Sensing small inductances

[bitrex]

On 8/26/19 11:24 AM, bitrex wrote:

On 8/26/19 5:25 AM, Jeroen Belleman wrote:

I don't see the point of negative capacitance. Increasing the
Q implies reducing or compensating losses. A reactive component
doesn't do that.

Jeroen Belleman

a negative capacitance has to be powered to operate; the charge goes
down but the voltage (and thus 1/2CV^2 energy) goes up, that requires
it to get some energy from somewhere the system didn't have before.
No such thing as a passive negative capacitance that behaves just
like a positive passive capacitance with its sign flipped and still
conserves the total energy of the system that I know of. so long as
it's not all returned to the source in a purely reactive system that
excess is then available to do work.

A negative capacitance indeed has to be a powered active circuit,

There are some naturally-occurring structures e.g. in ferroelectric
crystals that also exhibit negative capacitance they require energy from
somewhere, too

however, to provide nett energy, it is necessary to have a negative
real component in the impedance. A purely reactive impedance,
negative or not, does not provide or absorb nett work. That's the
*definition* of a reactive impedance.

who cares what the academic definition of a "purely reactive" negative
capacitance is, they don't exist, the electronic ones can compensate
tank circuit losses just fine just like a negative resistance but they
don't have DC gain which can be a nice feature to have

also it's easy to make a negative capacitance circuit tunable or
frequency-dependent by varying the gain of the negative feedback loop
portion so you can adjust the oscillator operating frequency that way
without a varicap, too

 

[Martin Brown]

On 25/08/2019 18:59, bitrex wrote:

On 8/25/19 1:14 PM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 24 Aug 2019 23:31:00 -0400, bitrex <user@example.net> wrote:

Microcontroller-based strategies like this work OK for high-Q
inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies that
microprocessors can easily provide from direct pin-switching system
clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm uncomfortable
using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a la
a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in some
implementation. Clock out to the tank and leading/lagging phase signal
back in to the uP to an onboard comparator/phase detector and
integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

The great little AADE meter has an oscillator circuit that always
oscillates, and a uP based frequency meter. It works very well for a
simple instrument, and does a few nH pretty well. The schematic is
online somewhere.

Making an oscillator is morally equivalent to boosting an inductor's
Q, and has the same problems at low Q.

Q is a function of frequency so whether the inductor needs its Q
boosted, or not, depends on what frequency you want it to oscillate at.

Most simple oscillator circuits don't tend to have enough loop gain to
make random scramble-wire inductance that might have a self-resonant
frequency in the hundreds of MHz ring reliably in a resonant tank down
at single-digit MHz frequencies a cheap uP could handle with its (cheap)
on-board peripherals.

Let it ring in the 10-100Mhz region, square it up to logic levels and
then use a divide by a suitable factor of 2^N to bring it into the
region where a uP can measure the frequency reliably.

However what I'm mainly interested in being able to detect relative
differences between random-wires inductances with precision, and less
concern about absolute accuracy as compared to some reference standard.

--
Regards,
Martin Brown

 

[bitrex]

On 8/26/19 3:33 AM, piglet wrote:

On 25/08/2019 04:31, bitrex wrote:
Microcontroller-based strategies like this work OK for high-Q
inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies
that microprocessors can easily provide from direct pin-switching
system clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm
uncomfortable using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a
la a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in
some implementation. Clock out to the tank and leading/lagging phase
signal back in to the uP to an onboard comparator/phase detector and
integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

What you could try is adapting the Boonton 72 capacitance meter topology
to measure inductance. A fixed frequency low current source feeds the
DUT into a calibrated resonant LC in the test instrument, amplify and
measure with quadrature synchronous detector. RF techniques rather than
time domain!

What's a "quadrature synchronous detector"?

It sounds a bit like my idea to use a phase detector

The Boonton 72 can resolve tiny capacitance changes so I expect a C to L
transformed version might also be capable of resolving tiny inductance
changes?

piglet

 

[John Larkin]

On Mon, 26 Aug 2019 01:41:18 GMT, Steve Wilson <no@spam.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

A high-Q LC oscillator follows the resonance equation even closer than
a low-Q one.

w = 1/root(LC)

With infinite Q.

For ordinary inductors, the ratio of the actual resonant frequency to the
thoretical resonant frequency is root(1 - 1/(4Q^2)). In practise, a Q of 10
gives an actual frequency of 99.87% of theoretical.

See Radiotron_Designers_Handbook_1954.pdf 90.6MB

http://preview.tinyurl.com/hmnpj2r

See EQ 3 on page 449. This explains the variation in resonant frequency
with Q. I have not been able to find this information anywhere else.

