More Math for the "SETUP"

Just a small correction, 55uh inductor.-

Thanks for all that, I'll print it out for further study. I need to review my numbers but at first look I followed it up to division of the Imaginary numbers, but I just have to do it to see how that works. The only thing I would want different is I start knowing the measurement at the sensing resistor. I want to work backwards to get ZL.

I have checked and swapped the probe/channels, the eye can see a difference when overlaid on each other.

Have you checked both

I have not checked against a known voltage, this has it's own problem, as you switch attenuation, I'd need several known voltage sources. But where I checked it, the channels read the same.

I use peak and also center the skirts around a centerline, ie. adjust vert position so two division down from peak the the waveform crosses equal distant from a center line. But, I admit this is an easy area for error to show.

No, I haven't. In fact while back I saw a method to check 3db points on the Q meter to test accuracy of the unit.

Don't trust 50-year-old standard inductors, But it's so pretty! I wonder if there is any helium left inside. :-)

Thanks, Mikek

Ya, where I used to work we used 7 element Chebychev low pass filters to make sure our harmonics were way down when doing these measurements.

Mikek

I by happy to give you all the information, might be a little tough characterizing 660/46 litz wire.

Ben Tongue was one of the founders of Blonder Tongue

He states that FEMM doesn't accommodate

I don't know, that paper was written in 2006, maybe it didn't then. Oh, it accomidates litz? :-) Ok 660/46 close wound. I'll send you an email and you can tell me what details you need. I think the rod is # 61, but I don't know for sure. According to Ben, some rods of the same material have much lower losses than others, even out of the same batch.

Thanks, Mikek

Typo, sorry, the math is still correct.

Division of complex numbers:

(a+jb)/(c+jd)

Multiply top and bottom by the conjugate of the denominator:

(a+jb)/(c-jd)/(c+jd)(c-jd) = (a+jb)(c-jd)/(c^2 -jcd +jcd -j^2d^2)

= (a+jb)(c-jd)/ (c^2 -j^2d^2)

j^2=-1, so we have (a+jb)(c-jd)/(c^2 + d^2)

Or (ac -jad +jbc -j^2bd) / (c^2 + d^2) = (ac +bd +j(bc - ad)) / (c^2 + d^2)

= ((ac +bd) / (c^2 +d^2)) +j((bc -ad) / c^2 + d^2))

We now have a real fraction, and an imaginary fraction, with only real numbers in the denominators.

E.&O.E.

That's left as an exercise :-)

What surprises me is that you didn't ask how to do series-parallel, and parallel-series impedance conversions. You now have the information to work it out, see if you can.

If you're going to do much of this sort of thing, I'd advise getting a calculator that handles complex numbers, it's so much easier.

Aha!

Helium diffuses through just about anything.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Oops, the eye "can't" see a difference when overlaid on each other.

I have a new set of probes, I did have a set that measure with a 6% error between the two.

Ya, that's I mentioned it.

Yes it does, and always has, AFAIK.

There are no pre-written litz models, you have to make your own, the "modify material" dialog has the necessary functionality. I guess the guy was in too much of a hurry to get started, rather than play around with FEMM, and find things out. I always play around with new software, to find out what it will, and wont, do. Found the litz within about half an hour, when I started.

Ok 660/46 close wound. I'll send you an email

What I need is winding length, thickness, and diameter. I'm not going to draw it a turn at a time, that's unnecessary and silly.

What diameter and length is the rod?

I usually make a properly dimensioned drawing with a CAD utility, then import it as a DXF into FEMM. FEMM's drafting is horribly unintuitive, but it imports DXFs properly.

I've got Fair-Rite 61 magnetic properties, from their site. No PDFs, just a web page. Silly people. I had to print it to postscript, and make a PDF from that. Engineers like to refer to stuff offline.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

I don't know about to much of a hurry, he wrote 29 original articles about crystal radio :-) Some pretty heavily on experimental data.

I always play around with new software, to find

It may be Tuesday before I can get you any info. I'm out of town. Mikek

I did a quick FEMM on it. Without knowing its exact dimensions, I guessed the OD of the wire at 1.5mm, since my tables don't go up to 660/46. I extrapolated it from 270/46.

90 turns, close wound, is 135mm. Internal 7/16" diameter = 11.113mm.

FEMM gives me 8.08 microhenries, and a Q of 253, with air core, at 472kHz.

8.075 microhenries, Q=138 at 3.85MHz.

Inductance is close. Q not so close. That reinforces my suspicion that your Boonton is out of cal. It's easy enough to check, it's really only two voltmeters.

Getting FEMM-acceptable B/H data for Fair-Rite 61 was a a pain. They use old-fashioned gauss and oersteds. FEMM wants tesla/amps per meter. I had to read points off the plot, convert, and import it.

