Impedance match a capacitor

Discussion in 'General Electronics Chat' started by Marv_92, Oct 9, 2018.

  1. Marv_92

    Thread Starter New Member

    Oct 9, 2018
    Hi everybody,

    I am currently trying to impedance match a capacitor as a load. In detail, it is an insulating thin film grown on top of a conductive layer (grounded). We now sputtered small Pt electrodes on top and want to apply a voltage to these. We expect the capacitance in the area of 0.5 - 1 pF and our source has a 50 Ohm impedance. We want to apply square shaped voltage pulses with < 50 ps rise time (20 GHz) and up to 20 ns (50 MHz) length. Thus, the impedance matching circuit has to be very broadband.

    Since I am not an electrical engineer, I would be very grateful if you explain any attempts to solve this problem as easy and detailed as possible.

    Thank you very much!

    Cheers, Marvin
  2. Ylli

    Active Member

    Nov 13, 2015
    You can not impedance match a pure reactive load to a resistive source.
  3. Marv_92

    Thread Starter New Member

    Oct 9, 2018
    Hi Ylli,

    thanks for the answer!

    If it is not possible, is there any way to at least minimize ringing and back-reflections (for example a 50 Ohm shunt resistor very close to the capacitor)?
    Or if not, lets assume I apply a pulse as described above to a 1 pF capacitor. Will the pulse be fully reflected and will the amplitudes of pulse and back reflection just sum up?

    Thanks again for helping me with this case.

  4. danadak

    Distinguished Member

    Mar 10, 2018
  5. crutschow


    Mar 14, 2008
    If you are driving the capacitor with a 50Ω generator through a 50Ω transmission line, and terminate it with a 50Ω resistor to ground than the pulse appearing at such a small capacitance should not have any significant distortion due to any mismatch.

    See LTspice simulation below of a 50Ω generator and transmission line (T1) with a 50Ω termination resistors driving a 1pF capacitor.
    Note that the voltage appearing across the capacitor is 1/2 the generator internal pulse (open circuit) value.

    For good signal fidelity, the connection between the transmission line, termination, and the capacitor must be as short as possible.

  6. Marv_92

    Thread Starter New Member

    Oct 9, 2018
    Thanks danadak and crutschow for the very helpful answers!

    I will try a 50 Ohm termination resistor in parallel to the capacitor. Crutschow, can you you explain why only 1/2 of the generator internal pulse appears across the capacitor? Assuming that the capacitor reflects the voltage, I would assume to have twice the amplitude there. And can you zoom in to the rise time of a pulse in the simulation? On this scale, I cannot see whether the rise time (high frequencies) is screwed up or not.

    Thanks again !!