Looking, QST construction article about measuring noise of a resistor - Page 2

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/19 9:16 PM, John Larkin wrote:
Quoted text here. Click to load it
There's a fun blog titled "Lies, Damned Lies, and Start-up PR".

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.
Re: Looking, QST construction article about measuring noise of a resistor
tirsdag den 26. februar 2019 kl. 03.23.25 UTC+1 skrev Phil Hobbs:
Quoted text here. Click to load it

https://www.kickstarter.com/projects/koleda/solus-the-most-efficient-radiator-in-the-world


Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/19 9:40 PM, Lasse Langwadt Christensen wrote:
Quoted text here. Click to load it
Yeah, like that. ;)

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.
Re: Looking, QST construction article about measuring noise of a resistor
On Mon, 25 Feb 2019 21:45:41 -0500, Phil Hobbs wrote:

Quoted text here. Click to load it

====snip====

Quoted text here. Click to load it
radiator-in-the-world
Quoted text here. Click to load it

 How, in Gawd's name, do they expect to get away with such bullshit!

 Oh, hang on, I know. It's the result of an education system designed to  
produce "Scam Fodder". :-(

--  
Johnny B Good

Re: Looking, QST construction article about measuring noise of a resistor
On Mon, 25 Feb 2019 18:40:43 -0800 (PST), Lasse Langwadt Christensen

Quoted text here. Click to load it


I'm sure there is a kickstarter lifestyle. Just invent something like
that every few months and live the good life. Many involve graphite or
nano somethings.

There's no legal limit to how many companies you can simultaneously be
the CEO of.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

We've slightly trimmed the long signature. Click to see the full one.
Re: Looking, QST construction article about measuring noise of a resistor
On Monday, February 25, 2019 at 4:31:40 PM UTC-5, John Larkin wrote:
Quoted text here. Click to load it
Yeah that's my only thought.  But whereas the voltage noise is a  
direct measure, for current noise you've got to subtract off the  
resistor noise.  One would also be tempted to use a large value  
resistor, which at some point is going to reduce the overall  
bandwidth.  
Quoted text here. Click to load it
Yeah the cross correlation trick gets rid of uncorrelated voltage noise
but now I've measured the sum of both current noises (from amps) and  
the resistor noise... so I'm not that much closer to measuring the  
current noise of one amp.  (One can also worry about equal bandwidths
in each channel.)  

George H.  
Quoted text here. Click to load it


Re: Looking, QST construction article about measuring noise of a resistor
On Tue, 26 Feb 2019 06:09:40 -0800 (PST), George Herold

Quoted text here. Click to load it

Don't you keep cryogens around? Freeze the resistor!

I think you can synthesize a room-temp active resistor that has below
Johnson noise.
  
Quoted text here. Click to load it

Seems like you could connect N opamps to one shared resistor, vary N,
very R, and do some math, to separate the resistor Johnson noise from
the opamp current-induced noise from the opamp voltage noise.

But you know the Johnson noise.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Looking, QST construction article about measuring noise of a resistor
On Tuesday, February 26, 2019 at 10:35:29 AM UTC-5, John Larkin wrote:
Quoted text here. Click to load it
Yabut, long wires going into cryogen adds C.  
(I know there are work arounds for that too.)  
Quoted text here. Click to load it

Right like with clocks, if you have three of them you can compare  
them in pairs and figure out a unique number for each.  

That is a lot of work, and measuring amp voltage noise is easy.

This says the same thing... pick the right resistor.
https://www.renesas.com/us/en/www/doc/application-note/an1560.pdf

George H.  
Quoted text here. Click to load it


Re: Looking, QST construction article about measuring noise of a resistor

Quoted text here. Click to load it

Good catch. Thanks.
  
Quoted text here. Click to load it

Re: Looking, QST construction article about measuring noise of a resistor
On 2/26/19 10:35 AM, John Larkin wrote:
Quoted text here. Click to load it

Sure.  A big resistor with positive feedback to the other end is the  
classical method, and at AC you can do it with transformers and so on.  
Good amplifiers are quieter than room-temperature resistors.

