Transformer thermal noise / *small* signal

Feb 01, 2005 4 Replies
J

Just a random thought, but amorphous (Metglas) cores aren't crystalline so may not have the grain boundaries that make core noise.

Where do you get the 200 pv/rthz parts?

Here's something similar:

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The PDF doesn't have schematics (those pages are blank) but the old printed manuals probably do.

Further random thought: a bank of narrowband, tuned step-up matching networks could drive a lot of separate amps, whose output was combined... but not this wideband, of course.

John

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J

On Tue, 1 Feb 2005 00:25:17 +0100, "Fred Bartoli" wroth:

Just a little. I'd recommend looking at amorphous, MetGlass, core material.

Jim

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F

Hello, For a wide band (0.1Hz-1MHz) low impedance small signal source I need to design a room temperature ultra low noise high gain preamp.



The specs are about 50nV rms (yes!) integrated over the 1MHz band, which is



50pV/root(Hz) noise density.

Some system specificities allow me to build the preamp like this :



.-----------------. | | | 200pV/root(Hz) | IN | | --------+-----------| G=10000 |----. .----------------. | | | | | | | | | | | | | | | | | '---| LP | | | '-----------------' | | | | | | | OUT --- |----------| SUM |----- --- .-----------------. | | | | | | .---| | | | 1:4 | 200pV/root(Hz) | | | HP | | | | | | | | | '---. ,-----| |----' | | | )|( | G=10000 | '----------------' )|( | | -' '- | | | '-----------------' === GND (created by AACircuit v1.28 beta 10/06/04

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having a 200pV/rt(Hz) noise density in the low frequency region and



50pV/rt(Hz) in the high frequency region (expected crossover about 10kHz).

At this level there are lots of noise sources to chase, one being the transformer thermal noise. Windings noise will be low but core magnetics thermal noise might be a pb (magnetic domain noise).



Any experience with this phenomena?



One hour googling didn't give anything useful (like orders of magnitude, PSD shape, ...)



Thanks, Fred.
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W

Those are nice parts, not the lowest e_n by any means, 0.8nV at 100kHz, but pretty damn low for a medium-sized JFET with Ciss only 10pF.

A nice reference. It should be noted that these fellows used ordinary Siemens N48 ferrite pot cores in their transformers. They also used Toshiba 2sk170 JFETs, with slightly-lower e_n, but 3x higher Ciss = 30pF.

Thanks, - Win
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J

In a low noise pre-amp I built some years ago, I used Vacuumschmelze Vitrovac amporhous alloy cores, both in the feedback and in the input noise matching transformer. The transformers did not contribute any noise that I could see.

The amplifier was a 300pV/rtHz 20kHz-75MHz device with a gain of 26dB and

50 Ohms input impedance. The input transistors where three Philips BF862 JFETs in parallel.

Maybe you can draw some inspiration from the description of the following

65pV/rtHz 5Hz-100kHz amplifier:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet Low-noise preamplifier with input and feedback transformers for low source resistance sensors Rev. Sci Instrum. 63(3), March 1992, p2089

Regards, Jeroen Belleman

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