real part of impedance

Discussion in 'General Electronics Chat' started by phystech, Aug 26, 2004.

  1. phystech

    Thread Starter New Member

    Aug 26, 2004
    I have always concidered the real part of a complex impedance to be the resistive part of the circuit. This means that a
    measured impedance must have a phase angle between +90 deg to -90 deg. We have an HP 4815A Vector Impedance meter
    and an HP 8508 Vector Voltmeter. It is possible to measure an impedance with the phase greater than +90 or less than -90.
    Even the HP manuals define inductance for 0 < angle < 180 and capacitance for -180 < angle < 0.
    A phase angle greater than +90 or less than -90 obviously, when expressed as real and imaginary parts, has the
    real part negative. This implies a negative resistance. It is confusing.
    Can anyone shed some light on this problem?

    Thank you
  2. beenthere

    Retired Moderator

    Apr 20, 2004

    You're correct in your supposition that no one impedance can exceed a phase shift of +/- 90 degrees. However, string a bunch of inductors together, and the combined phase shift can be hundreds of degrees. Same with multiple capacitors. This was handy in the days before digital graphics, where really wierd combinations of impedances were used to generate squiggles on CRT's.

    The formulas still give the real-world answers (one-over-2 pi F sub c and 2 pi F sub l). If you want a headache, analyze an old L-C delay line. The combination of inductance and capacitance should have your vector voltmeter gasping. Nevertheless, they passed pulses with a fixed delay from in to out for years and years.