Capacitive transformer as antenna-mixer coupling

Discussion in 'Wireless & RF Design' started by Elerion, Mar 24, 2018.

  1. Elerion

    Thread Starter Member

    Sep 11, 2017
    60
    1
    I'm trying to understand how a capacitive transformer works.
    After some days struggling with much reading and simulations, I now would like to ask for help.

    NE602 is a RF mixer, with 1.5k|3pf input impedance.
    Antennas are usually 50 or 75 ohm.
    ne602.png
    A capacitive network as shown, is suppose to raise the impedance of the antenna, so it matches more closely the higher impedance of the input circuitry.
    The only reason I see for this, is to improve power transfer.

    cap_txfm_simple_sim.png
    Now, if we set up a simple simulation, connecting a 50 ohm source to 3k pure resistive load, we see that power delivered to the load is 322uW, not 5mW, if the load was 50 ohm (matched to source).

    Now, let's see how the capacitive transformer helps.
    It seems to not help at all. Power is 260uW at 1.6 MHz, and outside a certain bandwidth, it obviuosly fades away.
    cap_txfm_simple_sim_graphs.png

    Now, the capacitive transformer is supposed to reflect 50 ohm into the source side. And that indeed happens if we look at the real impedance at low frequencies. But not the magnitude of the impedance, which is dominated by the imaginary component.

    At this point I think I'm just so confused to think about it the right way.
    I appreciate so much any guidance.
     
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