Figure 6. Highfrequency response to a rectangular wave
c. Effects of harmonics on pulse rise and decay times.
(1) A rectangular pulse with finite rise and decay times is
shown in Figure 6A. Practical circuits modify the shape of this
pulse. When a circuit has good, highfrequency response, the corners
of the pulse are rounded only slightly (Figure 6B), and the pulse
rise and decay times are not too greatly modified. When the circuit
has poor, highfrequency response (Figure 6C), the rise and decay
times of the pulse increase greatly and an undesirable rounding of
the corners occurs.
(2) In the composition of a square wave (Figure 2), the rise
and decay times of the resultant waves became shorter as the higher
order of harmonics were added. If a square wave is applied to a
circuit, the highfrequency response of the circuit determines the
shape of the output pulse during the rise and decay times. If the
circuit has good, highfrequency response, good reproduction is
developed at the output. If the circuit has poor, highfrequency
response, the higher order of harmonics are not reproduced, and the
rise and decay times are lengthened.
(3) The highest frequency that must be passed by a circuit can
be determined by using the formula fh = 1/2tr, where fh represents
the high frequency response or upper bandwidth limit of a circuit;
Assuming the rise time to be 0.5 usec, the highfrequency response of
the circuit must be fh = l/2tr = 1/2 (.5 x 106) = 1 x 106 or 1 MHz
(megahertz). The upper limit of
10
Previous Page
