltages being subtracted. That is done with trigoniometry, not simply by a straight subtraction of amplitude or peak to peak values. Using a voltmeter for this measurement is only possible when the voltmeter is capable of accurately measuring a voltage at a couple of MHz. My $150 multimeter for sure can't do that. The voltmeter's AC bandwidth is not that wide.

[@rdpecken]

Hi Kevin. As James Carrington noticed, the results were odd in that the impedance varied inversely with the test frequency. That bothered me, so I duplicated your procedure on my bench. While I never did see an inverse relationship of impedance to frequency, I did notice that the readings varied a lot from the expected when I got outside of the normal working range of the coils I tested. So it would be good to have some idea of reasonable parameters before making any impedance measurements. You can find my test procedure and results at my blog entry: www.wb0smx.net/?p=2215. Thanks for the video, I always enjoy them... Randy WB0SMX

[@loughkb]

Yes. I suspect that a clean sinusoidal signal source would help too. My signal was coming from a clock line on a digital chip, so it was a square wave, bent into something like a triangle wave. Very harmonic rich. I'll re-visit this method in the future whenever I finally obtain a real signal generator.

[@loughkb]

Just finished reading through your blog entry. Very extensive. And I agree that I was probably so far out of the comfort zone for that toroid I used that it probably caused the strange result. That torroid was a poor choice for the demo, it came out of an old power supply, so who knows what it's properties are. So hey, I learned something else here, thanks for the peer-review work!

[@jdmeaux]

Nice demo and great information. Thanks Kevin. 73 Moe k2jdm