Sensing small inductances

[Joseph Gwinn]

On Aug 30, 2019, Jeroen Belleman wrote
(in article <qkc66k$1kk$1@gioia.aioe.org&gt:

On 2019-08-30 18:26, Joseph Gwinn wrote:
On Aug 29, 2019, jlarkin@highlandsniptechnology.com wrote
(in article<c05hmelgktf15aj83l3akrpp53o2jjqp4u@4ax.com&gt:

On Thu, 29 Aug 2019 21:45:29 -0400, Joseph Gwinn
joegwinn@comcast.net> wrote:

On Aug 29, 2019, jlarkin@highlandsniptechnology.com wrote
(in article<hqofmet497f174cr4i1ddbrr64ipf7pao5@4ax.com&gt:
something going nonlinear before it goes nuclear.

I do have a question. One can build an oscillator using an EFDA amplifie

and a length of optical fiber in a ring connecting input to output. Wher

is the
negative resistance here?

It's not a 1-port electrical resonator, so I don't know.

OK. I agree that a one-port amp must present a negative effective resistance
to the resonant part of the circuit to support oscillation. But not all
oscillators are one-port.

One can also build an oscillator with an amplifier and a length of
twisted-pair transmission line connecting input to output. Someone got a
patent for putting a half-twist in the line, to get the 180 degree phase
shift. Nice and small, fits into an IC.

Joe Gwinn

The idea of negative resistance does not fit for such
oscillators, I agree. I've done that with ECL logic gates
as the amplifier. Very nice for snappy stop/start behaviour.
Gosh, that was in the late 1980's, over 30 years ago.

Yeah. I found the references to the twisted-pair oscillator, from 18 years
ago:

"Rotary traveling-wave oscillator arrays: a new clock technology", J. Wood,
T.C. Edwards, and S. Lipa, IEEE Journal of Solid-State Circuits, Volume: 36,
Issue: 11, Nov 2001, Page(s): 1654 - 166, DOI: 10.1109/4.962285
(https://doi.org/10.1109/4.962285). This is behind a paywall, but there are
full-text copies all over the internet.

Joe Gwinn

 

[Steve Wilson]

jlarkin@highlandsniptechnology.com wrote:

On Sat, 31 Aug 2019 00:03:59 GMT, Steve Wilson <no@spam.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

On Fri, 30 Aug 2019 10:44:00 GMT, Steve Wilson <no@spam.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

Jeroen's sim, run in time domain, clearly shows the negative
resistance that an emitter follower can present.

Jeroen's sym does not reflect reality. How do you control the
amplitude if an oscillator is running on negative resistance.

Any stable oscillator, with any gain mechanism, has to have a
nonlinear mechanism to set the oscillation amplitude.

Not true.

In a cc Colpitts, you set the amplitude by changing the current through
the emitter follower. This changes the energy delivered to the tank so
it is equal to the loss in the tank. This is a perfectly linear
operation.

What determines the amplitude of the voltage in the tank?

The amplitude of the current pulse from the emitter sets the tank voltage.
This dumps energy into the tank. The two capacitors across the tank are in
series. The tank voltage is monitored by the base of the transistor to tell
when to deliver the current pulse. The power is delivered by the emitter to
the bottom capacitor. The capacitors act as a voltage multiplier the same
as in an autotransformer.

If the gain element delivers constant output power regardless of
input, it's nonlinear.

In a Pierce, the resistor from the CMOS output to the tank sets the power
level. You are feeding a constant amplitude square wave into a constant
impedance tank. The power increases until the power delivered to the tank
equals the power lost in the tank. You set the power level by changing the
value of the resistor.

In a cc Colpitts, the power is set by the emitter current. This is adjusted
by changing the emitter resistor. As the resistance decreases, the current
pulse delivered to the tank increases and the voltage increases.

If it's linear, how does it know exactly how much gain to have to keep
the amplitude constant?

You set the value of the emitter current to reach the desired amplitude.
This is reached when the power delivered to the tank equals the power lost
in the tank.

Meanwhile, you watch the tank voltage to ensure it doesn't approach VCC,
which would forward bias the base-collector junction and cause clipping,
which increases the noise in the oscillator.

You also monitor the base-emitter voltage to ensure the voltage does not
approach the reverse breakdown junction voltage. This would also generate
noise and could harm the transistor.

