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How to test for Q of LC circuit unloaded

Discussion in 'Electronic Basics' started by Bill Bowden, Oct 9, 2007.

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  1. Bill Bowden

    Bill Bowden Guest

    I have a ferrite rod antenna that measures a Q of around 200 unloaded
    using a Q meter. This was done by someone else and I don't have a Q
    meter, so I tried to measure the Q using a generator and monitoring
    the BW on a scope. The generator is coupled to the tuned circuit using
    a single turn of wire in series with 50 ohms. This setup seems to
    present a load to the circuit since I only measure an 8KC bandwidth at
    600KHz which is a Q of less than 100.

    Question is, how can I couple the generator to the LC circuit without
    presenting any load and lowering the Q?

  2. John Larkin

    John Larkin Guest

    Just dangle the generator output, on a clip lead maybe, near the rod,
    and crank the generator output to max.

    Or make a bigger loop, a few inches in diameter, but keep it pretty
    far away. "Loose coupling."

    Use a 10:1 scope probe; 1:1 probes are pretty lossy.

  3. mike

    mike Guest

    I have even put a 100k to 1meg resistor in series with the probe, helps to
    isolate the 15pf of probe capacitance.
    For my measurements I adjust coupling and scope gain to get 7 units
    peak to peak at resonance. then adjust frequency until I have 5 units
    peak to peak. I do this above and below resonance.
    Note that .707 times 7(units) equals 4.949(units) Close enough to
    5 units for my eyes. (The .707 point is the 3db point.)
    Then upper 3db frequency minus lower 3db frequency equals difference
    So then resonant frequency / difference frequency=Q
    Example: Resonant frequency = 1,000,000
    Upper freq - lower freq --- 1,001,500 - 997,500 = 4000
    1,000,000 / 4000 = 250 So Q= 250
    I know you knew this but I added it for those who didn't.
  4. john jardine

    john jardine Guest

    Gives a shunt dynamic resistance about 250kohm at Q=200. For 5% error that's
    a signal source + meter loading in the >5Mohm area which is difficult.
    For a repeatable setup (like a Q meter), maybe series resonate the coil and
    For Q=200 at 600kHz the series coil loss is say 7ohm. If you short the
    generator output connector with 0.03ohms and drive the coil from that, then
    it's just a question of measuring the developed voltage across the
    capacitor, using a 10Mohm scope probe .
    (Say) "100mV" on generator dial, would give 70uV to the coil and 40mVpp
    across the cap at resonance, which gives a just about visible, one 'scope
    division at 50mV per div.
    (myself I'd drive from a function generator at say 10Vpp )

    .------------. The Coil 10:1 10Mohm probe
    | ___ | ___ .----------.
    | ,-|___|--o---o---------o--UUU-o-o---o<-------o .-----. |
    | | 50ohm | | | | | | |
    | / \ | .-. | | | ~~~ | |
    | ( ) | | | --- | '-----' |
    | \ / | | |0.03ohm --- | 50mV-div |
    | | | '-' | ,---o |
    | | | | | | | |
    | ===0V | ===0V ===0V ===0V| |
    '------------' '----------'
    600kHz Sig source 350pF tuning cap' Scope

    (created by AACircuit v1.28 beta 10/06/04
  5. Jamie

    Jamie Guest

    The last time I did something like that I used my old (OLD),
    HP R.F. voltage meter. I still have it btw :)
    Any ways, What I did on a case similar to what you're doing is
    use a 50 ohm R from the output of the generator to drive a
    series LC circuit with one end to the R and the other to common.
    Place the probe from your scope or an RF Vm like I did and
    find the lowest peak reading when moving the generator frequency.

    from there, do your calculations to determine the effective
    R in the LC circuit to be used the Q calculations.
    AT Semco (Capacitor Manuf), I serviced a few capacitor/induction
    value and Q meters in the LAB and on the floor. These units were
    old but very accurate to the standards.
    The theory of operation with these units were to resonate the
    tank with out the specimen first to get a center reading and adjust
    another dial which was nothing but a variable POT to get the reading
    at peak level.
    Then you insert the specimen. You then would offset the dial to
    give you the exact reading of the component and adjust the POT which
    also had an analog dial to get the signal back up to Peak again.
    THe offset value of the Pot via the scale would give you the Q of
    that component.
  6. Bill Bowden

    Bill Bowden Guest

    Ok, I used a few turns of wire from the generator spaced a few inches
    from the loopstick and I get a Q of 133. The turns don't encircle the
    stick, they just lay on the table. Scope probe is 10X. It's about
    twice a good as before but still a long way from Q=200.

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