# Looking, QST construction article about measuring noise of a resistor

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Hi all,
I'm looking for a QST construction article about a circuit designed
for measuring the noise generated by resistors. I recall the design used a
paint can as the enclosure.
My interest is in the low noise amp design.

Mike

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

All resistors of a given value generate the same Johnson noise
voltage, so there's no practical reason to measure that.

Practical value metal film resistors have almost no shot (Poisson)
noise. I have measured excess noise in Gohm range cermet resistors,
but it was difficult. I used a Danish butter-cookie can.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/19 10:57 AM, John Larkin wrote:

Not so bad if you use the correlation method, which gets rid of the
voltage noise of the measuring gizmo.  (It doesn't help the current
noise of course.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
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Re: Looking, QST construction article about measuring noise of a resistor

The old, bulk carbon media resistors were the most noisy.

They quell HV arcs without damage real good though.

Re: Looking, QST construction article about measuring noise of a resistor
It generates the same noise power not noise voltage.

Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/19 1:11 PM, snipped-for-privacy@columbus.rr.com wrote:

It's a resistor.  Power is V**2/R.  And noise is random, so it's only
the power spectrum and amplitude statistics that we care about (or that
are well-defined).

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 10:11:50 -0800 (PST), snipped-for-privacy@columbus.rr.com
wrote:

If it generates the same power, won't it generate the same voltage?

Resistors of the same value generate the same Johnson noise voltage
density. Stray capacitance can round off the high frequency part.

--

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 2:20:00 PM UTC-5, John Larkin wrote:

I missed that you said all resistors of the "same value".  My bad

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

Close!  Not QST, but...

http://web.mit.edu/8.13/www/JLExperiments/JLExp43.pdf

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

The MIT experiment uses an SRS preamp, the SR560, to measure
resistor noise.  In AoE III's low-noise chapter, 122-pages,
four pages (512-515) discuss in detail the SR560's design
principles.  We show how to make even more quiet amplifiers.

--
Thanks,
- Win

Re: Looking, QST construction article about measuring noise of a resistor
On 2019/02/25 7:21 a.m., amdx wrote:

ned

d a

Mike

Not exactly what you are looking for but Heathkit's Audio Signal tracer
kit had a Noisy Resistor test feature. It put a voltage across the
resistor in parallel with the audio probe and you could easily find the
noisier resistors. Not calibrated at all, other then your ear.

You could use that concept for testing resistors with an op amp for
amplifying the noise using a capacitor for dc isolation if you can't
find the article.

John :-#)#

Re: Looking, QST construction article about measuring noise of a resistor
On Monday, February 25, 2019 at 10:22:11 AM UTC-5, amdx wrote:

It's not all that hard if you are OK with ~audio frequencies, opamps
and largish (10 k ohm) resistors.
https://www.teachspin.com/noise-fundamentals

You do need a lot of gain.

Some sort of shielding will most likely be needed.

I've seen designs will all sorts of involved grounding schemes.
But it works fine if you just nail one end of the resistor to

I can talk more if you are interested.

George H.

Re: Looking, QST construction article about measuring noise of a resistor
On Mon, 25 Feb 2019 10:06:01 -0800 (PST), George Herold

A 10K resistor makes a lot more noise than a good jfet opamp.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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Re: Looking, QST construction article about measuring noise of a resistor
Am 25.02.19 um 20:51 schrieb John Larkin:

It is not so much more. Good FET opamps run out of steam at 5.x nVrt Hz
That is abt. 1K5.

When there was a dual op amp in SO-8 that does 1nV/rtHz I had one less
problem.

cheers,
Gerhard

Re: Looking, QST construction article about measuring noise of a resistor
On Monday, February 25, 2019 at 3:13:29 PM UTC-5, Gerhard Hoffmann wrote:

Right, it looks better if you plot noise density (V^2/Hz)
Say as long as the noise 'experts' are here.
How do you disentangle voltage noise and current noise in a
bjt amp?  I guess I'm asking how you measure the current noise?

George H.

Re: Looking, QST construction article about measuring noise of a resistor
On Mon, 25 Feb 2019 12:23:36 -0800 (PST), George Herold

Short the input to see the voltage noise.

Then use resistors to see the current noise, corrected for Johnson
noise.

Maybe you could connect two opamp inputs together to a resistor or a
cap. Voltage noises wouldn't correlate, current noise would. Maybe
that's what Phil was talking about.

That's a useful concept: use identical gadgets to measure themselves.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/19 4:31 PM, John Larkin wrote:

A couple of FET buffers measuring the same resistor, amplified,
digitized and cross-correlated.  A scope can compute the cross-power
spectrum.  The key thing is that the voltage noise of each amplifier
affects only its branch, so that their contributions are uncorrelated,
and so average to effectively zero if you wait long enough.

The current noise of the buffers of course just adds to the resistor
noise and shows up in both branches, so you really need to use FETs.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
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Re: Looking, QST construction article about measuring noise of a resistor
On Mon, 25 Feb 2019 17:11:42 -0500, Phil Hobbs

You can buy opamps with current noise measured in aA per root Hz. I
wonder how they measure that.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/19 6:02 PM, John Larkin wrote:

They don't.  They just publish sqrt(2 e I_bias).  Leads to some fairly
shocking underestimates for parts with leakage cancellation, not to
mention drain circuit noise coupling via C_dg.

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 20:10:44 -0500, Phil Hobbs

Lies, damned lies, and data sheets.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics