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Noiseless Damping?

Discussion in 'Electronic Design' started by Vladimir Vassilevsky, Mar 12, 2008.

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  1. There is a loop receive antenna. It is the LC circuit with the Q at
    the order of 100. It has a high resonant peak. The frequency and the
    amplitude of the peak are sensitive to the component values,
    temperature, particular parts, etc. I would like to damp the Q down to
    something like 10, so the parameters would be much more stable and
    predictable. This can be done by adding a series or a parallel resistor
    to LC.

    However the SNR of the antenna drops approx. proportionally to sqrt(Q)
    due to the thermal noise of the damping resistor. I wonder if it could
    be possible to implement the noiseless damping. Cooling down the
    resistor is not an option.

    One idea is to switch the damping resistor on-off with a high frequency
    and a low duty ratio. Or to switch or vary the value of the capacitor so
    the resonance peak will be dithered. It has many side effects, though.

    Any ideas, suggestions?

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
  2. De-tune the antenna?
  3. John Larkin

    John Larkin Guest

    What's the frequency?

    1. If this is below, say 30 MHz, ambient noise could well be high
    enough that the resistor doesn't do much additional harm, especially
    if you stop at Q=10.

    2. You can synthesize a "cold" resistor using a super-low-noise
    semiconductor, if you get the impedances right. Numbers like 40K might
    be possible. But a perfect resistor still eats signal energy.

    3. Or you could turn the loop into a bandpass filter; add a small
    additional LC resonator to make a double-hump resonance. Aren't you a
    filter guy?

    4... Oops, latte is gone, back to work.

  4. Joop

    Joop Guest

    If possible you can lower the L and increase the C both by a factor of
    5 or so.


  5. mpm

    mpm Guest

    I have it on good authority and experience that "Antennas make poor
  6. I thought that, too. Unfortunately, the antenna efficiency is too low so
    the resistor noise is dominating at Q = 10.
    Do you mean something like a small value resistor bootstrapped by the
    active circuit? Thank you for the good idea! That's what I am going to try.
    The values appear to be inconvenient for the passive inductors. GIC
    cound be the option, however got to watch for the noise.
    Thank you for your advice, John.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
  7. Mark

    Mark Guest

    Couple the loop more tightly to your receiver input?

  8. Wimpie

    Wimpie Guest

    Hello Vladimir,

    Your information is very limited but is it an option to generate the
    resistance by active parallel Feedback? As the feedback resistor is
    large enough, the impact on noise performance is minimal, but the
    effect is larger because of the gain of the input amplifier.

    Maybe you can do something with the size of the loop. When you can
    extend the loop in a third dimension, inductance reduces; hence
    current noise of amplifier will generate less noise. Try to use as
    much as volume you have available.

    Best regards,

    PA3DJS (Dutch).
  9. Fred Bloggs

    Fred Bloggs Guest

    snip duhhh

    see US2787704 .......
  10. Actually I have to go the opposite direction: generate a large
    resistance from the small resistance using the positive feedback
    (bootstraping or something like that).

    For this application, the noise of the input amplifier is not an issue;
    the dominant component is the thermal noise of R. The absolute values
    of the signal and the noise are not critical. The goal is to optimize
    the S/N of antenna while keeping the Q at 10.
    Understood. The energy caught in the loop is approximately proportional
    to the volume of the loop. However we have what we have.
    Thank you, Wim

  11. John Larkin

    John Larkin Guest

    If you had a high-impedance noiseless amplifier with a high inverting
    gain G, and use a negative feedback resistor R, the composite input
    impedance looks like R/(G+1), but has the current noise of the
    high-value R, so the apparent resistor noise temperature is below room
    temp. I've seen nuclear-sensor amps that use feedback transformers, to
    make a preamp that looks like a 50 ohm load to the detector but has
    much less Johnson noise; I may have a paper around here somewhere.

    You may as well combine the fake "cold" resistor with the preamp,
    namely design a preamp that kills your Q to the desired extent and
    looks like a cold resistor and has a basically low noise figure.

    Some of the phemts have noise figures of 0.4 dB, equivalent to about
    28 Kelvins, when properly matched. They tend to get noisier at low
    frequencies, for certain values of "low."

