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  • Tuned Circuit Scope Demo

    From G8MNY@21:5/101 to TECH on Mon Jul 15 06:00:18 2019
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    R:190715/0849Z @:GB7CIP.#32.GBR.EURO #:12500 [Caterham Surrey GBR]

    From: G8MNY@GB7CIP.#32.GBR.EURO
    To : TECH@WW

    By G8MNY (New Jan 10)
    (8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

    Recently I used an oscilloscope to demonstrate what is going on in a tuned circuit for an advanced Ham class.

    My 100MHz scope can do 4 traces using the 2 triggers as inputs, so I looked at using all 4 traces.

    ┌─/ o──┐
    ┌──────────┴──Rs──┴──┬───┬─────────────────┐
    ┌───────┴─────┐ /|\ │ (| /|\ │ Rs = 1k
    │ AF │ | C === (| L | ┌──┴───┐ Rc = 10R
    │ SIGNAL │ | │ (| | │AF Amp│ Rl = 10R
    │ GENERATOR │ Input ─┤ ├─ Output │ & LS │ C = 1uF Poly
    │Sine Triangle│ | /|\ │ │ /|\ | └──┬───┘ L = 88mH Pot core │ & Square │ | Ic Rc Rl Il | │ Freq = 536Hz └───────┬─────┘ \|/ \|/ │ │ \|/ \|/ │
    └────────────────────┴───┴─────────────────┘

    I used 10:1 scope probes for the larger Input & Output signals, & 1:1 probes on the smaller current sampling voltages. The Rs short is used when you sweep the frequency so you can better see the L & C currents levels varying. With Rs in circuit the selectivity can be shown & also heard on the LS.

    SCOPE DISPLAY

    Frequency Below Res Freq @ Resonance Frequency Above Res
    ┌───────────────────┐ ┌───────────────────┐ ┌───────────────────┐ Input │/~~\ /~~\ /~~│ │/~~\ /~~\ /~~│ │/~~\ /~~\ /~~│
    │ \__/ \__/ │ │ \__/ \__/ │ │ \__/ \__/ │
    │ │ │ │ │ │
    Ic │_ _--_ _--_ │ │~\ /~~\ /~~\ │ │~~\ /~~\ /~~\│
    │ ~--~ ~--~ ~-│ │ \__/ \__/ \│ │ \__/ \__/ │
    │ __ __ _│ │ │ │ │
    Il │ / \ / \ / │ │ /~~\ /~~\ /│ │ _--_ _--_ │
    │/ \ / \__/ │ │_/ \__/ \__/ │ │--~ ~--~ ~--~│
    │ ~~ │ │ │ │ │
    │--_ _--_ _--_│ │/~~\ /~~\ /~~│ │ _--_ _--_ _-│ Output│ ~--~ ~--~ │ │ \__/ \__/ │ │~ ~--~ ~--~ │
    └───────────────────┘ └───────────────────┘ └───────────────────┘

    Points to notice:
    Ic is always 90° leading the Output voltage.
    Il is always 90° lagging the Output voltage.
    Ic is always 180° out of phase with Il.
    Ic = Il at resonance, & it is why the circuit exhibit high Z at resonance.
    The Output is not only largest @ resonance but also in phase with the Input.

    WHAT IS Q
    The meaning of Q can also be discussed & measured with this arrangement.

    100% ─┤ .-. ├─ 0dB Fc
    │ / \ │ Q = --------------
    70.7% ─┤ ▐-----▌ ├─ -3dB -3dB Bandwidth
    50% ─┤ ▌-3dB ▐ ├─ -6dB
    │ / Band- \ │
    10% _│_.─ width -._│_ -20dB
    └───────┬───────┘
    Fc

    OTHER WAVEFORMS
    The filtering effect on Triangle & Square waves can be shown, & notice the high pulse shaped currents in the capacitor! This means that capacitors need low inductance leads (very short) if they are to filter out harmonics!

    The harmonics present in a Square wave can be demonstrated too by making the frequency 1/2, 1/3, 1/4 & 1/5 of 536Hz resonance! Sine wave harmonic Outputs @ 536Hz will be seen on the odd harmonics only.

    _ Level
    0dB │ │100%
    │ │
    │ │
    │ │ S Q U A R E W A V E S P E C T R U M
    -10dB_│ │ 33%
    │ │ | 20%
    -20dB-│ │ │ │ 14% 11% 9% 8% 7% 6% 5% 5% 4% 4% 3%
    └─┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴─>Hz
    1 3 5 7 9 11 13 15 17 19 21 23 25 27
    Harmonic


    See my Tech buls on "Scope RF Trick", "SSB Demo circuit", & "Oscilloscopes" & "Spectrum Harmonic Demo circuit".


    Why Don't U send an interesting bul?

    73 De John, G8MNY @ GB7CIP


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    * Origin: HAMRADIO telnet lu9dce.dynu.com (21:5/101)