A good diode or a diode-connected transistor has an effective noise  
temperature of T_j/2, which gives a useful method for making  
not-too-high frequency noise measurements.


Quoted text here. Click to load it

Yup.  A resistor from the summing node to the inverting input and  
another one in the noninverting input works, at least at frequencies  
where the resulting gain peaking isn't important.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.
Re: Looking, QST construction article about measuring noise of a resistor
On Mon, 25 Feb 2019 21:13:24 +0100, Gerhard Hoffmann

Quoted text here. Click to load it

But the noise voltages add as sqrt-of-sum-squared.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Looking, QST construction article about measuring noise of a resistor
Am 26.02.19 um 03:49 schrieb John Larkin:
Quoted text here. Click to load it

Yes, they do. And for amplifiers, they disappear with sqrt(N).

<  
https://www.flickr.com/photos/137684711@N07/46175592942/in/album-72157662535945536/lightbox/
   >

I get < 220 pV/rtHz from these 10 ADA4898 pairs. That's about 3 Ohms.
The problem is that the noise current starts to hurt.
With JFET opamps I would get 1 nV/rt Hz, just a BF862 :-(
Why does no one make a serious FET op amp? Some Interfet IF-37 ?

(And Phil, thanks to remember me about that xcorrelation fail)
There are also lots of analog switches, a SPI receiver in a
Coolrunner for control and prehistoric HP optocpouplers.

S.E.D. is quite interesting as long as we do not talk about politics.

I also have made a FET version with 2 IF3602 pairs
<  
https://www.flickr.com/photos/137684711@N07/46382531782/in/album-72157662535945536/lightbox/
   >

Features 190 pV/rtHz. It needs more current than promised in the
data sheet, but the coolers are only there for thermal mass.

It has heroic bootstrapping of the cascode to remove the FET
capacitances but stability is sensitive against everything,
moon phase included, already in simulation. Part of the problem
is that I insist in 1 MHz BW.

I did remove the bias loop. Input coupling C must be _much_ bigger
for noise reasons than for f-3dB. And the bias loop must be much
slower than input RC. One is constantly waiting.

A TL431 minus one BAV99 drop and 1/3 therefrom  is about
the right temp-compensated FET bias.

The right thing for rainy weekends.

Cheers,
Gerhard

Re: Looking, QST construction article about measuring noise of a resistor
Gerhard Hoffmann wrote:
Quoted text here. Click to load it


What's the low cut-off frequency you need?
Is it a high Zin or should it be 50 Ohms?

Jeroen Belleman

Re: Looking, QST construction article about measuring noise of a resistor
Am 26.02.19 um 11:23 schrieb Jeroen Belleman:
Quoted text here. Click to load it


Quoted text here. Click to load it

I'm open to suggestions.

The DUT impedance must be low against 2 or 3 Ohms, or such a
low noise amplifier would not make any sense from the beginning.

The input impedance would have to be 10 or 100 times larger, so that
it does not make the DUT noise collapse.

For cross correlation, one would have 2 amplifiers in par. and at
2*50 Ohms for example, the noise current would make heavy
xcorr / averaging pointless.

The lower frequency corner of the amplifier would not be required
by f-3dB but by the fact that the noise of the bias resistor must
be effectively shorted through the low impedance DUT.

At f-3dB, Xc and Rin are equal; that's a far cry from shorting.
And the non-shorted noise rises twice as fast as 1/f, so it is easy
to spoil the noise corner of the amplifier.

A real guru (Scott Wurcer of AD) saw in 100 ms that the input noise
of my first 20*ADA4898 was rising much too fast below 50 Hz.

The 20 10u WIMA foil caps in the first release were not enough; now
there is a 4700uF wet slug tantal.
$120.  :-(  AVX is much cheaper than Vishay.

