Sensing small inductances

[Steve Wilson]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

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

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.

Here is a 330Mhz Colpitts with split supplies. I had to add some base
inductance and capacitive feedback to get it to oscillate at a low enough
frequency for the 2N3904. That should not affect the question.

Where do you measure the negative resistance and how do you control it?

Version 4
SHEET 1 880 708
WIRE -96 -112 -160 -112
WIRE -32 -112 -96 -112
WIRE -32 -96 -32 -112
WIRE -160 -80 -160 -112
WIRE -464 -32 -528 -32
WIRE -336 -32 -384 -32
WIRE -240 -32 -336 -32
WIRE -224 -32 -240 -32
WIRE -528 -16 -528 -32
WIRE -336 -16 -336 -32
WIRE -32 0 -32 -16
WIRE -336 64 -336 48
WIRE -160 64 -160 16
WIRE -160 64 -336 64
WIRE -128 64 -160 64
WIRE -112 64 -128 64
WIRE -336 80 -336 64
WIRE -160 80 -160 64
WIRE -336 160 -336 144
WIRE -160 176 -160 160
WIRE -160 272 -160 256
FLAG -32 0 0
FLAG -336 160 0
FLAG -128 64 Vout
FLAG -240 -32 Q1B
FLAG -528 -16 0
FLAG -96 -112 VCC
FLAG -160 272 0
SYMBOL voltage -32 -112 R0
WINDOW 123 0 0 Left 2
WINDOW 0 33 33 Left 2
WINDOW 3 33 80 Left 2
SYMATTR InstName V2
SYMATTR Value 5V
SYMATTR SpiceLine Rser=0
SYMBOL npn -224 -80 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL cap -352 -16 R0
SYMATTR InstName C1
SYMATTR Value 10pf
SYMBOL cap -352 80 R0
SYMATTR InstName C2
SYMATTR Value 10pf
SYMBOL ind -368 -16 M270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
WINDOW 39 -16 53 VBottom 2
SYMATTR InstName L1
SYMATTR Value 20nh
SYMATTR SpiceLine Rser=1m
SYMBOL voltage -160 272 R180
WINDOW 123 0 0 Left 2
WINDOW 0 33 33 Left 2
WINDOW 3 33 80 Left 2
SYMATTR InstName V1
SYMATTR Value 5V
SYMATTR SpiceLine Rser=0
SYMBOL res -176 64 R0
SYMATTR InstName R1
SYMATTR Value 200
TEXT -400 -240 Left 2 !.tran 0 200n 0 50p
TEXT -408 -264 Left 2 ;'Negative Resistance

Cheers

Phil Hobbs

 

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:

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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

I started to cobble together a 'modernized' version a ways back
with variable frequency excitation to cover larger inductances,
and a discrete current source that was faster than the op-amp
version. But I diverted and never finished it, can't recall why.

The Kelvin drive scheme is elegant--no error from long leads.

Cheers,
James Arthur

 

[John Larkin]

On Tue, 27 Aug 2019 20:07:25 GMT, Steve Wilson <no@spam.com> wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

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

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.

Here is a 330Mhz Colpitts with split supplies. I had to add some base
inductance and capacitive feedback to get it to oscillate at a low enough
frequency for the 2N3904. That should not affect the question.

Where do you measure the negative resistance and how do you control it?

What Spice doesn't include in the transistor models is the wire bond
inductances.

 

[Martin Brown]

On 27/08/2019 18:27, John Larkin wrote:

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.

Jpegs do work OK for line art. Better than you might expect given that
they are optimised for continuous tone photographic images.

PNG is better though in terms of size if you optimise the palette.

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.

Though with many digital cameras it needs a stop or so over exposure if
the pure white isn't to come out as 18% grey.

--
Regards,
Martin Brown

 

[John Larkin]

On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

 

[Lasse Langwadt Christense]

tirsdag den 27. august 2019 kl. 22.50.59 UTC+2 skrev John Larkin:

On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

https://www.analog.com/en/products/ad5933.html ?