If you model the finite Q as a shunt resistor, and connect it to a
negative impedance to form an oscillator, the shunt resistance is
precisely canceled at the point of steady-state oscillation.

Anyhow, an oscillator is a simple way to measure inductance. At
moderate Q, the equation works.

 

[George Herold]

On Monday, August 26, 2019 at 12:56:44 PM UTC-4, bitrex wrote:

On 8/26/19 3:33 AM, piglet wrote:
On 25/08/2019 04:31, bitrex wrote:
Microcontroller-based strategies like this work OK for high-Q
inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies
that microprocessors can easily provide from direct pin-switching
system clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm
uncomfortable using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a
la a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in
some implementation. Clock out to the tank and leading/lagging phase
signal back in to the uP to an onboard comparator/phase detector and
integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

What you could try is adapting the Boonton 72 capacitance meter topology
to measure inductance. A fixed frequency low current source feeds the
DUT into a calibrated resonant LC in the test instrument, amplify and
measure with quadrature synchronous detector. RF techniques rather than
time domain!

What's a "quadrature synchronous detector"?

It sounds a bit like my idea to use a phase detector

Right but two synchronous detectors, one in phase and the other 90 degrees off
of that.

GH

The Boonton 72 can resolve tiny capacitance changes so I expect a C to L
transformed version might also be capable of resolving tiny inductance
changes?

piglet

 

[amdx]

On 8/26/2019 9:06 AM, Joseph Gwinn wrote:

On Aug 25, 2019, Steve Wilson wrote
(in article<XnsAAB6DCA0C5932idtokenpost@69.16.179.23&gt:

jlarkin@highlandsniptechnology.com wrote:

A high-Q LC oscillator follows the resonance equation even closer than
a low-Q one.

w = 1/root(LC)

With infinite Q.

For ordinary inductors, the ratio of the actual resonant frequency to the
thoretical resonant frequency is root(1 - 1/(4Q^2)). In practise, a Q of 10
gives an actual frequency of 99.87% of theoretical.

See Radiotron_Designers_Handbook_1954.pdf 90.6MB

http://preview.tinyurl.com/hmnpj2r

See EQ 3 on page 449. This explains the variation in resonant frequency
with Q. I have not been able to find this information anywhere else.

I have seen it elsewhere, in old textbooks. It may be in Terman.

Joe Gwinn

To the mathematically challenged an easy way to state this is the
frequency of a resonant circuit varies with the value of the loss
resistance, But not very damn much.
My physicist friend taught me that.

Would it affect the frequency differently if the R in the C or L or
external?

Also the resonant frequency is the geometric mean between the Âą 3db
points. I assume they are related.

Mikek

 

[Steve Wilson]

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Mon, 26 Aug 2019 01:41:18 GMT, Steve Wilson <no@spam.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

A high-Q LC oscillator follows the resonance equation even closer than
a low-Q one.

w = 1/root(LC)

With infinite Q.

For ordinary inductors, the ratio of the actual resonant frequency to
the thoretical resonant frequency is root(1 - 1/(4Q^2)). In practise, a
Q of 10 gives an actual frequency of 99.87% of theoretical.

See Radiotron_Designers_Handbook_1954.pdf 90.6MB

http://preview.tinyurl.com/hmnpj2r

See EQ 3 on page 449. This explains the variation in resonant frequency
with Q. I have not been able to find this information anywhere else.

If you model the finite Q as a shunt resistor, and connect it to a
negative impedance to form an oscillator, the shunt resistance is
precisely canceled at the point of steady-state oscillation.

After careful measurement, I have never been able to find any evidence of
negative resistance in a cc Colpitts or how to control it. Instead, I use
the Bode criteria. That is

1. Phase shift of 0 or multiples of 360 degrees
2. Loop gain equal or greater than 1

For examples, see Oscillator.zip at

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

Anyhow, an oscillator is a simple way to measure inductance. At
moderate Q, the equation works.

I never said it didn't. If you had read my post, or looked at the link, you
would find that Q has a minor effect on the frequency for Q of 4 or
greater.

"Q must, for example, be less than four to make it differ by 1%"

Radiotron Designer's Handbook Page 449

<http://preview.tinyurl.com/hmnpj2r>

A bigger problem is iron core inductors. As you pointed out earlier,
different measurement methods can give different results.

 

[John Larkin]

On Mon, 26 Aug 2019 08:33:48 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 25/08/2019 04:31, bitrex wrote:
Microcontroller-based strategies like this work OK for high-Q inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies that
microprocessors can easily provide from direct pin-switching system
clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm uncomfortable
using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a la
a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in some
implementation. Clock out to the tank and leading/lagging phase signal
back in to the uP to an onboard comparator/phase detector and integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

Only by 2:1. And it's easy to measure frequency to a part per million.

