Interesting inductor

[Phil Hobbs]

So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).

The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.

As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.

Searching on Digikey, I found this very interesting part:
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.

4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.

Pretty nifty, if true. (Parts on order.)

Cheers

Phil Hobbs

[John Larkin]

On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

I'm hassling with inductors now too, but at the other end of the speed
spectrum.

We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.

We could test all 2^n steps, make a list, and select the closest to
his request.

We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

[Phil Hobbs]

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant.

The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )

Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)

Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in. It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead.

There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability.

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain.

Post by John Larkin
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Fun. Analog computers forever!

Cheers

Phil Hobbs

[John Larkin]

On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant.
The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )
Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)
Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in. It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead.
There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability.

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain.

Post by John Larkin
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Fun. Analog computers forever!
Cheers
Phil Hobbs

We are about to publicly announce the P940, our modular power system.
It would be tragic if I make my fortune selling power supplies and
dummy loads that work in the single digits of KHz.

Making DACs with relays is humiliating.

[Phil Hobbs]

Post by John Larkin
On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant.
The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )
Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)
Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in. It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead.
There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability.

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain.

Post by John Larkin
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Fun. Analog computers forever!
Cheers
Phil Hobbs

We are about to publicly announce the P940, our modular power system.
It would be tragic if I make my fortune selling power supplies and
dummy loads that work in the single digits of KHz.

If that happens, I'll commiserate appropriately. ;)

Post by John Larkin
Making DACs with relays is humiliating.

Nah, relays are amazing. There are low-power muxes that come close,
e.g. the TMUX1511 (5 ohms R_on, 2 pF C_off), but nothing that will take
any sort of power.

Of course you can do similar things with tubes. ;)

Cheers

Phil Hobbs

[John Larkin]

On Wed, 13 Mar 2024 11:49:31 -0400, Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant.
The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )
Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)
Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in. It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead.
There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability.

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain.

Post by John Larkin
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Fun. Analog computers forever!
Cheers
Phil Hobbs

We are about to publicly announce the P940, our modular power system.
It would be tragic if I make my fortune selling power supplies and
dummy loads that work in the single digits of KHz.

If that happens, I'll commiserate appropriately. ;)

Buy me a beer that I can cry in.

Post by Phil Hobbs

Post by John Larkin
Making DACs with relays is humiliating.

Nah, relays are amazing. There are low-power muxes that come close,
e.g. the TMUX1511 (5 ohms R_on, 2 pF C_off), but nothing that will take
any sort of power.
Of course you can do similar things with tubes. ;)
Cheers
Phil Hobbs

If Ron * Coff is the figure of merit, in femtoseconds, no semi can
come within miles of a relay. We use a $1 DPDT telecom relay that is a
damned good 3 GHz 50 ohm switch.

Tubes don't score well by that standard, except krytrons maybe.

[Phil Hobbs]

Post by John Larkin
On Wed, 13 Mar 2024 11:49:31 -0400, Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant.
The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )
Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)
Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in. It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead.
There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability.

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain.

Post by John Larkin
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Fun. Analog computers forever!

We are about to publicly announce the P940, our modular power system.
It would be tragic if I make my fortune selling power supplies and
dummy loads that work in the single digits of KHz.

If that happens, I'll commiserate appropriately. ;)

Buy me a beer that I can cry in.

Post by Phil Hobbs

Post by John Larkin
Making DACs with relays is humiliating.

Nah, relays are amazing. There are low-power muxes that come close,
e.g. the TMUX1511 (5 ohms R_on, 2 pF C_off), but nothing that will take
any sort of power.
Of course you can do similar things with tubes. ;)

If Ron * Coff is the figure of merit, in femtoseconds, no semi can
come within miles of a relay. We use a $1 DPDT telecom relay that is a
damned good 3 GHz 50 ohm switch.
Tubes don't score well by that standard, except krytrons maybe.