 

[Lasse Langwadt Christense]

tirsdag den 27. august 2019 kl. 22.52.10 UTC+2 skrev John Larkin:

On Tue, 27 Aug 2019 13:30:16 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

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

And some software!

and a directional coupler, but once you have the arrays of complex data for
forward and reverse it is only a few lines of formulas

and if you have open/short/load measurements on the same frequencies
calibration is also easy

 

[John Larkin]

On Tue, 27 Aug 2019 13:30:16 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

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

And some software!

 

[John Larkin]

On Tue, 27 Aug 2019 22:36:29 +0100, Martin Brown
<'''newspam'''@nezumi.demon.co.uk> wrote:

On 27/08/2019 18:27, John Larkin wrote:
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.

Jpegs do work OK for line art. Better than you might expect given that
they are optimised for continuous tone photographic images.

PNG is better though in terms of size if you optimise the palette.

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.

Though with many digital cameras it needs a stop or so over exposure if
the pure white isn't to come out as 18% grey.

Drawings generally need more contrast too, and maybe some cropping,
sometimes sharpening. I use Irfanview.

 

[Steve Wilson]

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Tue, 27 Aug 2019 20:07:25 GMT, Steve Wilson <no@spam.com> wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

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

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.

Here is a 330Mhz Colpitts with split supplies. I had to add some base
inductance and capacitive feedback to get it to oscillate at a low enough
frequency for the 2N3904. That should not affect the question.

Where do you measure the negative resistance and how do you control it?

What Spice doesn't include in the transistor models is the wire bond
inductances.

You can add them.

 

On Tuesday, August 27, 2019 at 4:50:59 PM UTC-4, John Larkin wrote:

On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

I'm not sure those numbers work. For 10nH, for example, the L/R
constant is 200ps.

But you could digitally drive a constant current source, that works.
20mA 100kHz sine into 1uH makes 25mV p-p, or 25uV for 1nH.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

Cheers,
James

 

[Phil Hobbs]

On 8/27/19 4:07 PM, Steve Wilson wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

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

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.

Here is a 330Mhz Colpitts with split supplies. I had to add some base
inductance and capacitive feedback to get it to oscillate at a low enough
frequency for the 2N3904. That should not affect the question.

Where do you measure the negative resistance and how do you control it?

You measure it by watching the follower oscillate with no external
feedback. Put a small pot in series with the base and watch where the
oscillation stops.

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

 

[John Larkin]

On Tue, 27 Aug 2019 16:39:38 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 4:50:59 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

I'm not sure those numbers work. For 10nH, for example, the L/R
constant is 200ps.

But you could digitally drive a constant current source, that works.
20mA 100kHz sine into 1uH makes 25mV p-p, or 25uV for 1nH.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

Cheers,
James

1 volt across 50 ohms is 20 mA.

One would of course want a sine wave drive at a few (or many)
different frequencies. Hz or KHz for power magnetics, many MHz for
nanohenry inductors.

 

On Tuesday, August 27, 2019 at 8:08:45 PM UTC-4, John Larkin wrote:

On Tue, 27 Aug 2019 16:39:38 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 4:50:59 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

I'm not sure those numbers work. For 10nH, for example, the L/R
constant is 200ps.

But you could digitally drive a constant current source, that works.
20mA 100kHz sine into 1uH makes 25mV p-p, or 25uV for 1nH.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

Cheers,
James

1 volt across 50 ohms is 20 mA.

I know. I was thinking the signal level would be too small for
an ADC and the timing differences too small to resolve -- ADCs
aren't good at high-res 1MHz 25uV measurements.

But some appropriate gain stages fix that, and the 50 ohms is
enough larger than most interesting inductors' e.s.r. that 1V
drive becomes effectively a 40mA p-p current source. That's
not terrible.

One would of course want a sine wave drive at a few (or many)
different frequencies. Hz or KHz for power magnetics, many MHz for
nanohenry inductors.

The original scheme resolved 0.1nH with 100kHz x 20mA excitation.
That's pretty elegant. Not bad for two quad op-amps and one
MC1496.

A 'digital' version with a DAC, ADC, and a uC still needs the
gain stages, but saves the multiplier and a few discrete hairballs.

