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554 ⏐⏐⏐ SINUSOIDAL ALTERNATING WAVEFORMS
A student accustomed to watching TV might be confused when
first introduced to an oscilloscope. There is, at least initially, an
assumption that the oscilloscope is generating the waveform on the
screen—much like a TV broadcast. However, it is important to
clearly understand that
an oscilloscope displays only those signals generated elsewhere and
connected to the input terminals of the oscilloscope. The absence of
an external signal will simply result in a horizontal line on the screen
of the scope.
On most modern-day oscilloscopes, there is a switch or knob with
the choice DC/GND/AC, as shown in Fig. 13.70(a), that is often
ignored or treated too lightly in the early stages of scope utilization.
The effect of each position is fundamentally as shown in Fig. 13.70(b).
In the DC mode the dc and ac components of the input signal can pass
directly to the display. In the AC position the dc input is blocked by the
capacitor, but the ac portion of the signal can pass through to the
screen. In the GND position the input signal is prevented from reaching
the scope display by a direct ground connection, which reduces the
scope display to a single horizontal line.
(a)
AC
GND
DC
Oscilloscope
display
(b)
AC
GND
DC
FIG. 13.70
AC-GND-DC switch for the vertical channel of an oscilloscope.
13.9 APPLICATIONS
(120 V at 60 Hz) versus (220 V at 50 Hz)
Input signal
In North and South America the most common available ac supply is
120 V at 60 Hz, while in Europe and the Eastern countries it is 220 V
at 50 Hz. The choices of rms value and frequency were obviously made
carefully because they have such an important impact on the design and
operation of so many systems.
The fact that the frequency difference is only 10 Hz reveals that
there was agreement on the general frequency range that should be used
for power generation and distribution. History suggests that the question
of frequency selection was originally focused on that frequency
that would not exhibit flicker in the incandescent lamps available in
those days. Technically, however, there really wouldn’t be a noticeable
difference between 50 and 60 cycles per second based on this criterion.
Another important factor in the early design stages was the effect of frequency
on the size of transformers, which play a major role in power
generation and distribution. Working through the fundamental equations
for transformer design, you will find that the size of a transformer
is inversely proportional to frequency. The result is that transformers
operating at 50 Hz must be larger (on a purely mathematical basis about
17% larger) than those operating at 60 Hz. You will therefore find that
transformers designed for the international market where they can oper-