For crystal oscillators, you monitor the current through the crystal. Since
you know the ESR of the crystal, you also know the current through the ESR.

You calculate the power with the formula P = I^2 * R and compare this to
the recommended power level from the crystal manufacturer. You adjust the
emitter resistor until the power meets the manufacturer's value.

 

[Steve Wilson]

Steve Wilson <no@spam.com> wrote:

Hey John,

I am pleased you are asking these questions about the Colpitts oscillator.
There are a lot more questions to ask. The harmless looking Colpitts is one
of the most complex circuits in electronics.

To completely understand the operation, try thinking of how the oscillations
start and how they build up then level off. This should raise some more
questions to think about.

Some of these questions are hard to answer. You have to keep so many things
in your head simultaneously. It is very good mental exercise.

Thanks

 

[bitrex]

On 8/30/19 11:39 AM, jlarkin@highlandsniptechnology.com wrote:

On Fri, 30 Aug 2019 09:34:23 GMT, Steve Wilson <no@spam.com> wrote:

Steve Wilson <no@spam.com> wrote:

George Herold <gherold@teachspin.com> wrote:

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit.

There is no excuse for railing an oscillator.

Post one of your designs and I'll fix it for you.


Many oscillators inherently rail, for example common CMOS XOs. There
is no other amplitude limiting mechanism. Nothing wrong with that.

Some oscillators self-rectify at the gate or base and bias themselves
off at some not-hard-railed amplitude. I invented this one when I was
a kid; it flew on the C5A.

https://www.dropbox.com/s/gisv3uqrm5wb61m/LC_osc.JPG?raw=1

The p-p amplitude is almost exactly 2x V+, and it has a tiny flat on
the negative swing of the sine wave. At the negative swing peak, it
steals its own base bias. It even has a near-zero amplitude tempco.

that's used as an LCD backlight inverter circuit in many products from
the 1980s, except the collector winding in that drawing is instead
coupled to the base via the base capacitor, and the collector goes to
VCC thru an RF choke

 

[bitrex]

On 8/30/19 11:39 AM, jlarkin@highlandsniptechnology.com wrote:

On Fri, 30 Aug 2019 09:34:23 GMT, Steve Wilson <no@spam.com> wrote:

Steve Wilson <no@spam.com> wrote:

George Herold <gherold@teachspin.com> wrote:

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit.

There is no excuse for railing an oscillator.

Post one of your designs and I'll fix it for you.


Many oscillators inherently rail, for example common CMOS XOs. There
is no other amplitude limiting mechanism. Nothing wrong with that.

Some oscillators self-rectify at the gate or base and bias themselves
off at some not-hard-railed amplitude. I invented this one when I was
a kid; it flew on the C5A.

https://www.dropbox.com/s/gisv3uqrm5wb61m/LC_osc.JPG?raw=1

The p-p amplitude is almost exactly 2x V+, and it has a tiny flat on
the negative swing of the sine wave. At the negative swing peak, it
steals its own base bias. It even has a near-zero amplitude tempco.

e.g. <https://imgur.com/a/4691B1H>

 

On Wednesday, August 28, 2019 at 11:18:56 AM UTC-4, bitrex wrote:

On 8/28/19 10:03 AM, dagmargoodboat@yahoo.com wrote:

I have a power budget and a low supply voltage constraint (~3 volt) in
the project so speeding that oscillator up to where the test inductors
have a higher Q and I could just frequency count with a fast comparator
into a fast uP is gonna be a problem.

I feel my options are either to a) use a trick to boost the intrinsic
tank Q at lower frequency and use multiple measurements as Jan suggested
in his post, or use a suggestion like yours and not try to make the DUT
part of an oscillator circuit at all and do it indirectly.

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.

You're making it too complicated. A triangle-wave current drive
makes a squarewave voltage across your inductor, of amplitude
proportional to the unknown inductance. In-phase demodulating
removes the e.s.r. artifact.

An ideal inductor >> Lx makes a decent triangle-wave current
approximator. You could also consider it as an inductive voltage
divider.

Plop this into LTSpice with a 500kHz 5-ohm 3.3V CMOS squarewave drive,
and look at the voltages at Lx and Vout.