Cheers,
James Arthur

 

[bitrex]

On 8/27/19 4:50 PM, John Larkin wrote:

On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

the circuit as it is above is, as you say, awfully complicated...

 

On Tue, 27 Aug 2019 17:46:03 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 8:08:45 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 16:39:38 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 4:50:59 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

I'm not sure those numbers work. For 10nH, for example, the L/R
constant is 200ps.

But you could digitally drive a constant current source, that works.
20mA 100kHz sine into 1uH makes 25mV p-p, or 25uV for 1nH.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

Cheers,
James

1 volt across 50 ohms is 20 mA.

I know. I was thinking the signal level would be too small for
an ADC and the timing differences too small to resolve -- ADCs
aren't good at high-res 1MHz 25uV measurements.

Signal average. Noise is cheap and plentiful.

But some appropriate gain stages fix that, and the 50 ohms is
enough larger than most interesting inductors' e.s.r. that 1V
drive becomes effectively a 40mA p-p current source. That's
not terrible.

One would of course want a sine wave drive at a few (or many)
different frequencies. Hz or KHz for power magnetics, many MHz for
nanohenry inductors.

The original scheme resolved 0.1nH with 100kHz x 20mA excitation.
That's pretty elegant. Not bad for two quad op-amps and one
MC1496.

A 'digital' version with a DAC, ADC, and a uC still needs the
gain stages, but saves the multiplier and a few discrete hairballs.

If the frequency can be run up, you don't need so much gain.

Cheers,
James Arthur

 

[Steve Wilson]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 8/27/19 4:07 PM, Steve Wilson wrote:
Where do you measure the negative resistance and how do you control it?

You measure it by watching the follower oscillate with no external
feedback. Put a small pot in series with the base and watch where the
oscillation stops.

Cheers

Phil Hobbs

Thanks. This has been very illuminating. First, I find the term
negative resistance has nothing to do with the classical definition,
where an increasing voltage causes decreasing current. An example is
tunnel diode oscillators.

In this application, there is absolutely no negative resistance in
the classical sense. The term is a complete misnomer in this useage.

Second, I find a base resistance of 14.96 Ohms in the circuit I gave
you is sufficient to basically stop the oscillations.

Third, adding Darlington increases the required resistance to 46.58
Ohms.

These findings are of tremendous importance in everyday electronics.

It explains why a small bead or resistor in the base of a transistor
is so effective at stopping parasitic oscillations. It also explains
why parasitic oscillations are so hard to kill in Darlingstons.

The next problem is to find out exactly how the small base
resistance works. This opens a completely new field of investigation
where I am certain the new knowledge gained will be worth the
effort.

Thanks again.

 

On Wed, 28 Aug 2019 13:14:42 GMT, Steve Wilson <no@spam.com> wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 8/27/19 4:07 PM, Steve Wilson wrote:
Where do you measure the negative resistance and how do you control it?

You measure it by watching the follower oscillate with no external
feedback. Put a small pot in series with the base and watch where the
oscillation stops.

Cheers

Phil Hobbs

Thanks. This has been very illuminating. First, I find the term
negative resistance has nothing to do with the classical definition,
where an increasing voltage causes decreasing current. An example is
tunnel diode oscillators.

In this application, there is absolutely no negative resistance in
the classical sense. The term is a complete misnomer in this useage.

Given an LC tank or an equivalent 1-port passive resonator, only
seeing a negative resistance will make it oscillate. There is a class
of such oscillators that are analyzed based on negative resistance.

Second, I find a base resistance of 14.96 Ohms in the circuit I gave
you is sufficient to basically stop the oscillations.

Third, adding Darlington increases the required resistance to 46.58
Ohms.

These findings are of tremendous importance in everyday electronics.

It explains why a small bead or resistor in the base of a transistor
is so effective at stopping parasitic oscillations. It also explains
why parasitic oscillations are so hard to kill in Darlingstons.

The Mini-Circuits type MMICS are unconditionally stable. They are
Darlingtons.

