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Thinking about a 80MHz crystal - as a detector

Discussion in 'Electronic Design' started by Winfield Hill, Mar 31, 2006.

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  1. I'm thinking about the possibility of using high-Q 80MHz crystals
    in a sensitive electric-field detector.* You know, consider the
    usual amplifier in a high-performance crystal oscillator, but
    without ANY feedback path, and further modified so the amplifier
    doesn't excite the crystal, not even a little.

    Looking at crystal models, the loss-resistance element may be on
    the order of 10 ohms, which implies a Johnson noise density on the
    order of 0.4nV per root-Hz. So an optimum kT-sensitive amplifier
    would need a similar noise level. Hmm, that could imply a rather
    large JFET with excessively-high capacitances. Certainly the
    JFET will be part of the crystal's tuning capacitance, but I'm
    probably going to be limited to say 10pF or less. A 2sk146 JFET
    has 1nV noise, but has 40pF of capacitance. A 2sk152 has 1.8nV
    with 8pF, that's getting closer. But 1.8nV is 4.5 times kT for
    a 10-ohm crystal...

    Maybe a low-noise BJT amplifier would be better... I have some
    2sd786 transistors, which state 0.55nV on the datasheet, for
    Ic = 10mA. Oops, then r_e = 2.5-ohms and with a beta of 500 Zin
    would be only 1.2k, not good enough to avoid loading down a sensor
    with Q = 20,000. BJT base-current noise would be another issue.
    Hah, JFETs suffer from a bit of current noise at RF frequencies,
    according to AoE, but no doubt much less than a BJT amplifier.

    Another issue, fundamental vs overtone mode crystals. I've read
    that fundamental-mode crystals have much lower Qs than overtone
    mode, e.g., Corning lists a 4:1 improvement for 3rd versus 1st.

    * Don't ask about the application just now. It's a bit exotic,
    the same experiment that's getting the 10kV 1us precision ramp.
     
  2. Any possibility of inserting a transformer at the front?

    steve
     
  3. Fred Bloggs

    Fred Bloggs Guest

    Or low impedance series resonance...
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Prove it- what makes you think it is Johnson noise, because it is
    represented as a resistor symbol?
     
  5. Fred Bloggs wrote...
    Acckk, how am I going to do that? I'm making an assumption.
    Maybe someone else can give us an answer, references, etc.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    It's back to basics for you. The book by Nye on the physics of crystal
    should have the answer. Physical Properties of Crystals, Oxford, still
    going through late editions, will be in nearly any technical library.
     
  7. Tim Shoppa

    Tim Shoppa Guest

    Just speculating, not sure what you're actually detecting, but do some
    Google searches for "microcalorimeter" and see if this is what you're
    trying to do. To oversimplify they are looking for energy deposited
    into a crystal giving some tiny phonons of quantum vibration, and
    looking for this at extremely low noise levels.

    I know the nuclear physics side (hint: these experiments are usually
    buried deep in mines or inside mountains), but always got lost in the
    solid-state phonon stuff.

    Tim.
     
  8. John  Larkin

    John Larkin Guest


    I don't understand this. Is the crystal itself going to be exposed to
    the e-field, or is there an antenna of some sort, connected to the
    crystal?

    If the crystal is used directly, is its metallization the antenna?

    Is there any advantage over using the crystal in the front-end, as
    opposed to matching+amplification followed by a narrowband, maybe
    crystal, filter? A well-matched fet amp can have a noise figure well
    below 1 dB.

    And of course, the crystal series resistance behaves, from a Johnson
    noise perspective, just like any other resistor. Conservation of
    energy, again. Ignoring microphonics.

    John
     
  9. Wouldn't 80MHz fundamental crystals be mesa crystals?
    How does their Q compare to bulk crystals?

    P.S. I think you'll get more grant money if you use MEMS resonators
    instead of boring old crystals. Nanotech would be even better.
     