 

[whit3rd]

On Monday, August 26, 2019 at 10:09:46 AM UTC-7, George Herold wrote:

On Monday, August 26, 2019 at 12:56:44 PM UTC-4, bitrex wrote:

What's a "quadrature synchronous detector"?

It sounds a bit like my idea to use a phase detector

Right but two synchronous detectors, one in phase and the other 90 degrees off
of that.

It's a one-component Fourier meter, doing a multiply/accumulate on
a periodic signal.

 

[Lasse Langwadt Christense]

mandag den 26. august 2019 kl. 18.56.44 UTC+2 skrev bitrex:

On 8/26/19 3:33 AM, piglet wrote:
On 25/08/2019 04:31, bitrex wrote:
Microcontroller-based strategies like this work OK for high-Q
inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies
that microprocessors can easily provide from direct pin-switching
system clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm
uncomfortable using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a
la a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in
some implementation. Clock out to the tank and leading/lagging phase
signal back in to the uP to an onboard comparator/phase detector and
integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

What you could try is adapting the Boonton 72 capacitance meter topology
to measure inductance. A fixed frequency low current source feeds the
DUT into a calibrated resonant LC in the test instrument, amplify and
measure with quadrature synchronous detector. RF techniques rather than
time domain!

What's a "quadrature synchronous detector"?

It sounds a bit like my idea to use a phase detector

https://www.analog.com/-/media/images/analog-dialogue/en/volume-48/number-4/articles/synchronous-detectors-facilitate-precision/synchronous-detectors-facilitate-precision-fig09.png

 

[Steve Wilson]

Steve Wilson <no@spam.com> wrote:

After careful measurement, I have never been able to find any evidence
of negative resistance in a cc Colpitts or how to control it. Instead, I
use the Bode criteria. That is

1. Phase shift of 0 or multiples of 360 degrees
2. Loop gain equal or greater than 1

Bode should be Barkhausen

 

On Sun, 25 Aug 2019 18:18:12 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (25 Aug 2019 10:38:32 -0700) it happened Winfield Hill
winfieldhill@yahoo.com> wrote in <qjuh2o0v42@drn.newsguy.com>:

jlarkin@highlandsniptechnology.com wrote...

Jan Panteltje wrote:
amdx wrote
Jan Panteltje wrote:
bitrex wrote

http://panteltje.com/panteltje/pic/lc_pic/

Print and put in values.

Get a real monitor and /or see an eye doctor.

Your schematics are unreadable. I don't even try.

I'm often curious, but can not read them.
Jan, why are they so dark, with such a poor
contrast ratio? How do you even do that?

I dunno, I have no problem reading those in my browser,
or Linux 'xv' viewer or whatever,

Often I wonder (video is my background) if people even know how to adjust a monitor,
These pictures are of schematics I use to build the stuff,
so 100% of info is there or it would have to be so obvious I omitted it.
I do not sell kits.
All the babble .. read the asm source it clearly shows in ASCII what is connected to what.
If anybody actually builds this thing they can ask.
The explanation text in the link I gave is 100% simple for anybody known in the art,

And if you cannot read it or understand it, so be it,

As to the art, if you cannot read a component value and understand what sort of magnitude say 'resistor' must be there
you need to do more experimenting, not so many silly simulations.

I definitely not going to set-up my monitor for every odd picture. I
once set it up for color balance and background illumination levels.

Get yourself a decent flatbed scanner and scan it to proper B/W GIF
(not JPEG). Flatbed scanners are available in multifunction printers
for less than 50 €.

 

[piglet]

On 26/08/2019 22:26, John Larkin wrote:

On Mon, 26 Aug 2019 08:33:48 +0100, piglet <erichpwagner@hotmail.com
A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

Only by 2:1. And it's easy to measure frequency to a part per million.

Yes of course but in one of his post's the OP was complaining their
hardware had problems measuring small differences in frequency.

piglet

 

[Steve Wilson]

piglet <erichpwagner@hotmail.com> wrote:

On 26/08/2019 22:26, John Larkin wrote:
On Mon, 26 Aug 2019 08:33:48 +0100, piglet <erichpwagner@hotmail.com
A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

Only by 2:1. And it's easy to measure frequency to a part per million.

Yes of course but in one of his post's the OP was complaining their
hardware had problems measuring small differences in frequency.

piglet

There are some tricks you can use. These make it trivial to measure to 1e-12
in 1 second. 1e-14 is a bit tougher. 1e-16 is for the pros. 1e-18 is
theoretically possible but difficult.

