SMD resistor reactance

Discussion in 'Wireless & RF Design' started by toughspeaker, Jul 11, 2010.

  1. toughspeaker

    Thread Starter Active Member

    Jan 28, 2009
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    Hi boys and girls!

    In RF electronics, not only capacitances and impedances can change with temperature, time etc, but also resistances. As a rule of thumb, engineers do not use carbon film resistors but rather metal film etc.

    Now my problem is that i have a batch of smd resistors that i would like to test their reactance on.

    How do one know if a smd resistor is carbon film or metal film? How can one test the reactance of the smd resistor in a reliable way?

    Any help would be appriciated.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. toughspeaker

    Thread Starter Active Member

    Jan 28, 2009
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    Thanks Bertus! A very informative link.

    So this means that one should use as small SMD resistances as possible and that there is no simple way to check the reactance of the SMD resistor other than empirically by building the resistor into the circuit and hope for the best?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The influence of the inductance and capacitance of the resistor is depended on the frequency of use.
    At very high frequencies (above 100 MHz ) the influence will rise.

    I think the attached PDF will give you even more info.

    Bertus
     
  5. Ghar

    Active Member

    Mar 8, 2010
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    The inductance and capacitance of a resistor will be influenced by the PCB layout you use, probably more so than whatever the resistor contributes itself.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    A long while back I had to build a M derived filter for work. I had access to both SMT and through hole components, so I build both, then tested them. The rolloff freq was somewhere in the neighborhood of 130Mhz I think. The curves were identical except the notch, the SMD version had a 10db better rejection than the through hole version.
     
  7. toughspeaker

    Thread Starter Active Member

    Jan 28, 2009
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    Wow Bill! Increadible! Now, would you mind telling me a bit more about that experiment of yours? the through hole resistors, were they carbon- or metal film based? Which size were your SMD components 1206, 0805 or 0402?

    Thanks for all inputs!
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    That I can't answer, these were standard parts for our shop, I just snagged some and made it. A M derived filter doesn't use resistors per se, except for the terminations. My point is the worst SMT will beat a through hole version, since the parasitics are drastically reduced.
     
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