LED reference current source

If you remote the AC bridge stuff, you've got to shield a lot of wires. If you don't, you get a capacitively coupled quadrature signal that the PSD turns into an offset vs phase error. All those wires and shields leak a lot of heat.

If a Kelvin resistor makes an AC voltage, you've got to subtract something from that to make an AC error signal. The thing that you subtract has to come from somewhere and has to track the RTD drive current. If it's outside the oven, it will have big TC errors. A bridge with all the resistors inside the oven doesn't have that problem; the resistors have to be stable but don't need especially low TCs.

Hey, put all that AC drive and detection electronics... in another oven!

If the amplifier has some given bandwidth, it doesn't matter if it's a lowpass or integrator after a PDS, or just after a DC error amplifier. There's no magic advantage to be had from modulating and then demodulating. But there is complexity and more things to make errors.

Chopamps have drifts in single-digit nanovolts per degree C, and, with the amp next to the RTD inside the oven, even that's just a tiny change of slope on top of the big RTD signal. So keep the DC excitation small. A little self-heating is no big deal unless it boils helium.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Try hunting down patents relating to "resolver tracking bridge" they should be well expired by now.

?-)

al

ensor,

This is the crucial advantage. Whit3rd understands it, even if John Larkin doesn't.

John Larkin doesn't appreciate the significance of the 1/f noise corner - for pretty much every real device the 1/f noise kicks in some frequency between 100Hz and 10Hz and starts rising as you lower the frequency.

-- Bill Sloman, Nijmegen

An area where Jamie has some expertise.

The 1988 project where I put AC excitation and demodulation on a 2" diameter board was driving and detecting the output from an LVDT in a high precision weight sensor in the Cambridge Instrument/Metals Research GaAs crystal puller.

AC excitation has advantages for a resistive bridge, which is why Larsen - at the US National Standards Lab - used it in his micro- degree thermostat set-up

Larsen N T 1968 Rev. Sci. Instrum. 39 1=9612

You'd have to be able to read complicated stuff to get the idea, and that does seem to be beyond you.

I wonder what Jamie intended to mean by that? It's nonsense, like most of his output.

So you do want to eliminate it

That seems to be John Larkin's delusion too. It's wrong.

Tell Larsen about it. I'm not surprised that AC excitation is complicated enough to frighten the life out of you - you can't process even slightly complicated ideas.

It's a polite way of saying that John Larkin is posting rubbish. You don't understand that kind of implication, do you.

You don't know much, and while what you do know may work for you, the rest of the world has to cope with more complicated ideas.

you

It's not raining at the moment, so your prayer does not seem to have been answered. You are probably praying to a god who doesn't happen to exist.

-- Bill Sloman, Nijmegen

All gibberish.. 100%.. your degrading mental faculties are showing.

Jamie

I

you

All too complicated for Jamie to process. Presumably he's been dyslexic all his life, so his mental faculties are sea-green incorruptible - further degradation isn't possible.

-- Bill Sloman, Nijmegen

Sloman doesn't want to discuss electronics. He just needs an audience where he can say how smart he is and how stupid and uneducated everyone else is. He has no doubt exhausted all available pools of people where he can do that in person. His ancient temperature controller paper was the pinnacle of his career. He is depressed, and richly deserves to be.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Chopamps have no 1/f noise. Metal film resistors and and platinum RTDs make no 1/f noise, except that they do sense temperature.

TSSOP-packaged chopamps weren't available 30 or 40 years ago, when you were actually designing electronics.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Lark> >

One nice thing about making downhole instruments is that their lifetimes are short and difficult, so you get to sell lots of copies.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

The DC approach requires one sensor, four metal film resistors, one cap, and one SC70 chop amp. (Do look at the OPA387 datasheet, it's a good read, and actually tells the truth AFAICT.) That will easily fit in one square centimetre, which makes putting it inside the controlled volume very attractive, and that gets rid of the thermocouple offsets. Good luck getting anywhere near that with an AC approach. That's what I meant by AC being much more complicated.

One crummy temperature control loop is a nit compared to the actual complexity of an instrument, regardless of how it's built.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Lark>> >

The gear they used to instrument underground nuclear tests were even better. They sent their data up a cable microseconds before they turned to plasma.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

As long as you're in the thermal-mass limit (where diffusion is fast compared with the one-pole response), adding mass doesn't hurt you much, because the phase shift is constant on the slope of the rolloff, so you can add high frequency gain to compensate.

