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NNTP-Posting-Date: Wed, 13 Mar 2024 15:49:32 +0000
Subject: Re: Interesting inductor
Newsgroups: sci.electronics.design
References: <f6aba241-3e20-db1f-36ea-8641ca6fac49@electrooptical.net> <rf92vitlvsi39u3blfak7papobugd8p68g@4ax.com> <uss7c0$v3a8$1@dont-email.me> <ijf3viln425o1sul0cv61b3tg6m71c22ug@4ax.com>
From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Message-ID: <c39e4436-7ae9-3645-af15-60bde447d303@electrooptical.net>
Date: Wed, 13 Mar 2024 11:49:31 -0400
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On 2024-03-13 10:59, John Larkin wrote:
> On Wed, 13 Mar 2024 12:49:04 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> John Larkin <jl@997PotHill.com> wrote:
>>> On Tue, 12 Mar 2024 23:17:57 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> 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
>>>
>>> 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.
>>
>>>
>>> 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.
>>>
>>> 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. ;)
> 
> 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

-- 
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