 

On Tuesday, August 27, 2019 at 11:17:04 PM UTC-4, jla...@highlandsniptechnology.com wrote:

On Tue, 27 Aug 2019 17:46:03 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 8:08:45 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 16:39:38 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 4:50:59 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

I'm not sure those numbers work. For 10nH, for example, the L/R
constant is 200ps.

But you could digitally drive a constant current source, that works.
20mA 100kHz sine into 1uH makes 25mV p-p, or 25uV for 1nH.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

Cheers,
James

1 volt across 50 ohms is 20 mA.

I know. I was thinking the signal level would be too small for
an ADC and the timing differences too small to resolve -- ADCs
aren't good at high-res 1MHz 25uV measurements.


Signal average. Noise is cheap and plentiful.



But some appropriate gain stages fix that, and the 50 ohms is
enough larger than most interesting inductors' e.s.r. that 1V
drive becomes effectively a 40mA p-p current source. That's
not terrible.

One would of course want a sine wave drive at a few (or many)
different frequencies. Hz or KHz for power magnetics, many MHz for
nanohenry inductors.

The original scheme resolved 0.1nH with 100kHz x 20mA excitation.
That's pretty elegant. Not bad for two quad op-amps and one
MC1496.

A 'digital' version with a DAC, ADC, and a uC still needs the
gain stages, but saves the multiplier and a few discrete hairballs.


If the frequency can be run up, you don't need so much gain.

Gain's cheap, speed isn't.

I understand the urge to clean up all the discretes, but it seems a
bit campy to throw a million transistors + software at it.

I fell victim to that cleaning urge with my 'upgrade.' I drove the
inductor with a triangle-wave current excitation, since that was
stable, easily calibrated, and easily generated from my
variable-frequency digital source. No DAC required.

Triangular current-drive changes the inductor voltage to a squarewave
proportional to inductance, with e.s.r. ramps instead of flat tops
and bottoms.

The e.s.r. ramp starts at -i excitation and ends with +i excitation,
so if you in-phase demodulate, the e.s.r. component cancels and you're
left with the pure inductive component.

I replaced the original Jim Thompson(?) MC1496 analog multiplier with
CMOS switches. That saved a bunch of biasing and tweaking. De-modulating
in-phase eliminated the earlier design's quadrature phase-shifters and
associated adjustments.

(I'm basically in software hell at the moment, trying to get a
daisychain of undocumented, layered, script-kiddie IDE abstraction-heaps
going, so three analog ICs and a probe-able discrete hairball seems
pretty attractive at the moment, along with a carburetor, points, and
a distributor.)

Cheers,
James Arthur

 

[bitrex]

On 8/27/19 11:16 PM, jlarkin@highlandsniptechnology.com wrote:

On Tue, 27 Aug 2019 17:46:03 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 8:08:45 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 16:39:38 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 4:50:59 PM UTC-4, John Larkin wrote:
On Tue, 27 Aug 2019 12:57:51 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Tuesday, August 27, 2019 at 3:10:58 PM UTC-4, John Larkin wrote:
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.

50 ohms? The scheme needs a current source. That way the resistive
and reactive components are easily measured.

But 50 ohms is easier, and an accurate wideband 50 ohm resistor costs
under 1 cent. The ADC can measure stuff and a uP or PC can do the
math.

Actually, the ADC can sample at F or F/N, as long as its trigger phase
can be shifted around in steps of 90 degrees. That needs a couple
flipflops.

If you sample both ends of the 50 ohm resistor, you know the vector
current.

I'm not sure those numbers work. For 10nH, for example, the L/R
constant is 200ps.

But you could digitally drive a constant current source, that works.
20mA 100kHz sine into 1uH makes 25mV p-p, or 25uV for 1nH.

That would be a cool instrument, something that would plot the vector
impedance and equivalent R/L/C components vs frequency. A cheap little
USB thing. I'd like to go down to 1 Hz for power magnetics.

Cheers,
James

1 volt across 50 ohms is 20 mA.

I know. I was thinking the signal level would be too small for
an ADC and the timing differences too small to resolve -- ADCs
aren't good at high-res 1MHz 25uV measurements.


Signal average. Noise is cheap and plentiful.

I would do a moving average and math in software, do they even make that
Motorola quadrature detector chip anymore?