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Impedance

Discussion in 'Electronic Basics' started by Music Man, May 6, 2005.

  1. Music Man

    Music Man Guest

    How does impedance mismatches alter signals?
    I know you lose HF but always wanted to know how different frequencies
    are affected by different circuitry.

    Thanks
     
  2. Impedance is an expansion of the concept of resistance to include
    time. Resistance relates voltage across something to the current
    through it. Another way to say ohms is volts per ampere. But that
    description of the relationship between voltage and current does not
    involve time. It describes stuff that only consumes energy when
    voltage is connected across it, and does so, regardless of how long
    the voltage is applied, or how fast it changes.

    When you include the possibility of energy storage as well as
    consumption, time becomes an important factor. For example,
    capacitance stores energy in proportion to the square of the voltage
    across it (E=(1/2)*(V^2)*C), but it takes time for a given current to
    build up that voltage (I=C*(dv/dt) or the capacitive current is
    proportional to the time rate of change of voltage across the
    capacitor). So once energy storage is involved, you need a two
    dimensional description of the relationship between voltage and current.

    Back to the frequency effects: If an impedance has a capacitive
    component, then the current that passes through it will increase when
    the voltage changes faster. If the current through that impedance
    arrives through a series resistance, then as frequency rises, more of
    the total applied voltage will get used up across the resistance
    (because the current is rising) and less will appear across the
    capacitive impedance (because the total of the resistive and
    capacitive voltages must add up to the applied voltage). This forms a
    basic low pass filter if the signal across the capacitor is the output.
     
  3. Easy, first of all this applys only if the dimensions of your circuit
    are much much smaller than the wavelenght... High Frecuency... on easy
    words. When you send a wave trough a transmission line (e.g. coaxial
    cable) and the other end is not impedance matched, you get a stationary
    wave patern on you transmission line, this makes some of the transmited
    power to return to your transmitter. On High Power RF trasnmitter you
    burn the Tx. On your little circuit it just reduces the power you get
    at the other end. The stationary wave patern depends of the frecuency
    and the lenght of the line.
    You can find anything about it on a "Transmission Lines" book.
     
  4. Bob Eldred

    Bob Eldred Guest

    First off, maximum power is transferred when impedances match. A mismatch
    will cause less power to be transmitted to a load than when matched. This is
    especially important when the power is extremely low like signals from an
    antenna where every little bit of signal counts. Also getting the maximum
    power to a load from an amplifier or transmitter can be an important reason
    to have matched impedances.

    Secondly, part any signal moving down a line will reflect off of an
    impedance mismatch and reverse direction back toward where it came from. It
    reflects back because maximum power is not transferred across the mismatch
    and that not transferred gets reflected. The reflected signal will
    interfere with the forward signal and produce peaks and valleys of
    amplitude. These peaks and valleys forms a standing wave (doesn't move in
    position) on the line that can interfere with the forward transmission of
    signal. A measurement in voltage of the peaks and valleys is called the
    Voltage Standing Wave Ratio, VSWR and is an indication of the amount of
    mismatch.

    One consequence of a mismatch condition can be seen on a TV screen when
    sharp vertical edges like lettering show ghosting or multiple lines near
    each other where there should only be one line. This ghosting is caused by
    multiple reflections back and forth on a line or in the air. Similarly, data
    transmissions can be compromised by reflections that smear out the data
    timing.

    These effects occur at all frequencies and are not frequency dependant.
    However, if the wavelength on a line is large compared to the dimensions of
    the line, or length of mismatch spacing, there is rarely a problem. That's
    why this usually becomes an issue at RF frequencies but not audio
    frequencies or below.

    However there are many times when an impedance mismatch is desirable even
    necessary. For example when you plug a light into the wall, the impedance
    must be mismatched because you do not want maximum power transferred, you
    want all of the power required to light the lamp transferred but not all of
    the power the generator can deliver. The impedance of the wall is near zero
    and the lamp is, maybe 140 ohms, a definite mismatch, but it's the only
    practical way to deliver power without the source wasting half of it as
    happens when impedances are matched. This is called constant voltage and is
    the way most circuits work. In other words, most connections are not
    impedance matched.
    Bob
     
  5. I may be wrong, but, in strictly technical terms, isn't it when the
    resistive components are equal and the reactive components are equal
    but opposite, complex conjugates (?) of one another?

    Tom
     
  6. Bob Eldred

    Bob Eldred Guest

    Yes, you are correct but I'm trying to keep it simple and not bring
    reactance into it at this level. First things first is a basic understanding
    of the underlying principles and why line reflections occur and are
    important. If the basic concepts are not there, complex conjugates are
    going to fall on deaf ears.
    Bob
     
  7. Music Man

    Music Man Guest

    So are you saying,basically,that the high capacitance can't deal with the
    faster frequencies?
    Thanks
     
  8. Capacitance deals with high frequencies just fine, by sucking current
    out of it. ;-)

    Seriously, the effect is that the higher the frequency, the lower the
    impedance (volts per ampere) the capacitor exhibits. This can be a
    bad thing if you are trying to avoid it (like sending audio through a
    long chunk of high capacitance cable from a high impedance source like
    a guitar pickup, and the cable soaks up the higher frequencies) or
    real handy if you are making use of the effect (like you do in a tone
    control circuit).
     
  9. Music Man

    Music Man Guest

    So what is the relation to frequency and capacitance John?
    Could you explain on how audio signals are "expressed" in electronic
    circuits.
    What is need to creat a clean signals?How resistors creat noise and add to
    signal?
    What I mean is why would a Neve mixing console sound better than a cheapo
    desk.

    Thanks
     
  10. Buddy Smith

    Buddy Smith Guest

  11. The effect of capacitance (the ability of the capacitance to pass
    current per volt across it) is proportional to frequency. Pick any
    capacitance and this formula tells you the volts needed to drive 1
    ampere through it. But volts per ampere are called ohms, so the
    formula is Xc = 1/(2*pi*f*C)
    where Xc is the capacitive impedance in ohms (or volts per ampere), pi
    is 3.14159, f is frequency in hertz, and C is capacitance in farads.
    Either as a voltage that represents the signal or as a current that
    represents the signal. If a capacitor is in series with the signal,
    it passes it better as the frequency goes up. If the capacitor is
    between the signal and ground, it drains more and more of the signal
    to ground as the frequency goes up.
    Whole different subject. Resistors (the gas of electrons in
    resistors, actually) make noise just to be in thermal equilibrium with
    their surroundings much like gas molecules bang around just from
    thermal energy. Then when you pass current through resistors, it
    bumps and bangs and surges a bit, because the current is composed of
    finite charges, not a smooth fluid, adding a different spectrum of
    noise to that from the unbiased resistor. You should probably read a
    bit on this and come back with questions.
    http://zone.ni.com/devzone/conceptd.nsf/webmain/8DE42E13BD089D8B86256816006545CD?OpenDocument
    There are noisy resistors (noisier than can be explained by thermal
    noise), low quality capacitors that pick up vibrations and change
    capacitance as the signal voltage swings, opamps that are noisier or
    quieter, well shielded and poorly shielded designs, etc.
    Understanding all that, including the etceteras can take a lot of
    study and experience.
     
  12. Rich Grise

    Rich Grise Guest

    Xc = 1 / (2 * pi * f * C)

    Xc = capacitive reactance in Ohms
    f = frequency in Hertz
    C = capacitance in Farads.

    Cheers!
    Rich
     
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