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Q of this circuit?

Discussion in 'Electronic Repair' started by Albert, Apr 1, 2005.

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  1. Albert

    Albert Guest

    I need to have an estimate of the Q for a parallel tuned circuit
    consisting of a 10 microhenry air wound coil and an 8.2 microfarad
    electrolytic cap.

    Coil parameters are below.

    DC Resistance 0.16 Ohms
    Wire Gauge 24 AWG
    Wire Diameter 20.1 mils (1 mil = .001 in)
    Coil Length 1 in
    Coil Inner Diameter 0.5 in
    Coil Outer Diameter 0.54 in
    Average Turn Diameter 0.5 in
    Wire Length 6.02 feet
    Copper Weight 0.01 pounds
    Turns 46
    Levels 0.92
    Turns/Level 49.75

    The circuit will feed into an mk484 AM radio chip (or it's very
    similar brother, the LMF501) which has an input impedance of (about)
    100 K ohms.

    I'm sure this is easy to do, but I can't figure out how the ac
    resistance of the cap impacts the Q calculation.

    Thanks,

    A
     
  2. April 1 post?
     
  3. Albert

    Albert Guest

    No, not really................

    Did I leave out some information??
     
  4. Resonates at 18 kHz and still seems like April Fool to me!
     
  5. When the homework is due? :)

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  6. NSM

    NSM Guest

    <Albert> wrote in message
    .... 10 microhenry air wound coil and an 8.2 microfarad electrolytic cap...

    Say what? That's completely Looney tunes.

    N
     
  7. Stan

    Stan Guest

    Albert <> wrote:

    }I need to have an estimate of the Q for a parallel tuned circuit
    }consisting of a 10 microhenry air wound coil and an 8.2 microfarad
    }electrolytic cap.

    An "8.2 microfarad" cap in an RF tuned circuit?

    Please re-check...perhaps you meant 8 uuf?

    Stan.
     
  8. NSM

    NSM Guest

    Unless he wants to create a circuit with negative Q.
     
  9. Albert

    Albert Guest

    No, that is not an error. I meant 8.2 uf, not 8.2 uuf.

    The 10 uh coil is easier to make, so I wanted to use a small inductor
    and a large capacitor. It should resonate at 17.8 Khz, which is the
    frequency I am building a receiver for.

    Why does everyone think this is a joke and/or an error? This is a
    legitimate question. I gave (what I thought) was the specifications
    needed to arrive at a value for loaded Q....or at least a rough
    estimate.

    Thanks to all,

    A
     
  10. NSM

    NSM Guest

    Simple. A resonant circuit is like a baseball on a foot of rubber band.

    Your circuit is like a piece of bread attached to a piece of taffy.

    Which one do you think is 'bouncier'?
     
  11. Asimov

    Asimov Guest

    "NSM" bravely wrote to "All" (02 Apr 05 23:30:41)
    --- on the heady topic of "Re: Q of this circuit?"

    NS> From: "NSM" <>
    NS> Xref: aeinews sci.electronics.repair:44791


    NS> <Albert> wrote in message
    NS> > No, that is not an
    NS> error. I meant 8.2 uf, not 8.2 uuf. >
    NS> Simple. A resonant circuit is like a baseball on a foot of rubber
    NS> band.
    NS> Your circuit is like a piece of bread attached to a piece of taffy.

    NS> Which one do you think is 'bouncier'?

    No matter which reactance is greater it would still oscillate. The
    problem is being able to drive it adequately since the inductor and
    capacitor each has an impedance of only 1.1 ohms. For example, it
    would require about 15 amps just to get 10 volts across either
    element. IOW this circuit is more suited to being a trap than a tank.

    The more desirable strategy is to use as much inductance as possible
    without the resistance losses of the wire becoming significant. The
    ratio of the inductive reactance to the resistive losses is called the
    quality or Q-factor of the coil. Generally it is the coil which has
    the worst quality factor thus usually only the coil's Q is considered.
    Look up inductance calculation tables or programs to help design it.

    An alternative to using a coil is to synthesize a really large
    inductance using a gyrator or what is termed a GIC or General
    Immitance Converter. This is often seen in audio equalizers and active
    crossovers for the low frequency region but with a fast opamp it
    should easily work at 17.8Khz.

    A*s*i*m*o*v

    .... Electrical Engineers do it with more frequency and less resistance.
     
  12. Bill Jeffrey

    Bill Jeffrey Guest

    Albert -

    Electrolytic caps are really not very good in resonant circuits.

    First, you must not allow the cap voltage to go negative - which means
    you must ensure that there is a DC bias at least as great as the peak
    voltage of the sine wave that will exist there.

    In addiion, they have a fair amount of series resistance (which will
    drag down the Q of the overall circuit). And because of their
    construction, they also have a fair amount of parasitic inductance -
    probably well above 10 uH. Just as a general statement, at KHz
    frequencies, you probably should be thinking about mH coils and nF caps.

    Bill
    --------------------
     
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