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Unformatted text preview: s travel path. In addition, there is also the
delay in amplifying the signal at the detector diode. Since it is difficult to figure this into the calculation,
we can effectively eliminate the cable and circuitryrelated delay simply by taking two different readings of
position (and the corresponding time reading on the oscilloscope), since both readings have the same delay
in common: c= x 2− x 1
t 2− t 1 It is also possible to determine the speed of light through a medium other than air by placing that material
between the laser and detector. Note that the speed of light in other media varies from c =3 x 108 m / s
based on its index of refraction n: n= c
v In air, n = 1.0003.
Impedance
As mentioned previously, impedance (represented by Z )is the AC equivalent of resistance. AC circuits
differ from DC circuits in that their behavior is dependent on the frequency of oscillation (for DC circuits
the frequency =0 ). The equivalent of Ohm's law is now: V = IZ , where Z is the impedance of an AC circuit. For the RLC circuit (and only for an RLC circuit), the impedance is: Z = R 2 X L − X C 2
R is the usual resistance of the resistor, X L = L is the inductive reactance of an inductor
1
and X C =
is the capacitive reactance of the capacitor. As the inductive reactance and capacitive
C
where reactance are clealy frequency dependent, so is the overall impedance of the RLC circuit. If you examine
the inductive reactance more closely, you'll notice that it increases as the frequency increases; it is for this
reason that inductors are employed to attenuate (reduce the amplitude of) the highfrequency response in so
called lowpass circuits. Conversely, the capacitive reactance increases as the frequency decreases;
capacitors are therefore used to attenuate the lowfrequency response in so called highpass circuits.
Every transmission line (a coaxial cable is one example) has a characteristic impedance which is defined as
the ratio of the amplitudes of a voltage and current signal running through it; it only depends on the cable's
geometry and its material co...
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This document was uploaded on 12/27/2012.
 Summer '09
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