 

On Tue, 27 Aug 2019 07:08:09 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 26/08/2019 22:26, John Larkin wrote:
On Mon, 26 Aug 2019 08:33:48 +0100, piglet <erichpwagner@hotmail.com
A problem with measuring inductance by making the DUT part of an
oscillator tank and measuring frequency is the square root relationship
works against you by compressing sensitivity.

Only by 2:1. And it's easy to measure frequency to a part per million.


Yes of course but in one of his post's the OP was complaining their
hardware had problems measuring small differences in frequency.

piglet

He could buy a cheap counter and get to 1 PPM and be done.

 

[John Larkin]

On Tue, 27 Aug 2019 07:44:43 +0300, upsidedown@downunder.com wrote:

On Sun, 25 Aug 2019 18:18:12 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (25 Aug 2019 10:38:32 -0700) it happened Winfield Hill
winfieldhill@yahoo.com> wrote in <qjuh2o0v42@drn.newsguy.com>:

jlarkin@highlandsniptechnology.com wrote...

Jan Panteltje wrote:
amdx wrote
Jan Panteltje wrote:
bitrex wrote

http://panteltje.com/panteltje/pic/lc_pic/

Print and put in values.

Get a real monitor and /or see an eye doctor.

Your schematics are unreadable. I don't even try.

I'm often curious, but can not read them.
Jan, why are they so dark, with such a poor
contrast ratio? How do you even do that?

I dunno, I have no problem reading those in my browser,
or Linux 'xv' viewer or whatever,

Often I wonder (video is my background) if people even know how to adjust a monitor,
These pictures are of schematics I use to build the stuff,
so 100% of info is there or it would have to be so obvious I omitted it.
I do not sell kits.
All the babble .. read the asm source it clearly shows in ASCII what is connected to what.
If anybody actually builds this thing they can ask.
The explanation text in the link I gave is 100% simple for anybody known in the art,

And if you cannot read it or understand it, so be it,

As to the art, if you cannot read a component value and understand what sort of magnitude say 'resistor' must be there
you need to do more experimenting, not so many silly simulations.

I definitely not going to set-up my monitor for every odd picture. I
once set it up for color balance and background illumination levels.

Get yourself a decent flatbed scanner and scan it to proper B/W GIF
(not JPEG). Flatbed scanners are available in multifunction printers
for less than 50 €.

A clearly hand-drawn schematic can be photographed with a cheap
camera, or even a cell phone camera, and be perfectly legible.

Jpegs work fine.

I think this circuit works:

https://www.dropbox.com/s/hpyu6ndeimrbjvs/24_to_dual_15.JPG?raw=1

Ink on grid paper, cheap camera.

All my sketches have a title and a date, so if anyone sees it years
later they know what it is.

A whiteboard can be nicely photographed too.

 

[Phil Hobbs]

On 8/26/19 2:53 PM, Steve Wilson wrote:

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Mon, 26 Aug 2019 01:41:18 GMT, Steve Wilson <no@spam.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

A high-Q LC oscillator follows the resonance equation even closer than
a low-Q one.

w = 1/root(LC)

With infinite Q.

For ordinary inductors, the ratio of the actual resonant frequency to
the thoretical resonant frequency is root(1 - 1/(4Q^2)). In practise, a
Q of 10 gives an actual frequency of 99.87% of theoretical.

See Radiotron_Designers_Handbook_1954.pdf 90.6MB

http://preview.tinyurl.com/hmnpj2r

See EQ 3 on page 449. This explains the variation in resonant frequency
with Q. I have not been able to find this information anywhere else.

If you model the finite Q as a shunt resistor, and connect it to a
negative impedance to form an oscillator, the shunt resistance is
precisely canceled at the point of steady-state oscillation.

After careful measurement, I have never been able to find any evidence of
negative resistance in a cc Colpitts or how to control it.

Simple. Make a 2N3904 emitter follower running at 25 mA or so from
split supplies. Ground the base via a couple of inches of wire, bypass
the collector, and watch it oscillate. No tapped tank required.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

On Saturday, August 24, 2019 at 11:31:06 PM UTC-4, bitrex wrote:

Microcontroller-based strategies like this work OK for high-Q inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies that
microprocessors can easily provide from direct pin-switching system
clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm uncomfortable
using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a la
a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in some
implementation. Clock out to the tank and leading/lagging phase signal
back in to the uP to an onboard comparator/phase detector and integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

R.F. Design magazine featured a circuit some years ago that drives
an inductor with a 100kHz sinusoidal current, and measures the
resulting in-phase and quadrature voltages.

