Frequency sweep for a high-pass filter???

Discussion in 'General Electronics Chat' started by Lainya, May 24, 2008.

  1. Lainya

    Thread Starter New Member

    Apr 13, 2008
    :confused::confused: We had a basic resistive-capacitive high-pass filter. In the manual frequency sweep the output voltage across the resistor increased with frequency as it was supposed to until we reached 1MHz, then the voltage dropped. What could cause this? Would the capacitor beaking down cause this? We are working with really old capacitors.:confused::confused:
  2. mik3

    Senior Member

    Feb 4, 2008
    Capacitors are not perfect as well as resistors and wires. Your capacitors may be broken but what i suspect is that after 1 MHz the stray inductance of the capacitors leads and the inductance of your wires (if they are long) cause the voltage to drop because the stray inductance in series with the capacitor form a band-pass filter.
  3. hgmjr

    Retired Moderator

    Jan 28, 2005
    Can you post a schematic of the circuit so that we can study it? It is always easier to discuss a circuit when a schematic of the circuit in question is present.

  4. SgtWookie


    Jul 17, 2007
    "Ideal" capacitors consist of only capacitance - but they exist only in simulations.
    "Real" capacitors, along with capacitance, have relatively small parasitic values of resistance and inductance. The higher the frequency, the more apparent these parasitics become. See the attached.

    All electronic components have parasitic values besides their main attribute. Even resistors have parasitic inductance and capacitance.
    Last edited: May 25, 2008