Using NanoVNA To Measure Inductor Q

How good are those inductors you just wound? What about those inductors you just bought from your favorite online store? Don’t know? Well knowing the Q of the part is going to help you know whether they are going to be useful in that bandpass filter or oscillator and it could not be simpler if you have a nanoVNA.

So what I have here is a T68-6 toriod with 19 turns of enamel wire on it. 19 turns gives a theoretical inductance of 1.7uH, the actual measured inductance with my cheap LCR meter is 1.747uh yeah that is surprisingly close and not something that I typically find that happens. Usually 1 or 2 turns either way is required to get to the value I want.

For this measurement you want the inductor to be in series. So you can see from the picture above my test jig the inductor goes from the center conductor to the shield of the coax.

So the most important part is how to make the measurement. First is to re-calibrate the nanoVNA for the frequency range of interest for the part. For me, Captain HF, 3 to 30Mhz is where I keep a saved calibration for. Next, the measurement is an S11 1 port measurement and the plot you require is R+jX.

So from here the math is rather simple, Q = X / Rseries for the frequency of interest. So for the above at 14Mhz, for simplicity sake lets call it Q = 200 / 1 || Q = 200. In reality the Rs is less than 1 and the X is less than 200, but thankfully NanoVnaSaver does the actual calculations for you and displays them in this case the actual Q was 150. Which is pretty damn good for a hand wound toriod inductor. Now the good thing is, you know how to measure Q and you can start checking all sorts of parts you might have sitting around.

Take these cheap ebay variable inductors as an example. I built a filter with them for 40m and it was crap. Q at 7Mhz was about 20. But at 14Mhz though to 150Mhz the Q was about 50 to 75 on average. So they would be useful to use at higher frequencies and certainly not useful at lower ones. Unless you have a datasheet that tells you what the Q is for a given frequency range is, you will never know unless you measure them.

And lets face it, measurement is king. Even a non perfect measurement with a non perfect instument like the nanVNA is vastly superior to having no measurement at all. Oh and one last nugget, now that you know the Q of your actual parts, you can then use that value in Eslie when designing filters. Being able to measure things can really improve everything you do.

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Op Amp Gain Bandwidth NanoVNA

I have been thinking for a while about using high speed op amps as IF amps in a receiver. As mental as that might sound to some, it actually makes practical sense in someways. Gain is easy to set, impedance is easy to set, being that the IF is at a fixed frequency you are not worried about being broad banded and can tailor the circuit to suit by using a suitable op amp with sufficient bandwidth to do the job.

And that is where the problem lies, op amp gain bandwidth is given at unity gain, IE a gain of 1 and as soon as you start adding gain, you start losing bandwidth. This means that you need a unity bandwidth of Gain x MHz to be somewhere close and then you also need a op amp with a fast enough slew rate to deliver the waveform amplitude you desire.

Now there are what are called current controlled op amps that give much better gain bandwidths above unity, but they are kind of expensive and so that leaves using voltage controlled op amps and working around all its limitations, but as you start to get up there in unity gains above 300 MHz even they start becoming non cheap items also.

So a few weeks ago I was on one of the Chinese parts sellers just looking at all the different crap they have and for some reason I ended up in the op amp section and found an op amp with a few hundred MHz unity gain bandwidth for pretty cheap. And by cheap, i am talking in the 40c each kind of space. So i bought a few to try out.

So i built up the non inverting circuit as shown above. Which is quite simple to set the gain and the impedance’s just by changing the value of a few resistors.

This is the circuit built on the test board. While the op amp is an SMD part, its SOIC 8 so its big enough that even a dummy with coke bottle glasses could hand solder, but I am kind of slack in that regard so I used paste and hot air, i mean why not. LOL

You can see from the S21 gain plot that there is usable gain from 40m to 10m. I am not sure what that notch is, but i suspect that its an artifact from the nanovna, because a manual sweep of that section of spectrum using a function generator and oscilloscope did not show that dip.

Oh I should say that I have the gain set to 6x for this test. And slew rate was not an issue for 7MHz to 30MHz, with the op amp able to deliver 1.3v peak to peak quite happily. Below that, particularly around 80m, the op amp could not deliver much more than 500mV peak to peak. So for a 9MHz or 12MHz IF amplifier, the op amp might be a credible option.

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Measuring Gain Bandwidth With NanoVNA

The NanoVNA is more than just a fancy SWR meter for checking your antenna. Its much more and a very useful tool for the home brewer building amps and filters and the like. Now i have been buying things like crazy for various projects that I would like to build in the future. Often these parts are spec’ed for bands not of interest to me. So what do you do? Well you measure them at the frequency of interest and see how they work yourself.

Below is a MMIC amplifier part that I found for really cheap. By cheap I am talking in the 40 cents per range, so i bought 100 of them. I mean why not, they are spec’ed for 100mhz to 3gig with +30db of gain. Worst case senario they are kind of useless at HF and I will have parts for when I actually want to build things at VHF and up.

So anyway I had a test board built with 4 different circuits on it for testing out various parts I have here and it includes Op Amp, Mosfet and BJT amp circuits. So i decided to start with the MMIC and see what it can do.

The good thing about MMIC gain blocks is the fact that they have such a low parts count. 2 blocking caps an inductor, bypass cap and gain setting resistor. Initially i set the bias resistor a little to high and was getting a lot of distortion, so I halved the value and boom it was providing 24dB of gain at 7MHz, which is my go to frequency for all these sorts of tests. Next though, I wanted to see how the gain bandwidth was. My bandwidth of interest is HF so 3 to 30MHz, so just for shits and giggles I measures it out to the 6m band.

The test setup was Port 1 of the NanoVNA to the input of the test board, the output of the test board to the RF Sampler i made the other day, which was also connected to a dummyload and then back to Port 2 of the VNA. Then an sweep of was performed and the S21 Gain was measured.

Things actually looked quite nice and rather flat. A few dB down at 80m and 1 dB down at 6m. That is pretty good for a part with a minimum frequency of 100MHz. So all in all, this part is a winner and something I can use in a project sometime soon.

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