That method is robust, yielding independent measurements of inductance
and effective series resistance.

I have a scan of the original article somewhere(*), but this seems to
be a mostly-faithful reproduction:

https://www.qsl.net/va3iul/L_meter/L_meter.htm

You'll notice some interesting compound op-amp arrangements,
which the original article explains provide a 2nd-order
frequency compensation, which drastically improves the phase
response. (That's important, otherwise phase errors eat into
the I-Q scheme's accuracy.)

(*) Ah yes, here it is:
"Simple Digital Inductance Meter with 0.1nH Resolution"
Roger A. Williams, LTX Corp., R.F. Design, October 1987, p50-55


Cheers,
James Arthur

 

[John Larkin]

On Tue, 27 Aug 2019 11:38:43 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Saturday, August 24, 2019 at 11:31:06 PM UTC-4, bitrex wrote:
Microcontroller-based strategies like this work OK for high-Q inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies that
microprocessors can easily provide from direct pin-switching system
clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm uncomfortable
using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a la
a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in some
implementation. Clock out to the tank and leading/lagging phase signal
back in to the uP to an onboard comparator/phase detector and integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

R.F. Design magazine featured a circuit some years ago that drives
an inductor with a 100kHz sinusoidal current, and measures the
resulting in-phase and quadrature voltages.

That method is robust, yielding independent measurements of inductance
and effective series resistance.

I have a scan of the original article somewhere(*), but this seems to
be a mostly-faithful reproduction:

https://www.qsl.net/va3iul/L_meter/L_meter.htm

You'll notice some interesting compound op-amp arrangements,
which the original article explains provide a 2nd-order
frequency compensation, which drastically improves the phase
response. (That's important, otherwise phase errors eat into
the I-Q scheme's accuracy.)

(*) Ah yes, here it is:
"Simple Digital Inductance Meter with 0.1nH Resolution"
Roger A. Williams, LTX Corp., R.F. Design, October 1987, p50-55


Cheers,
James Arthur

A more modern version could use a 50 ohm sine source, digitize at 4F,
and do some math.

 

[Lasse Langwadt Christense]

tirsdag den 27. august 2019 kl. 21.10.58 UTC+2 skrev John Larkin:

On Tue, 27 Aug 2019 11:38:43 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Saturday, August 24, 2019 at 11:31:06 PM UTC-4, bitrex wrote:
Microcontroller-based strategies like this work OK for high-Q inductances:

http://www.pa3fwm.nl/technotes/tn11b.html

But don't work too good for little random-wire very lossy inductances,
of values around 0.5uH to 5uH, at the lower excitation frequencies that
microprocessors can easily provide from direct pin-switching system
clock-derived outputs. e.g. inductaors that have self-resonant
frequencies in the 100s of MHz.

I was thinking the small inductance could have its effective Q boosted
via boostrapping, perhaps (I'm kinda down on negative impedance
circuits, now, you can make some cute circuits with them but they all
obv. tend towards being unstable and are "fiddly" and I'm uncomfortable
using them in "real work")

and then you could measure a certain range of small inductances by
applying a clock to a tank circuit thru a resistor, and putting the
original clock plus the output from the tank into a phase detector a la
a PLL and look at the integrated leading or lagging phase "up/down"
signal to infer the inductance.

It might need little external hardware other than the Q-booster in some
implementation. Clock out to the tank and leading/lagging phase signal
back in to the uP to an onboard comparator/phase detector and integrator.

For my particular solution needs whatever form it takes, it would be
best to trade of absolute accuracy for precision/repeatability.

R.F. Design magazine featured a circuit some years ago that drives
an inductor with a 100kHz sinusoidal current, and measures the
resulting in-phase and quadrature voltages.

That method is robust, yielding independent measurements of inductance
and effective series resistance.

I have a scan of the original article somewhere(*), but this seems to
be a mostly-faithful reproduction:

https://www.qsl.net/va3iul/L_meter/L_meter.htm

You'll notice some interesting compound op-amp arrangements,
which the original article explains provide a 2nd-order
frequency compensation, which drastically improves the phase
response. (That's important, otherwise phase errors eat into
the I-Q scheme's accuracy.)

(*) Ah yes, here it is:
"Simple Digital Inductance Meter with 0.1nH Resolution"
Roger A. Williams, LTX Corp., R.F. Design, October 1987, p50-55


Cheers,
James Arthur

A more modern version could use a 50 ohm sine source, digitize at 4F,
and do some math.

ADC/DAC and an FPGA and you have all you need for a vector analyser