robf@mcs213k.cs.umr.edu (Rob Fugina) (03/30/91)
I'm confused as to the importance of impedance matching... Since I may not know the whole scope of this subject, I'll say I'm referring to things like stereo line-level, headphone, microphone, antenna inputs and outputs. I can see that if you have a high impedance source, you need to have a high impedence load or input so as not to load down the source. So what is the need, when you have a low impedence source, to have a low impedence load? Me
raoul@eplunix.UUCP (Nico Garcia) (03/30/91)
In article <2532@umriscc.isc.umr.edu>, robf@mcs213k.cs.umr.edu (Rob Fugina) writes: > I'm confused as to the importance of impedance matching... Since I may > not know the whole scope of this subject, I'll say I'm referring to things > like stereo line-level, headphone, microphone, antenna inputs and outputs. There are two factors here: one is a problem of power dissipation, the other is reflection and transmission problems. For a given voltage source, with a series resistance before the load, the maximum power dissipation in the load occurs when both the load and the line have the same resistance. Try the equations yourself if you don't believe me. However, real circuits have parallel capacitance and series inductance, the characteristics of a transmission line. These yield an impedance of sqrt(L/C), which describes the relationship between high frequency voltage and current or between the transmission line's electric and magnetic fields. It doesn't *dissipate* power, but it does describe how a transmission line carries power/signal to your target. Now, if you connect this circuit to another one with a different impedance, it has a different relationship, but they both have to match current and voltage at the junction. Nature cleverly matches this boundary condition by reflecting another signal back down the original circuit. This reflection can bounce back and forth along a real circuit, causing serious signal grief in the process. The only way to avoid it completely for a wide range of frequencies is to match these impedances. You can avoid this for a pre-specified set of frequencies by matching the length of an intermediate transmission line to fractions of the wavelengths, but this is a serious pain in the keister. It's much easier to use all 50 ohm or 75 ohm cable and devices. Does that answer your question? -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
whit@milton.u.washington.edu (John Whitmore) (03/30/91)
In article <2532@umriscc.isc.umr.edu> robf@mcs213k.cs.umr.edu (Rob Fugina) writes: >I'm confused as to the importance of impedance matching... Since I may >not know the whole scope of this subject, I'll say I'm referring to things >like stereo line-level, headphone, microphone, antenna inputs and outputs. >I can see that if you have a high impedance source, you need to have a high >impedence load or input so as not to load down the source. So what is the >need, when you have a low impedence source, to have a low impedence load? The power transferred to the load is greatest when the impedances are matched (to be accurate, when the load impedance is the complex conjugate of the source impedance). So, if you don't want to waste the gain of the previous stage of the amplification chain, you should match impedances. IF you have gain to burn, however, you might deliberately mismatch impedances for some sort of effect (like making a voltmeter input higher impedance than the circuits it measures, or making an ammeter input much lower impedance than the circuits it measures). Try a few numbers; if a 1000 Ohm output carries 1 Volt of signal, the voltage level is 1V(no load), or 0.5V (1000 Ohm load), or 0.001V (into a 1 Ohm load). So the load power is 0 (into no load...) or 0.25 mW into 1000 Ohms, or 0.001 mW into 1 Ohm. In a very real sense, the power transferred is always competing with some noise power; higher power transfer minimizes the degradation of the signal due to noise. Lastly, some common elements of a circuit (the long wires) have odd frequency-dependent properties UNLESS a particular impedance drives (and receives) the signals. Usually RF cables are connector- coded so that the wrong impedance wiring literally cannot be connected where it doesn't belong. John Whitmore
lrk@k5qwb.UUCP (Lyn R. Kennedy) (03/30/91)
robf@mcs213k.cs.umr.edu (Rob Fugina) writes: > I'm confused as to the importance of impedance matching... Since I may > not know the whole scope of this subject, I'll say I'm referring to things > like stereo line-level, headphone, microphone, antenna inputs and outputs. > I can see that if you have a high impedance source, you need to have a high > impedence load or input so as not to load down the source. So what is the > need, when you have a low impedence source, to have a low impedence load? > > Me If you don't need to get all the power from the source into the load there is no problem. --------- lrk@k5qwb.UUCP lrk%k5qwb@kf5iw.UUCP 73, utacfd.utarl.edu!letni!rwsys!kf5iw!k5qwb!lrk Lyn Kennedy K5QWB @ N5LDD.#NTX.TX.US P.O. Box 5133, Ovilla, TX, USA 75154 -------- "We have met the enemy and they are us." Pogo -----------------
joeld@hpnmdla.hp.com (Joel Dunsmore) (04/03/91)
In sci.electronics, lrk@k5qwb.UUCP (Lyn R. Kennedy) writes: robf@mcs213k.cs.umr.edu (Rob Fugina) writes: >> I'm confused as to the importance of impedance matching... Since I may . . . >> need, when you have a low impedence source, to have a low impedence load? >> Me > If you don't need to get all the power from the source into the load > there is no problem. One tiny problem, (ususally not important in low power, but maybe in dorm blasters, or ham 1kW amplifiers, and other high power devices) That is, if the output match is a hi-Z, the voltage across the last stage can be 2x the rated, and blow the last stage (RF voltage, not DC). This can be a problem, and some fixes include reverse power protection circuits...but, as I said, is only of major concern if you're talking high power. (or low power 20 GHz GaAs FETs with only 9 volts VDS breakdown :-( ). The effects of mismatch can actually damage cables, if they are rated at right where they are being used, and the match goes bad (some enemy fighter shoots off your antenna), the VSWR (Voltage standing wave ratio) can cause your matched voltage to exceed the dielectric strength of your coax (really!). Joeld