1 MHz Bandpass filter

Discussion in 'Wireless & RF Design' started by peter_morley, Jul 12, 2011.

  1. peter_morley

    Thread Starter Member

    Mar 12, 2011
    I am having trouble making an effective bandpass filter for an antenna receiver. I am using a function generator to test out my circuits but when I get to about 400Khz nothing works. I tried the RLC bandpass filter and that seems to only be effective at lower frequencies. I also tried a low and high pass RC configuration and I'm getting the same frustrating results. I know how bandpass filter works when jwL + 1/(jwC) = 0 the circuit is purely real resistant so all of the voltage should be across the resistor (ignoring inductor resistance which a real inductor acts as an inductor and resistor in series). I also notice when my sig gen goes to high frequencies my meter reads lower and lower ac voltages. Could this be due to a slow multimeter that can't track high freq signals. Any advice is greatly appreciated! Thanks
  2. bertus


    Apr 5, 2008

    Can you post a schematic of your intended filter?
    It would also be nice to have a schematic of the test setup.

  3. Adjuster

    Late Member

    Dec 26, 2010
    Filters based on LC resonance have been used for radio since the days of Marconi. They get more difficult at higher frequencies, but the practical limit is some orders of magnitude higher than a few hundred kHz.

    That said, satisfactory results can only be achieved by using components suitable for the given frequency range. In particular, some types of ferromagnetic cored inductor are only suitable for low frequencies. Similarly, only certain types of capacitor are suitable for high-frequency tuning.

    Other limiting factors are the parasitic capacitances, inductances and resistances that affect all practical components. This limits the ranges of component values that we can use in practice. For instance, an inductor of some milli-Henries will not be much use for tuning at 100MHz, because practical minimum circuit capacitances of the order of picofarads will impose a resonance at some much lower frequency.