-. 47uH
| .-.-.-.
|----||---' ' ' '-----+-----> Vout
| C1 L1 |
-' 470nF .-.
| | R.esr
'-'
|
)
) Lx
) 10-500nH
|
===

All you need is some gain, and a few CMOS switches for a demodulator.

Cheers,
James Arthur

 

[Michael Terrell]

On Sunday, August 25, 2019 at 2:18:22 PM UTC-4, Jan Panteltje wrote:

Often I wonder (video is my background) if people even know how to adjust a monitor,

I know how to calibrate my monitors, and I ignore anything that is so sloppy that it can't be viewed on a properly setup display. To me, it indicates that anything related to crap images was only posted to make the owner think he is smarter than he really is.

A Video professional would be quite embarrassed to post that crap. I suppose you maladjusted every monitor you touched at the TV studio rather than correct a video problem? I would have fired anyone who did that, and I was the chief engineer at two of the tree TV stations I worked at. I turned down the job of chief at a forth station, that was looking for their third Chief in under a year for going on air.

 

On Sun, 1 Sep 2019 17:32:19 -0400, bitrex <user@example.net> wrote:

On 8/30/19 11:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 30 Aug 2019 09:34:23 GMT, Steve Wilson <no@spam.com> wrote:

Steve Wilson <no@spam.com> wrote:

George Herold <gherold@teachspin.com> wrote:

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit.

There is no excuse for railing an oscillator.

Post one of your designs and I'll fix it for you.


Many oscillators inherently rail, for example common CMOS XOs. There
is no other amplitude limiting mechanism. Nothing wrong with that.

Some oscillators self-rectify at the gate or base and bias themselves
off at some not-hard-railed amplitude. I invented this one when I was
a kid; it flew on the C5A.

https://www.dropbox.com/s/gisv3uqrm5wb61m/LC_osc.JPG?raw=1

The p-p amplitude is almost exactly 2x V+, and it has a tiny flat on
the negative swing of the sine wave. At the negative swing peak, it
steals its own base bias. It even has a near-zero amplitude tempco.

that's used as an LCD backlight inverter circuit in many products from
the 1980s, except the collector winding in that drawing is instead
coupled to the base via the base capacitor, and the collector goes to
VCC thru an RF choke

That's a heap of "except for."

I did that oscillator somewhere in the late '60s. The intent was to
get a very stable amplitude and frequency sine wave, to excite a
magnetic-pendulum inclinometer, to measure the level-ness of the
aircraft.

 

[bitrex]

On 9/3/19 7:27 PM, jlarkin@highlandsniptechnology.com wrote:

On Sun, 1 Sep 2019 17:32:19 -0400, bitrex <user@example.net> wrote:

On 8/30/19 11:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 30 Aug 2019 09:34:23 GMT, Steve Wilson <no@spam.com> wrote:

Steve Wilson <no@spam.com> wrote:

George Herold <gherold@teachspin.com> wrote:

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit.

There is no excuse for railing an oscillator.

Post one of your designs and I'll fix it for you.


Many oscillators inherently rail, for example common CMOS XOs. There
is no other amplitude limiting mechanism. Nothing wrong with that.

Some oscillators self-rectify at the gate or base and bias themselves
off at some not-hard-railed amplitude. I invented this one when I was
a kid; it flew on the C5A.

https://www.dropbox.com/s/gisv3uqrm5wb61m/LC_osc.JPG?raw=1

The p-p amplitude is almost exactly 2x V+, and it has a tiny flat on
the negative swing of the sine wave. At the negative swing peak, it
steals its own base bias. It even has a near-zero amplitude tempco.

that's used as an LCD backlight inverter circuit in many products from
the 1980s, except the collector winding in that drawing is instead
coupled to the base via the base capacitor, and the collector goes to
VCC thru an RF choke

That's a heap of "except for."

I did that oscillator somewhere in the late '60s. The intent was to
get a very stable amplitude and frequency sine wave, to excite a
magnetic-pendulum inclinometer, to measure the level-ness of the
aircraft.

There's a paper somewhere showing all tuned indutively-coupled
one-transistor feedback oscillator topologies are essentially equivalent
in function, where you decide to inject the feedback prolly depends on
your desired frequency range and output power requirements/active device
characteristics