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Transformer thermal noise / *small* signal

Discussion in 'Electronic Design' started by Fred Bartoli, Jan 31, 2005.

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  1. Fred Bartoli

    Fred Bartoli Guest

    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 www.tech-chat.de)

    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, ...)
     
  2. James Meyer

    James Meyer Guest

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

    Jim
     
  3. John Larkin

    John Larkin Guest


    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:

    http://www.thinksrs.com/downloads/PDFs/Manuals/SR554m.pdf

    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
     
  4. 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
     
  5. Jeroen Belleman wrote...
    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.
     
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