Not identical things, just similar. Dragging a grid up to +200V quickly
and then leaving it there, with no turn-off charge injection and nearly
no capacitive loading, is a job for a tube. (I used an 811A for that
BITD--it even had a B battery for the plate and a C battery for the grid
bias.) :)

Cheers

Phil Hobbs

[john larkin]

On Wed, 13 Mar 2024 12:27:08 -0400, Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Wed, 13 Mar 2024 11:49:31 -0400, Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)
Cheers
Phil Hobbs

Couldn't you have a high tail voltage and a big resistor, or maybe a
string of smaller inductors? Or something. We've made super wideband
inductors from a string of various values.

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A
follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That
means that the noise of the follower and the second stage is not
insignificant.
The second stage is a VCVS active lowpass using an OPA818 at a gain of 10,
and the output stage is an OPA695 CFA inverter, to make the overall circuit
noninverting and provide a gain adjustment. (TE now makes a low-inductance
pot that’s nearly as good as the old Murata PVA2 ones that you use. )
Keeping the supplies simple is important, and so is avoiding ground loops.
The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then
using regulating cap multipliers. (The second and third stages’ supplies
are followers running off the quiet ones, to prevent unwanted feedback.)
Sooo, I want to run the follower on +7/0 if possible, which is where the
inductor comes in. It doesn’t save any power, on account of the
railsplitter, so I can probably use the -5 rail instead.
There’s no overall feedback in this version, because it’s hard to do
without trashing the noise performance and/or stability.

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.

We did something similar for choosing resistor taps in a low noise PGA.
Works okay, but is a bit of a pain.

Post by John Larkin
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Fun. Analog computers forever!

We are about to publicly announce the P940, our modular power system.
It would be tragic if I make my fortune selling power supplies and
dummy loads that work in the single digits of KHz.

If that happens, I'll commiserate appropriately. ;)

Buy me a beer that I can cry in.

Post by Phil Hobbs

Post by John Larkin
Making DACs with relays is humiliating.

Nah, relays are amazing. There are low-power muxes that come close,
e.g. the TMUX1511 (5 ohms R_on, 2 pF C_off), but nothing that will take
any sort of power.
Of course you can do similar things with tubes. ;)

If Ron * Coff is the figure of merit, in femtoseconds, no semi can
come within miles of a relay. We use a $1 DPDT telecom relay that is a
damned good 3 GHz 50 ohm switch.
Tubes don't score well by that standard, except krytrons maybe.

Not identical things, just similar. Dragging a grid up to +200V quickly
and then leaving it there, with no turn-off charge injection and nearly
no capacitive loading, is a job for a tube. (I used an 811A for that
BITD--it even had a B battery for the plate and a C battery for the grid
bias.) :)
Cheers
Phil Hobbs

My first paying job (50 cents per hour) was at LSUNO, a summer job
working with a physicist doing Stark effect microwave spectroscopy. I
built two high-voltage square wave generators for him, one with
thyratrons and one with giant transmitting tubes. The upper tube was
driven by a pulse transformer and as you say, only needed short grid
blips.

One thing we used for calibration was OCS, which has a giant Stark
effect and is deadly stuff.

https://en.wikipedia.org/wiki/Carbonyl_sulfide#Toxicity

Life was cheap in those days. Everybody played with mercury and such.

I wasn't a registered student (my plan was to go to Tulane, which had
an engineering school) and the rule was that only students at LSUNO
could be paid. So the dean of physics made a call and they assigned me
student number 20,000 on the theory that they'd never get there, which
I'm sure they have by now. It's now called UNO on the lakefront in
New Orleans.

[Clive Arthur]

On 13/03/2024 04:18, John Larkin wrote:

<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

[john larkin]

On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?

A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

[Clive Arthur]

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

Yes, I got part way down the road of designing a gyrator to block
telemetry signals on a power line comms device. Soon realised it would
need lots of power.

Just thinking out loud, and not really a serious suggestion, but would a
variac with a fixed inductor on the secondary work as a variable
inductor? I guess 500:1 would be impossible.