Using #61 data, both linear, and nonlinear B-H gets nowhere near your

247uH, higher than that, in fact. That's with a 5/16" diameter rod.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

That explains why I'd never heard of him. I stopped playing with crystal radios when I got my first tubes, at the age of about 12, nearly 60 years ago. To me, that's like being an expert in buggy whips ;-)

His company appears to be into consumer/entertainment stuff, which explains why I'd never heard of them, either.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

I'll get some time tonight to get you some numbers. I think I said my inductor measures 8uh to 247uh so your 8.0xxuh is close my measurement. I don't understand what you got for a maximum inductance, higher than

247uh? And I think my ferrite rod is 5/16". Mikek

Yes.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Ok, here's the data on my ferrite rod inductor.

Ferrite rod---8.160" x 0.337" Coil length---6.125" Coil OD---0.563" Coil ID---0.4375" Wire diameter---0.0625"

91 turns

The 660/46 litz is wound over a polypropylene tube with an OD of 0.04375"

The inductance measured 250uh at 475kHz with the rod centered in the winding. I measured the Q with an air cap and my scope using the three db method. I made 3 measurements at 475 kHz, I got 350, 361 and 385. I also used the 3db method on my Boonton and got the same 550 I get when using the normal method.

See Calibration on this page. I don't see where this could be wrong.

I am using a frequency counter on the Boonton , not the frequency dial readings.

I wonder if my variable cap has an extra 0.7 ohms of loss over the cap in the Boonton.

Mikek

That's pretty much what I'd guessed.

In air, I get 8.30257uH, Q=239.6593

With an 8" x 5/16" #61 rod, placed on-axis, and centered symmetrically, I get 632.399uH, Q=621.164

All at 472kHz.

*One thing I've noticed: your Boonton and scope readings are in almost the exact ratio 2/pi (or pi/2). Have you mixed some peak and average readings?*

Use the procedure in the Boonton manual. Pages 17 & 18.

What sort of variable capacitor is it? Lab grade, or commercial? 0.7 ohms isn't much in a commercial capacitor. My GR1422CD is specced at dissipation factor

s/TF1275/TF1245

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

633uh, I think there is some info that didn't make it into the calculation. I'm sure the inductance is not that high.

I don't think this analysis includes a resonating capacitor, is that correct? I'd have to pull out someone else's derivation to back into the Q using a Q of 3000 to 5000 for the capacitor. The Boonton cap may be higher than that, I don't know. Including the cap loses may lower the Q to my 550.

Two completely different measurements. The only common element is the inductor.

I'll look to see what that is, but I use the machine as it was designed to measure Q. Then as a check, because of your doubt about the

550 number, I tried the 3db method on the Boonton and got very good agreement.

That's with a Fair-Rite #61 model, derived from published data. I suspect your rod either isn't #61, or has been roughly handled / subjected to excessive field strength.

Your results imply a ferrite with permeability around 79, for the dimensions I used. #61 is quoted as 125, although I used proper hysteresis curves, rather than just a blanket permeability figure. Modeling was done at 100 milliamps.

It was a finite element magnetic model. No resonance. FEMM Modeling doesn't work that way, it derives inductance, resistance, and flux, from field equations.

Like this: Total current = 0.1 Amps Voltage Drop = 0.30193+I*187.548 Volts Flux Linkage = 6.32399e-005-I*9.93076e-008 Webers Flux/Current = 0.000632399-I*9.93076e-007 Henries Voltage/Current = 3.0193+I*1875.48 Ohms Real Power = 0.0150965 Watts Reactive Power = 9.37741 VAr Apparent Power = 9.37742 VA

That translates to an inductance of 632.399uH, Q=621

At 10 amps, I get this: Total current = 10 Amps Voltage Drop = 5.6904+I*7731.37 Volts Flux Linkage = 0.00260696-I*1.66867e-06 Webers Flux/Current = 0.000260696-I*1.66867e-07 Henries Voltage/Current = 0.56904+I*773.137 Ohms Real Power = 28.452 Watts Reactive Power = 38656.8 VAr Apparent Power = 38656.8 VA

Inductance now 261uH, Q=1359.

That (extreme) example shows the difference, measuring at different currents, with a nonlinear core.

The manual for the Marconi TF1245 is on BAMA. Get it. It includes a good treatment of errors arising from instrument strays in Q meters.

Capacitor losses would not account for the discrepancy.

Don't overlook the significance. Investigate. It's too close to be likely coincidental. pi/2, to within less than 1%.

I'd expect that.

It'd have to be a pretty lousy cap to produce the results given. Look into the effects of injection impedance, yet again, the Marconi manual covers this in depth. The impedance of your signal generator will have an effect on the perceived Q.

Herein lies where I think your discrepancy is: Generator impedance in series with your tuned circuit.

As an example, a 250uH inductor, having a true Q of 100, resonated at

450kHz, will have an apparent Q, by the "center frequency over bandwidth" method, of only about half that, with a generator impedance of 8 ohms. Don't assume that, because your generator is calibrated into a 50 ohm load, that its looking-backwards output impedance is also 50 ohms, unless the spec actually says so, specifically.