It's distance that kills you, because once you're in the diffusion-dominated case, every 1/e of rolloff gets you another radian of phase shift, and there's no way to recover from that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

John Lark> >> >

Yeah, but they still use drillholes!

Did you ever watch the Operation Ivy videos on archive.org? They show some of that stuff, e.g. analogue instrument panels watched by TV cameras, with a radio link to the control centre on the USS Estes.

The Castle Bravo shot was one of those disasters that happen when you're too successful for your own good: it was a lithium deuteride bomb that was supposed to be about 5 MT, but ran away to 15 MT because both lithium isotopes turned out to contribute, instead of just one as they thought. A bunch of Pacific Islanders and Japanese fishermen got irradiated due to the huge fallout cloud. :(

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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Compare the performance of a Blumlein bridge with a Wheatstone bridge. There are more advantages to AC excitation that just getting rid of thermocouple voltages and other 1/f noise sources.

H K P Neubert knew that back in the 1940's and his book - "Instrument Transducers" - which I read as graduate student in the Melboure Unversity Physics library around 1968 - spells out the advantages in some detail.

Modern surface mount components and programmable logic parts let a skilled electronic engineer squeeze the more complicated circuitry into a fairly small compass. John Larkin doesn't seem able to comprehend this.

That depends on the instrument.

-- Bill Sloman, Nijmegen

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Not so much the sort of trivial electronics that you like to boast about.

I much prefer talking electronics to people who do know what they are talking about. This does happen here, but it doesn't lead to long and acrimonious threads. The under-educated can take a while to realise that they've got it wrong.

Far from it. I'm part of a happy three-way e-mail conversation at the moment between my ex-boss in London and my confocal microscope mate in Melbourne. Most of it is way over my head but it is really very interesting.

1996 isn't ancient. And it certainly wasn't the pinnacle of my career, which had happened a few years earlier with the multi-flash stroboscopic electron beam tester. Bits of that project did get into the literature, but the project got canned after we'd we'd got the prototype working, but before we'd sold any of them, and hardly anybody cites the papers.

Because I don't praise John Larkin's dubious skills sufficiently enthusiastically? Probably a rather subjective assessment.

-- Bill sloman, Nijmegen

ensor

or.

ostat)

I could get it all onto a 2" diameter board back in 1988 using through hole components. You may not be able to imagine doing as well, but that a defect in your imagination.

Think about a Blumlein bridge. It's appreciably better than a Wheatstone bridge, but requires AC excitation.

An incompetent engineer might want to take that approach.

-- Bill Sloman, Nijmegen

So, twist the exciting pair, and the sensing pair. At 1 kHz, they don't need elaborate shielding, and pickup (millihertz bandwidth!) isn't a problem. Figure 100 ohms for RTD, 5 pF stray capacitance, 1 kHz frequency... the resistor wins. Any capacitive part of current is out-of-phase for the synchronous rectification, doesn't get sensed anyway. Minimize heat leak by long zigzag section of the cable.

Not big; it can be a bridge, and tempco of multiple similar resistors will be small, and mis-tracking smaller. More to the point, it's a big signal after amplification and rectification.

Or at least near a heat spreader, or inside a windshield. Those sensitive parts, though, aren't nearly as sensitive as the RTD. Keep 'em away from open flame, though...

cryostat)

It doesn't get sensed if the phase angle is precisely 90 degrees. It gets sensed big-time if the phase drifts a tiny bit for any reason.

Why not put a tiny chopper opamp right next to the RTD, *inside* the oven?

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

I have an HP 5246L and use an HP 5248M nixie tube counter. I also have a mess of somewhat functional plug-ins:

2nd box from the bottom on the right. The 4256L is buried under a pile of things to fix, someday, maybe...

The 5248M works fine, but the 5246L probably needs all new caps. Basically, it's a 100MHz counter, with front end mixer to downconvert to below 100MHz. For it to read correctly, I have to know the frequency within 100MHz. That's usually not a problem unless I have to deal with a comb line spectral mess.

Notice what's sitting in the middle of this pile of e-Waste:

I should have grabbed it when nobody was looking.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558