[Phil Hobbs]

Post by Clive Arthur

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

Yes, I got part way down the road of designing a gyrator to block
telemetry signals on a power line comms device. Soon realised it would
need lots of power.
Just thinking out loud, and not really a serious suggestion, but would a
variac with a fixed inductor on the secondary work as a variable
inductor? I guess 500:1 would be impossible.

Not a dumb idea at all. To avoid using a motor to turn it, one or more
transformers with binary-weighted windings and relays, maybe.

The inductance of the transformer needs to be large enough, of course.

I’ve occasionally considered using a transformer to make an isolated
version of a dpot, but it’s never been quite the right solution, mostly on
account of limited inductance.

Cheers

Phil Hobbs

[John Larkin]

On Thu, 14 Mar 2024 10:42:57 +0000, Clive Arthur

Post by Clive Arthur

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

Yes, I got part way down the road of designing a gyrator to block
telemetry signals on a power line comms device. Soon realised it would
need lots of power.
Just thinking out loud, and not really a serious suggestion, but would a
variac with a fixed inductor on the secondary work as a variable
inductor? I guess 500:1 would be impossible.

There may be a cae for using a tapped transformer to front-end a
single inductor, or two. That would need thinking, not my favorite
activity at 7:30 in the morning.

[Jasen Betts]

Post by Clive Arthur

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

Yes, I got part way down the road of designing a gyrator to block
telemetry signals on a power line comms device. Soon realised it would
need lots of power.
Just thinking out loud, and not really a serious suggestion, but would a
variac with a fixed inductor on the secondary work as a variable
inductor? I guess 500:1 would be impossible.

it's probably easier to just use the output terminals as a variable inductor.

[john larkin]

On Fri, 15 Mar 2024 05:56:38 -0000 (UTC), Jasen Betts

Post by Jasen Betts

Post by Clive Arthur

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

Yes, I got part way down the road of designing a gyrator to block
telemetry signals on a power line comms device. Soon realised it would
need lots of power.
Just thinking out loud, and not really a serious suggestion, but would a
variac with a fixed inductor on the secondary work as a variable
inductor? I guess 500:1 would be impossible.

it's probably easier to just use the output terminals as a variable inductor.

I need a surface-mount motarized variac!

General Radio used to make Variacs. I think they had the patent. The
brush was tricky.

GR was interesting. Long gone now.

[Bill Sloman]

Post by john larkin
On Fri, 15 Mar 2024 05:56:38 -0000 (UTC), Jasen Betts

Post by Jasen Betts

Post by Clive Arthur

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

<snip>

Post by john larkin

Post by Jasen Betts

Post by Clive Arthur
Just thinking out loud, and not really a serious suggestion, but would a
variac with a fixed inductor on the secondary work as a variable
inductor? I guess 500:1 would be impossible.

it's probably easier to just use the output terminals as a variable inductor.

I need a surface-mount motorized variac!

Actually what you need is a multi-tapped ratio transformer and a
multiplexer that can let you select the tap you need.

Litz wire might randomise it's conductor-distribution well enough to let
you use single strands for specific taps.

B.P.Kibble and G.H. Rayner's "Coaxial AC Bridges" ISBN 0-85274-389-0
talks about the construction of the transformer. B.P. Kibble is Brian
Kibble of the Kibble Balance.

https://www.amazon.com.au/Coaxial-AC-Bridges-B-Kibble/dp/0852743890

You'd need to stack up several of them to make a DAC-like structure.

The elements are precise and stable to about 1 part in 10^7 so you could
go three deep.

It wouldn't be an off-the-shelf part. If you excite the core with a
separate winding you can avoid first order resistive error, but there
would still be capacitative currents circulating in the sense windings.

[piglet]

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

I wonder if you could reduce the power supply needs a bit by switchmoding
the incoming current into big storage capacitors so the gyrator does some
energy storage and could make flybacks up to some limit?

[John Larkin]

On Thu, 14 Mar 2024 13:03:07 -0000 (UTC), piglet

Post by piglet

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

I wonder if you could reduce the power supply needs a bit by switchmoding
the incoming current into big storage capacitors so the gyrator does some
energy storage and could make flybacks up to some limit?