    Hey, how about this?

  12. Guest

    Would a parallel loop that intercepted the flux of the main loop lower
    the Q if this parallel loop were terminated?
  13. John Larkin

    John Larkin Guest

    Sure, but the termination will be a noise source.

  14. Tim Wescott

    Tim Wescott Guest

    I suggest you do some basic analysis, because I'm not sure that you're
    going to do yourself any good at all.

    Even if you had a dewar full of liquid nitrogen handy to keep your
    loading resistor in, all the resistor is going to do is burn up signal
    that would have otherwise reached the detector. At no point in the
    spectrum will the amount of energy reaching your detector be greater
    with loading than without -- on the contrary, the resistive loading will
    just lower the response of the antenna where it had previously been more

    The only place that I could see such resistive loading being an
    advantage is if you are attempting to receive a signal so broad that it
    is filtered by the antenna -- then you may gain something.

    The only two choices that I can suggest are to use an RF amplifier that
    itself loads the antenna down, or take the suggestion of one of your
    other respondents and change the antenna to more efficiently capture
    energy over a broader band.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  15. Wimpie

    Wimpie Guest

    Based on your information, that is not obliged. Generating a certain
    low noise input impedance/resistance (to damp or load a circuit) can
    be done with or a low value resistor with series feedback or a large
    resistor with parallel feedback.

    I did the same in a 8 MHz circuit to generate a prediscribed load for
    a 2 resonator LC filter (to avoid resonance and keep the curve).
    Best regards,

    PA3DJS (Dutch)
  16. You are mistaking me for some other idiot, pehaps. Of course, I have
    accounted for the tradeoffs.
    So what? The noise of the electronics is well under the other sources of
    noise. I trade off a tiny part of signal to get the robust operation.
    And this is actually bad. The sharp resonance peak can slip off the
    frequency of interest and pick up some interference. Hence the dynamic
    range has to be increased by Q times, which is problematic.

    BTW, I have tried the automatic tuning with the GIC, too. The simple
    straightforward solutions are too noisy, the good solutions take too
    many parts. Varactors are inapplicable.

    -- on the contrary, the resistive loading will
    The absolute sensitivity is not a problem. The S/N and the dynamic range
    is what matters.
    In the addition to the above mentioned reasons, the gain and the phase
    shift of the loaded antenna are very stable and predictable.
    That was considered, too. The total noise balance is going to be worse.
    This misses the point. There is no problem with capturing enough of energy.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
  17. John Larkin

    John Larkin Guest

    Thermodynamics proves that the universe is cruel.

  18. Tim Wescott

    Tim Wescott Guest

    No, Mr. arrogant-but-human, I'm mistaking you for someone who is asking
    for assistance. Just because you're Russian doesn't mean you have to act
    like the stereotype.

    But it's good that you're thinking.
    I'm not sure how this is consistent with your other statements about the
    total noise. Is this one of the things that you've analyzed so
    thoroughly that you can diss me for suggesting that you analyze it, or is
    it one of the things that you know you don't have to do your homework on
    because Everything is Bigger in Russia?
    Well, why didn't you say so in your original post? You're perfect, so it
    can't be that you forgot to mention it.

    Knowing your frequency of interest would help, too, and whether you're
    interested in sky waves, ground waves, or whatever happens to impinge
    upon your antenna.

    At 300kHz, a one-meter capacitive probe connected to the gate of a JFET
    works nicely -- it receives enough atmospheric noise that any energy loss
    to inefficiencies is negligible, and it's quite small compared to a

    Tim Wescott
    Control systems and communications consulting

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
  19. What a shame.

    I used to be of better oppinion about you, Mr. Wescott. Never mind.

  20. Precisely. However I am more interested in the opposite trick: if a
    small resistor is bootstrapped by a low noise amplifier with the gain
    close to unity, the input impeadance is multiplied while retaining the
    voltage noise of the small resistor.
    Yes. The simple math shows that it is possible to reduce the noise
    temperature by about of the order of magnitude.
    It works, although there are some pitfalls because of the phase shifts
    and the stability concerns.
    They basically do the same thing plus the C1/C2 transformer feedback
    coupling. This allows for the additional flexibility, not sure about the
    stability margin though.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
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