The fat tantalum has proved to be dangerous for delicate DUTs; I have
zapped a LED with stored charge. Now there are shorting relays
that are sequenced properly by a BBB.

FET amplifiers make that easier. Their 22 Meg bias resistor may
produce more noise voltage absolutely - but the shorting is easier
without the sqrt, so the situation is a win after all.


BTW, there is another fail in cross correlation. For measuring the
added phase noise of an amplifier or sth. like that, a clean oscillator
signal is split (sp?) with a Wilkinson divider and one path gets the
DUT. The 2 paths then are cross-correlated.

Now, there is a 200 Ohm resistor typically in the Wilkinson splitter
and its noise goes into both outputs. But it anti-correlates
because at each instant one output has the + side of the resistor
and the other the - side.

cheers,
Gerhard

Re: Looking, QST construction article about measuring noise of a resistor
On Tue, 26 Feb 2019 10:59:25 +0100, Gerhard Hoffmann

Quoted text here. Click to load it

I guess there is an optimum drain current (hence Vgs) for gate current
noise. Zero gate current may not correspond to zero gate current
noise!

Vibrating reed electrometers have pretty low gate current!

Quoted text here. Click to load it

Most people don't have much to say about electronics.

Quoted text here. Click to load it

The NMR people like cooled jfets for their preamps. They have lots of
liquid helium, and like to keep their amps around 20K.

This is the opposite, FTMS preamp

https://www.dropbox.com/s/rgsv2hezpfgr4he/DSC03582.JPG?dl=0

jfet opamps in a vacuum with no cooling, and 1Meg gate resistors. It
was designed by a couple of chemists.  


Quoted text here. Click to load it

What signal source needs such low-noise gain? Something that doesn't
last, can't be signal averaged?

FTMS uses lots of signal averaging, but an ion in orbot eventually
collides with some residual gas molecule and gets wrecked, so
averaging time is limited.

Quoted text here. Click to load it

Spice is great for rainy weekends too.

Quoted text here. Click to load it

--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Looking, QST construction article about measuring noise of a resistor

Quoted text here. Click to load it

  It should be called Larkin noise, because you sure are guilty of  
spewing enough of it here despite claiming to be all about pure  
electronics, you have a way with weilding and using the shit pot  
stirring stick.

Re: Looking, QST construction article about measuring noise of a resistor
On Tue, 26 Feb 2019 15:55:16 +0000 (UTC), DecadentLinuxUserNumeroUno

Quoted text here. Click to load it

If you had a name, we could name something after you.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Looking, QST construction article about measuring noise of a resistor
On 2/26/19 4:59 AM, Gerhard Hoffmann wrote:
Quoted text here. Click to load it

Quoted text here. Click to load it

Quoted text here. Click to load it

Quoted text here. Click to load it

If you have a lot of BF862s in stock, they parallel very nicely if you  
use them near I_DSS.  I have about 30,000 of them for my retirement  
savings. ;)

The BF862's 1/f corner is near 1 kHz, but milliamp for milliamp it's  
hard to beat.  The current-production CPH3910 is about 1 dB quieter, but  
its I_DSS is inconveniently high.  I don't have enough experience with  
it to know how well it parallels.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.
Re: Looking, QST construction article about measuring noise of a resistor
amdx wrote...
Quoted text here. Click to load it

 You should read the 122 page chapter in AoE III, all
 about noise and low-noise amplifiers.  It's easy to
 make amplifiers be far more quiet than resistors.


--  
 Thanks,
    - Win

Re: Looking, QST construction article about measuring noise of a resistor
On Monday, February 25, 2019 at 2:05:48 PM UTC-5, Winfield Hill wrote:
Quoted text here. Click to load it

You can do pretty well with just opamps these days.  
On the practical front, we used FET opamps with 'no'  
(or negligible) current noise.  This makes it easy to  
separate the opamp noise from the resistor noise, by  
just changing the input resistor.  

(But your low noise chapter is great.. and well worth my  
re-reading.)

George H.  

Site Timeline