Flicker noise voltage distribution.

I measured it using a 9V battery, a low noise preamp (homemade, about

I'm not claiming that anyone's measurements are wrong, only that there are lots of other sources of low frequency junk that are not flicker noise. Temperature drift is one very strong candidate.

Millikelvin temperature changes at low audio frequencies aren't that far fetched. The forcing is fairly wideband and the thermal mass is small. I have a bunch of really nice 1 ppm/K resistors that I'll try out in the next few days--bug me if I forget.

Cheers

Phil Hobbs

Phil,

Getting an amplifier with flicker noise that low near DC is pretty good. Any chance to post a schematic?

Thanks,

Mike Monett

That was several years ago...I probably still have it kicking around in a drawer in my lab.

It used iirc 3x Toyo-Rohm low noise transistors (2SD786) in parallel. The 1/f corner was around 1 Hz or a bit below (good BJTs are amazing).

Cheers,

Phil Hobbs

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Geesh Phil, Give Phil H. a break. He certainly knows more than me about noise. (Which isn't saying much, I only claim to be able to measure it at some level.) Isn't there some chance that the resistors he looked at had lower noise?

George Herold

"Phil Allison" "Phil Hobbs"

Geesh Phil, Give Phil H. a break.

(Which isn't saying much, I only claim to be able to measure it at some level.) Isn't there some chance that the resistors he looked at had lower noise?

All the results are *entirely consistent* with maker's published data and the laws of physics.

The direct opposite is true for Hobbs.

Planet Hobbs is not in this universe.

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Yeah, well my junk was just a 4nV/rtHz op-amp... and the rest home built electronics. And I'll claim nothing about my battery noise. But say can't I estimate the resistors thermal time constant. I've done this for pieces of metal and at least got within a factor of two or so. I guess I need to know the heat capacity and thermal diffusion coef. I have no idea what materials they make resistors out of.

George H.

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Phil, I'm certainly not questioning your results. But if there are lower niose MF resistors out there don't you want to know of them. A resistor with lower excess noise does not break any law of physics.

George H.

"Phil Allison"

Phil, I'm certainly not questioning your results.

But if there are lower niose MF resistors out there don't you want to know of them.

A resistor with lower excess noise does not break any law of physics.

Go away.

Plane Hobbs is waiting for you.

..... Phil

Heh, I'm having problems finding a datasheet in Engish. There's some tantalizing fragments, such as

en = 0.55nV / sqrt(Hz) (at 10Hz, 10mA)

There's also some discussions of the low noise at:

There's probably more but I didn't take time to check.

The 2SD786 seems to be available for $26.98, but there's no date so it's not clear if that price is still valid:

The Analog Devices MAT02 and SSM-2210 are less expensive at about $6.00/1k:

The Mat02 and SSM-2210 are also supposed to have a fairly low corner frequency, depending on the collector current. But it doesn't look to be as low as 1Hz for the 2SD786:

The SSM-2210 datasheet describes three devices in parallel, giving 500pV/SQRT(Hz) and a corner of 1.5Hz. It uses a red LED as a bias current reference voltage. This is also described in the Analog Devices app note AN-102:

So it would be very interesting to take a peek at your implementation if you have time to post it.

Thanks,

Mike Monett

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Say Phil H. I was thinking about this last night. What is the current noise from your low noise pre-amp? (I assume the typical 1 to 2 pA/ rtHz.) When looking at a 10 kOhm resistor isn't this going to swamp the wonderful low voltage noise?

George Herold

Mike,

Yes, those were nice parts. There was a discussion on this group awhile back about available alternative devices, but I don't have time to track it down just now.

I'll post the schematic once I find the gizmo and trace it out. I didn't write down the schematic anywhere, I don't think...for lab one-offs, I usually just do a few lines of algebra and start soldering, because near enough is usually good enough. There's a differential current amp based on those parts in my book, though.

I also want to find the lab book the results are written in before I get too dogmatic about the performance details. I might be confusing two different gizmos--the 0.55 nV gizmo with the very low 1/f corner would have had to have at least a few pA/sqrt(Hz) of current noise, and the current noise 1/f corners are usually a bit higher in frequency.

Cheers,

Phil Hobbs

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I've seen the Analog devices circuit reproduced in lots of places. You might notice that it uses the LED and transistor Vbe voltage reference trick mentioned by Walt Jung in an earlier post.

George Herold

Hi Phil,

Thanks for the info. I'll try to find the discussion you mentioned.

I'm really interested in your schematic. Often, a circuit will appear in an app note, but may have a typo, bug or error that could be hard to find. Or the component values could be non-optimal. So a schematic of an actual, working circuit could be very valuable.

This is more than idle curiosity - I'm trying to make a phase noise analyzer with emphasis on flicker noise. I'm hoping it might help indicate precision crystal oscillators that are prone to phase bumps.

These can occur randomly over intervals of a couple of days to six months or more, and it would be very nice to find some method of identifying oscillators that are likely to have the problem. This could save months of tying equipment up in precision phase measurements that could still miss an event.

I was also wondering about the current noise in your amplifier, and I'm glad you mentioned it. Wouldn't this be a problem trying to measure flicker noise in high value resistors? How do you handle something like this?

Regards,

Mike Monett

Try here:

--=20 Regards, Howard snipped-for-privacy@ix.netcom.com

That's great! Thanks, Howard.

Mike Monett

I have a number of NOS of these fine devices around, and $26.98 sounds like a *very* fair price. ;-)

Digikey Canada lists the MAT02FHZ-ND for $12.53CAD in singles

That's for a matched pair with max offset voltage drift of 0.1uv/c.

Which is pretty good when you want to measure flicker at DC.

The 2SD786 is a single, so you need two. That makes $53.96. The match and tempco is anyone's guess.

For $53.96, I can get 4 MAT02's and have enough left over for a beer and some resistors:

This should give about 1/SQRT(4) = 1/2 the rms noise of the 2SD786.

With the above analysis, it's hard to figure a fair price for the 2SD786. My guess is $3.00CAD might be in the right ballpark, as long as you pay the GST/PST, and the beer:)

Best Regards,

Mike Monett

I recently scored a bag of *66* 1N2929a germanium tunnel diodes as part of a surplus assortment. Probably >$1k worth if I sold them in onesies...maybe a retirement project, 20 years from now. ;) (If anyone wants one to play with, send me an email.)

I have probably 20 2SD786s left--I bought a bunch back in about 1992. More useful but less fun than the TDs.

Cheers,

Phil Hobbs

Phil,

Is there any chance the 1N2929a's might be used as a horizontal trigger in a Tek 465 or 475 scope, or perform some other function in an older piece of test equipment? Could they be adapted to replace an existing diode that has gone bad?

If so, the guys on the Yahoo forums

and

would sure like to talk with you.

If they are used in any HP equipment, the guys at

would also love to know.

RF Parts lists them for $13.90 each as of 5 November, 2008, with a minimum order of $25.00:

so you might be able to charge $15.00 + shipping & handling for one.

I hope you get rich quick:)

Now I have a question. Unlike the MAT02, the 2SD786 is a single transistor, and you need two for a differential amplifier. There is no guarantee of matching or offset tempco, so how do you use them in a circuit to measure flicker noise near DC? It would seem the drift would kill you.

Thanks,

Mike Monett