  10. Phil Hobbs

    Phil Hobbs Guest

    The crystal's mechanical properties are actually somewhat anharmonic,
    which is why the overtones aren't in exact harmonic relationship. This
    could be used as a detection mechanism. Otherwise it sounds more or
    less like it's being used as a high-Q tank circuit for impedance
    transformation, which isn't altogether unreasonable.

    Providing that there's a dissipative process going on, this is right,
    and that's what's going on in the crystal. The fluctuation-dissipation
    theorem again.

    <dim-memory>
    There was that guy ten years or so ago who made a big stir with a
    "lossless resistor" for switching circuits. I never really learned the
    details, but he got a big IEEE medal or something for it. Does anybody
    remember?
    </dim-memory>

    Cheers,

    Phil Hobbs
     
  11. Tim Wescott

    Tim Wescott Guest

    AFAIK Johnson noise happens to any dissipative 'thing' that shows up as
    an electrical effect -- all that matters is the apparent resistance and
    the temperature. AoE actually mentions this.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  12. Tim Wescott

    Tim Wescott Guest

    Or an impedance matching network. One adjustable coil and a pair of
    caps will do almost anything you want them to, and you don't have to
    worry about the assembly being too narrowband compared to the crystal.
    If that doesn't float your boat then you can get 3:1 trifilar broadband
    transformers from Mini-Circuits that'll bring that optimal match up to
    90 ohms -- still not good, but better at least.

    Check out "Radio Frequency Design" by Wes Hayward -- he goes into detail
    about noise matching in an RF environment (which you should already
    know) but also about using transformer feedback to get a power match at
    the same impedance as a noise match.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  13. Ken Smith

    Ken Smith Guest

    Is there gas around the crystal? There will be noise from the atoms
    smacking into it, if there is.

    Crystals aren't very linear at very small or very large signals will this
    be a problem?

    Someone at Berkeley was using a tiny ring core to detect the magnetic
    field that the electrostatic one implies. You may want to google on this
    to see if it worked.
     
  14. Terry Given wrote...
    Puny, under 1000 watts? Some of us resemble remarks like that.
     
  15. Terry Given

    Terry Given Guest

    :)

    I wouldnt want a belt from one of your 10kV gadgets though
    (electroporesis perhaps?)

    I just scanned thru the last 6 years of IEEE professional comics, and
    couldnt find a mention of it. but upon further recollection I think my
    explanation is roughly correct, and the examples he gave were about 50W
    or so. the lack of mention this century suggests it wasnt so great after
    all though.

    I'm doing some design work at the moment for a 200KVA converter. Only
    problem is, there is nowhere to plug them in at home :( Even my
    neighbour (a joinery place) only has a 60A 400V outlet. So I'm going to
    make a 2kVA prototype first....

    fault currents get quite exciting with the big stuff. IGBTs limit fault
    current to around 10x their rated current (depending, of course, on Vg)
    so my 300Arms converter has to cope with a peak fault current of about
    4,000A. turning that off without going bang is a good trick.

    I tried to estimate last night how much money has been spent training me
    over the last 15 years, its a LOT. Tens of millions of dollars. No
    wonder smart companies work hard to retain staff. I once destroyed
    $1,000,000 worth of fancy flywheels, one at a time :) (OK, its not that
    bad, I tried to operate them at their rated load etc, and they all
    failed in various ways)

    on an analogous note:

    Air NZ is looking at laying off a whole lot of engineering workers in NZ
    (they do all their own aircraft maintenance, and contract to others
    too). I wondered why, until a friend who is in the military set me
    straight. A few years back, we neutered our air force - we no longer
    have planes with guns; any offensive air capability will only come about
    through hiring saudi pilots ;) so the air force no longer trains all the
    associated personnel. So Air NZ cant hire pre-trained technical staff,
    but has to train them itself. which it cant afford....

    Cheers
    Terry
     
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