Avionics News June 2019 - 49

FIGURE 2: Various ways to measure a sinusoidal waveform

Another way to produce AC is with an oscillator,
which takes DC and creates AC electronically. Oscillators
can create AC at higher frequencies than alternators or
generators. In fact, oscillators can produce frequencies as
high as 100 GHz (100,000,000,000 Hz). Frequencies up
to 20 kHz are called audio frequencies, and the noises,
voices, and music we hear are made with frequencies
between 20 Hz and 20 kHz. Frequencies above 20 kHz are
considered radio frequencies.
In Figure 2, we can see there is a time associated with each
cycle, called period (t). Time is the reciprocal of frequency.
If one knows either period or frequency of the wave form, he
or she can calculate the other (1/f=t and 1/t=f) (1/60=0.0166
seconds). In other words, the period of household current
is 0.0166 seconds, or 16.66 mS. In contrast, the period of
aircraft AC systems is 1/400, or 2.5 mS. As you may have
guessed, as frequency increases, time decreases.
The point at which the voltage reaches its maximum
positive value is referred to as the positive peak voltage.
Later, the voltage reaches its maximum negative value,
known as the negative peak voltage. The total change in
voltage from positive to negative is referred to as the peak
to peak voltage. The voltmeter in your tool kit or garage
measures none of these. Instead, it measures volts RMS.
RMS stands for root mean squared. In order to avoid filling
up the rest of my allotted space with math, I'll define RMS
voltage simply as the voltage that has the same effect as
DC voltage. In other words, 12 VAC and 12 VDC do the
same amount of work. Typically, when you see VAC, it's
actually VAC RMS. Fortunately, if you know the RMS

voltage, you can calculate peak or peak to
peak values easily. Peak is 1.414*Vrms. Peak
to peak, as Figure 2 shows, is twice peak. So,
the 120 VAC outlet in your home is peaking
at positive and negative 169.68 V. The full
peak to peak value is 339.36 V. Those are big
numbers, but the amount of work the wave
can do is based on 120 V, the RMS value of
the wave form.
Another measurement of AC is shown
in Figure 3. A sine wave can be divided
into 360 degrees. Figure 3 shows a 2
Hz waveform divided into degrees. The
waveform reaches peak value at 1/8 of a
second, at 90 degrees. The zero-crossing
point occurs at 180 degrees, the negative peak at 270
degrees, and finally back to 0 V at 360 degrees, which
is the zero degree point for the next cycle. Measuring
waves in degrees allows waves to be compared with
other waves. For example, the electricity supplied by the
power company to your neighborhood comes on three
wires, in three phases, each 120 degrees different. If you
live in a typical house, you are using two of those phases
to run high-energy items like an oven or a clothes dryer.
The 120 VAC outlets are using a single phase. Figure 4
(page 50) shows two waveforms, 120 degrees apart. The
green waveform is 120 degrees later, or lagging, the red
wave. Understanding how waves can be compared using
degrees becomes helpful when learning about the behavior
of inductors and capacitors in AC circuits. Furthermore,
this understanding is important for technicians servicing
Continued on following page

FIGURE 3: A sinusoidal pattern can be divided into degrees.
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Avionics News June 2019

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