One of my engineers, in her job interview, suggested using a
buck-booster switcher to scale up/down one big inductor. That gives
continuous inductor value tuning. That was clever and got her hired,
but it's probably not practical.

[John Larkin]

Post by John Larkin
On Thu, 14 Mar 2024 13:03:07 -0000 (UTC), piglet

Post by piglet

Post by john larkin
On Wed, 13 Mar 2024 22:32:27 +0000, Clive Arthur

Post by Clive Arthur
<snip>

Post by John Larkin
I'm hassling with inductors now too, but at the other end of the speed
spectrum.
We want a programmable inductor, from maybe 1 mH to 500 mH or so,
maybe 100 mA. Sounds like an inductive DAC, a series string of
inductors with shorting relays. If the step inductance ratio were,
say, 1.8:1 we could have some hidden bits, more than the customer
sees, so we could get pretty close to his requested value.
We could test all 2^n steps, make a list, and select the closest to
his request.
We're simulating loads to an engine control computer, torque motors
and solenoids and steppers.

Gyrator?

We just yesterday had a brainstorm session about that. How can one
make a programmable electronic fake inductor?
A real inductor stores energy, and can do things like high voltage
flyback. So a fake inductor should store energy, or pretend to. It
could be done with a current shunt, a fast ADC, some math in an FPGA,
a fast DAC, and a big power amplifier with big power supplies. Too
much work.

I wonder if you could reduce the power supply needs a bit by switchmoding
the incoming current into big storage capacitors so the gyrator does some
energy storage and could make flybacks up to some limit?

One of my engineers, in her job interview, suggested using a
buck-booster switcher to scale up/down one big inductor. That gives
continuous inductor value tuning. That was clever and got her hired,
but it's probably not practical.

Or a buck-boost could scale a capacitor to look like an inductor.

[Bill Sloman]

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz).  The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
  That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true.  (Parts on order.)

Anything over 1uH has a ferrite core - probably a nickel-zinc ferrite at
those sorts of frequencies.

Minimising parallel capacitance is supposed to demand spacing the
winding wires by their own diameter, but that doesn't show up on the
drawing (and probably wouldn't even if they were doing it).

Definitely interesting.

[Cursitor Doom]

On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)

As you say, nifty. Do you have some means of verifying that Fo claim,
Phil? Even a NanoVNA would give a pretty good idea if it's really that
high.

Post by Phil Hobbs
Cheers
Phil Hobbs

[Phil Hobbs]

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)

As you say, nifty. Do you have some means of verifying that Fo claim,
Phil? Even a NanoVNA would give a pretty good idea if it's really that
high.

Sure, SRF measurements aren’t super subtle.

Cheers

Phil Hobbs

[Cursitor Doom]

On Thu, 14 Mar 2024 00:03:54 -0000 (UTC), Phil Hobbs

Post by John Larkin
On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs

Post by Phil Hobbs
So I'm doing a new lab amp product.
Our existing one is 500 Hz -- 20 MHz, 1.1 nV/sqrt(Hz).
The new one is aiming to be 10 kHz -- 200 MHz, 0.25 nV/sqrt(Hz). The
spherical cows love it, so we'll see when the test boards arrive later
this week.
As part of the design, I wanted to make an emitter follower with a
decent amount of inductance in series with its tail resistor, to avoid
the transistor turning off on fast negative edges and causing linearity
problems.
<https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82498F1472J000/697521>.
4.7 uH 0805 wirewound, with a self-resonant frequency of _210 MHz_,
which is several times higher than many other parts of that description.
That corresponds to an effective parallel capacitance of 0.12 pF,
about that of a resistor of the same size, despite all the copper windings.
Pretty nifty, if true. (Parts on order.)

As you say, nifty. Do you have some means of verifying that Fo claim,
Phil? Even a NanoVNA would give a pretty good idea if it's really that
high.

Sure, SRF measurements aren’t super subtle.
Cheers
Phil Hobbs

Good show. I'd be interested to know the result.