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Measuring AC impedance/resistance — Parallax Forums

Measuring AC impedance/resistance

Bobb FwedBobb Fwed Posts: 1,119
edited 2010-10-21 20:56 in General Discussion
So, I have a very weak AC signal that I am trying to determine the impedance/resistance on. It's an antenna of sorts. I'm guessing it's in the 10K-100K-ohm range. But beyond that, it's guess work. Is there an accepted way, to measure it. It doesn't have to be too precise, within 15-20% is good enough.

Comments

  • LeonLeon Posts: 7,620
    edited 2010-10-21 10:16
    What sort of antenna? You could use an antenna modeling program, or an antenna analyser at HF.
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-10-21 10:23
    That's why I say, "of sorts". I just have a wire with which I am measuring bleed-off from a device that generates lots of EMI. I want to figure out the impedance so I can build a circuit that amplifies/buffers it so it can be read by an ADC.
  • LeonLeon Posts: 7,620
    edited 2010-10-21 10:48
    For measuring those sort of emissions you need a broadband antenna and a spectrum analyser.
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-10-21 10:57
    Leon wrote: »
    For measuring those sort of emissions you need a broadband antenna (my 16 gauge wire) and a spectrum analyser (my ADC and some software).

    So the answer is, there isn't any easy way? No, "hook up a 100Meg resistor and read the voltage difference with a $5 radio shack multimeter" or anything?
  • LeonLeon Posts: 7,620
    edited 2010-10-21 11:12
    No, the results will be meaningless. If you need to establish emission levels for CE or FCC conformity you need to go to a specialist company and pay them quite a lot of money.
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-10-21 11:40
    No, its just for a small project. I guess I'll just try different values in my filter until I get the right range of values.
  • FranklinFranklin Posts: 4,747
    edited 2010-10-21 20:36
    How will you know when you have "the right range of values"?
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-10-21 20:56
    The output is somewhat predictable, so I just need to find the max and min input and keep that between the voltages my ADC will handle (with some margin for error).

    My idea for a circuit was a capacitor-voltage divider circuit as a high pass and to keep the input in the range of a rail-to-rail op amp in voltage follower configuration, followed by a LC band pass filter with another divider to keep it in ADC range. I will post a schematic tomorrow if no better suggestions come in.

    To get the right range, I should just need to change the divider circuit before the op amp.
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