There's another error that can arise, which only becomes significant at low Q values (less than about 10), due to measuring the resonant voltage across the variable capacitor, which all Q meters do. The peak you get isn't actually resonance, but a bit off, if you fine tune with the capacitor, rather than by tweaking the frequency. Another reason for using the "incremental frequency" method. You don't need worry about that, with the Q you have.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

All that could be correct, but a circuit Q of 550 is pretty high for a circuit Q. This would include capacitor losses.

Near as I can figure the Boonton is measuring with about 4 microamps.

That's what I thought, no resonating capacitor.

Ran into this in one of my early electronics classes. Each person in the class measured a 5 milliHenry inductor and got something like

150uhs. Way off. The instructor just ignored it. I dug into it and learned about the BH curve and the slope. Our measurement current was way down on the slope. It was sloppily put together experiment by a grad student. But I learned something even if no one else in the class cared about the discrepancy.

So does the Boonton manual.

I think it would. I'll let you do the math, but if you use a resonating cap with a Q of 4500, this brings the circuit Q down to 547. I used your 621uh, even though it is not what my inductor is. I'll rerun the numbers just to see what happens at 250uh.

I can't use the Boonton and a separate circuit to measure Q on my bench at the same time. Just a strange coincidence.

Ya, that could be right. As a thought check, using my numbers,

250uh Q=550. I assumed the Boonton Cap has a Q of 6000(?). That would mean the inductor has a loss R of 1.225 ohms. My "pretty lousy" second test cap would need a loss R of 0.815 ohms, That's only a Q of 915. That would be pretty lousy. I'll have to run that again. I'll found out which of my caps are really good!

Look into

Yes lots of corrections, for strays and other losses, but they are all small and under normal circumstances you don't need to run the calculations.

I don't think we are far of on Q if you put in the tuning capacitor losses. I have not found a number for that in the Boonton Manual. I will keep looking. But then again if I used 621uh, our Q numbers probably won't be in close agreement.

I'm not connecting my frequency generator to the tuned circuit. I'm driving a separate air core coil with my generator. This is placed as far as I can get it away from my variable inductor and still induce enough signal to be able to read the voltage on my scope. it's about

8" away. I have a better (higher) load than my scope probe, I'll pull it out and run some tests with all my supposedly "good" caps. Should be interesting.

Thanks for the feedback, Mikek

PS. see if you can figure anything about our 250uh/621uh discrepancy. The rod is out of an AM radio, I don't know of any that used other than #61 material, but?

Oh, I also learned something about the connection points. I'm using banana pin connection on my coil, but the threaded connectors still need to be tight or there is a loss. After tightening my Q=550 increased to Q=610 I'll even try removing the banana plugs and see if the Q goes over

610.

With Cap 1 average Q = 721

With Cap 2 average Q = 630

With Cap 3 average Q = 701 First 5 @ 475kHz

With Cap 4 average Q = 517

With Cap 5 average Q = 672

With Cap 6 average Q = 689 @ 500kHz

With Cap 7 average Q = 713 @ 600kHz

After removing the banana plugs from my inductor and connecting with the compression thumb nuts, The Q measured on the Boonton. Q = 618. It needs some work, I would think it equal or better my 3db testing. My first thought is the wiper connection on the capacitor.

So it was a worthwhile exercise, I have marked my caps and know which are best.

Your thoughts, Mikek

17 "somethings". My newsreader doesn't support those silly Windows characters. Please either use ", and ', or "in", "ft", "mm", "m", etc. I can't tell whether it was inches or feet ;-)

For best accuracy, fine tune the frequency, not the capacitor. I did a proof thirty years ago, when I liked calculus more than I do today.

That's why Marconi's Dielectric Test Set, a huge beast, with a 2 foot scale mirror galvanometer, glass insulators, and micrometer capacitors, used parallel, not series,resonance. Cost as much as a car did, back then.

5/7 isn't quite -3dB. How much bandwidth error you'll get depends on how sharp, or flat the peak response is.

That's a rather Byzantine way of going about things.

Why not use a parallel connection method on the Boonton? Measure at several frequencies, and tabulate the results.

You don't say what capacitance the capacitors were, or the frequency.

Lab grade capacitors have multiple wipers, usually bearing on the vane edges.

Firstly, without capacitor values, the figures aren't much use. I will say that they suck as measurement capacitors. Boonton (carefully?) don't state parameters for their main capacitor. Marconi, however, did. I'll post their curves to A.B.S.E. Suffice it to say here that, at 500pF, and 1Mhz, Q is greater than 20000.

Do the capacitor testing on the Boonton, in parallel with the main capacitor, measure the C, and Q at various settings, and tabulate the results. I can do curves with gnuplot, from tables, if you don't have plotting software.

I refer you to my last reply, where I wrote about Boonton test current. Using 15 milliamps, calculated from Boonton's voltmeter figures, FEMM gives Q at 620. Close enuf?

Inductance given by FEMM is still high, at 633uH. I still suspect that your ferrite isn't what you think it is. Might it be #67?

I'm beginning to believe your Boonton ;-)

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)